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A study of okaite and associated rocks near Oka, Quebec Davidson, Anthony 1963

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A  STUDY  OF  OKAITB  AID  ASSOCIATED  ROCKS  NEAR  OKA,  QUEBEC  by/  ANTHONY B . S c ,  A  The  Thesis of  the  DAVIDSON  University  of  Submitted  i n  B r i t i s h  P a r t i a l  Requirements MASTER In  the  Columbia,  for  OF  1959  Fulfilment:,  the  Degree  of  SCIENCE  Department:  of  GEOLOGY  We  accept; t h i s  required  THE  thesis  as  conforming  to  standard  UNIVERSITY  OF  BRITISH  February/,  1963  COLUMBIA  the  In presenting  t h i s thesis i n p a r t i a l f u l f i l m e n t o f  the requirements f o r an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y available f o r reference and study.  I further agree that permission  for extensive copying of t h i s thesis f o r scholarly purposes may be granted by the Head of my Department or by his  representatives.  It i s understood that copying or publication of t h i s thesis f o r f i n a n c i a l gain s h a l l not be allowed without my written permission.  (Anthony Davidson)  Department of  GEOLOGY  The University of B r i t i s h Columbia, Vancouver 8,^ Canada. Date  March 1963  ABSTRACT  T h e a l k a l i n e  f r a c t u r e s . s i t u a t e d  o f  a l o n g  c o m p l e x .  i s  m i n e r a l o g y  i t e  and  s u c h t o  I t  as  i n  i s  and  t h e  O k a i t e s e c t i o n  X - r a y s ,  i s  t h e  a n d  m a i n  h a u y n e ,  a n d  t h e  r o c k  g r a n i t i c  t e x t u r e .  r i m s  e x s o l u t i o n  i n t o  r o c k  t h e  o k a i t e ,  t h e  were  s i m i l a r  i l i t e  c a n  t y p e s  s o d i u m  m e l i l i t e t h e s e  R o c k  be  i n  t y p e  O p t i c a l  r o c k s  b a s i s  m i n e r a l , and  o f  r o c k ,  a r e  O k a  t o  I t  j a c u p i r a n g -  t o  s t u d i e d  m a i n l y  by  a r e  what  h a s  were  r e p r e s e n t e d m e l i l i t e , a n d I n  were  c o m p l e x , be  r e l a t e d  s u c h  g e h l e n i t e ,  a n d  i n  r o c k s  c o m p o s i t i o n  a p p r o x i m a t e were  r a t i o  I t  o t h e r 35  m e l i l i t e  *  d e t e r m i n e d ;  s e r i e s .  f r o m  o f  r o c k s o f  m e l i l i t e  n a t u r a l  m e l -  m o l e c u l a r i s  f o u n d  l o c a l i t i e s 10  T h e s e  p r o p e r t i e s  o f  t h r e e  a k e r m a n i t e . f r o m  d i v i d e d  o k a i t e  t h o s e  a  i g n e o u s  c a r b o n a t i t e .  O p t i c a l  t h e  o f  o k a i t e ,  m i n e r a l s  w i t h  have  r e a c t i o n  a r e  opaque  p r o p o r t i o n . o f  t i t a n a u g i t e  r o c k s  t h e  c h e m i c a l  s i m i l a r  e v i d e n c e  o k a i t e ,  d e t a i l .  a c c e s s o r y  m a g m a t i c  t h e  c o m p a r e d  t h e  t h e  M e l i l i t e  r o c k s  c a l l e d  a n d  a  as  p a r t i  t i . t a n a u g i t e ,  T h e  b e e n  i n  b y  o f  F r e s h  c a l c - s i l i c a t e  s t u d i e d  The  amounts  c o n c l u s i v e  a n d  I n  e s t a b l i s h e d .  a p a t i t e .  p y r o x e n e  t h i n  i d e n t i f i e d ,  p r e s e n t .  r e l a t i o n s ,  h e r e  a r e  were  a r e  a n d  a n d  t i t a n a u g i . t e ,  o f  t h e  r e l a t e d  a p p e a r  s t r u c t u r e s  m i n e r a l  o c c u r r e n c e s .  p r o p e r t i e s  c o m p l e x .  o f  n o t  v a r y i n g  c a l c i t e  j a c u p i r a n g i t e ,  m o l e c u l e s  h a l f  t h e s e  h a v e  t h e  t h e  t h e m  a n d  p e r o v s k i t e ,  d e t e r m i n e d ,  o k a i t e  w i t h  w i t h i n  m i n e r a l s  b e t w e e n  i n t e r g r o w t h s  s e r i e s  a r e  r o c k s  c l o s e l y  r o c k s do.  were  f o r m i n g  M i c r o s c o p i c  c o n s t i t u t e  o k a i t e  m e l i l i t e f r o m  r e l a t e d  b i o t i t e ,  M e l i l i t e , t h e  O t h e r  a r c u a t e  a n t i c l i n a l  c o n t a c t -  among  t h e  t h e s e  g r a n i t i c  a r e  by:  a l o n g  n o r t h - w e s t  t h a t  m e l t e i g i t e ,  r e l a t i o n s  t y p e s  t h e  r o c k s  r o c k s  t r e n d i n g  o f  o c c u r r e n c e ;  and  m e l i l i t e - c a l c i t e  t y p e s  w i t h i n  f o r m e d  c o m p l e x e s ;  a n d  f e e t  l o n t e r e g i a n  way.  m a g n e t i t e ,  On  o f  r i n g  g n e i s s i c  h u n d r e d  t h e  i n t r u s i o n  n o r t h - w e s t  w i t h  o f  c a r b o n a t i t e  c a r b o n a t i t e .  f o r m i n g  a r e  o r i g i n .  and  o n l y  mode  same  w e s t e r l y  c o m p o s i t e  t h e  few  u r t i t e ,  n e p h e l i n e ,  a n d  o f  f o u n d  i n  m i n e r a l s  o f  a  e x a m i n a t i o n .  b y  a  most  o v e r l a p p i n g  a s s o c i a t e d  i j o l i t e ,  t h e  P r e c a m b r i a n  c a l c - s i l i c a t e  o k a i t e  r o c k  two  w i t h i n  O k a i t e  i s  c r e s t  a x i s .  I s  a l k a l i n e  a r e  t h e  f e n i i t i z e d  I t  b a s i c  T h e r e  B e a u h a r n o i s  i n  c o m p l e x  i n t r u s i o n s .  i n t r o d u c t i o n  b e e n  O k a  * 55  t h a t c o n t a i n s  r e s p e c t i v e l y .  c h e m i c a l ,  a n a l y s i s  shows that titanaugict.e from j a c u p i r a n g i t e i s a h i g h l y aluminous pyroxene c o n t a i n i n g more calcium than i s normal. were studied i n p o l i s h e d s e c t i o n s .  The opaque  E x s o l u t i o n intergrowths  minerals  "between  magnetite and a s p i n e l , i d e n t i f i e d as hercynit.e, are present  in all  rocks o f the okaite s e r i e s , and suggest a high temperature o r i g i n . Titanium  minerals  are not exsolved  assumed t o be a low-titanium  from magnetite which i s t h e r e f o r e  variety.  Many of the o k a i t i c rocks are a l t e r e d .  This i s p r i m a r i l y  due t o hydrothermal replacement of m e l i l i t e and hauyne by a v a r i e t y of secondary minerals.  A l t e r a t i o n was accomplished by* hydrothermal  f l u i d s derived from c r y s t a l l i z i n g magma. are described;  Pour types of a l t e r a t i o n  these f o u r assemblages are g r a d a t i o n a l and represent  d i f f e r i n g temperature c o n d i t i o n s . involved i n a l t e r a t i o n .  L i t t l e o r no chemical- changes were  Common secondary minerals  are vesuvianilte,  diopside, c a l c i t e , c e b o l l i t e , z e o l i t e , and garnet. The  okaite i n t r u s i o n i s compared with other occurrences of  m e l i l i t e bearing rocks.  The rocks at Iron H i l l ,  Colorado, at; T u r j a ,  Kola Peninsula,  and near Gwasi, Kenya, are s i m i l a r t o those at Oka  both chemically  and i n mode of occurrence.  There i s a notable  sim-  i l a r i t y between the rocks of the o k a i t e s e r i e s and the b a s i c a l k a l i n e rocks that are known t o have been formed by limestone syntexis at Scawt H i l l , Northern I r e l a n d . Consideration  of the s t a b i l i t y of m e l i l i t e leads t o the  conclusion that the okait.e s e r i e s magma c r y s t a l l i z e d at temperatures between 700°C. and 800°C. at a depth of l e s s than two k i l o m e t e r s . Temperatures at which a l t e r a t i o n took place v a r i e d from above 600°C. to below 300°C. it  The sequence of i n t r u s i o n i s not a l t o g e t h e r c l e a r ;  appears that s p e c i f i c rock types were not n e c e s s a r i l y associated  i n time. ijolite,  The okaite s e r i e s does not- seem t o be d i r e c t l y r e l a t e d to u r t i t e , and m e l t e i g i t e ; these rocks may represent  that have undergone rheomorphism.  I t i s postulated that the okaite  s e r i e s was derived from a gabbro o r an essexite magma that limestone, carbonate. was  first  by r e a c t i o n with,  fenites  and then by s o l u t i o n of,  assimilated, calcium  I t i s p o s s i b l e that the ultimate e f f e c t of t h i s process  t o produce a carbonatiite magma.  iv  TABLE OP CONTENTS Page 1  INTRODUCTION Scope o f I n v e s t i g a t i o n  1  Location and Topography  2  Acknowledgements  4  GEOLOGY OP THE OKA HILLS Regional Geology  .  5 5  ..  Precambrian Rocks  5  P a l e o z o i c Rocks  7  The Oka Marbles  8  Monteregian A l k a l i n e I n t r u s i v e s  9 10  General Structure THE GRENVILLE LIMESTONE - CARBONATITE CONTROVERSY  12  Evidence Discussion  12  '  THE FIELD OCCURRENCE OF OKAITE AND ASSOCIATED ROCKS  13 16  Areal Extent  16  The Nature and Appearance o f Outcrops  16  Determination of M i n e r a l s i n Hand Specimen  18  Melilite  18  Nepheline  20  Hauyne  20  Pyroxene  20  Biotite  20  Calcite  21  Apatite  21  Magnetite  21  Perovskite  21  Pyrrhotite  22  A l t e r a t i o n o f Okaite i n Hand Specimen  22  Rock Types Cropping Out on Husereau H i l l  23  The Occurrence of Okaite South and South-East- o f Husereau H i l l  24  Other Rock Types Associated with Okaite  25  Structure  26  Page PETROGRAPHY OF THE OKAITE IM'RUSIONS Method of Study  ;  28  .  28  C l a s s i f i c a t i o n - o f Okaite and Belated Rocks  28  Mineral C h a r a c t e r i s t i c s i n Thin Section  30  Melilite  31  Biotite  31  Calcite  34  Apatite  34  Magnetite  34  Perovskite  34  Titanaugite  •  36>  Hauyne  36  Nepheline  38  Spinel  38  Montieellite  38  Olivine  39  Augite  39  Hornblende  39 40  Microscopic Textures Pyroxene Okaite Okaite  4° ....  M e l i l i t e Okaite and Pegmatite Phases  43 43  Primary C a l c i t e  47  Sequence o f C r y s t a l l i z a t i o n and Development of the Okaite Series  47  Monticellite Alnoite  53  DETAILED MINEBALOGIC STUDIES  57  Methods  57  Melilite  57  Titanaugite  69  The  75  Opaque Minerals  THE ALTERATION OP ROCKS OP THE OKAITE SERIES  87  The Nature of A l t e r a t i o n  87  The  A l t e r a t i o n of M e l i l i t e  88  The  A l t e r a t i o n o f Hauyne  97  The  A l t e r a t i o n of Other Rock Forming Minerals  103  VI  Page THE ALTERATION OP ROCKS.OP THE OKAITE SERIES - continued The A l t e r a t i o n of. l e l i l i t e - C a l c i t e Rock  106  Sequence of A l t e r a t i o n  108  Chemical Changes i n A l t e r a t i o n  110  ...  112.  COMPARISON OP OKAITE AND SIMILAR ROCKS Introduction  112  Iron H i l l ,  112  Colorado  113  Kusnavolok, T u r j a , K o l a P e n i n s u l a Gwasi, Kenya  114  •  Similarities  114  Comparison of Chemical Compositions  115  M e l i l i t e Rocks at Scawt H i l l , Northern I r e l a n d  11-7  Chemical Nature o f M o n t i c e l l i t e A l n o i t e at Husereau H i l l PETROGENESIS  .  121 124  The Okaite S e r i e s  124  Considerations o f Temperature and Pressure  125  Sequence of I n t r u s i o n  130  R e l a t i o n of the Okaite S e r i e s to Other Rocks of the Oka Complex  131  Hypothetical- O r i g i n o f the Oka Complex  133  SUMMARY  138  CONCLUSIONS  141  SELECTED BIBLIOGRAPHY  143  ADDENDUM  142a  vii.  LIST OF ILLUSTRATIONS Figure  Page  1  L o c a t i o n map  3  2  Photograph of hand specimen of m e l i l i t e okaite pegmatite  3  Photograph of okaite hand specimens showing g r a n i t i c 17  texture 4  Photograph of a l t e r e d okaite hand specimen  5  Photograph of hand specimen showing m e l i l i t e  17  19 crystals 19  exposed by weathering 6:  Photomicrograph of m e l i l i t e c r y s t a l s enclosed i n c a l c i t e  7  Photomicrograph showing p o i k i l i t i c texture of m e l i l i t e enclosing a p a t i t e Photomicrograph of vermicular magnetite on m e l i l i t e cleavage planes Photomicrograph of r a d i a t i n g a p a t i t e c r y s t a l s i n : m e l i l i t e o k a i t e pegmatite  33  10  Photomicrograph of l a m e l l a r twinning i n p e r o v s k i t e  35  11.  Photomicrograph of o s c i l l a t o r y zoning i n t i t a n a u g i t e  12  Photomicrograph of hauyne i n pyroxene okaite  13  Photomicrograph of opaque i n c l u s i o n g r i d In hauyne  14  Photomicrograph of euhedral t i t a n a u g i t e In m e l i l i t e  15  Photomicrograph of d e t a i l i n resorption, corona around t i t a n a u g i t e i n pyroxene okaite  41  Photomicrograph of p a r t l y resorbed t i t a n a u g i t e c r y s t a l i n pyroxene okaite  42  8 9  16 17  32 32  33  ...  35 37  .....  37  ....  41  Photomicrograph of 'ghost* of completely resorbed 42  t i t a n a u g i t e c r y s t a l i n okaite 18  Photomicrograph of subhedral m e l i l i t e i n okaite  44  19  Photomicrograph of subhedral hauyne i n okaite  44  20  Photomicrograph of hauyne rims around nepheline i n coatact to m e l i l i t e , plane l i g h t Photomicrograph of hauyne rims around nepheline i n contact to m e l i l i t e , crossed N i c o l s  21 22  45 45  Photomicrograph showing two generations of a p a t i t e 46  i n m e l i l i t e okaite pegmatite 23  Photomicrograph of m e l i l i t e - m o n t i c e l l i t e intergrowth  24  Photomicrograph showing d e t a i l i n m e l i l i t e - m o n t i c e l l i t e intergrowth  48  Photomicrograph of a l t e r e d m e l i l i t e c r y s t a l s i n c a l c i t e  49  25  ...  48  Figure 26  Page Photomicrograph of hauyne c r y s t a l p a r t l y enclosed i n 51  titanaugite 27  Photomicrograph of b i o t i t e rim around magnetite  28  Sequence of c r y s t a l l i z a t i o n f o r rocks of the o k a i t e series Photomicrograph of r e a c t i o n rim around t i t a n a u g i t e in monticellite alnoite  54  Photomicrograph of o l i v i n e and t i t a n a u g i t e i n monticellite alnoite  55  29 30 31  51 52  Photomicrograph showing fragmental nature of 55  monticellite alnoite 32  X-ray powder photograph of m e l i l i t e from m e l i l i t e okaite  33  Graphs showing d-spacing v a r i a t i o n s i n akermanitegehlenite s o l i d solutions T r i a n g u l a r diagram showing v a r i a t i o n of r e f r a c t i v e i n d i c e s and b i r e f r i n g e n c e i n the sodium m e l i l i t e gehlenite-akermanite s e r i e s  34  35  O p t i c o r i e n t a t i o n of t i t a n a u g i t e from pyroxene okaite  36  X-ray powder photograph of t i t a n a u g i t e from pyroxene okaite  38  Photomicrograph showing exsolved h e r c y n i t e lamellae i n magnetite Photomicrograph showing exsolved h e r c y n i t e blebs and lamellae i n magnetite  41  68 .  ...  Photomicrograph of a p a t i t e i n c l u s i o n with p e r o v s k i t e rim i n magnetite with exsolved h e r c y n i t e lamellae  76 79 79  ..  81  Photomicrograph showing replacement of h e r c y n i t e by 81  magnetite 42  Photomicrograph of magnetite exsolved from h e r c y n i t e  43  Photomicrograph showing a l t e r a t i o n of magnetite to maghemite or hematite Photomicrograph of type I a l t e r a t i o n of m e l i l i t e to  44  7® 74  X-ray powder photographs of hercynite and magnetite  40  62  •  37  39  61  ..  83 83 90  mineral l a 45  Photomicrograph of a l t e r e d m e l i l i t e c r y s t a l i n c a l c i t e  46  Photomicrograph showing cebollii.e pseudomorphous a f t e r  90  91  melilite crystals i n calcite • 47  Photomicrograph of c e b o l l i t e from Oka  92  48  Photomicrograph of c e b o l l i t e from Iron H i l l ,  49  Comparison of X-ray powder photographs of pure v e s u v i a n i t e and mixed v e s u v i a n i t e and c e b o l l i t e from Oka and from Iron H i l l , Colorado  Colorado  .  92  S$J  ix  Figure  Page  50  Photomicrograph of type I I I a l t e r a t i o n o f m e l i l i t e  ... 96  51  Photomicrograph of type I I I a l t e r a t i o n of m e l i l i t e  ... 98  52  Photomicrograph of s.pherulitic a l t e r a t i o n of m e l i l i t e  98  53  Photomicrograph showing replacement of m e l i l i t e by andradite .  99  54  Photomicrograph of a l t e r e d hauyne i n pyroxene okaite  . 101  55  Photomicrograph o f a l t e r e d hauyne i n pyroxene o k a i t e  . 101  56  Photomicrograph o f a l t e r e d hauyne i n okaite  57  Photomicrograph of okaite a l t e r e d along f r a c t u r e s  .... 104  58  Photomicrograph showing andradite a s s o c i a t e d with ragged magnetite i n a l t e r e d okaite  104  Diagram t o show the change i n formation of m e l i l i t e a l t e r a t i o n minerals with temperature  109  Diagram t o shov; the modal v a r i a t i o n w i t h i n the okaite s e r i e s , based on p l o t t e d modes  126  T r i a n g u l a r composition diagram  137  59 60 61  102  Plate I  The geology o f the Oka area, Quebec  i n pocket  II  D e t a i l e d geology of the okaite occurrence near Oka, Quebec  i n pocket  X  LIST OF TABLES Table  Page C l a s s i f i c a t i o n of rock types  29  II  X-ray powder data f o r m e l i l i t e from Oka  61  III  Comparison of r e f r a c t i v e i n d i c e s f o r various m e l i l i t e s  63  IV  Chemical  64  I  V VI VII  analyses of n a t u r a l m e l i l i t e s  T h e o r e t i c a l chemical contents of various m e l i l i t e compounds  65  D e r i v a t i o n of the type molecule r a t i o of m e l i l i t e from Oka  65  V a r i a t i o n i n e x t i n c t i o n angle and pleochroism f o r 69  titanaugite VIII  Comparison of o p t i c a l p r o p e r t i e s of various pyroxenes  IX  Chemical a n a l y s i s and d e r i v a t i o n of the calciummagnesium-iron r a t i o and percentage substitutionof  X  XIII  aluminum f o r s i l i c o n of t i t a n a u g i t e from Oka  X-ray powder data f o r h e r c y n i t e and magnetite '  ........  Types of a l t e r a t i o n of m e l i l i t e Comparison of X-ray powder data f o r v e s u v i a n i t e and  94 95  X-ray powder data f o r c e b o l l i t e  XV  C o r r e l a t i o n of the types of m e l i l i t e and hauyne alterations Approximate chemical formulae of m e l i l i t e and I t s a l t e r a t i o n minerals Comparison of the average modes of okaite and s i m i l a r rocks  XVIII XIX XX XXI  77 88  XIV  XVII  73 74  mixtures of v e s u v i a n i t e and c e b o l l i t e  XVI  ...  X-ray powder data f o r t i t a n a u g i t e and diopside  XI XII  71  99 Ill 113  Comparison of chemical analyses of o k a i t e and s i m i l a r rocks  116  Comparison of chemical analyses of m e l i l i t e rocks from Scawt H i l l and Oka  119  Comparison of chemical analyses of m e l i l i t e t i t a n a u g i t e from Scawt H i l l and Oka  120  and  Comparison of chemical analyses of m o n t i c e l l i t e from Husereau H i l l , Oka, and a l n o i t e from Alnti I s l a n d , Sweden  122  1  INTRODUCTION  Scope of I n v e s t i g a t i o n . Mining a c t i v i t y has followed the recent; d i s c o v e r y of niobium and r a r e - e a r t h minerals near Oka,  Quebec.  MineralIzatiom  lis confined to rocks of the Oka complex, the geology of which has t h e r e f o r e r e c e i v e d renewed Interest..  The complex c o n s i s t s of c a l c -  s i l i c a t e and carbonate rocks that are i n t i m a t e l y a s s o c i a t e d with b a s i c a l k a l i n e i n t r u s i v e rocks.  The l a t t e r i n c l u d e the o k a i t e  i n t r u s i o n s that form l a r g e d i k e - l i k e bodies at the north-west of the area u n d e r l a i n by the Oka  end  complex.  Dr. J.A. Gower, of Kennco E x p l o r a t i o n s (Western) L t d . , In 1955  suggested the Oka complex as a t h e s i s t o p i c . considerable time mapping In the f i e l d at Oka, two hundred specimens.  he spent a  and c o l l e c t e d some 1961,  The author spent a week there im May  and i n that time c o l l e c t e d eighty a d d i t i o n a l specimens while becomingf a m i l i a r with the general f i e l d r e l a t i o n s .  Because rock outcrop; i s  r e l a t i v e l y sparse i n the area u n d e r l a i n by the complex, and because the main o k a i t e body a f f o r d s more complete f i e l d evidence than  any  other part: of the complex, the present, work has been r e s t r i c t e d i n i d e t a i l to a study of okai.t.e and rocks a l l i e d to i t Im l o c a t i o n and. composition.  At l e a s t one h a l f of the specimens c o l l e c t e d from the  complex are from the area where o k a i t e and r e l a t e d rocks crop out. T h i s study deals mainly with the d e t a i l e d mineralogy  and  petrography of both the f r e s h and a l t e r e d rocks of the o k a i t e i n trusions.  Much of the work has been done by the examination  t h i n s e c t i o n s ; s e v e n t y - f i v e s e c t i o n s were prepared by the  of  author,  r e p r e s e n t a t i v e rock specimens being chosen f o r t h i s purpose where possible.  X-ray powder d i f f r a c t i o n photographs were used, f o r the  i d e n t i f i c a t i o n of minerals, and, where a p p l i c a b l e , f o r the d e t e r mination of composition.  The i n t r u s i o n of o k a i t e i s compared to  other occurrences of a similar, nature, and a method of petrogenesis Is  suggested.  2  L o c a t i o n and Topography Twenty-five miles west of Montreal i s an area of high ground known as the Oka H i l l s .  These h i l l s r i s e q u i t e sharply above  the surrounding p l a i n s of the St. Lawrence Lowlands.  Much of the  t h i r t y - f i v e square miles of h i l l y country i s p l e a s a n t l y wooded. Farming f o r f r u i t  and d a i r y products i s c a r r i e d out i n the v a l l e y s ,  and the area i s served by numerous roads.  The v i l l a g e of Oka i s  located to the south of the h i l l s on the north bank of Lac  des  Deux Montagnes. The Oka H i l l s may  be roughly d i v i d e d i n t o f o u r areas of  high ground, s i t u a t e d north, east, south, and west of the centre of the h i l l y area.  A broad, low v a l l e y cuts o f f the northern area from  the r e s t , and i s drained by the R i v i e r e du Chene which i n i t i a l l y flows westwards before curving round the northern h i l l s to j o i n the R i v i e r e des l i l i e s l i e s at St. Eustache, north of Montreal.  Another  stream r i s e s between the western and southern h i l l areas, f l o w i n g eastwards before t u r n i n g south between the eastern and southern h i l l s ; it  flows past La Trappe Monastery and empties i n t o Lac des Deux  Montagnes. Between these two Hills,  streams, and at the centre of the  Oka  there i s a s m a l l , steep h i l l , standing about 250 feet above  the surrounding country.  T h i s h i l l a f f o r d s the type l o c a l i t y f o r  okaite; i t l i e s on the farm of M. Husereau, and f o r t h i s reason been r e f e r r e d to as Husereau H i l l .  The low area between the  and eastern h i l l s i s u n d e r l a i n by rocks of the Oka  has  southern  complex f o r the  f o u r miles between Husereau H i l l and Lac des Deux Montagnes south of La Trappe Monastery, a b e l t about one and a h a l f miles wide. Topography i n the area has been considerably modified by Pleistocene g l a c i a t i o n .  Areas of high ground, reaching e l e v a t i o n s  between 500 and 800 f e e t , are u n d e r l a i n by hard Pre-Cambrian gneiss and anorthosite; they are scoured f r e e of s o i l , and o f f e r f a i r l y p l e n t i f u l rock exposures. Rocks of the Oka  The v a l l e y s are mostly f i l l e d with g l a c i a l  drift.  complex, being r e l a t i v e l y s o f t , occupy such a v a l l e y ,  and the major part of the complex i s thus unexposed. P a l e o z o i c sedimentary  S o f t e r Lower  rocks, which u n d e r l i e the surrounding  lowlands,  Figure  l i  The  location  of  the O k a a r e a  with  rsrncet  and t h e i n t e r n a t i o n a l border. Vap A i s g i v e r , o n p l a t e on p l a t e IT  to  Montreal  T; map B i a  crop= out;, or are present at= shallow depth, only, on the south s i d e of the Oka H i l l s .  G l a c i a l d r i f t i s e s p e c i a l l y t h i c k : i n the area  surrounding the northern s e c t i o n of the h i l l s .  Acknowledgements The author i s indebted to Dr.- J.A. Gower, of Kennco Explorations (Western) L t d . , who t h e s i s t o p i c , and who from the area.  suggested the Oka complex as a  made a v a i l a b l e a l a r g e c o l l e c t i o n of samples  A p p r e c i a t i o n i s also expressed to Mr.  and Mr. G i l l e s Joncas, of Quebec Columbiumi Ltd..,. who  S.B.  gave a great;,  deal of t h e i r time to the author during h i s v i s i t to Oka; Professors R.M.  Thompson and K.C.  B r i t i s h Columbia, who  Bond  to  McTaggart, of the U n i v e r s i t y of  o f f e r e d t h e i r help and encouragement during  the l a b o r a t o r y study and i n the p r e p a r a t i o n of the  manuscript;  to Mr. J.A.. Donnan under whose able s u p e r v i s i o n the t h i n and p o l i s h e d s e c t i o n s were made. Special, thanks are due to Mr. D.P. U n i v e r s i t y , who  has r e c e n t l y made a comprehensive general study  of the Oka complex. geologic map his  Gold, of M c G i l l  He very k i n d l y allowed the author to use h i s  of the Oka complex, and a l s o made a v a i l a b l e some of  chemical analyses of rocks and minerals.  GEOLOGY OP THE OKA HILLS Regional Geology F i f t e e n miles north of Oka l i e s the southern edge of the Canadian S h i e l d , which here trends n o r t h - e a s t e r l y .  South of t h i s  pronounced contact are the e s s e n t i a l l y f l a t - l y i n g Lower P a l e o z o i c sedimentary rocks o f the St. Lawrence Lowlands.  Intrusive into  these sediments are stocks, plugs, dikes, s i l l s ,  and diatremes o f  the Monteregian a l k a l i n e igneous s e r i e s .  The Oka H i l l s  represent  an i n l i e r o f Precambrian rocks, surrounded by P a l e o z o i c sediments, and intruded by Monteregian rocks. general geology  P l a t e I ( i n pocket) shows the  of the Oka H i l l s .  Precambrian Rocks A l l f o u r groups of h i l l s north of Oka are u n d e r l a i n by Precambrian rocks, most of which are g n e i s s i c o r g r a n i t i c i n texture. Banding o r f o l i a t i o n i s pronounced i n many areas. such rocks i s predominantly  The s t r i k e of  north-east, with steep dips v a r y i n g  about the v e r t i c a l s t h i s t r e n d i s maintained  throughout the area  with only minor l o c a l v a r i a t i o n .  Between the south and east  i s a complex mixture  and c a l c - s i l i c a t e rocks, i n t i m a t e l y  of carbonate  associated with b a s i c and a l k a l i c  hills  intrusives.  The S h i e l d area, some f i f t e e n miles to the north o f Oka, i s made up o f G r e n v i l l e limestone, metasediments, and metavolcanics, associated with i n t r u s i v e g r a n i t e , g r a n o d i o r i t e , s y e n i t e , and granite-gneiss.  I n t r u s i v e to these rocks are a n o r t h o s i t e , gabbro,  g r a n i t e , and syenite of the Morin s e r i e s .  Basic dikes cut a l l these  rocks, and are i n t u r n cut by g r a n i t i c rocks of the Chatham-Grenville stock s e r i e s .  The general trend of the G r e n v i l l e rocks and the 1944).  f o l i a t e d i n t r u s i v e s i s n o r t h - e a s t e r l y (Dresser and Denis, In the past, geologic examination  of the Oka i n l i e r has  always produced the o p i n i o n that the carbonate rocks are G r e n v i l l e limestones.  and c a l c - s i l i c a t e  Dresser and Denis (1944) a p the Oka m  o u t l i e r as c r y s t a l l i n e limestone of the G r e n v i l l e s e r i e s , with g r a n i t e , g r a n i t e - g n e i s s , and sedimentary states that "The  gneiss.  Clark (1952, p. 15)  rocks exposed i n c l u d e a v a r i e t y of s o - c a l l e d G r e n v i l l e  metasediments, mostly r e c r y s t a l l i z e d limestones, with which are a s s o c i a t e d paragneisses and q u a r t z i t e s , together with a number of igneous types, among which g r a n i t e s , s y e n i t e s , and quartz d i o r i t e s predominate."  Maurice (1958) i n c l u d e s a l l the rocks, other than  the limestones and the a l k a l i n e i n t r u s i v e s , i n the Morin S e r i e s . His p r e l i m i n a r y map  shows the north and west h i l l s to be u n d e r l a i n  by a n o r t h o s i t e , gabbro, and d i o r i t e .  These are s i m i l a r to the type  Morin i n t r u s i v e rocks i n the S h i e l d area about 30 miles north of  Oka.  The southern and eastern h i l l s , where many outcrops near the Oka complex were examined by Dr. Gower and the author, are u n d e r l a i n mainly by g r a n i t e gneiss, hornblende g n e i s s , garnet g n e i s s , q u a r t z - f e l d s p a r gneiss, and minor q u a r t z i t e and pegmatite.  Maurice  (1958)> although acknowledging the presence of some t r u e gneisses, b e l i e v e s these rocks to be an a c i d phase of the i n t r u s i v e Morin Series. Apart from the carbonate  and c a l c - s i l i c a t e rocks of the  complex between the south and east h i l l s , Maurice maps two areas as G r e n v i l l e limestone.  small  These l i e i n the t o p o g r a p h i c a l l y  low area between the north and west h i l l s , hills,  Oka  and the south and east  c l o s e tp the unexposed contact between the a n o r t h o s i t e and  gabbro and the more a c i d i c g r a n i t e gneiss.  In between these  two  occurrences, and s i t u a t e d at the crossroads just west of Husereau H i l l , there i s a shallow road cut i n a p e r t h i t e - f o r s t e r i t e - c a l c i t e marble.  This rock i s q u i t e u n l i k e any rock found i n the Oka  i n that i t contains an appreciable q u a n t i t y of f e l d s p a r .  complex  These three  areas of limestone l i e i n a l i n e which p a r a l l e l s the t r ^ n d of the neighbouring g n e i s s i c rocks, and they may  w e l l represent a discontinuous  band of metamorphic limestone of G r e n v i l l e type.  I f t h i s supposition  i s c o r r e c t , i t induces the i d e a that adjacent g n e i s s i c rocks, e s p e c i a l l y those of the south and east h i l l s ,  are not a l l Morin  7  I n t u s i v e s , but may  be G r e n v i l l e metasediments and  metavolcanics  that are r a t h e r more h i g h l y metamorphosed than i s t y p i c a l of G r e n v i l l e rocks.  More c o n c l u s i v e evidence f o r t h i s theory would  be provided by the occurrence of diabase dikes c u t t i n g the limestone. Such dikes are p l e n t i f u l i n a l l the g n e i s s i c and g r a n i t i c rocks of the area, and g e n e r a l l y e x h i b i t a s i m i l a r n o r t h - e a s t e r l y trend. A feature of the g n e i s s i c and g r a n i t i c rocks at or near the contact of the Oka  complex i s that a f r e s h surface commonly  shows a b l u i s h or g r e e n i s h c o l o u r a t i o n . Thin s e c t i o n  examination  shows t h i s to be due to the presence of r i e b e c k l t e or aegerine-augite. The l a t t e r mineral occupies small v e i n l e t s and rims quartz g r a i n s . Biebeckite has formed at the edges of b i o t i t e , hornblende,  or pyroxene  grains; i t i s seen bordering o l i v i n e In the marble c l a s s e d G r e n v i l l e limestone i n the road cut just west of Husereaui H i l l . these minerals are developed more than 1,000 contacts with the Oka complex.  N e i t h e r of  f e e t away from the  T h i s a l t e r a t i o n has been c a l l e d  f e n i t i z a t l o n by Br^gger (1.921), and i s a widely recognized contactdevelopment i n a l k a l i n e and c a r b o n a t i t e i n t r u s i o n s . Paleozoic locks The Precambrian surrounded  rocks of the Oka H i l l s form an i n l i e r  by Lower P a l e o z o i c sedimentary  rocks.  Although nowhere  seen i n contact, the lowest member of the St. Lawrence Lowland P a l eozoic sequence, the Upper Cambrian Potsdam snadstone, to crop out.  i s the nearest  I t i s known i n very close proximity to the  Precambrian  rocks from well boreholes, e s p e c i a l l y on the south side of the h i l l s i n the v i c i n i t y of Oka.  I t s c l o s e s t approach to the surface i s 32  feet one h a l f mile west of Oka; feet of s u r f i c i a l d e p o s i t s . to the Potsdam sandstone near La Trappe monastery.  normally i t i s beneath 100 to  Fragments of quartz sandstone  are found i n Monteregian  200  similar:-  explosive b r e c c i a s  The succeeding formation i s the Lower  Ordovician Beekmantown dolomite; i t does not crop out near  Oka.  The Oka Marbles L o c a l l y r e f e r r e d to as marbles, the carbonate and c a l c s i l i c a t e rocks of the Oka complex are c o n s i s t e n t l y a s s o c i a t e d with s u b - s i l i c i c a l k a l i n e i n t r u s i v e rocks.  These i n t r u s i v e s show marked  s i m i l a r i t i e s i n nature and composition to rocks of the Monteregian igneous s e r i e s t o the east. coarse grained.  The marbles themselves  are commonly  They c o n t a i n v a r y i n g amounts of such  silicate  minerals as o l i v i n e , pyroxene, m o n t i c e l l i t e , m e l i l i t e , and b i o t i t e . Melanite garnet and w o l l a s t o n i t e are o c c a s i o n a l c o n s t i t u e n t s . Two e s s e n t i a l minerals are a p a t i t e and magnetite, proportions i n a few rocks.  which occur i n large  A common rock type i s composed predom-  i n a n t l y of c a l c i t e with l e s s than f i v e percent each of a p a t i t e , magnetite,  and b i o t i t e .  Such a rock has the composition of sovi.te,  as described by von Eckermann (I.948»• a l k a l i n e complex i n Sweden.  13) from the Alno I s l a n d  At Oka, the carbonate i s normally  c a l c i t e ; i t i s s l i g h t l y y e l l o w i s h i n colour, and contains a very small percentage  of strontium.  Dolomite occurs i n a few p l a c e s .  Most of the Oka marbles d i s p l a y some form o f d i r e c t i o n a l texture.  F o l i a t i o n may be imparted to the rock by the presence of  biotite.  More common are banding due to t e x t u r a l changes, such as  g r a i n s i z e v a r i a t i o n , and l a y e r i n g due to pronounced compositional changes.  Wherever a t t i t u d e s of these f e a t u r e s are measured, i t i s  found that dips are v e r t i c a l or s t e e p l y i n c l i n e d , and that s t r i k e s commonly p a r a l l e l the nearest contact to g n e i s s i c o r igneous rocks, g i v i n g a crudely arcuate s t r u c t u r e about the c e n t r a l part of the complex.  Although outcrop i s sparse, recent t r e n c h i n g and d r i l l i n g  have added a great deal of i n f o r m a t i o n t o support t h i s i n t e r p r e t ation.  I t i s safe to say that the a t t i t u d e s o f these rocks do not  conform t o the north-east trend of the surrounding g n e i s s i c rocks. Thus i f the surrounding gneisses and the discontinuous limestone horizon are G r e n v i l l e rocks, then the Oka marbles must be explained i n some other way.  Monteregian  Alkaline Intrusives  Of the a s s o c i a t e d igneous rocks, the o k a i t e at the n o r t h west end of the complex i s the most obvious, both by i t s prominent topographic expression and by i t s s p e c t a c u l a r appearance. of  Outcrops  the other a l k a l i n e i n t r u s i v e s are more patchy i n occurrence,  are t h e r e f o r e l e s s e a s i l y recognized and i n t e r p r e t e d .  The  and  first  mention of the Husereau H i l l outcrops i s contained i n a report on. road b u i l d i n g m a t e r i a l s i n the Montreal area (Gauthier, 1916,  p. 200);  reference i s made to "... a large outcrop of a l n o i t e on Husereaus farm f o u r miles north of Oka." camptonite  Howard (1922, p. 78)  s t u d i e d both  and coarse grained 'nepheline a p l i t e ' from the same  occurrence. by Gauthier.  The camptonite 'Nepheline  i s the same rock as the a l n o i t e described  a p l i t e ' cannot be explained unless some  m e l i l i t e was mistaken f o r nepheline; evidence i n support of t h i s i s the a l t e r a t i o n of 'nepheline* along f r a c t u r e s and g r a i n to  the "... h i g h l y b i r e f r i n g e n t z e o l i t e , thomsonite."  boundaries Melilite  alters  i n t h i s f a s h i o n to a h i g h l y b i r e f r i n g e n t mineral, c e b o l l i t e ; thomsonite  i s not h i g h l y b i r e f r i n g e n t , e x h i b i t i n g only f i r s t order colours  i n t h i n s e c t i o n s of normal t h i c k n e s s .  Howard's 'nepheline a p l i t e ' Is  i n a l l p r o b a b i l i t y a nepheline bearing o k a i t e . S t a n s f i e l d (1923) was the f i r s t to recognize the t r u e nature of the rocks at Husereau H i l l .  I t was he who  named o k a i t e  and nepheline o k a i t e as igneous rocks composed mainly of m e l i l i t e , hauyne, and b i o t i t e , and these three minerals plus nepheline r e s p e c t ively.  He also f i r m l y e s t a b l i s h e d the okaite i n t r u s i o n as part of  the Monteregian  petrographic province.  Other than o k a i t o , important, igneous rocks of the  Oka  complex i n c l u d e i j o l i t e , u r t i t e , m e l t e i g i t e , and j a c u p i r a n g i t e ; a minor amount of nepheline s y e n i t e i s reported south-east of La Trappe monastery (Maurice, 1958). of  the complex with the g n e i s s i c country rocks, e s p e c i a l l y on the  eastern s i d e . of  Explosive b r e c c i a s occur along the contact  These b r e c c i a s contain a v a r i e t y of angular fragments  rocks both from the complex i t s e l f and from the surrounding gneisses;  rocks of the l a t t e r category are more abundant.  S i z e of fragments  ranges from l e s s than sand-sized g r a i n s to blocks g r e a t e r than s i x inches across. both.  E x p l o s i v e b r e c c i a t i o n probably represents the l a t e s t phase  of Monteregian of  The cementing m a t e r i a l i s c a l c i t e or dolomite, or  a c t i v i t y i n the Oka  area, except f o r the i n t r u s i o n  small lamprophyre dikes. Specimens of b r e c c i a from near La Trappe monastery contain  fragments of a well s o r t e d and well rounded quartz sandstone, i n appearance to the Potsdam sandstone.  Two  similar  small areas of banded,  f i n e grained, grey marbles are known w i t h i n the complex, both just south of Husereau H i l l .  One  i s predominantly  minor p e r i c l a s e and magnetites  the other i s c a l c i t i c , and contains  b i o t i t e and pyroxene i n a d d i t i o n to p e r i c l a s e . l i k e the t y p i c a l Oka marbles.  dolomite c o n t a i n i n g  These rocks are not  I t i s p o s s i b l e that they are septa or  i n c l u s i o n s of e i t h e r G r e n v i l l e or P a l e o z o i c limestone; they appear to be s i m i l a r to the Lower Ordovician Beekmantown dolomite, though t h i s resemblance i s probably f o r t u i t o u s . Monteregian  I t has been deduced at other  igneous centres that a much t h i c k e r Paleozoic cover  e x i s t e d at the time of i n t r u s i v e a c t i v i t y than e x i s t s at present. General S t r u c t u r e A north-south t r e n d i n g a n t i c l i n a l s t r u c t u r e known as the Beauharnois  a x i s u n d e r l i e s the Oka H i l l s ( C l a r k , 1952).  The  Pal-  eozoic sediments dip g e n t l y away from t h i s a x i s , succeedingly higher formations being exposed to the east and west. presumably been eroded from the Oka  area.  These rocks have  Precambrian  rocks are  exposed only along the l i n e of t h i s axis north and south of Lac des Deux Montagnes i n the v i c i n i t y of It  Oka.  i s d i f f i c u l t to estimate the o r i g i n a l t h i c k n e s s of the  Paleozoic cover.  I t i s p o s s i b l e that the Oka i n l i e r was  an i s l a n d  during the times of i n i t i a l P a l e o z o i c sedimentation; i f t h i s was case, i t was  probably covered by l a t e Ordovician time.  hand, i t may  have been s o l e l y the product of u p l i f t  nois axis during the times of Monteregian  the  On the other  along the Beauhar-  igneous a c t i v i t y , subsequent  erosion producing the present topographic r e l i e f .  I t i s thought more  l i k e l y , f o r reasons given i n a l a t e r s e c t i o n , that the l a t t e r theory i s the c o r r e c t one.  The Beauharnois  axis appears to have e x e r c i s e d  some c o n t r o l over the l o c a l i z a t i o n of the Monteregian i n t r u s i v e s of the Oka area, since they occupy i t s core.  12  THE GRENVILLE LIMESTONE - CARBONATITE CONTROVERSY  Evidence Maurice (195$) wa.s the f i r s t person to consider the p o s s i b i l i t y that the carbonate and c a l c - s i l i c a t e rocks of the Oka complex may have been the product of a carbonatite magma, s i m i l a r to those which produced the well known carbonatites of East A f r i c a . He f a l l s back to the i d e a , however, that these rocks are G r e n v i l l e limestones, and s t a t e s (p. 3) that they had acquired t h e i r present l i t h o l o g i c a l , t e x t u r a l , and s t r u c t u r a l  "...  characteristics  long before the advent of Monteregian igneous a c t i v i t y . " In favour of the Oka marbles being G r e n v i l l e limestone Maurice o f f e r s the f o l l o w i n g evidence: 1) There i s an absence of zoning or r i n g s t r u c t u r e that i s commonly found associated with c a r b o n a t i t e s ; there i s also a lack of the "... c i r c u l a r form g e n e r a l l y t y p i c a l of them." (p. 4 ) . 2) C a l c i t e i s not t y p i c a l l y associated with Monteregian igneous rocks, as f a r as i s known. 3) There i s a considerable amount of G r e n v i l l e limestone i n the Precambrian S h i e l d area north of Oka. 4) Apophyses and dikes of Morin igneous rocks i n t r u d e the marbles. In favour of c a r b o n a t i t e o r i g i n , Maurice considers: 1) The accessory minerals (magnetite, a p a t i t e , p e r o v s k i t e , pyrochlore, and r a r e - e a r t h minerals) are s i m i l a r to those found i n ; many c a r b o n a t i t e s i n A f r i c a . 2) Diabase dikes, numerous enough i n the Precambrian rocks bordering the complex, are nowhere seen to cut the marbles. To these p o i n t s may be added the f o l l o w i n g : 3) The a t t i t u d e s of the marbles, shown by banding, d i s p l a y c r o s s - c u t t i n g r e l a t i o n s to the surrounding gneisses. 4) Monteregian i n t r u s i v e s , except f o r minor lamprphyre dikes and explosive diatreme b r e c c i a s , are associated only with the marbles. 5) Potassium-argon d a t i n g on b i o t i t e from marble gives an age  of  95 m i l l i o n years, which i s i n good agreement with ages given for.  other rocks of the Monteregian province (Hurley et a l . , i960). 6) Explosive b r e c c i a s at the contact, between gneisses and rocks of the complex, and i s o l a t e d w i t h i n the gneisses, are cemented by c a l c i t e or dolomite, or both.  Such b r e c c i a s c a r r y fragments of what  are i n a l l p r o b a b i l i t y P a l e o z o i c  sediments.  Discussion The evidence i n favour of the Oka marbles being G r e n v i l l e limestones i s somewhat negative i n approach. s t r u c t u r e i s absent i s to the contrary.  Is questionable, as f i e l d evidence i n many places The complex i t s e l f i s not c i r c u l a r ; i t may,  however, be made up of two overlap one another. examination,  That zoning or r i n g  (or more) s u b - c i r c u l a r complexes that  T h i s idea was  considered at the time of f i e l d  and has since been confirmed by Gold (1962, personal  communication) on the b a s i s of m i n e r a l o g i c a l d i s s i m i l a r i t i e s north and south of La Trappe monastery.  The statement  that c a l c i t e Is not  normally a s s o c i a t e d with Monteregian igneous rocks i s a l s o questionable.  Clark (1952, p. 106)  s t a t e s that; the s a t e l l i t i c rocks of the  Mount Royal i n t r u s i o n at Montreal are s i m i l a r to the main type except that they have a "... l a r g e content, of COg other f u g i t i v e c o n s t i t u e n t s i n t h e i r magmas." may  igneous  and H2O and,  The presence  of what  well be G r e n v i l l e limestone, distinct., from the Oka marbles, has  already been discussed; thus there i s some conformity to the S h i e l d rock types f i f t e e n miles north of Oka. for  No evidence whatever was  i n t r u s i o n of the Oka marbles by Morin-igneous  seen  types.  Much of the evidence i n favour of a c a r b o n a t i t e o r i g i n i s convincing.  Maurice (1958, p. 5) apparently attaches no importance  to the absence of diabase dikes i n the marbles.  He b e l i e v e s the rare  accessory minerals to have been introduced at the time of Monteregian igneous a c t i v i t y , a l l o w i n g that the marbles were there before t h i s time.  I f t h i s i s the case, b i o t i t e must also have been introduced,  or r e c r y s t a l l i z e d , at t h i s time In. order to give a Cretaceous  age.  Since b i o t i t e and bands of accessory minerals c o n t r i b u t e so much to  the "... l i t h o l o g i c a l , t e x t u r a l , and s t r u c t u r a l c h a r a c t e r i s t i c s of the marbles  (Maurice, 1958> P* 3 ) , i"t i s hard to understand  ..." how  these f e a t u r e s can antedate, yet m i n e r a l i z a t i o n be a product o f , Monteregian  activity.  The change i n a t t i t u d e between the g n e i s s i c country rocks and the Oka marbles i s e s p e c i a l l y marked at the western contact to the north of La Trappe monastery.  I f the marbles represent G r e n v l l l  limestones, they must have moved at some time i n . order to; g i v e t h e i r present c r o s s - c u t t i n g r e l a t i o n s h i p to the country rocks.  During  Monteregian i n t r u s i o n seems the obvious time to suggest f o r t h i s movement. The a l t e r n a t i v e ways of e x p l a i n i n g the Oka marbles are: 1)  that they represent G r e n v i l l e limestone changed only by  m i n e r a l i z a t i o n accompanying Monteregian i n t r u s i v e  activity;  2) , that they represent, G r e n v i l l e limestone that has been moved to i t s present p o s i t i o n by means of p l a s t i c flow, and f o l l o w e d by r e c r y s t a l l i z a t i o n and m i n e r a l i z a t i o n ; 3 ) that they are the product of c o n s o l i d a t i o n of a c a r b o n a t i t e magma; 4) that they are the r e s u l t of replacement: of p r e - e x i s t i n g rocks by carbonates and other minerals of metasomatic o r i g i n . The author b e l i e v e s that the Oka marbles are a product of e i t h e r or both of the two  latter  possibilities.  The main d i f f i c u l t y i n the acceptance of c a r b o n a t i t e s as such has been the b e l i e f that l i q u i d s of carbonate composition could not e x i s t , or t h a t , i f they d i d , then very s p e c i a l c o n d i t i o n s were necessary.  Added to t h i s must be both the comparative r a r i t y  small- s i z e of c a r b o n a t i t e i n t r u s i o n s . notably by Wyllie.and T u t t l e ( 1 9 5 9 >  Recent  and  experimental work,  I 9 6 0 ) has shown that l i q u i d s i n  the systems CaO-C0 -H 0, CaO-MgO-C0 -H 0, and Ca3(P0 ) -CaC03-Ca(0H) 2  2  2  2  4  2  can e x i s t at temperatures between 600°C. and 7 0 0 ° C , a t moderate pressures.  With t h i s knowledge, i t i s f a i r l y easy to envisage hot;,  b a s i c Monteregian magmas r i s i n g from great depth through G r e n v i l l e limestones t o t h e i r s i t e of s o l i d i f i c a t i o n or even eruption; p a r t i a l  15  i n c o r p o r a t i o n of limestone at depth, by a b a s i c magma could produce many of the unusual rocks, such as o k a i t e , found i n : the complex; t h i s process could have proceeded  so f a r as t o r e s u l t  i n a magma of c a r b o n a t i t e composition. I f the Oka marbles were t o be proven beyond doubt t o be c a r b o n a t i t e s , they would not be unique i n Eastern Canada; several c a r b o n a t i t e s , some a s s o c i a t e d with a l k a l i n e igneous rocks, occur i n the S h i e l d north and east of Lake Superior (Parsons, 1961). It i s not w i t h i n the scope of t h i s study t o probe i n t o t h i s f a s c i n a t i n g problem any more deeply.  Okaite and the rocks  of Husereau H i l l are, however, i n t i m a t e l y a s s o c i a t e d with carbonate rocks, and evidence w i l l be presented f o r the existence o f primary c a l c i t e i n these rocks.  I t w i l l also be shown that rocks of  o k a i t i c composition grade d i r e c t l y i n t o c a l c - s i l i c a t e  marbles.  The presence of carbon dioxide may even be necessary f o r the existence o f a magma capable of producing o k a i t i c rocks.  Suffice  i t t o say that the a l k a l i n e rocks at Oka cannot be f a i r l y considered i f the carbonate rocks are ignored, and that the s o - c a l l e d Oka marbles  show many f e a t u r e s that r a i s e a large question mark a f t e r  the statement  that they are G r e n v i l l e limestones.  16.  THE FIELD OCCURRENCE OF OKAITE AND  ASSOCIATED ROCKS  Areal Extent Okaite and nepheline o k a i t e were f i r s t f u l l y described by S t a n s f i e l d (1923) a f t e r examination Husereau H i l l .  of the rocks cropping out  on  Okaite and c l o s e l y r e l a t e d types u n d e r l i e the whole  h i l l , which i s crudely crescent shaped, convex to the north-west. The h i l l i s about 4,000 feet long and 2,000 feet across at i t s widest p o i n t , covering an area of a l i t t l e over a quarter of a square mile. Okaite a l t e r s to a rock of e n t i r e l y d i f f e r e n t aspect, and because of  t h i s , i t s a r e a l extent remained undiscovered f o r some time.  to  recent g e o l o g i c a l i n t e r e s t i n the area, however, i t has now  Owing been  found i n s c a t t e r e d exposures and i n trenches over a considerably l a r g e r area south and south-east of Husereau H i l l .  The t o t a l area  which o k a i t e and a l l i e d rocks crop out i s approximately of  a square mile.  throughout  three quarters  Furthest removed from Husereau H i l l i s an  occurrence  i n a trench on the east slope of the southern h i l l area, a distance of one and a quarter miles from the type  locality.  The Nature and Appearance of Outcrops Husereau H i l l i s the only place i n the Oka a f f o r d s good rock exposure.  complex that  The h i l l i t s e l f i s steep and wooded; the  land i s c l e a r e d on the south side where s o i l cover i s present, outcrops here are few and f a r between.  and  Elsewhere the l a r g e r outcrops  are g e n e r a l l y prominent, though many smaller exposures are only seen by t u r n i n g back the surface l a y e r of dead leaves. Okaite and a l l i e d rock types d i s p l a y a great v a r i a t i o n i n t e x t u r e , although they are s i m i l a r i n composition. r a r e l y f i n e grained; g r a i n s i z e v a r i e s from 1 mm and averages between 5 and 10 mm. appear to be a pegmatite  Fresh o k a i t e i s  to s e v e r a l centimetres,  Coarser grained v a r i e t i e s , which  phase, are i r r e g u l a r i n composition, and  u s u a l l y enriched i n minerals that are normally accessory ( f i g .  2).  They show a t e x t u r e that can be described as blotchy, on account i r r e g u l a r patches of d i f f e r e n t coloured minerals.  are  Average g r a i n  of  Figure  Figure  2: Pegmatite phase mineral i s apatite.  3s  Two s p e c i m e n s texture.  of m e l i l i t e okaite; the The s c a l e i s i n i n c h e s  of okaite The s c a l e  fibrous  showing the t y p i c a l i s i n inches  granitic  s i z e d specimens have a g r a n i t i c t e x t u r e , and are medium grey i n c o l o u r ( f i g . 3). hues.  F i n e r g r a i n produces dark grey to almost  In rare cases banding  due to v a r i a t i o n i n g r a i n s i z e or  o r i e n t a t i o n of minerals i s present.  Where banding  i s seen i t ; tends  to be p a r a l l e l to the length or curvature of the h i l l . t h i s banding  black  The dip of  i s r a r e l y measurable because i t i s i n d i s t i n c t , but;  where measured i t i s found to be almost  vertical.  Okaite i s commonly e i t h e r p a r t l y or completely  altered.  A l t e r a t i o n i s accompanied by l i g h t e n i n g i n c o l o u r and clouding of the c l e a r rock forming minerals.  In t o t a l l y a l t e r e d okaite a very f i n e  grained, cream coloured groundmass contains u n a l t e r e d accessory minerals ( f i g . 4).  Unweathered a l t e r e d rocks of t h i s nature are very  tough and hard to break. Weathering of f r e s h okaite produces a t h i n l a y e r of l i g h t e r colour, and i n outcrop the rock appears medium grey to brown.  The  more r e s i s t a n t minerals stand i n r e l i e f above the surface of the rock, imparting a rough t e x t u r e . as much as an i n c h t h i c k .  A l t e r e d types weather, to a crumbly surface Harder v e i n l e t s p e n e t r a t i n g such  rocks  form l i t t l e r i d g e s on the surface. Determination of Minerals i n Hand Specimen Ten minerals are d i s t i n g u i s h a b l e i n hand specimens of f r e s h okaite and r e l a t e d rock types, although correct i d e n t i f i c a t i o n i s not always easy.  Some of the minerals have very s i m i l a r appearances,  others are comparatively  r a r e , and some minerals that are common" i n  more ordinary rocks here d i s p l a y unusual characters. Melilite M e l i l i t e i s by f a r the most abundant m i n e r a l .  I t may  be  recognized with the a i d of a hand lens by i t s translucent, y e l l o w i s h grey colour, and by i t s somewhat greasy v i t r e o u s l u s t r e on conchoidal f r a c t u r e surfaces and p o o r l y developed are transparent honey-yellow.  cleavage planes.  Thin s l i v e r s  I t i s both hard and tough, and makes  rocks which bear large proportions of I t d i f f i c u l t to break. coarser grained pegmatitic rocks i t may  In the  he recognized by i t s c r y s t a l  Figure 4: A l t e r e d okaite; b i o t i t e and accessory minerals are in. a groundmass of a l t e r e d m e l i l i t e and hauyne. The scale i s i n inches  Figure $:  M e l i l i t e c r y s t a l s exposed by weathering of the c a l c i t e . The s c a l e i s i n inches  enclosing  form, stubby t e t r a g o n a l c r y s t a l s with many p r i s m a t i c faces by a basal p i n a c o l d .  terminated  Such c r y s t a l s are u s u a l l y embedded i n c a l c i t e ;  the l a t t e r may d i s s o l v e away on weathered surfaces, l e a v i n g the m e l i l i t e c r y s t a l s w e l l d i s p l a y e d In what look l i k e vugs ( f i g . 5)' More r a r e l y m e l i l i t e may form large anhedral, p o i k i l i t i c to; four centimeters  c r y s t a l s up;  i n diameter, i n c l u d i n g smaller g r a i n s of other  rock forming minerals. Nepheline Nepheline i s very hard to d i s t i n g u i s h from m e l i l i t e , except, that when c l e a r i t does n o t normally melilite.  show the y e l l o w i s h c o l o u r of  F i e l d i d e n t i f i c a t i o n , however, has proven q u i t e  inaccurate  as f a r as a s s i g n i n g d e f i n i t e percentages f o r these two minerals. Nepheline i s u s u a l l y i n t e r s t i t i a l , t o m e l i l i t e . Hauyne Hauyne i s d i f f i c u l t  to recognize,  c l e a r and r e l a t i v e l y f r e e from i n c l u s i o n s .  e s p e c i a l l y where i t i s Presence of the l a t t e r ,  give hauyne a d u l l grey, l u s t r e l e s s appearance on a f r e s h broken surface.  I t occurs  as rounded g r a i n s or i r r e g u l a r masses, r a r e l y as  much as t e n m i l l i m e t e r s i n diameter. Pyroxene Pyroxene i s i d e n t i f i e d by i t s brownish black c o l o u r and by i t s cleavage which i s u s u a l l y w e l l developed.  Rocks c a r r y i n g a high  percentage o f pyroxene are black, although t h i s type i s not common. T y p i c a l o k a i t e does not contain pyroxene, although pyroxene grains o f a d i f f e r e n t nature occur i n some pegmatite phases, where small, deep yellow-green, p r i s m a t i c c r y s t a l s are present  i n c a l c i t e i n contact,  with m e l i l i t e . Biotite B i o t i t e i s e s s e n t i a l l y normal i n character, and i s e a s i l y recognized. are present  Lustrous black f l a k e s o r stubby pseudo-hexagonal, prisms i n a l l rocks.  Irregular p o i k i l i t i c present  Thin f l a k e s have a greenish brown colour.  c r y s t a l s up t:o three centimeters  I n some rocks.  i n diameter are  Calcite C a l c i t e i s present i n small q u a n t i t i e s i n most specimens. In coarser grained v a r i e t i e s I t i s i n t e r s t i t i a l the spaces between the c r y s t a l s .  to m e l i l i t e ,  filling  Such c a l c i t e i s p o i k i l i t i c , en-  c l o s i n g s o l i t a r y m e l i l i t e , magnetite, p e r o v s k i t e , b i o t i t e , and a p a t i t e c r y s t a l s ; the same cleavage  face may be seen to f l a s h  d i s c o n t i n u o u s l y over as much as f i v e  centimeters.  Apatite Apatite i s u s u a l l y pale lemon yellow i n colour, and coarse hexagonal prisms may be as long as f o u r centimeters pegmatite phases of o k a i t e .  i n length i n  In such rocks, a p a t i t e forms r a d i a t i n g  bursts of c r y s t a l s enclosed i n c a l c i t e ( f i g . 2 ) . A p a t i t e u s u a l l y occurs as small, c o l o u r l e s s needles that are enclosed i n s e v e r a l d i f f e r e n t g r a i n s of other rock forming  minerals.  Magnetite Magnetite i s c o n s i s t e n t l y present i n a l l rocks.  as an accessory  mineral  U s u a l l y i n s i n g l e rounded g r a i n s between one and  eight m i l l i m e t e r s i n diameter, i t shows good development of l u s t r o u s c r y s t a l faces with an i r r i d e s c e n t t a r n i s h i n some of the coarser pegmatite rocks.  I t s c r y s t a l habit i s unusual;  (201) form i s commonly present  tetrahexahedral  alone, although t h i s may be modified  by dodecahedral ( l l O ) o r octahedral ( i l l ) forms.  I n general,  crystal  faces are poorly developed, and r e c o g n i t i o n of forms i s impossible. Perovskite Perovskite i s black and shiny, and may be r e a d i l y  identi-  f i e d by c r y s t a l form i n pegmatite rocks; here i t i s u s u a l l y w e l l c r y s t a l l i z e d as cubes between one and f o u r m i l l i m e t e r s i n diameter. The cubic faces tend to bulge outwards, and may be modified by octahedral faces.  Where perovskite i s not w e l l c r y s t a l l i z e d , as i n :  normal o k a i t e s with g r a n i t i c t e x t u r e , i t may be d i s t i n g u i s h e d from magnetite by being non-magnetic.  22  Pyrrhotite P y r r h o t i t e i s commonly present i n very small quantity, e i t h e r as small i s o l a t e d g r a i n s or intergrown with magnetite.  In a  few specimens t i n y "blebs of c h a l c o p y r i t e are seen a s s o c i a t e d with i t . A l t e r a t i o n of Okaite i n Hand Specimen In a t y p i c a l specimen of completely a l t e r e d o k a i t e , b i o t i t e , c a l c i t e , a p a t i t e , magnetite, content of b i o t i t e may  and p e r o v s k i t e remain u n a l t e r e d .  The  be s u b s t a n t i a l l y increased by the a l t e r a t i o n  of pyroxene; s i m i l a r l y c a l c i t e i s more abundant, presumably being formed by the breakdown of m e l i l i t e .  The minerals which make up. the  cream coloured, f i n e grained groundmass a l t e r a t i o n are not i n hand specimen.  identifiable  M e l i l i t e , nepheline, and hauyne are the minerals  that have been a l t e r e d . The p a t t e r n of a l t e r a t i o n may  be t r a c e d by the  examination  of a s e r i e s of rock specimens showing v a r i o u s degrees of a l t e r a t i o n . I t i s noted that a l t e r a t i o n i s very patchy i n i t s occurrence; average s i z e d hand specimen may quite f r e s h at the other.  an  be completely a l t e r e d one end,  and  Although the f i n a l products are apparently  the same, a l t e r a t i o n has taken place i n e i t h e r or both of two ways. In some rocks pronounced f r a c t u r i n g has allowed a l t e r a t i o n to take place on e i t h e r s i d e of t i n y v e i n l e t s that f i l l  the f r a c t u r e s .  The  extent of f r a c t u r i n g v a r i e s a great d e a l , and i t i s g e n e r a l l y pronounced i n one d i r e c t i o n only. absent.  A l t e r a t i o n has pervaded  In other rocks v e i n i n g appears to be the whole rock, or patches of i t .  In the e a r l y stage of pervasive a l t e r a t i o n , hauyne or nepheline appear cloudy white, and m e l i l i t e i s a l t e r e d along b a s a l cleavage planes.  In some rocks nepheline remains c l e a r and m e l i l i t e i s  completely a l t e r e d .  Advanced a l t e r a t i o n renders i t impossible to  d i s t i n g u i s h between m e l i l i t e , hauyne, and nepheline i n hand specimen.  Where v e i n i n g occurs i n completely a l t e r e d o k a i t e , an  extremely f i n e grained, d u l l b l u i s h green a l t e r a t i o n may adjacent to f r a c t u r e s .  be  seen  In a l t e r e d m e l i l i t e - c a l c i t e rock t h i s b l u e -  green a l t e r a t i o n occurs i n i r r e g u l a r patches.  23  Rock Types Cropping Out on Husereau H i l l On the b a s i s of mineralogy as seen i n hand specimen, the f r e s h rocks o f Husereau H i l l f a l l n a t u r a l l y i n t o two c a t e g o r i e s : those that contain an appreciable quantity of pyroxene, and those that do not.  The pyroxene bearing type i s s i m i l a r t o o k a i t e i n  many respects; i t does not, however, d i s p l a y a pegmatite  phase,  and appears to have between medium and coarse g r a i n s i z e only. A combination o f s e v e r a l v i s u a l estimates g i v e s the f o l l o w i n g modal variation: Pyroxene M e l i l i t e , nepheline Hauyne Biotite Accessory minerals In  15-40 $ 35-55 • 10-20 5-I5 10-15  the category of.rocks that do not contain pyroxene  are o k a i t e and nepheline o k a i t e , as reported by S t a n s f i e l d (1923,  p.440).  He gives the f o l l o w i n g composition f o r o k a i t e : Melilite Hauyne Biotite Magnetite Apatite Calcite Perovskite  66-63 $ 19^20 8-10 2' 2 2 1.  This mode i s based on separations o f the c o n s t i t u e n t minerals.  He  does not g i v e a mode f o r nepheline o k a i t e , but s t a t e s that the nepheline "... i s q u i t e subordinate i n amount t o m e l i l i t e . "  (p.433).  Another type not mentioned by S t a n s f i e l d c a r r i e s approximately the same amounts o f b i o t i t e and accessory minerals, but both nepheline and hauyne are absent; m e l i l i t e c o n s t i t u t e s up t o 85$ o f the rock. The c a l c i t e - r i c h , or pegmatite, phase of o k a i t e f a l l s f r e e category a l s o .  i n t o the pyroxene-  Such rocks do not appear t o c a r r y hauyne or  nepheline, and t h e i r composition i s very i r r e g u l a r .  Of note here i s  that they may c o n t a i n a small amount o f pyroxene o f a d i f f e r e n t nature to the pyroxene already mentioned i n that i t i s yellow-green i n s t e a d of brownish black, and occurs i n c a l c i t e as s m a l l , w e l l formed  24 c r y s t a l s rather that i n the groundmass as l a r g e i r r e g u l a r g r a i n s .  A  v i s u a l l y estimated modal range i s j 4O-65 <$> 10-25 10-15 5-10 5-15 3-10 0-2  Melilite Apatite Calcite Biotite Magnetite Perovskite Pyroxene  I t has already been mentioned that previous workers described a l n o i t e as o c c u r r i n g on Husereau H i l l . at  the south-west  end o f the h i l l  The steep slope  exposes a small area o f t h i s  rock between outcrops o f rocks o f the types already described. The a l n o i t e i s f r e s h , dense black i n c o l o u r , and very hard. Scattered i n a f i n e grained, even textured groundmass are phenocrysts and c r y s t a l fragments o f hornblende, b i o t i t e , and pyroxene.  The  a l n o i t e body i s about 25O feet wide near the base o f outcrop, and appears t o narrow considerably at the top o f the h i l l .  I t s outcrop  cannot be t r a c e d any f u r t h e r than t h i s , and because rock exposures are f a i r l y p l e n t i f u l i t i s assumed that t h i s d i k e - l i k e body does not continue beyond the top of the h i l l .  The rock i s coarser i n g r a i n  s i z e towards the c e n t r a l part o f i t s outcrop area; here phenocrysts of hornblende up t o 1 cm across may be seen.  I n c l u s i o n s , mainly o f  o k a i t e , a r e present near the edges o f i t s exposed area. outcrop c r y s t a l s and small angular fragments  I n weathered  stand out on the s u r f a c e .  The Occurrence o f Okaite South and South-East o f Husereau H i l l For a d i s t a n c e o f n e a r l y one mile south and south-east o f Husereau H i l l  s c a t t e r e d rock exposures may he found.  Many o f these  are s i m i l a r to the a l t e r e d o k a i t e s already described, and some are fresh.  At several l o c a l i t i e s v a r i e t i e s between pyroxenite and  pyroxene bearing o k a i t e are found.  As w e l l as the types s i m i l a r t o  those found on Husereau H i l l , a few low and weathered outcrops of banded marble, i j o l i t e ,  and b i o t i t e rock are found.  An i n t e r e s t i n g occurrence i s found on a small k n o l l three quarters of a mile due south of the high point of Husereau H i l l .  25  Here o k a i t e which i s cut by a dike o f pyroxenite has been h i g h l y a l t e r e d and m i n e r a l i z e d with r a r e elements.  B i o t i t e has changed t o  v e r m i c u l i t e , and pyroxene has developed t h i c k rims of, aegerine-augite; present i n the r e d d i s h brown groundmass i s b r i t h o l i t e , a r a d i o - a c t i v e cerium bearing phosphate r e l a t e d t o a p a t i t e . On the north-east slope of.the southern h i l l , area considerable amounts o f banded and m i n e r a l i z e d marbles are exposed i n trenches. M e l i l i t e r i c h rocks, and t h e i r a l t e r e d e q u i v a l e n t s , are i n t e r l a y e r e d with the marbles.  These rocks are s i m i l a r i n appearance t o the o k a i t e s  of Husereau H i l l , one and a quarter miles t o the north. amounts o f c a l c i t e , a p a t i t e , and magnetite, of  With i n c r e a s i n g  and with the i n t r o d u c t i o n  soda-pyroxene and m o n t i c e l l i t e , these rocks grade i n t o t y p i c a l  c a l c - s i l i c a t e bearing c a r b o n a t i t e s .  These i n t u r n grade i n t o the  more t y p i c a l s o v i t e which i s composed o f c a l c i t e with l e s s than 10$ each o f b i o t i t e , magnetite,  and a p a t i t e .  I n some exposures the  m e l i l i t e bearing rocks are b r e c c i a t e d and cemented by veins o f c a r bonate and c a l c - s i l i c a t e . ijolite  I n t h i s area also are d i k e - l i k e masses o f  and b i o t i t i z e d i j o l i t e ,  much of which has a l s o been b r e c c i a t e d ,  i n a similar fashion. Other Rock Types Associated with Okaite Small q u a n t i t i e s o f two other rock types are found i n the area south o f Husereau H i l l .  One i s the f i n e grained, banded, grey  rock (see p.10) that has the appearance o f a sedimentary  limestone.  Two outcrops are known: one i s on the south s i d e o f Husereau H i l l , close t o outcrops o f o k a i t e ; the other occurs near the road j u n c t i o n just south o f the b r i t h o l i t e l o c a l i t y , and i s bordered by coarse grained marbles.  The second rock type i s an exceedingly f i n e grained  dark grey-green rock i n which may be seen patches and v e i n l e t s o f green pyroxene, probably aegerine. ijolite,  I t looks l i k e a f i n e grained  and may be a h i g h l y f e n i t i z e d country rock.  I t crops out  on the low r i d g e 2000 feet south-west of the high point o f Husereau Hill.  I t i s s i m i l a r to the i j o l i t e  and b r e c c i a t e d i j o l i t e  found i n  the trenches on the north-east slope o f the southern h i l l area; s e c t i o n s o f s i m i l a r rock have been found i n many d r i l l cores.  26;  Structure Okaite g e n e r a l l y l a c k s d i r e c t i o n a l f e a t u r e s , u n l i k e the carbonate  rocks to the south of Husereau H i l l .  the d i f f e r e n t types of pyroxene-free pattern.  The d i s t r i b u t i o n of  o k a i t e does not show an o r d e r l y  A l t e r a t i o n does not seem to f o l l o w any p a r t i c u l a r horizon.  The pyroxene bearing o k a i t e , with which are grouped rocks  having  more the nature of pyroxenite or j a c u p i r a n g i t e , appear as c e r t a i n horizons that f o l l o w the general curvature of Husereau H i l l , i n d i c a t i n g an arcuate s t r u c t u r e .  T h i s i s i n general keeping with the trend of the  banding i n the marbles to the south, and may of  ring structure.  t h e r e f o r e i n d i c a t e a type  In places the contact between pyroxene bearing  rocks and okaite i s f a i r l y d i s t i n c t ; i n places i t i s g r a d a t i o n a l . it  is difficult  to say from f i e l d evidence  Thus  alone whether the pyroxene  bearing rocks represent dikes, or whether they are bands formed by segregation of pyroxene. T e x t u r a l banding i n o k a i t e i s seen at two  outcrops.  Fractures  are commonly present i n the rocks of Husereau H i l l , and tend to be pronounced i n one d i r e c t i o n , e s p e c i a l l y where they have c o n t r o l l e d alteration.  Rarely, pegmatite phases of o k a i t e are seen i n elongate,  l e n s - l i k e bodies.  A t t i t u d e s of these three features conform to the  trend e s t a b l i s h e d f o r the pyroxene bearing horizons. or b r e c c i a t e d o k a i t e , i s found  Where o k a i t e ,  as d i k e - l i k e masses w i t h i n marbles,  as on the north-east slope of the southern h i l l s , i t i s seen that t h e i r trends p a r a l l e l the banding i n the marbles and the contact to g n e i s s i c rocks.  neighbouring  Dips i n a l l cases are steep, v a r y i n g on  e i t h e r s i d e of the v e r t i c a l . I n t r u s i o n s of okaite. and r e l a t e d rocks occur only i n the north-west part of the Oka  complex.  that they form crescent-shaped  I t i s thought from f i e l d  evidence  dikes that f o r c i b l y " i n t r u d e d Precambrian  g n e i s s i c and igneous rocks along s u b - v e r t i c a l arcuate f r a c t u r e s . i n t u r n have been i n t r u d e d and b r e c c i a t e d by c a r b o n a t i t e dikes.  These The  r e l a t i o n s h i p of the i j o l i t e and the r e l a t e d u r t i t e and m e l t e i g i t e to the o k a i t e s i s not c l e a r from f i e l d evidence.  Carbonatite dikes cut  21 both these groups of rock types; the l a y e r i n g i n such rocks appears to flow round i n c l u s i o n s of okaite or i j o l i t e . i j o l i t e group may  represent  Rocks of the  rheomorphic f e n i t i z e d gneisses.  I n t r u s i o n has been l o c a l i z e d i n the crest of the Beauharnois axis.  Evidence from surface exposure and from d r i l l cores suggests  that the Oka  complex was  formed by a continuing succession of r i n g -  dike i n t r u s i o n s of a l k a l i n e and carbonatite rocks. geologic map  The d e t a i l e d  ( P l a t e I I , i n pocket) gives an i n t e r p r e t a t i o n of the  northern part of the complex.  A more d e t a i l e d account of the g e o l o g i c  i n t e r p r e t a t i o n i s given i n a l a t e r s e c t i o n .  PETROGRAPHY OP THE OKAITE INTRUSION Method of StudyThin s e c t i o n s were made from 56 specimens chosen as r e p r e s entative of okaite and r e l a t e d rocks. two o r more t h i n s e c t i o n s were made. unaltered rocks.  For coarse grained specimens 33 specimens were found to be  T h e i r corresponding hand specimens were checked  again  f o r u n i f o r m i t y of t e x t u r e and composition, and eight were d i s c a r d e d because they showed too great a v a r i a t i o n i n mineralogy t o be sure that they t r u l y represented the outcrop from which they were taken. The f o l l o w i n g c l a s s i f i c a t i o n i s thus based on 25 specimens of the rocks from Husereau H i l l and v i c i n i t y .  I t w i l l be shown that the  o r i g i n a l mineralogic composition o f the a l t e r e d rocks can be deduced once the a l t e r a t i o n minerals are recognized. compositions  Such reconstructed  agree with the c l a s s i f i c a t i o n proposed  below.  A f t e r microscopic i d e n t i f i c a t i o n of the rock forming minerals, between 1,000 and 1,500 point counts were made on each of the f r e s h r e p r e s e n t a t i v e t h i n s e c t i o n s .  These proportions were  reduced to percentages, c o r r e c t e d to the nearest h a l f of one percent.. C l a s s i f i c a t i o n of Okaite and Related Rocks Excluding the a l n o i t e specimens, there are found to be three d i s t i n c t rock types w i t h i n the o k a i t e i n t r u s i o n s .  These form  a continuously v a r y i n g s e r i e s , with the three types r e p r e s e n t i n g stages w i t h i n the s e r i e s that seem to be f a r more prevalent i n occurrence than do the types with intermediate compositions.  Sub-  ordinate amounts of the intermediate types are present, however, and i t i s t h e i r existence that betrays the continuous v a r i a t i o n w i t h i n the okaite s e r i e s . Each of the three types c o n t a i n m e l i l i t e and b i o t i t e and the accessory minerals a p a t i t e , magnetite,  and p e r o v s k i t e . The  d i a g n o s t i c minerals are hauyne and t i t a n a u g i t e .  Pyroxene o k a i t e  i s c h a r a c t e r i z e d by the presence of t i t a n a u g i t e , m e l i l i t e , and hauyne. Okaite lacks t i t a n a u g i t e , and contains hauyne and/or nepheline i n addition to m e l i l i t e .  M e l i l i t e predominates i n m e l i l i t e okarte,  29  both nepheline and hauyne being absent. variation  Table I gives the  average  i n the modes o f these three rock types. Table I Classification  of Rock Types  Pyroxene Okaite Titanaugite Hauyne Nepheline Melilite Biotite Calcite Apatite Magnetite Perovskite  15 15 -  -  15 5 0 2 4 1  45 30  Okaite  M e l i l i t e Okaite  _•  -  12 0 45 3 1 3 4 1  - 40 - 20 3 7 - 12 8  -  28 10 65 20 5 7 10 5 -  -  55 3 5 3 4 1  -  1  75 20 25 10 10 5  1) Average range of nine specimens 2) Average range of seven specimens 3) Average range of f i v e specimens Stansfield The two  (1923) described o k a i t e and nepheline o k a i t e .  are not d i s t i n g u i s h e d here because they represent  s i m i l a r phases of the same rock type.  essentially  Hauyne ( 6 N a A l S i 0 4 * C a S O ^ )  and  nepheline (NaAlSiO^)  d i f f e r chemically only i n the presence of calcium  sulphate i n hauyne.  The a v a i l a b i l i t y of calcium sulphate i s a l l that  governs which of these two minerals separates from the magma, and the r e l a t i v e l y small amount r e q u i r e d does not s i g n i f i c a n t l y magmatic composition.  affect  the  I t should be noted here that nepheline has  not yet been found i n a specimen o f pyroxene o k a i t e , and that i n nepheline o k a i t e i t i s i n v a r i a b l y  a s s o c i a t e d with hauyne.  Nepheline  i s most commonly present i n rocks intermediate i n composition between okaite and m e l i l i t e o k a i t e , that i s , i n rocks whose combined hauyne and nepheline content i s between 15  and 0 $.  I t should be noted that b i o t i t e i s a c o n s t a n t l y v a r i a b l e component i n a l l three types; the average okaites i s about 10$,  content of b i o t i t e i n the  although i t tends to be somewhat higher than  t h i s f o r types intermediate between pyroxene o k a i t e and o k a i t e . The  content of accessory minerals i s c o n s i s t a n t l y steady, accounting  f o r about 15$ of the modes.  Calcite  i s not c l a s s e d as an accessory  mineral i n the same way as a p a t i t e , magnetite, may p l a y a more important rock types.  and p e r o v s k i t e , as i t  r o l e i n the development of these  unusual  The content of c a l c i t e i s higher i n m e l i l i t e okaite than  i n other types.  The m e l i l i t e - c a l c i t e rock, described on page 16 as  a pegmatitic phase o f o k a i t e , f a l l s i n t o the category o f m e l i l i t e okaite.  I t has a lower m e l i l i t e content than i s t y p i c a l , and I s  enriched i n accessory minerals, notably a p a t i t e and magnetite.  In  places i t appears to grade i n t o t r u e c a r b o n a t i t e i n which c a l c i t e i s the main rock forming mineral. The a l n o i t e body, on Husereau H i l l i s undoubtedly  related  i n mineralogy to the o k a i t e s , but t e x t u r a l l y i t i s t o t a l l y different., and contains some minerals that are r a r e l y found i n o k a i t e s . s e c t i o n examination  Thini  r e v e a l s that the a l n o i t e s o l i d i f i e d while i t was  i n a most unstable s t a t e ; t h i s s i t u a t i o n w i l l be discussed a f t e r a d e t a i l e d t e x t u r a l d e s c r i p t i o n i s given.  Because of the f i n e g r a i n  of the a l n o i t e , and the extremely v a r i a b l e composition  throughout  the body, i t i s p r a c t i c a l l y impossible to give an accurate mode.  An  approximate mode, averaged from three t h i n s e c t i o n s , i s s Melilite Olivine Monticellite Titanaugite Hornblende Biotite Calcite Apatite Magnetite Perovskite Hercynite  45$ 4 8 7 6 5 4 1 9 5 6  M i n e r a l C h a r a c t e r i s t i c s i n Thin S e c t i o n The important, rock forming minerals that are present, i n a l l of the okaites are m e l i l i t e , b i o t i t e , c a l c i t e , a p a t i t e , and p e r o v s k i t e .  Important  magnetite,  but present only i n c e r t a i n rock types  are t i t a n a u g i t e , hauyne, and nepheline.  Occurring i n small,  q u a n t i t i e s i n a few specimens are s p i n e l ( h e r c y n i t e ) , p y r r h o t l t e , c h a l c o p y r i t e , o l i v i n e , m o n t i c e l l i t e , augite, and hornblende.  'These minerals are described i n d i v i d u a l l y as they appear i n t h i n s e c t ion.  M i n e r a l o g i c d e t a i l s of some of the more important  ones are given  i n another s e c t i o n . Melilite M e l i l i t e i s c o l o u r l e s s i n - t h i n s e c t i o n , and i s c h a r a c t e r i z e d by moderate p o s i t i v e r e l i e f and a f a i r l y good b a s a l cleavage. It i s o p t i c a l l y negative i n a l l . rocks, and extinguishes p a r a l l e l to cleavage between crossed N i c o l prisms.  B i r e f r i n g e n c e In the okaites  i s low, showing f i r s t order grey to yellow; i n alnoiite i t i s exceedi n g l y low, rendering m e l i l i t e almost negative i n t e r f e r e n c e f i g u r e s may  i s o t r o p i c , although  s t i l l be obtained.  optically  Interference  colours are s l i g h t l y anomalous, g i v i n g b l u i s h and yellowish, t i n t s to grey; t h i s Is e s p e c i a l l y n o t i c a b l e where m e l i l i t e i s seen adjacent to nepheline.  M e l i l i t e g r a i n s are normally anhedral to subhedral,  l o c k i n g to give t y p i c a l l y a hypautomorphic t e x t u r e .  inter-  In pegmatitic  rocks i t i s commonly euhedral, and d i s p l a y s r e c t a n g u l a r cross s e c t i o n s of c r y s t a l s that are elongate p a r a l l e l to the cleavage ( f i g . 6).  Mel-  i l i t e i s t e t r a g o n a l , and cleaves along the b a s a l plane; thus the c r y s t a l s are stubby prisms whose C-axes are much s h o r t e r than t h e i r A-axes.  Coarse grained m e l i l i t e may  be conspicuously p o i k i l i t i c ,  enclosing accessory minerals ( f i g . 7).  Cleavage planes may  f l a t , v e r m i c u l a r growths of an opaque mineral, probably  contain  magnetite,  which i s t r a n s l u c e n t dark brown i n t h i n edges ( f i g . 8). Biotite B i o t i t e i s e s s e n t i a l l y normal, i n appearance, with strong pleochroism, though v a r i a b l e i n colour.  In f r e s h o k a i t e s b i o t i t e  grains are g e n e r a l l y o l i v e - g r e e n to khaki-brown when the C-axis, p a r a l l e l to the microscope stage, i s north-south, and pale yellow to yellow-brown when the C-axis i s east-west.  straw-  V a r i e t i e s are  deep chestnut-red to yellow-brown, or pale sage-green to c o l o u r l e s s i n the same o r i e n t a t i o n s .  Uneven c o l o u r zoning i s not uncommon, with  greener, c o l o u r a t i o n towards the edges of the g r a i n s .  B i o t i t e grains  appear ragged i n t h i n s e c t i o n , r a r e l y showing c r y s t a l form, even though such i s apparent  i n hand specimen.  Commonly i n c l u d e d w i t h i n  b i o t i t e g r a i n s are unoriented needles of a p a t i t e .  32  F i g u r e 6;  M e l i l i t e c r y s t a l s enclosed i n c a l c i t e i n c a l c i t e r o c k ( c r o s s e d N i c o l s , x20)  melilite-  F i g u r e 7* needles;  P o i k i l i t i c texture of m e l i l i t e enclosing a p a t i t e the grey mineral i s b i o t i t e (plane l i g h t , x20)  33  Figure 9*  Radiating a p a t i t e c r y s t a l s i n m e l i l i t e o k a i t e pegmatite (crossed N i c o l s , x 2 0 )  Calcite C a l c i t e i s i n v a r i a b l y present i n small, amounts, reaching a maximum content i n the m e l i l i t e o k a i t e s . and i n t e r s t i t i a l .  I t i s always anhedral  A s i n g l e .crystal of c a l c i t e may  appear as sep-  arate g r a i n s with the same o p t i c o r i e n t a t i o n , a m a n i f e s t a t i o n of i t s p o i k i l i t i c nature mentioned on page  21.  Apatite Apatite i s present as an accessory mineral in: every rock examined.  I t i s always anhedral, and occurs as thin, hexagonal, prisms  that penetrate every other mineral present.  It i s i t s e l f quite clear  and f r e e of i n c l u s i o n s , except that i n a few s e c t i o n s i t contains t i n y bubbles that are f i n e l y elongate p a r a l l e l to the l e n g t h of the g r a i n s . Apatite i s o p t i c a l l y negative, l i k e m e l i l i t e ; i t may  be d i s t i n g u i s h e d  from m e l i l i t e by i t s c l e a r e r appearance and i t s s l i g h t l y higher r e fringence; i t does not show the anomalous i n t e r f e r e n c e c o l o u r s of melilite. easily. of  Where c r y s t a l form Is w e l l developed, d i s t i n c t i o n i s made In pegmatite  phases of m e l i l i t e o k a i t e a p a t i t e forms groups  radiating crystals ( f i g . 9);  these may  a t t a i n a r a d i a l length of  several centimeters, as shown i n f i g u r e 2.  !  Magnetite  :  Magnetite occurs as euhedral. or subhedral, rounded g r a i n s . It i s i n c l u d e d i n a l l . minerals except a p a t i t e .  Commonly a s s o c i a t e d  with i t are small, amounts of p y r r h o t i t e and t i n y specks of chalcop y r i t e ; these three minerals may reflected light.  be d i s t i n g u i s h e d i n t h i n s e c t i o n by  Some rocks c o n t a i n magnetite  small equant grains of green to brownish-black Perovskite  i n which are included spinel. 1  i  Perovskite i s normally anhedral and somewhat ragged, except . i n pegmatitic rocks where i t shows cubic form.  I t i s recognized i n  t h i n s e c t i o n by i t s deep orange-brown colour and i t s extreme  relief.  It has very low b i r e f r i n g e n c e and i n v a r i a b l y shows complicated'- l a m e l l a r twinning ( f i g . 1 0 ) .  In some rocks a t h i n rim of p e r o v s k i t e may  be  Figure l i t  O s c i l l a t o r y zoning i n t i t a n a u g i t e ( c r o s s e d N i c o l s , x 2 0 )  seen bordering magnetite g r a i n s : t h i s i s commonly p a l e r than the l a r g e r equant g r a i n s . Titanaugite T i t a n a u g i t e i s present only In. pyroxene o k a i t e and It may  alnoite.  be euhedral i n rocks i n which i t c o n s t i t u t e s a h i g h p r o p o r t i o n  of the mineral content, but i t i s u s u a l l y subhedral with borders.  ragged  A corona s t r u c t u r e i s commonly present, formed by magmatic  r e a c t i o n to give m e l i l i t e and bilotiite.  It i s noticably pleochroic  i n shades of pinkish-brown  Most g r a i n s show o s c i l l a t o r y /  and yellow.  zoning towards the outer edges ( f i g . 1 1 ) , may  show 'hour-glass' zoning.  and, the c e n t r a l p o r t i o n s  Maximum i n t e r f e r e n c e colours i n sec-  t i o n s of normal thickness are second order yellow and orange. b i r e f r i n g e n c e colours are extremely  anomalous on account  The  of strong  i n c l i n e d d i s p e r s i o n ( r e d g r e a t e r than v i o l e t ) ; t i t a n a u g i t e does n o t e x t i n g u i s h i n c e r t a i n o r i e n t a t i o n s , showing i n s t e a d a t r a n s i t i o n from deep B e r l i n blue to reddish-brown, or from v i o l e t - g r e y to yellow.  Simple twinning on ( 1 0 0 ) i s not uncommon.  angle Z against C ranges between 5 0 ° a* ^ 6 0 . 1  moderate and p o s i t i v e .  (110)  a  The  straw-  extinction  The o p t i c angle i s  cleavage i s w e l l developed.  Central  p o r t i o n s of t i t a n a u g i t e grains are commonly seen to i n c l u d e a network of t i n y opaque needles o r i e n t e d along the ( 0 0 1 )  and  (lOO)  crystallographic directions. Hauyne Hauyne i s present i n o k a i t e and pyroxene o k a i t e . normally euhedral i n pyroxene o k a i t e .  I t s pseudo-hexagonal o u t l i n e  i n t h i n s e c t i o n i s i n d i c a t i v e of i t s usual dodecahedral In o k a i t e i t i s subhedral to anhedral, and may  c r y s t a l form.  be i n t e r s t i t i a l to  m e l i l i t e , e s p e c i a l l y where a s s o c i a t e d with nepheline. optically isotropic (fig. 1 2 ) ,  Hauyne i s  and has moderate negative  ?/hich d i s t i n g u i s h e s i t from nepheline.  It i s  relief  It i s very r a r e l y c l e a r ,  however, and the many t i n y equant opaque i n c l u s i o n s w i t h i n i t serve for this distinction.  The i n c l u s i o n s are commonly o r i e n t e d i n l a y e r s  along the ( 1 0 0 ) planes, forming a g r i d ( f i g . 1 3 ) .  The outer rims of  hauyne c r y s t a l s i n pyroxene o k a i t e lack the opaque i n c l u s i o n s . Between crossed N l c o l prisms t h i s outer zone Is seen to be crowded  Figure 12: T y p i c a l appearance o f hauyne i n pyroxene o k a i t e . Note the o u t e r zone of a n i s o t r o p i c i n c l u s i o n s ( c r o s s e d B i c o l s , x20)  F i g u r e 131  G r i d of opaque i n c l u s i o n s i n hauyne (plane l i g h t , x 4 8 )  with o r i e n t e d b i r e f r i n g e n t i n c l u s i o n s i n s t e a d , except f o r the extreme outer edge, which i s c l e a r . Nepheline Nepheline i s only present i n the nepheline hearing of o k a i t e . melilite. relief.  phase  I t i s subhedral to anhedral, and i s i n t e r s t i t i a l to I t i s c l e a r and  Hauyne may  c o l o u r l e s s , and has very low negative  rim grains of nepheline at contacts  with m e l i l i t e .  Spinel S p i n e l i s most n o t i c a b l e where i t occurs as small within magnetite.  inclusions  Here i t i s seen as bright green grains with high  p o s i t i v e r e l i e f that are i s o t r o p i c between crossed N i c o l s .  Eeflected  l i g h t shows that some magnetite grains have e i t h e r a c e n t r a l p o r t i o n , or a rim, or both, of opaque s p i n e l : i t appears black i n r e f l e c t e d l i g h t , and  does not show the rough, s t e e l - b l u e r e f l e c t i o n of magnetite.  Closer i n s p e c t i o n r e v e a l s that t h i n edges are translucent brown i n plane l i g h t .  S p i n e l i s not v i s i b l e i n many of the  examined, and i s not observed i n pyroxene o k a i t e . a large p r o p o r t i o n  dark greenspecimens  It constitutes a  of the opaque m a t e r i a l i n a l n o i t e .  Monticellite M o n t i c e l l i t e i s present i n small amounts i n three of m e l i l i t e okaite examined i n t h i s study, and  also in. a l n o i t e .  the former rock type, i t Is abundant only i n one i t makes up about 1 5 $ of the t o t a l rock.  specimens In  specimen, i n which  I t occurs here  Intimately  intergrown with m e l i l i t e , from which i t i s d i s t i n g u i s h e d by higher b i r e f r i n g e n c e and b i a x i a l negative character; firmed by an X-ray powder photograph.  The  i t s i d e n t i t y was  intergrowth  and has the appearance of a large s c a l e myrmekite.  con-  i s irregular,  Large c r y s t a l s of  m e l i l i t e are i n t e r l o c k e d with e q u a l l y large c r y s t a l s of m o n t i c e l l i t e , associated with small amounts of c a l c i t e ; the o p t i c o r i e n t a t i o n of one may  be continuous throughout more than, one  and v i c e versa.  c r y s t a l of the  other,  In a l n o i t e , m o n t i c e l l i t e occurs as d i s c r e t e ragged  grains or groups of subhedral c r y s t a l s , and  also as a l t e r a t i o n rims  around o l i v i n e or pyroxene. Olivine O l i v i n e i s present only i n a l n o i t e .  Here i t forms rounded  grains with, high p o s i t i v e r e l i e f and high b i r e f r i n g e n c e . angle i s about ition.  I t s optic  90°» i n d i c a t i n g that i t i s near f o r s t e r i t e i n compos-  I t i s resorbed, and may  be seen as rounded g r a i n s enclosed  i n t i t a n a u g i t e , or rimmed by m o n t i c e l l i t e .  I t has not been observed  i n any of the o k a i t e s . Augite Augite i s present i n some m e l i l i t e o k a i t e s , notably i n the pegmatitic types.  Rarely amounting to more than two percent of th.e  t o t a l rock, i t replaces m e l i l i t e c r y s t a l s at t h e i r edges, or i s embedded as s i n g l e c r y s t a l s i n c a l c i t e and f i b r o u s a p a t i t e .  It i s  q u i t e d i s t i n c t from t i t a n a u g i t e i n having a pale greenish-yellow colour, and i n l a c k i n g pleochroism and anomalous absorption: the e x t i n c t i o n angle Z against C i s about 45°• Hornblende Hornblende i s present i n few rocks. okaite i t i s seen a s s o c i a t e d with b i o t i t e .  In one specimen of  I t i s present i n one  specimen of pyroxene o k a i t e where i t forms anhedral g r a i n s at the borders of t i t a n a u g i t e .  Both these amphiboles are s t r o n g l y  p l e o c h r o i c i n green and brown, and show o p t i c a l p r o p e r t i e s s i m i l a r to normal hornblende.  Hornblende i s present i n a l n o i t e as  ragged  g r a i n s whose c e n t r a l p a r t s are s t r o n g l y p l e o c h r o i c greenish-brown to reddish-brown; the outer p o r t i o n s are c r y s t a l l o g r a p h i c a l l y  continuous  with d i f f e r e n t e x t i n c t i o n , and are only f a i n t l y p l e o c h r o i c pale yellow to pale brown.  These outer zones are crowded with i n c l u s i o n s  which i n c r e a s e i n s i z e and quantity towards the ragged outer rims.  40  Microscopic  Textures  Pyroxene Okaite Fresh, pyroxene okaite i s c h a r a c t e r i z e d i n t h i n s e c t i o n by subhedral  t o euhedral  c r y s t a l s of zoned, p l e o c h r o i c t i t a n a u g i t e , and  euhedral  c r y s t a l s of hauyne with zones of i n c l u s i o n s .  minerals  are set i n a groundmass of anhedral  t o subhedral  b i o t i t e , and minor i n t e r s t i t i a l c a l c i t e ; euhedral accessory minerals  These two melilite,  t o subhedral  are included i n the main rock forming minerals.  M e l i l i t e forms most of the groundmass; i t may be c o a r s e l y c r y s t a l l i n e l i k e hauyne and pyroxene, or may be i n the form of f i n e grained subhedral l a t h s that show t r a e h y t i c alignment. small euhedral  T i t a n a u g i t e may occur as  c r y s t a l s that form a s u b - o p h i t i c texture with l a r g e r  e n c l o s i n g m e l i l i t e g r a i n s ( f i g . 14)•  B i o t i t e may form l a r g e p o i k i l -  i t i c c r y s t a l s enclosing t i t a n a u g i t e , m e l i l i t e , hauyne, and accessory minerals. With i n c r e a s i n g content  of t i t a n a u g i t e , pyroxene okaite  grades towards j a c u p i r a n g i t e i n composition.  J a c u p i r a n g i t e i s the  name given by Derby (l89l) to an igneous rock composed of 80$ t i t a n augite, 18$ magnetite, and 2$ of p e r o v s k i t e , a p a t i t e , and nepheline. Of the rocks used i n the present  study, the nearest  approach to it., i s  a rock c o n t a i n i n g 65$ t i t a n a u g i t e , 10$ hauyne, 8$ m e l i l i t e , 5$ b i o t i t e , 6$ magnetite, 4$ p e r o v s k i t e , and 1$ each'of a p a t i t e and c a l c i t e .  Gold  (personal communication) reports a s i m i l a r rock with 75$ t i t a n a u g i t e . With decreasing  content of t i t a n a u g i t e , pyroxene okaite  grades i n t o true o k a i t e , as defined on page 29. type Is r e l a t i v e l y common a t Husereau:. H i l l .  The intermediate  I t s most noteworthy  feature i s that, t i t a n a u g i t e has coronas of m e l i l i t e and b i o t i t e , apparently  formed by magmatlc r e a c t i o n ( f i g s . 1.5 and 16). The o r i g -  i n a l boundaries of these p a r t i a l l y resorbed  pyroxene g r a i n s are shown  by r i n g s of small magnetite and perovskite g r a i n s , deposited a t the time of r e s o r p t i o n .  The r e p l a c i n g m e l i l i t e i s t e x t u r a l l y  different  than that i n the groundmass, commonly being formed of unoriented i n a medium grained groundmass of o r i e n t e d m e l i l i t e .  This  grains  striking  t e x t u r e may be i d e n t i f i e d even where the whole o f the o r i g i n a l pyroxene  Figure 14: Small euhedral c r y s t a l s of t i t a n a u g i t e enclosed i n large anhedral c r y s t a l s of m e l i l i t e . The opaque mineral i s magnetite. (Plane l i g h t , x20)  Figure 1^: D e t a i l of a r e s o r p t i o n corona around t i t a n a u g i t e . Small f l a k e s of b i o t i t e are enclosed i n m e l i l i t e . The former edge of the t i t a n a u g i t e c r y s t a l i s marked by a concentration of small grains of magnetite and perovskite (plane l i g h t , x 4 8 )  42  F i g u r e 16: P a r t l y resorbed t i t a n a u g i t e c r y s t a l i n pyroxene okaite. The o r i g i n a l c r y s t a l i s o u t l i n e d b y a b a n d o f s m a l l p e r o v s k i t e and magnetite grains. The groundmass o f o r i e n t e d m e l i l i t e l a t h s c o n t a i n s s m a l l g r a i n s o f p e r o v s k i t e , a p a t i t e , m a g n e t i t e , hauyne, and biotite. ( P l a n e l i g h t , x20)  F i g u r e 17: "Ghost" o f completely resorbed t i t a n a u g i t e c r y s t a l , r e p l a c e d a l m o s t s o l e l y by m e l i l i t e . Large c r y s t a l at lower l e f t i s hauyne. T h e g r o u n d m a s s i s t h e same a s i n f i g u r e 16. (Plane  light,  x20)  43  {  g r a i n has been resorbed ( f i g . 1 7 ) .  In some of these 'ghosts' small  disconnected pyroxene remnants are seen to be o p t i c a l l y even to the extent that the same composition zone may s e v e r a l remnants.  continuous,  be t r a c e d through  The pyroxene 'ghosts' are commonly crowded with  small, b i o t i t e c r y s t a l s that may  form a crudely r a d i a l s t r u c t u r e ; there  i s a g r e a t e r p r o p o r t i o n of b i o t i t e here than i n : t h e surrounding groundmass.  Rocks with resorbed pyroxene consistently, contain about 20$  hauyne, and grade d i r e c t l y to o k a i t e .  Rocks with resorbed pyroxene  are not found d i r e c t l y t r a n s i t i o n a l , to m e l i l i t e o k a i t e . Okaite Okaite i s l a r g e l y made up of m e l i l i t e , hauyne, and b i o t i t e , and i s u s u a l l y coarse grained.  M e l i l i t e i s the dominant, m i n e r a l , and  i s normally subhedral ( f i g . 1 8 ) .  Hauyne occurs as l e s s w e l l formed  c r y s t a l s than In pyroxene o k a i t e , and does not show the marked zoning of i n c l u s i o n s ( f i g . 1 9 ) .  C a l c i t e and b i o t i t e are i n t e r s t i t i a l , the  l a t t e r mineral commonly being found i n c o n t a c t with, or surrounding, magnetite  grains.  Magnetite  c r y s t a l form; both may  and p e r o v s k i t e vary i n the development of  be anhedral, even ragged, although magnetite  form rounded subhedral. g r a i n s .  may  A p a t i t e i s I n v a r i a b l y euhedral.  Where nepheline i s present, hauyne i s reduced i n amount, though r a r e l y t o t a l l y absent. a l l y i n close association.  Where both are present they are norm-  Hauyne may  form a rim around nepheline so  that i n places i t appears that hauyne grains have nepheline cores ( f i g s . 20 and  21).  M e l i l i t e Okaite and Pegmatite  Phases  As the content of s p e c i f i c sodium minerals decreases, the content of m e l i l i t e i n c r e a s e s u n t i l the rock, can be designated as m e l i l i t e o k a i t e , f r e e of both nepheline and hauyne.  With t h i s t r a n -  s i t i o n m e l i l i t e tends to become euhedral, and the c a l c i t e  content  increases; c a l c i t e f i l l s the spaces between m e l i l i t e c r y s t a l s as large p o i k i l i t i c crystals.  T h i s i s r e f e r r e d to as m e l i l i t e - c a l c i t e  M e l i l i t e okaite also grades i n t o a pegmatite  rock.  phase In which magnetite,  p e r o v s k i t e , and a p a t i t e are i n t e r s t i t i a l to m e l i l i t e as groups of  Figure 18: T y p i c a l l y subhedral m e l i l i t e i n okaite. Other minerals are a p a t i t e (A), b i o t i t e (B), c a l c i t e ( c ) ; the opaque mineral i s magn e t i t e (crossed N i c o l s , x20)  Figure 19: T y p i c a l l y subhedral hauyne i n okaite. The opaque i n c l u s i o n s here are not o r i e n t e d . A i s a p a t i t e , B i s b i o t i t e , and M i s m e l i l i t e (plane l i g h t , x20)  45  Figure  21:  The  same  as  figure  20  (crossed  MIcols,  x20)  euhedral c r y s t a l s , apparently as a second generation to those that are enclosed w i t h i n m e l i l i t e .  A good example of t h i s i s shown i n  f i g u r e 22, i n which two m e l i l i t e c r y s t a l s e n c l o s i n g a s i n g l e  needle  of a p a t i t e are themselves surrounded by second generation a p a t i t e .  Figure 22: Two generations of a p a t i t e i n m e l i l i t e okaite pegmatite. A f i n e c r y s t a l of a p a t i t e ( A i ) enclosed i n p a r t l y a l t e r e d m e l i l i t e (M) i s cut by coarser grained massive a p a t i t e ( A ) (plane l i g h t , x20) 2  Pale yellow augite i s present i n pegmatite phases i n small amounts.  I t i s u s u a l l y i n contact with, or r e p l a c i n g , m e l i l i t e  c r y s t a l s , but i n some s e c t i o n s i t forms groups of euhedral  crystals  with orange andradite garnet and small amounts of pale green b i o t i t e i n coarsely c r y s t a l l i n e c a l c i t e .  I t i s probable that such an  assemblage i s formed by high temperature a l t e r a t i o n (see pages 106- 108).  In one t h i n s e c t i o n z e o l i t e s form a mass of f i b r o u s  c r y s t a l s w i t h i n a hexagonal o u t l i n e ; t h i s i s due to the  alteration  of hauyne, as t r a c e s of i n c l u s i o n g r i d s are present i n some g r a i n s . Hauyne i s not g e n e r a l l y present i n the pegmatite phase of m e l i l i t e okaite.  A rare mineral i n m e l i l i t e okaite Is m o n t i c e l l i t e ; i t has only been found i n i two  specimens of f r e s h m e l i l i t e o k a i t e .  In these  rocks, which come from d i f f e r e n t p a r t s of the o k a i t e i n t r u s i o n , the subhedral m e l i l i t e i s extremely coarse grained, s i n g l e c r y s t a l s exceeding three centimeters across. g r a i n s , and may  comprise  M e l i l i t e forms i n t e r l o c k i n g  80$ of the t o t a l , rock forming minerals.  M o n t i c e l l i t e i s present i n . s m a l l amounts a s s o c i a t e d with c a l c i t e . These two minerals form a coarse grained intergrowth with m e l i l i t e . Figures 23 and 24 show the t y p i c a l , form of t h i s intergrowth.  Patches  of disconnected m o n t i c e l l i t e show o p t i c a l c o n t i n u i t y , though contained i n more than one c r y s t a l of m e l i l i t e . m o n t i c e l l i t e may rim  The o p t i c a l c o n t i n u i t y of  change w i t h i n one m e l i l i t e g r a i n .  i n c l u s i o n s i n m e l i l i t e , such as a p a t i t e .  Monticellite  I t may  may  p a r t l y rim  m e l i l i t e i n contact with c a l c i t e . Primary  Calcite  C a l c i t e i s found without, exception In a l l rocks examined from the o k a i t e i n t r u s i o n s .  Normally i t forms c l e a r ,  g r a i n s between the other rock forming minerals. texture i n pegmatite described.  interstitial  Its p o i k i l i t i c  phases of m e l i l i t e okaite has already been  Secondary c a l c i t e i s present i n a l t e r e d o k a i t e ; I t has  a t o t a l l y d i f f e r e n t appearance to the i n t e r s t i t i a l - c a l c i t e of f r e s h rocks, o c c u r r i n g e i t h e r as a very f i n e grained groundmass c o n t a i n i n g other a l t e r a t i o n minerals, or as a f i n e grained aggregate fractures.  The coarse i n t e r s t i t i a l  filling  c a l c i t e of f r e s h o k a i t e s ( f i g s .  6 and 18) remains u n a f f e c t e d i n a l t e r e d rocks, and i t s presence preserves the c r y s t a l o u t l i n e s of completely a l t e r e d m e l i l i t e ( f i g . 25).  crystals  Thus i t appears that coarse c a l c i t e was present i n the  rocks before secondary  alteration.  I t was probably the l a s t mineral  to be formed during the c r y s t a l l i z a t i o n of the o k a i t e s . Sequence of C r y s t a l l i z a t i o n and Development of the Okaite S e r i e s In most of the rocks of the okaite s e r i e s there Is good evidence a v a i l a b l e f o r determining the sequence of c r y s t a l l i z a t i o n . Although i t i s r e a l i z e d that the presence of one* mineral i n c l u d e d i m another i s not always r e l i a b l e f o r such a deduction, c e r t a i n i n c l u s i o n  Figure 23t Intergrowth of m e l i l i t e (dark and medium grey) and m o n t i c e l l i t e ( l i g h t grey) (crossed M l c o l s , x20)  Figure 24:  D e t a i l of m e l i l i t e (M) m o n t i c e l l i t e (Mo) C i s c a l c i t e (crossed N i c o l s , 148)  intergrowth;  Figure 25:  Primary c a l c i t e i n t e r s t i t i a l t o a l t e r e d m e l i l i t e (plane l i g h t , z20)  crystals  sequences i n the okaite s e r i e s are so consistent that, t a k i n g i n t o account other determinative t e x t u r e s , an i n t e r p r e t a t i o n can be made with reasonable c e r t a i n t y . Apatite may be seen i n c l u d e d i n every other mineral  present,  and i s i t s e l f i n v a r i a b l y c l e a r ; i t i s therefore thought t o have been the f i r s t mineral deposited.  I t s presence as f i b r o u s bursts i n pegmatitic  rocks a l s o i n d i c a t e s that i t was one o f the l a s t minerals formed i n such rocks.  Magnetite and perovskite c r y s t a l l i z e d next; both these  minerals are seen included i n a l l other minerals except a p a t i t e . Perovskite i s r a r e l y i n c l u d e d i n magnetite, and commonly borders magnetite g r a i n s .  In some rocks, e s p e c i a l l y pyroxene o k a i t e , small  c r y s t a l s o f perovskite i n the groundmass, and t h i n rims on magnetite, both of p a l e r colour than the l a r g e e a r l i e r perovskite c r y s t a l s , i n d i c a t e that perovskite continued t o c r y s t a l l i z e i n small q u a n t i t i e s a f t e r the formation o f the l a r g e r g r a i n s .  Where s p i n e l i s present, i t i s e a r l i e r  than magnetite. The  e a r l i e s t t i t a n a u g i t e of pyroxene okaite may have  c r y s t a l l i z e d , at centres  of  about t h e  large titanaugite  but i n some c r y s t a l s o n l y at  same t i m e as m a g n e t i t e  the  edges.  crystals  magnetite  include crystals  and p e r o v s k i t e  Large pyroxene c r y s t a l s  grown a r o u n d hauyne c r y s t a l s ,  r e a c t e d w i t h the  stage o f c r y s t a l l i z a t i o n . crystals  at  this  (fig.  of t i t a n a u g i t e .  magnetite  R e a c t i o n c o r o n a s a r e absent  and t i t a n a u g i t e  of  original titanaugite,  of  its  titanaugite  i n pyroxene r i c h  not exceed  15$ It  of the t o t a l  the  rocks;  crystallize  at t h e b o r d e r s o f  In rocks that  show o n l y  the p r o p o r t i o n of t i t a n a u g i t e  c r y s t a l l i z a t i o n , o r at  this  the extent o f r e s o r p t i o n  forming l a t e  grains.  and o k a i t e  e n v e l o p i n g magma a t  d i r e c t l y r e l a t e d to  s o l i d i f i c a t i o n , with b i o t i t e  before  around t i t a n a u g i t e  h a u y n e , m e l i l i t e , and a c c e s s o r y m i n e r a l s c o n t i n u e d t o until  have  26).  The p r o p o r t i o n o f magma t o  stage i s  apatite,  so hauyne had s t a r t e d t o f o r m  i n r o c k s t r a n s i t i o n a l between p y r o x e n e o k a i t e titanaugite  at  'ghosts* completion  s t a g e o f magmatic r e a c t i o n , d i d  rock forming m i n e r a l s .  seems t h e r e f o r e t h a t  melilite first  formed i n t h e magma  by means o f magmatic r e a c t i o n w i t h s o l i d t i t a n a u g i t e .  It  however,  melilite,  that  this  r e a c t i o n was n o t t h e  b e c a u s e more m e l i l i t e i s p r e s e n t f o r by t h i s  reaction alone.  all.  sole  source of  i n t h e r o c k s t h a n c a n be  Although traces  f o u n d i n a few r o c k s o f t r u e o k a i t e no p y r o x e n e r e l i c s a t  the  are found i n c l u d e d  The c o r o n a s o f m e l i l i t e and b i o t i t e  suggest t h a t  of  are a l s o seen t o  t h e c r y s t a l l i z a t i o n o f p y r o x e n e was c o m p l e t e  crystals  and p e r o v s k i t e ;  appears,  accounted  o f r e s o r b e d pyroxene  composition, the m a j o r i t y  are  contain  A l s o , most o f t h e m e l i l i t e i n p y r o x e n e  o k a i t e has c r y s t a l l i z e d d i r e c t l y f r o m t h e magma, and n o t by r e a c t i o n . It set  i s p o s s i b l e that of  t h e o r i g i n a l magma d e p o s i t e d p y r o x e n e u n d e r one  c o n d i t i o n s , and m e l i l i t e u n d e r a n o t h e r ;  'ghosts'  of titanaugite  represent  w i l l be d i s c u s s e d more f u l l y  a change o f  i n another  I n rocks which d i d not f i r s t melilite  the  conditions.  At t h i s  This idea  section. crystallize  and hauyne c r y s t a l l i z e d t o g e t h e r u n t i l  magma l e f t .  c o r o n a s and  titanaugite,  t h e r e was  little  s t a g e , when t h e magma was p r e s u m a b l y i n t e r s t i t i a l ,  b i o t i t e was d e p o s i t e d between e x i s t i n g m i n e r a l g r a i n s . a l s o f o r m e d by r e a c t i o n o f t h e magma w i t h m a g n e t i t e ,  B i o t i t e was  as i s  evidenced  Figure  27:  B i o t i t e (B) rims magnetite i n o k a i t e (crossed N i c o l s , x20)  by the common occurrence of b i o t i t e rimming magnetite g r a i n s ( f i g . 2 7 ) • Biotite-rimmed magnetite g r a i n s are somewhat ragged i n o u t l i n e ; where magnetite i s euhedral, b i o t i t e rims are not present. Where nepheline i s found i t i s u s u a l l y i n t e r s t i t i a l to m e l i l i t e , i n d i c a t i n g that i t was m e l i l i t e was w e l l advanced. more or l e s s hauyne.  formed when the c r y s t a l l i z a t i o n of  Nepheline i s i n v a r i a b l y a s s o c i a t e d with  As has been stated on p. 29» the a v a i l a b i l i t y  of CaSC-4 i s the f a c t o r that governs which o f hauyne or nepheline w i l l form.  When the a v a i l a b l e sulphate i s removed by the d e p o s i t i o n o f  hauyne, nepheline continues i n p l a c e of hauyne as the product of crystallization.  F i g u r e s 20 and 21 show nepheline to have f i l l e d the  c e n t r a l p a r t s of i n t e r s t i t i a l spaces i n which hauyne had s t a r t e d t o crystallize. Figure 2 8 , below, shows the courses of c r y s t a l l i z a t i o n of pyroxene  o k a i t e , o k a i t e , and m e l i l i t e o k a i t e pegmatite.  l i n e i n d i c a t e s the major p e r i o d o f c r y s t a l l i z a t i o n ;  A solid  a dashed l i n e i s  used to denote minor i n i t i a l o r c o n t i n u i n g c r y s t a l l i z a t i o n i n some cases.  Pyroxene Okaite  M e l i l i t e Okaite s Pegmatite t t  Titanaugite Hauyne Melilite Nepheline Biotite Augite Calcite Apatite Magnetite Perovskite  Figure 28t Sequence of c r y s t a l l i z a t i o n f o r rocks of the o k a i t e s e r i e s . Temperature decreases with time to the r i g h t f o r each column  C l o s i n g the sequence o f c r y s t a l l i z a t i o n i n a l l cases i s c a l c i t e ; t h i s c o n c l u s i o n i s reached by c o n s i d e r a t i o n of i t s i n v a r i a b l y  interstitial  nature.  In the pegmatite phases of m e l i l i t e o k a i t e ,  a second generation of a p a t i t e , p e r o v s k i t e , and magnetite helped t o fill  the spaces between m e l i l i t e c r y s t a l s .  The term second  generation r e f e r s to the s i t u a t i o n apparent i n t h i n sections a t r u e r r e p r e s e n t a t i o n i s probably given by the continued c r y s t a l l i z a t i o n of these accessory minerals a f t e r m e l i l i t e had ceased to  form.  Minor amounts of augite were also formed at t h i s stage,  p a r t l y by r e a c t i o n o f already formed m e l i l i t e with the remaining fluid.  Again, the l a s t mineral t o form was c a l c i t e , s i n g l e  anhedral c r y s t a l s o f which b i n d together the l a t e concentration of  accessory minerals.  Monticellite Alnoite The t e x t u r a l f e a t u r e s of the a l n o i t e of Husereau are  t o t a l l y d i f f e r e n t from those of the o k a i t e s .  Hill  The primary  compositional d i f f e r e n c e i n these rocks i s the presence of o l i v i n e , m o n t i c e l l i t e , and hornblende i n a l n o i t e .  T h i n s e c t i o n examination  reveals that much o f the a l n o i t e i s made up o f fragments of these minerals plus fragments of t i t a n a u g i t e , m e l i l i t e , hauyne, b i o t i t e , c a l c i t e , p e r o v s k i t e , a p a t i t e , magnetite, and melanite garnet. Some of the fragments are d i s t i n c t l y l i t h i c , and may be recognized as i n c l u s i o n s of o k a i t e or pyroxene okaite.  These broken c r y s t a l  and l i t h i c fragments are set i n a very f i n e grained groundmass of m e l i l i t e , magnetite, and dark green s p i n e l .  Melilite  crystals  may be t r a c h y t i c a l l y a l i g n e d , o r may form a mosaic of i n t e r l o c k i n g equant g r a i n s .  The former texture i s seen i n specimens from the  edges o f the a l n o i t e body. Reaction rims are extremely common around g r a i n s of o l i v i n e , pyroxene, and hornblende ( f i g .  29).  rims o l i v i n e , and hornblende may rim pyroxene.  M o n t i c e l l i t e commonly M o n t i c e l l i t e and  hornblende appear to have c r y s t a l l i z e d w i t h i n the a l n o i t e , but have themselves become corroded and resorbed.  Biotite,  calcite,  and m e l i l i t e form a f i n e intergrowth around these mafic mineral grains.  C r y s t a l fragments o f a p a t i t e , m e l i l i t e , p e r o v s k i t e ,  Figure  29t  Eeaction  rim around t i t a n a u g i t e i n m o n t i c e l l i t e a l n o i t e (plane l i g h t , x20)  magnetite, and garnet do not have r e a c t i o n rims. o l i v i n e i s uncertain.  The o r i g i n o f  I t may have been an e a r l y product o f c r y s t -  a l l i z a t i o n o f a l n o i t e magma.  A l t e r n a t i v e l y , i t may have been derived  from an o l i v i n e bearing rock at depth, such as the o l i v i n e - p e r t h i t e marble that crops out about 2,000 f e e t west of the a l n o i t e exposure. In one fragment of t i t a n a u g i t e i s included a rounded g r a i n o f o l i v i n e with a r e a c t i o n rim to the t i t a n a u g i t e ( f i g . 30). Many fragments are grouped i n what appear t o be l a r g e r fragments several centimeters i n diameter; the outer edges of the l a r g e r fragments are d i s t i n g u i s h e d i n t h i n s e c t i o n by a concentration material  of f i n e opaque  ( f i g . 31). The  groundmass m e l i l i t e has very low b i r e f r i n g e n c e ,  suggesting that i t has a d i f f e r e n t composition t o the m e l i l i t e i n rocks of the okaite s e r i e s .  I t i s almost o p t i c a l l y i s o t r o p i c , but  negative u n i a x i a l i n t e r f e r e n c e f i g u r e s may be obtained.  It i s  thought to be nearer to akermanite i n composition than o k a i t e m e l i l i t e , i n d i c a t i n g that a l n o i t e i s r i c h e r i n magnesium than okaite.  55  Figure 3 0 : O l i v i n e (o) surrounded by t i t a n a u g i t e (Px) i n m o n t i c e l l i t e a l n o i t e ; Mo i s m o n t i c e l l i t e , B i s b i o t i t e (plane l i g h t , x 2 0 ) .  Figure 3 1 : Part of large fragment i n m o n t i c e l l i t e a l n o i t e ; i t contains broken c r y s t a l s of t i t a n a u g i t e (Px), hornblende (Hb), m e l i l i t e (M), and c a l c i t e (C) (plane l i g h t , x 2 0 ) .  This i s shown t o be the ease by the chemical  analyses given i n a  later section. Included fragments vary from microscopic  s i z e t o angular t o  rounded blocks o f o k a i t e and pyroxene okaite g r e a t e r than 10 cm across. The  l a r g e r included c r y s t a l fragments o f t i t a n a u g i t e , m e l i l i t e , a p a t i t e ,  p e r o v s k i t e , and magnetite are i d e n t i c a l i n appearance t o t h e same minerals  i n rocks o f the okaite s e r i e s ; melanite garnet  derived from a l t e r e d o k a i t e .  has been  The b r e c c i a t e d nature of the weathered  rock surface has been described on page 24* A l n o i t e i s l a t e r than the main o k a i t e i n t r u s i o n . have been introduced when the surrounding order t o e x p l a i n i t s f i n e groundmass.  I t must  o k a i t e was q u i t e c o o l i n  The f i n e r g r a i n s i z e near the  margins i s i n d i c a t i v e o f marginal c h i l l i n g ; the contact i s not d i r e c t l y exposed, so t h i s could not be v e r i f i e d i n the f i e l d .  Brecciation i n  a rock o f t h i s nature has been explained i n many cases by explosion. The  chemical  rock.  a n a l y s i s shows an appreciable p r o p o r t i o n o f GO2 i n the  I t i s p o s s i b l e that the r e l e a s e o f c o n f i n i n g pressure  as the  magma rose through the earth* s c r u s t caused expulsion and expansion of CO2 i n gas form; t h i s i n t u r n may have caused explosive b r e c c i a t i o n of the w a l l rocks, fragments of which were i n c l u d e d i n the a l n o i t e magma.  Repeated explosion may have caused b r e c c i a t i o n o f already  s o l i d i f i e d a l n o i t e , fragments being cemented by remaining i n t e r s t i t i a l f l u i d s , o r by a new i n f l u x o f magma from more l i q u i d p a r t s below. F r e e z i n g must have taken place q u i c k l y i n order t o preserve rims around included g r a i n s .  The v a r i e d and unstable mineral  d i d not have time t o reach e q u i l i b r i u m conditions before solidification.  the r e a c t i o n assemblage  complete  DETAILED MIHBRALOOIC STUDIES  Methods Pyroxene, m e l i l i t e , and hauyne are the most important forming minerals i n the o k a i t e s .  rock  B i o t i t e i s not considered i n t h i s  category because i t i s present i n a l l the various types of o k a i t e i n widely v a r y i n g p r o p o r t i o n s .  Pyroxene and hauyne are both used to  define rock types: m e l i l i t e i s present i n a l l types.  Hauyne was  not  studied i n d e t a i l , p a r t l y because i t does not vary much i n chemical composition, and p a r t l y because i t i s r a r e l y f r e e from numerous t i n y i n c l u s i o n s and p a r t i a l a l t e r a t i o n to z e o l i t e s . Pyroxene and m e l i l i t e can be obtained i n a r e l a t i v e l y pure and u n a l t e r e d s t a t e . by X-ray methods.  Both have been studied with the microscope  and  R e f r a c t i v e i n d i c e s were measured by immersion i n  o i l s of known i n d i c e s .  O p t i c a l data f o r t i t a n a u g i t e were measured  u s i n g a u n i v e r s a l stage. The occurrence of s p i n e l i n magnetite provoked the opaque minerals i n p o l i s h e d s e c t i o n . magnetite,  a study of  The opaque minerals i n c l u d e  s p i n e l , p y r r h o t i t e , c h a l c o p y r i t e , and p e r o v s k i t e .  data f o r s p i n e l and magnetite  X-ray  are given.  Melilite Natural m e l i l i t e i s commonly r e f e r r e d to as being composed predominantly  of the two end members akermanite  (Ca2MgSi207) and  g e h l e n i t e (CagAlgSiOy), with minor s u b s t i t u t i o n by sodium, and both d i v a l e n t and t r i v a l e n t i r o n .  potassium,  The r o l e of the monovalent  elements has been a subject of mystery f o r some time.  Berman ( 1 9 2 9 )  acknowledged the prevalent content of between 2 and 5$ sodium i n n a t u r a l m e l i l i t e s , and also recognized that many specimens show a higher p r o p o r t i o n of s i l i c a than can be accounted f o r by s o l i d s o l u t i o n between akermanite new  and g e h l e n i t e alone.  He proposed  end members, s o d a - m e l i l i t e (Na2Si307), and s u b - m e l i l i t e  two (CaSi30f).  He suggested that a l l n a t u r a l m e l i l i t e s could be chemically described as proportions o f these f o u r end members, allowing minor s u b s t i t u t i o n of potassium f o r sodium, f e r r i c I r o n f o r aluminum, and f e r r o u s i r o n and manganese f o r magnesium; excess calcium might s u b s t i t u t e f o r magnesium a l s o .  In a l a t e r paper Barman (1937> P«394) s t a t e d that  "Ma enters i n t o the composition t o a r a t h e r l i m i t e d extent with the composition CaHaAlSigQf as a probable l i m i t o f the Ca-Na s e r i e s . " Goldsmith (1948), i n a study o f s y n t h e t i c m e l i l i t e  solid  s o l u t i o n s , s t a t e d that only 15$ N a 2 S i 3 0 7 , which i s equivalent t o 3*85$ N a 2 0 , could be accomodated i n g e h l e n i t e , and that none could be accomodated i n akermanite.  He found that t h i s d i d not f u l l y  explain the sodium content o f n a t u r a l m e l i l i t e s , as some may contain as much as 6$ Na^O.  He was unable t o prepare the proposed soda-  m e l i l i t e , ^28130-7. A recent study o f s y n t h e t i c m e l i l i t e s by Nurse and. Midgley (1953) i n v o l v e d the p r e p a r a t i o n o f m e l i l i t e end members.  Apart from  akermanite and g e h l e n i t e , they produced i r o n - g e h l e n i t e (Ca2FeAlSi07), iron-akermanite (CagFeSigOy), and sodium m e l i l i t e (NaCaAlSI^Oy).  The  members iron-akermanite and sodium m e l i l i t e are only known a r t i f i c i a l l y , and on heating, both d i s s o c i a t e below t h e i r m e l t i n g p o i n t s t o give w o l l a s t o n i t e plus k i r s c h s t e i n i t e (CaFeSiO-4)  o r nepheline r e s p e c t i v e l y .  Nurse and Midgley produced a d d i t i o n a l evidence that up t o 20$ of the potassium analogue o f sodium m e l i l i t e (KCaAlSi207) can e x i s t i n s o l i d s o l u t i o n with g e h l e n i t e , although i t cannot be prepared s y n t h e t i c a l l y . A l l the m e l i l i t e s i n v e s t i g a t e d by Nurse and Midgley, with the  exception o f potassium m e l i l i t e , e x h i b i t complete s o l i d  w i t h i n each other at temperatures near t h e i r melting p o i n t s .  solution Incomplete  s o l i d s o l u t i o n may e x i s t below p r e s e n t l y unknown temperatures, the presence o f one end member p o s s i b l y l i m i t i n g the extent o f s o l i d s o l u t i o n o f another, and r e s u l t i n g i n e x s o l u t i o n i n some m e l i l i t e s at temperatures lower than that o f formation.  Such an explanation i s  thought to account f o r the peg s t r u c t u r e found i n some n a t u r a l m e l i l i t e s . It should be noted that peg s t r u c t u r e has not been observed i n m e l i l i t e from Oka.  59  N a t u r a l m e l i l i t e may c o n t a i n a p p r e c i a b l e q u a n t i t i e s o f i r o n and a l k a l i s .  A g e n e r a l formula f o r m e l i l i t e may be w r i t t e n : A  (2+x) (l-x) 2°7 C  B  where: A = Na, K, Ca, Mg, Mn, F e " B = A l , Fe"' C = Al, Si T h i s can be f u r t h e r broken down i n t o t h e t h r e e type molecules sodium m e l i l i t e (Na), g e h l e n i t e (Ge), and akermanite ( A k ) , a l l o f which may c o n t a i n s u b s t i t u t i o n o f m e t a l l i c i o n s as shown: (Na,K)CaAlSi207  -  sodium m e l i l i t e type molecule (Na)  Ca2(Al,Fe* )AlSiO«jr  -  g e h l e n i t e type molecule (Ge)  Ca2(Ca,Mg,Fe )Si207  -  akermanite type molecule (Ak)  M  M  T h i s i s done here s o l e l y f o r the sake o f c l a r i t y .  These end members  must not be c o n s i d e r e d t o be present as m o l e c u l a r e n t i t i e s , as t h e d i f f e r e n t s u b s t i t u t i n g i o n s are d i s t r i b u t e d s t a t i s t i c a l l y throughout a continuous c r y s t a l l a t t i c e .  The sodium m e l i l i t e type molecule i s  used t o account f o r t h e a l k a l i content o f n a t u r a l m e l i l i t e s . be d e r i v e d from t h e s o d a - m e l i l i t e molecule used by Berman  I t can  (1929)  by  the a d d i t i o n o f g e h l e n i t e a c c o r d i n g t o t h e f o l l o w i n g e q u a t i o n : ^281307 + C a A l S i 0 7 2  2  = 2NaCaAlSi207  The excess s i l i c a i n some n a t u r a l m e l i l i t e s i s so f a r u n e x p l a i n e d , except by Berman's s u b - m e l i l i t e (CaSi307)} t h i s molecule does not represent a t r u e m e l i l i t e , and i t i s d o u b t f u l i f i t c o u l d be c o n t a i n e d i n s o l i d s o l u t i o n i n m e l i l i t e .  W i n c h e l l and W i n c h e l l  (1951) suggest t h a t t h e double m e l i l i t e molecule C a 3 A l 2 S i 4 0 i 4 may be present i n some n a t u r a l m e l i l i t e s .  The p r o p o s a l o f t h i s molecule,  although i t c o u l d be used t o e x p l a i n excess s i l i c a , does not answer the problem, as i t can be d e r i v e d by adding g e h l e n i t e t o s u b - m e l i l i t e a c c o r d i n g t o t h e equation: CaSi3(>7 + C a 2 A l 2 S i 0 = Ca3Al Si40i4 7  2  Another e x p l a n a t i o n f o r excess s i l i c a , as y e t u n i n v e s t i g a t e d , i s t h a t a d e f e c t s t r u c t u r e may be present i n m e l i l i t e , i n v o l v i n g t h e absence of m e t a l l i c i o n s .  I t i s observed i n t h i n s e c t i o n t h a t t h e m e l i l i t e  contained  i n any member o f t h e o k a i t e s e r i e s has very s i m i l a r p r o p e r t i e s t o t h e m e l i l i t e i n any o t h e r member, i n d i c a t i n g t h a t i t does not v a r y much i n composition from one rook type t o another.  I n order t o v e r i f y  t h i s , specimens o f m e l i l i t e from each o f pyroxene o k a i t e , o k a i t e , and m e l i l i t e o k a i t e pegmatite were used f o r X-ray powder photographs.  The  three specimens gave i d e n t i c a l p a t t e r n s w i t h i n t h e l i m i t s o f e x p e r i mental e r r o r .  I t was expected t h a t t h e m e l i l i t e from m o n t i c e l l i t e  a l n o i t e would g i v e a s l i g h t l y d i f f e r e n t p a t t e r n t o those o f m e l i l i t e from t h e o k a i t e s , as i t shows a much lower b i r e f r i n g e n c e i n t h i n section.  I t s p a t t e r n , however, i s not a p p r e c i a b l y d i f f e r e n t . Table I I  g i v e s t h e r e l a t i v e i n t e n s i t i e s and i n t e r p l a n a r spacings f o r t h e f o u r m e l i l i t e samples.  The powder p a t t e r n o f m e l i l i t e from o k a i t e i s  reproduced i n f i g u r e 32. E r v i n and Osborn (1949) s t u d i e d t h e  akermanite-gehlenite  system, and produced diagrams t o show l i n e a r v a r i a t i o n o f c e r t a i n i n t e r p l a n a r spacings w i t h composition.  They estimated  t h a t t h e composition  could be determined t o an accuracy o f 2j$> f o r m e l i l i t e s w i t h i n t h e akermanite-gehlenite  s e r i e s by measuring these spacings and p l o t t i n g  them on a v a r i a t i o n graph.  T h i s method i s a p p l i e d t o m e l i l i t e from  the o k a i t e s and from a l n o i t e , and t h e r e s u l t i s shown i n f i g u r e 33. Because t h e s l i g h t d i f f e r e n c e s i n t h e f o u r X-ray powder photographs are w i t h i n t h e l i m i t s o f e r r o r i n reading t h e f i l m s , average values are used f o r the spacings chosen as d i a g n o s t i c by E r v i n and Osborn. I t i s seen t h a t t h e r e i s l i t t l e o r no agreement shown between t h e spacings and composition w i t h i n t h e akermanite-gehlenite  series. I t  i s t h e r e f o r e assumed t h a t m e l i l i t e from Oka c o n t a i n s an a p p r e c i a b l e p r o p o r t i o n o f at l e a s t one o t h e r end member o f t h e m e l i l i t e group. A l l m e l i l i t e s are t e t r a g o n a l , t h e r e f o r e o p t i c a l l y u n i a x i a l . A l l a r e o p t i c a l l y negative  except akermanite.  S o l i d solutions with  a p p r e c i a b l e amounts o f akermanite, such as Ak5oOe40, are t h e r e f o r e optically isotropic.  M e l i l i t e from o k a i t e i s i n v a r i a b l y o p t i c a l l y  n e g a t i v e , w i t h b i r e f r i n g e n c e between 0.004 and 0*007.  Compositional  zoning i s present i n m e l i l i t e from p e g m a t i t i c phases o f m e l i l i t e o k a i t e s i n g l e c r y s t a l s showing s l i g h t l y h i g h e r b i r e f r i n g e n c e towards t h e i r  Table I I X-Hay Powder Data f o r M e l i l i t e from Oka 1  /  / / / / /  /  2  3  hkl  I  dm  I  110 001 101 111 210 201 211 002 310 102 301 212 400 410 330 312 420  10 30 10 30 5 30 100 10 20 25 20 20 10 5 10 50  5-52 5.03 4.22 3.70 3.47 3.07 2.85 2.511 2.452  10 20 10 30 5 40 100 5 10 20 15 20 10 5 10 50  322 113  / 213 223 / 521  5 10 10 20 20 20  2.394  2.293 2.037 1.939 1.875 1.827 1.755 1.731 1.636 1.602 1.510 1.429 1.384  5 10 10 20 20 20  m  I  5.52 5.02 4.21 3.70 3-47 3.07 2.85 2.514 2.452 2.389 2.295 2.030 1.945 1.877 1.826 1.754 1.731 1.638 I.605 1.509 1.427 1.386  20 30 20 40 10 40 100 5 25 30 20 30 20 5 10 50 10 10 10 30 30 30  d  4 d  m  5.49 5.03 4.22 3.69 3.48 3.06 2.85 2.511 2.450 2.385 2.288 2.030 1.935 1.877 1.820 1.753 1.729 1.633 1.599 1.502 1.426 1.381  I  dm  10 20 20 30 10 30 100 5 20 20 15 30 10 5 10 50  5.54 5.07 4.19 3.68 3.47 3.06 2.85 2.508 2.455 2.392 2.298 2.032 1.930 1.877 1.821 1.752 1.727 1.632 1.596 I.506 1.429 1.384  5 5 10 20 20 20  /  I n t e r p l a n a r spacings suggested by E r v i n and Osborn ( 1 9 4 9 ) f o r the determination o f composition i n the s e r i e s akermanite-gehlenite  1. 2. 3. 4.  Melilite Melilite Melilite Melilite  F i g u r e 32»  from from from from  pyroxene o k a i t e okaite m e l i l i t e okaite monticellite alnoite  X-ray powder photograph o f m e l i l i t e from m e l i l i t e  okaite  $2  F i g u r e 33; Graphs showing the change i n d - s p a c i n g s o f p l a n e s ( h k l ) i n a k e r m a n i t e - g e h l e n i t e s o l i d s o l u t i o n s , as g i v e n by E r v i n and Osborn •(1949,. p.719). The a v e r a g e d - s p a c i n g s o f t h e f o u r m e l i l i t e samples given i n t a b l e I I are p l o t t e d  63  o u t e r edges.  I f m e l i l i t e w i t h t h i s o p t i c a l character i s assigned t o  the a k e r m a n i t e - g e h l e n i t e s e r i e s , i t s r e f r a c t i v e i n d i c e s should he about 1.661 f o r H p.473).  e  and I.656 f o r IT© ( l i n e h e l l and W i n o h e l l , 1951,  The measured r e f r a c t i v e i n d i c e s o f o k a i t e m e l i l i t e do not  agree w i t h these v a l u e s , a g a i n i n d i c a t i n g t h a t a t l e a s t ©ne o t h e r member o f the m e l i l i t e group i s p r e s e n t . R e f r a c t i v e i n d i c e s f o r a s e l e c t i o n o f m e l i l i t e s from d i f f e r e n t rock types o f t h e o k a i t e s e r i e s were measured by immersion i n o i l s o f known i n d i c e s .  They are compared i n t a b l e I I I t o t h e  r e f r a c t i v e i n d i c e s g i v e n by Hurse and Midgley (1953) f o r t h e synt h e t i c end members, and a l s o t o those o f m e l i l i t e from uncompahgrite ( L a r s e n and Hunter, 1914» P»473)t t h e r e i s c l o s e agreement w i t h t h e latter.  The i n d i c e s o f akermanite are comparable, but the o p t i c  sign d i f f e r s .  The i n d i c e s o f sodium m e l i l i t e are the o n l y ones t h a t  are lower than those o f akermanite.  I t appears, t h e r e f o r e , t h a t t h e  two n a t u r a l m e l i l i t e s whose i n d i c e s are g i v e n below are b a s i c a l l y s o l i d s o l u t i o n s between akermanite, g e h l e n i t e , and sodium m e l i l i t e .  Table I I I Comparison o f R e f r a c t i v e I n d i c e s f o r V a r i o u s M e l i l i t e s  He  No  Iron-gehlenit e  1.723  1.726  -0.003  Iron-akermanite  1.673  1.690  -0.017  Gehlenite  I.658  1.669  -0.011  Akermanite  1.639  1.632  +0.007  Sodium m e l i l i t e  1.575  I.58O  -0.005  Okaite m e l i l i t e  1.627-1.630  1.632-1.636  -O.OO5-O.OO7  Uncompahgrite m e l i l i t e  1.625-1.627  1.632-1.634  -0.007  He-Ho  There are s e v e r a l o t h e r l o c a l i t i e s where i n t r u s i v e rocks r i c h i n m e l i l i t e are found.  Rocks v e r y s i m i l a r t o o k a i t e occur a t  t h r e e o f these l o c a l i t i e s , which are: T u r j a , K o l a P e n i n s u l a , where  64  the rock t u r j a i t e was f i r s t d e s c r i b e d (Ramsay, 1921); I r o n H i l l , Colorado, where t h e m e l i l i t e rock i s c a l l e d uncompahgrite  (Larsen  and Hunter, 1914)$ Gwasi a r e a , Kenya, where t u r j a i t e and uncompahg r i t e are a s s o c i a t e d w i t h c a r b o n a t i t e ( M c C a l l , 1959)*  The m e l i l i t e  at each o f these l o c a l i t i e s i s c o a r s e l y c r y s t a l l i n e , i s o p t i c a l l y n e g a t i v e , and shows i n t e r f e r e n c e c o l o u r s o f f i r s t o r d e r grey-white t o p a l e y e l l o w ; these p r o p e r t i e s are i n c l o s e agreement w i t h those observed f o r o k a i t e m e l i l i t e .  The r e f r a c t i v e i n d i c e s f o r m e l i l i t e  from I r o n H i l l and from Oka have been shown t o be remarkably s i m i l a r . A chemical a n a l y s i s o f m e l i l i t e from o k a i t e has been very k i n d l y made a v a i l a b l e by D.P. Gold o f M c G i l l U n i v e r s i t y .  This  a n a l y s i s has been c o r r e c t e d f o r the i m p u r i t i e s a p a t i t e and c a l c i t e , and i s compared i n t a b l e I V t o analyses f o r m e l i l i t e from T u r j a (Kranck, 1928) and from I r o n H i l l (Larsen and Hunter, 1914). T h e o r e t i c a l chemical contents f o r the end members g e h l e n i t e , akermanite, and sodium m e l i l i t e , and f o r f o u r t h e o r e t i c a l s o l i d s o l u t i o n s are g i v e n i n t a b l e V f o r comparison.  Table IV Chemical Analyses o f N a t u r a l  Si0 A1 0 Pe 03 PeO MnO MgO CaO Na 0 K 0 H 0 2  2  2  2  2  2  3  Melilites  1  2  3  44.13 10.80  41.93 8.34 1.54 3.40  41.99 11.03 O.50 1.97 0.38 6.93 32.93 4.26  -  2.04 0.16 4.35 34.63 3.40 tr 0.49  -  6.26 31.93 4.O8 O.48 1.08  -  0.01  1.  M e l i l i t e from uncompahgrite; W.T. S c h a l l e r , a n a l y s t . Hunter, 1914 > p.476  2.  M e l i l i t e from t u r j a i t e * P. E s k o l a , a n a l y s t .  3.  M e l i l i t e from o k a i t e ; H. U l k , a n a l y s t . communication  Larsen and  Kranck, 1928, p.19  D.P. G o l d , 1962, p e r s o n a l  Table V T h e o r e t i c a l Chemical Contents of Various M e l i l i t e Compounds  Si0  2  AI2O3  MgO CaO Na20  1  2  3  4  5  6  7  21.90 37-22  44-12  40.88  46.51 19.77  14.70 41.18  —  —  45.28 9.62 7.55 31.70 5.85  42.75 II.48 7.51 33.61 4.65  40.27 13.31 7.46 35-54 3.47  42.54 9.52 8.95 35-52 3.47  -  -  21.70 12.02  1) Gehlenite (Ge); 2) akermanite (Ak); 3) sodium m e l i l i t e (Na); 4) Na5oAk ; 5) N a 4 o G e A k o ; 50  10  5  6)  ^ C ^ C ^ Q :  7) N a o G e A k 6 . 3  10  0  Table VI D e r i v a t i o n of the Type Molecule E a t i o of M e l i l i t e from Oka ( a f t e r Hess, 1949)  Al  Al  Al  C  Cations  Fe"»  Al  B  to  A  seven 0  Na  Si0 AI2O3 Pe20 2  FeO MnO MgO CaO Na 0 2  3  41.99 11.03 0.50 1.97 0.38 6.93 32.93 4.26 99-99$  S i 703 Al  215  Fe"' 6 Fe" 26 Mn 6 Mg 170 Ca 589 Na 139  6  139  35  f  744  2.005  35 A+B 1110  2.992  2598  7.000  139  0 2598  $ type molecule r a t i o : -  Na3YGe2iAk^  2  66  f a b l e YI shows a method of d e r i v i n g the type molecule r a t i o of m e l i l i t e from the chemical a n a l y s i s .  T h i s method i s an  a d a p t a t i o n of the method used by Hess (1949) f o r pyroxenes. m e l i l i t e (A+B)  In  must equal 3, and C must equal 2, i f the oxygen 0  i s t o equal 7 i n the g e n e r a l formula A ( 2 + x ) ( l - x ) 2 ° 7 * B  c  F  e  r  r  i  c  i  r  o  n  can occupy o n l y the B p o s i t i o n because i t s i o n i c r a d i u s (O.64X i n s i x - f o l d c o o r d i n a t i o n ) i s too l a r g e f o r i t t o s u b s t i t u t e f o r s i l i c o n ( 0 . 3 9 ! i n f o u r - f o l d c o o r d i n a t i o n ) i n the C p o s i t i o n .  Aluminum can  occupy e i t h e r the B o r C p o s i t i o n t o a l i m i t e d e x t e n t ; i t s i o n i c r a d i u s i n s i x - f o l d c o o r d i n a t i o n i s 0 . 5 l i , and i n f o u r - f o l d c o o r d i n a t i o n i t i s O.48X (Ahrens, 1952).  I n o r d e r t o a s s i g n the c o r r e c t  p r o p o r t i o n o f aluminum t o each p o s i t i o n , the f o l l o w i n g procedure i s c a r r i e d outt l ) the a n a l y s i s i s reduced t o atomic p r o p o r t i o n s ; 2) the a l k a l i s (Ha and K) are equated w i t h a p r o p o r t i o n a t e amount of aluminum i n the B p o s i t i o n , a c c o r d i n g t o the sodium m e l i l i t e type m o l e c u l a r formula NaCaAlSi207; 3) f e r r i c i r o n i s equated w i t h an e q u i v a l e n t p r o p o r t i o n of aluminum i n the C p o s i t i o n ,  according  to t h e i r o n - g e h l e n i t e molecule Ca2FeAlSi<>7; 4) the remaining  aluminum  i s d i v i d e d e q u a l l y between the B and C p o s i t i o n s , a c c o r d i n g t o the g e h l e n i t e type molecule C ^ A l A l S i O y .  I f the chemical a n a l y s i s has  been c o r r e c t l y made on a pure sample of m e l i l i t e , the t o t a l number of i o n s i n the C p o s i t i o n w i l l be two t h i r d s the t o t a l number o f i o n s i n the A and B p o s i t i o n s .  S i m i l a r l y , the r a t i o between ( A + B ) ,  C, and the t o t a l number o f oxygen i o n s w i l l be 3s 2s  7*  The r a t i o Has Ges Ak i s e a s i l y d e r i v e d from t h i s t a b l e . The p r o p o r t i o n of sodium m e l i l i t e type molecule i s the atomic p r o p o r t i o n of sodium p l u s potassium.  The p r o p o r t i o n of t h e  gehlen-  i t e type molecule i s the t o t a l atomic p r o p o r t i o n o f aluminum i n the C position.  The p r o p o r t i o n o f akermanite type molecule i s one  of the t o t a l remaining d i v a l e n t i o n s .  third  T h i s i s reduced t o a percent  r a t i o f o r convenience of e x p r e s s i o n . T h i s treatment of the analyses g i v e n f o r m e l i l i t e from the t u r j a i t e o f K o l a P e n i n s u l a and from the uncompahgrite of I r o n H i l l , Colorado, y i e l d s r e s u l t s very s i m i l a r t o t h a t d e r i v e d f o r  67  m e l i l i t e from o k a i t e i n t a b l e V I . for  these t h r e e m e l i l i t e s aret M e l i l i t e from o k a i t e  -  Sa^Q^iiik^  M e l i l i t e from t u r j a i t e  -  Ha^gGe g Ak^g  M e l i l i t e from uncompahgrite  -  1^230*2.3^59  The m e l i l i t e from the uncompahgrite to  The compositions by type molecule  (approx.)  o f I r o n H i l l , Colorado, appears  c o n t a i n a c o n s i d e r a b l e excess of s i l i c a , a f e a t u r e t h a t caused  Berman (1929, P«398) t o a s s i g n t o i t n e a r l y 10$ ©f h i s proposed s u b - m e l i l i t e molecule. U s i n g the o p t i c a l data p r o v i d e d by W i n c h e l l and W i n c h e l l (1951, P«473-475) and Hurse and Midgley (1953), a t r i a n g u l a r diagram has been drawn t o show the v a r i a t i o n of r e f r a c t i v e i n d i c e s and b i r e f r i n g e n c e w i t h i n the system sodium m e l i l i t e - g e h l e n i t e - akermanite (fig.  34)*  The t h r e e n a t u r a l m e l i l i t e s t h a t have been c o n s i d e r e d i n  t h i s study are p l o t t e d a c c o r d i n g t o the compositions g i v e n above. I t i s seen t h a t , a c c o r d i n g t o t h i s diagram, the r e f r a c t i v e i n d i c e s and b i r e f r i n g e n c e f o r m e l i l i t e from both Oka and I r o n H i l l are s l i g h t l y lower than those g i v e n i n t a b l e I I I .  This i s readily  e x p l a i n e d by the f a c t t h a t both these m e l i l i t e s c o n t a i n a c e r t a i n amount of i r o n , which s u b s t i t u t e s i n akermanite o r g e h l e n i t e , depending on i t s valence.  S u b s t i t u t i o n o f i r o n i n b o t h these  molecules probably i n c r e a s e s the r e f r a c t i v e i n d i c e s and the b i r e f ringence by an amount approximately p r o p o r t i o n a l t o the degree o f substitution. I t may be s a i d i n summary t h a t n a t u r a l m e l i l i t e s are not simply represented by s o l i d s o l u t i o n i n the g e h l e n i t e - a k e r m a n i t e series.  The presence of a l k a l i s i s e x p l a i n e d by the sodium  m e l i l i t e molecule, the presence of i r o n by s m a l l amounts o f i r o n - g e h l e n i t e and i r o n - a k e r m a n i t e , and excess c a l c i u m by subs t i t u t i o n f o r magnesium ( o r f e r r o u s i r o n ) i n akermanite.  Using  the proposed type m o l e c u l e s , the m e l i l i t e s from the i n t r u s i v e a l k a l i c rocks o k a i t e , t u r j a i t e , and uncompahgrite  c o n t a i n sodium  m e l i l i t e , g e h l e n i t e , and akermanite i n the approximate p r o p o r t i o n s Ba^ t ^ ^ o A k t j ^ .  68  1. 3.  M e l i l i t e from uncompahgrite M e l i l i t e from t u r j a i t e M e l i l i t e from okaite  Titanaugite As has already been s t a t e d , there are two pyroxenes present i n the o k a i t e s .  The only one that i s important  as a major rock forming  mineral i s the t i t a n a u g i t e i n the pyroxene o k a i t e s .  The  optical  p r o p e r t i e s of t h i s mineral have been studied i n d e t a i l . V a r i a t i o n i n composition i n t h i s mineral has produced  zoning  which i s made apparent by d i f f e r e n c e s i n pleochroism and i n e x t i n c t i o n angles f o r d i f f e r e n t zones.  Boundaries between zones may  but are more commonly q u i t e sharp.  be g r a d a t i o n a l ,  E x t i n c t i o n angles Z against C were  measured f o r 36 g r a i n s , chosen from nine t h i n s e c t i o n s .  Grains  twinned  on (lOO) were.used wherever p o s s i b l e so that double measurements were obtained; such g r a i n s were r o t a t e d on the u n i v e r s a l stage u n t i l Y was p a r a l l e l to the i n c i d e n t l i g h t beam.  A direct relation exists  between the e x t i n c t i o n angle and the pleochroism, such that the p l e o c h r o i c colours i n t e n s i f y as the e x t i n c t i o n angle i n c r e a s e s .  This  r e l a t i o n s h i p i s e s p e c i a l l y n o t i c a b l e i n zoned c r y s t a l s .  angle  The  Z against C averages about 5 6 ° , but v a r i e s between the l i m i t s given below. Table VII V a r i a t i o n i n E x t i n c t i o n Angle and Pleochroism f o r T i t a n a u g i t e  Z against C  X  Y  Z  5.2°  pale p i n k i s h b u f f  straw  56°  pale v i o l e t tan  tan  yellow  61°  v i o l e t brown  p i n k i s h brown  g r e e n i s h yellow  pale yellow  The p o s i t i v e o p t i c angle does not vary appreciably with composition. of 49° to 5 6 ° ,  U n i v e r s a l stage measurements on 16 g r a i n s show a range with 12 g r a i n s g i v i n g values between 5 ° °  and  53°.  Extremely strong i n c l i n e d d i s p e r s i o n with red g r e a t e r than v i o l e t ( r > v) -hindered r e a l l y accurate measurement of e x t i n c t i o n  angles and the o p t i c angle ( 2 V ) .  A s i n g l e g r a i n twinned on ( 1 G 0 )  r o t a t e d on a u n i v e r s a l stage so that Y was beam.  p a r a l l e l to the  was  incident  E x t i n c t i o n angles Z against. C were measured as c l o s e l y as  p o s s i b l e f o r red and v i o l e t : l i g h t , u s i n g corresponding f i l t e r s on a Z f o r v i o l e t against  carbon arc l i g h t source. to be 5 9 ° ,  Z f o r red against  and  C (Z AC) 54°. r  d i f f e r e n c e i n e x t i n c t i o n angle, with ZyA C > Z A r  A p e r f e c t l y centred  C (ZvA  Optic angle measurements made with f i l t e r s and = 56°,  When the g r a i n was  2V  V  = 48 , 0  found  Thus there i s a 5 ° c.  The  the 4 5 ° p o s i t i o n showed quite d i f f e r e n t degrees of  r  was  acute b i s e c t r i x f i g u r e was  from a s i m i l a r g r a i n i n the same t h i n s e c t i o n .  source gave 2 V  c)  two  obtained isogyres  dispersion.  carbon arc  light  a d i f f e r e n c e of 8 ° , with r > v.  r o t a t e d to give an o p t i c axis f i g u r e with  d i s p e r s i o n , whose isogyre was by plane l i g h t revealed  strong  red on the convex s i d e , examination  that both ( 1 1 0 )  making an angle of about. 75°'  at  cleavages were v i s i b l e ,  S i m i l a r l y , on centering the  isogyre  showing very weak d i s p e r s i o n , blue on the convex s i d e , plane l i g h t examination showed only one  cleavage d i r e c t i o n .  Figure 3 5 shows  the o p t i c o r i e n t a t i o n of t i t a n a u g i t e from pyroxene okaite as compiled from the information  Figure 35t  The  given above.  o p t i c o r i e n t a t i o n of t i t a n a u g i t e from pyroxene okaite  Another f e a t u r e of note i s that, many of the specimens of t i t a n a u g i t e examined contain a f i n e network of opaque n e e d l e - l i k e i n c l u s i o n s , probably magnetite or i l m e n i t e . the (010)  These are o r i e n t e d  in  plane along the A and C c r y s t a l l o g r a p h i c d i r e c t i o n s .  The  occurrence of these i n c l u s i o n s i s patchy w i t h i n i n d i v i d u a l g r a i n s of t i t a n a u g i t e .  Many grains do not  contain i n c l u s i o n networks.  R e f r a c t i v e i n d i c e s f o r t i t a n a u g i t e were measured by o i l immersion methods.  The measured i n d i c e s and other p r o p e r t i e s  compared w i t h those of other pyroxenes i n t a b l e V I I I .  are  Data f o r the  other pyroxenes are taken from Winchell and Winchell (1951, p.416).  Table VIII Comparison of O p t i c a l P r o p e r t i e s  2V  Z  Z  AC  n  of Various Pyroxenes  x  n  y  njj  n -»x z  Augite  59  41  1.687  1.694  1.713  0.026  Ferroaugite  49  44  1.726  1.732  1.753  0.027  Titanaugite  51  50  1.721  1.725  1.746  0.025  Oka  52  56  titanaugite  I t may Oka ZAC  0.030  I.724-I.726 I.728-I.730 1.752-1.758  be seen from the above data that the i n d i c e s f o r  t i t a n a u g i t e match both those f o r t i t a n a u g i t e and f o r f e r r o a u g i t e . and  2V  Z  are most s i m i l a r to those of t i t a n a u g i t e .  Ferroaugite  i s so c a l l e d because i t contains a high proportion  of f e r r o u s i r o n ,  which t i t a n a u g i t e commonly does not.  augite have  Z A C < Z v A C, r  Diopside and  whereas a high i r o n content i n augite produces the  opposite e f f e c t (Winchell  and Winchell,  1951,  P ' 4 1 4 ) , as has  shown to e x i s t i n t i t a n a u g i t e from pyroxene o k a i t e . p.355) has  shown that an increase  produces an increase  s i x - f o l d coordination  Dana (1896,  i n i r o n content i n pyroxenes  i n the e x t i n c t i o n angle  Titanium may  been  ZAC.  s u b s t i t u t e f o r f e r r i c i r o n i n pyroxene. f e r r i c i r o n has  an i o n i c radius of 0. 6 4 i ,  and t i t a n i u m has an i o n i c radius of 0.682..  Titanium cannot  In  s u b s t i t u t e d i r e c t l y f o r s i l i c o n , which has an i o n i c radius of 0.39A i n f o u r - f o l d coordination.  S u b s t i t u t i o n of t i t a n i u m f o r f e r r i c  iron  must t h e r e f o r e be accompanied by s u b s t i t u t i o n of aluminum f o r s i l i c o n i f an e l e c t r o v a l e n t e q u i l i b r i u m i s to be maintained. be present  Aluminum  i n both f o u r - f o l d and s i x - f o l d c o o r d i n a t i o n , with  e c t i v e i o n i c r a d i i of 0. 4&1 and 0.51A  (Ahrens,  Pyroxenes such as augite may aluminum; t h i s i s present  be present  resp-  1952).  contain a c e r t a i n amount of  i n both f o u r - f o l d and s i x - f o l d  coordin-  a t i o n , as shown by the molecule CaAlAlSi06, f o r example. i r o n may  may  Ferric  i n the s i x - f o l d c o o r d i n a t i o n p o s i t i o n only,  and  gives r i s e to a formula such as CaFe'" AlSi06; i t s i o n i c radius i s too large f o r i t t o occupy the f o u r - f o l d p o s i t i o n .  I f titanium  s u b s t i t u t e s f o r f e r r i c i r o n , s i l i c o n i s absent from the u n i t pyroxene molecule, whose formula becomes CaTiAl206. not n e c e s s a r i l y represent  These u n i t molecules do  end members of the pyroxene s e r i e s , and  are used here merely to show how  s u b s t i t u t i o n takes place.  The treatment given to the chemical  a n a l y s i s of t i t a n a u g i t e  from pyroxene o k a i t e i n t a b l e IX i s that employed by Hess h i s chemical  and o p t i c a l s t u d i e s of many d i f f e r e n t  I t i s a r a p i d method f o r determining  (1949)  in  clino-pyroxenes.  the o v e r a l l calcium-magnesium-  i r o n r a t i o and the percentage s u b s t i t u t i o n of aluminum f o r s i l i c o n . I t i s used here to show two from pyroxene o k a i t e . of calcium;  One  remarkable features of the t i t a n a u g i t e f e a t u r e i s the r e l a t i v e l y high  proportion  i f t h i s pyroxene were p l o t t e d on a t r i a n g u l a r diagram  between CaSiO^, M g S i 0 3 , and F e S i 0 3 , i t would l i e on the G a S i 0 3 s i d e of the l i n e j o i n i n g diopside and hedenbergite.  The  other f e a t u r e i s  the considerable amount of s u b s t i t u t i o n of aluminum f o r s i l i c o n ,  24.4$,  and indeed the t o t a l percentage of aluminum.  Hess  (1949,  p.625) s t a t e s that "Very few analyses have been made showing A l r e p l a c i n g S i to an extent g r e a t e r than 12$." content  The normal aluminum  of augite and f e r r o a u g i t e i s between 2$ and 8$.  It i s  thought that a combination of the high p r o p o r t i o n o f aluminum and the t i t a n i u m content  causes Z A 0 < Z v A C , whereas the reverse of r  t h i s equation i s the case f o r normal augite. too low to account f o r Z A C < Z y A C . r  The  i r o n content  is  Table IX Chemical A n a l y s i s o f T i t a n a u g i t e from Oka; D e r i v a t i o n o f the Calcium-Magnesium-Iron Ratio and Percentage S u b s t i t u t i o n of Aluminum f o r S i l i c o n ( a f t e r Hess, 1949)  2A1  Analyst H. Dlk; H 0 ignored, PgOtj subtracted as a p a t i t e . 2  Si0 A1 0 2  2  3  Pe G3 PeO MnO MgO CaO Ha 0 K 0 2  2  2  Ti0  2  39.89 14.94 5.62 2.01 0.21  10.14 24.18 0.10  0.03 2.30  99.42$  Pe»«  Ti  Cations  Al Al Fe"«  to  Al W  665  Si Al  293  Pe"« Fe" Mn Mg Ca Ha K Ti  70 28 3 254 432  0  ( 1  3  84  58-  I T  2  874  six 0 2.00  84  3  67  W+X+Y 874  2.00  3 29  29 2617  Ca5 .iMg32.iPe .8; 5  12  $ Al i n Z t 2 4 . 4  X-ray powder photographs were taken o f t i t a n a u g i t e I n order t o see i f any unusual features i n the powder p a t t e r n were present.  The p a t t e r n i s s i m i l a r t o the general pyroxene p a t t e r n ,  d i f f e r i n g only i n minor d e t a i l .  The s i g n i f i c a n c e o f these small  d i f f e r e n c e s i s probably o f a complex nature, and no attempt has been made t o i n t e r p r e t them.  I t i s thought, however, that the  X-ray powder data should be included f o r the sake o f completeness ( t a b l e X and f i g . 3 6 ) . I t has already been s t a t e d that the o p t i c a l p r o p e r t i e s of t i t a n a u g i t e vary from one rock t o another, and that many t h i n s e c t i o n s show that the pyroxene has marked compositional The  zoning.  chemical a n a l y s i s , and the data d e r i v e d from i t , must t h e r e f o r e  Table X Comparison o f the X-Eay Powder Data f o r T i t a n a u g i t e and Diopside 1 I 5 20 5 7 40 100 40 20  2  dm  I  d  hkl  I  4.68  1 3 3 3 11 25 100 25 30 1  4.69 4.47 4.41 3.66 3.35 3.23 2.991 2.952 2.893 2.837  200 020 111 111 021 220 221 310 311 130  3 20  1.870 1.836  30  1.744  20 40 30 3  2.566 2.528 2.518 2.392 2.304 2.218 2.200 2.157 2.134 2.109 2.077 2.043 2.016 2.009 1.970  131 002 112 131 311 112 022 330 331 421 420 041 402 202 132  10 10 30 10 5 10 15  1.675 1.634 I.625 1.613 I.586 1.565 1.544  15 10  1.528 1.512  10  I.48I  50  1.423 1.407  4-42 3.55 3.31 3.22 2.983 2.938 2.888  60  2.805 2.550  60  2.515  20 10B  2.307 2.226  30 30 10  2.145 2.123 2.108  40B  2.033  5  1.972  5B  1  2  15 13 11 9 15 7 1 13 9 9 7  30  d  m  I  d  hkl  3 5 3 3 1 11 1 1 1 5 5 25 5 3 3 3 1 9 11 1 3 1 1 3 13 7  1.862 1.838 1.832 I.815 1.777 1.755 1.720 1.713 I.685 1.674 1.659 1.625 1.618 I.588 1.565 1.551 1.529 1.526 I.504 1.494 I.488 I.468 1.463 1.447 I.424 1.410  331 510 222 132 421 150 512 113 151 042 313 223 440 530 600 550 602 402 133 242 060 333 441 513 531 352  1.  T i t a n a u g i t e from Oka, Quebec  2.  Diopside from Schwarzenstein, A u s t r i a (ASTM Card II-654)  F i g u r e 36t  X-ray powder photograph of t i t a n a u g i t e from pyroxene okaite  be considered to represent an average.  Zoning i s probably due to  f l u c t u a t i o n i n the t i t a n i u m content, the g r e a t e r amount of which i s present, the more n o t i c a b l e the pleochroism, and the l a r g e r the e x t i n c t i o n angle and the d i s p e r s i o n .  There does not seem to be a  r e g u l a r p a t t e r n of zoning; i n some g r a i n s the e x t i n c t i o n angle increases towards the outer edges, and i n others the c e n t r a l zone has the g r e a t e r e x t i n c t i o n angle. In summary i t may  be s a i d that the pyroxene i n pyroxene  o k a i t e i s a h i g h l y aluminous v a r i e t y c o n t a i n i n g an appreciable but v a r i a b l e amount of t i t a n i u m , and a l i t t l e more calcium than i s normal.  In terms of the recognized nomenclature f o r pyroxenes i t  should be c a l l e d an aluminous t i t a n i a n s a l i t e .  I t s chemical nature  i s i n keeping with the c a l c i u m - r i c h , s i l i c a - d e f i c i e n t nature of the rocks of the o k a i t e s e r i e s .  The Opaque Minerals Bottle-green s p i n e l i s i n c l u d e d i n magnetite sections.  i n nine t h i n  Examination of other t h i n s e c t i o n s with r e f l e c t e d  r e v e a l s the presence of opaque s p i n e l a s s o c i a t e d with  light  magnetite,  i t s o p a c i t y preventing i t s r e c o g n i t i o n by t r a n s m i t t e d l i g h t . most rocks s p i n e l i s present i n minute proportions; i n two of o k a i t e , however, i t comprises  In  specimens  about two percent of the rock  forming minerals, and i n m o n t i c e l l i t e a l n o i t e i t makes up about s i x percent. X-ray powder photographs were made of both s p i n e l magnetite.  Magnetite was  taken from a specimen of m e l i l i t e o k a i t e ,  and was known t o c o n t a i n i n c l u s i o n s of green s p i n e l . was  and  Opaque s p i n e l  taken from a specimen of p a r t l y a l t e r e d o k a i t e ; magnetite  was  removed by passing a magnet over crushed m a t e r i a l which had been sieved between 80 and 100 mesh; u s i n g a carbon a r c , s p i n e l  was  separated from p e r o v s k i t e with a needle, these minerals being i d e n t i f i e d by t h e i r r e s p e c t i v e green and deep orange i n t e r n a l reflections.  The estimated v i s u a l i n t e n s i t i e s and d-spacings  are  compared i n t a b l e XI to those of magnetite and h e r c y n i t e .  The  X-ray powder photographs are reproduced i n f i g u r e 37The s p i n e l f i l m i s almost i d e n t i c a l to that of h e r c y n i t e from Ronsberg, Bohemia, the type l o c a l i t y .  In a d d i t i o n , i t shows  weak l i n e s that correspond to the strong l i n e s of magnetite, i n d i c a t i n g that magnetite i s probably included w i t h i n h e r c y n i t e .  The  c e l l edge f o r the s p i n e l , c a l c u l a t e d and averaged from the d-spacings, i s 8.162..  This value i s s l i g h t l y higher than the 8.1192. given f o r  pure h e r c y n i t e by Clark et a l . ( 1 9 3 1 , P« 539) > and may be accounted for  by the presence of f e r r i c i r o n s u b s t i t u t i n g f o r aluminum i n  hercynite, FeAl204«  R u s s e l l et a l . ( 1 9 5 4 » P« 2 0 3 ) report i n c l u s i o n s  of green s p i n e l i n magnetite from a carbonatite i n Transvaal. state that t h i s s p i n e l contains  They  aluminum, i r o n , and magnesium, with  a very small amount of z i n c ; the c e l l edge i s given as 8 . 1 0 3 -  0.0022.  Magnesium as a major constituent i s responsible f o r the c e l l edge being smaller than that of h e r c y n i t e , and the s p i n e l i s pleonaste. The magnetite f i l m shows weak l i n e s corresponding to the strong l i n e s of h e r c y n i t e , i n e x a c t l y the reverse manner to the hercynite f i l m . present  This was expected because s p i n e l was known to be  i n the specimen of magnetite used f o r the X-ray powder  photograph.  The c e l l  edge f o r magnetite from m e l i l i t e o k a i t e i s  8 . 4 0 2 , which i s not appreciably l a r g e r than the c e l l edge f o r pure magnetite, 8 . 3 9 l 2 ( C l a r k et a l . , 1 9 3 1 , p.  Figure 37t  539).  X-ray powder photographs of h e r c y n i t e ( l ) and magnetite (2) from Husereau H i l l , near Oka, Quebec  Table XI X-Ray Powder Data f o r Hercynite and Magnetite  I  I  *m  30  d  m  4.86  30  4-73  5 60  2.97 2.88  60 5  2.97 2.88  40  2.71  5  2.71  15  2.53  100  2.52  100  2.46  15 5  2.45 2.42  50 10  2.10 2.03  5  4-78  50  2.87  I  hkl  dm  30  4.84  j-111  60  2.96  j-022 impurity  100  100  2.53  10  2.42  70  2.09  2.46  ) , , ,  r  222  5 60  2.10 2.04  30  1.922  impurity  5  1.727  impurity  20  1.665  5 70  1.618 1.571  65 10  10 90  I.482 I.440  80 15  30  10 30 20  10  1.714  1.093  50  1.063  30  1.020  Hercynite Magnetite Hercynite Magnetite  1.614 1.565  50  1.570  1.483 1.435  70  1.284 1.268  10  1.212  5  1.292  20  1.244  5 30  1.124  70  1.094  40  1.712  80  1.611  90  I.48I  1.443  1.332  5  from from from from  2.04  1.670  1.180  20  30  20  I.246;  40  1. 2. 3. 4.  I.  5  I.416  10  1.327  40  1.280  10  I.264  20  1.211  5  1.175  1.177  20  1.090  50  1.063  40  1.020  I.050  40  1.122  80  1.094  50  1.050  004  j-224 ]-115,333 }"044 135 ^-026 }-335 226  j-444 117,155  j-246 ]-137,355 008  Husereau H i l l , near Oka, Quebec Husereau H i l l , near Oka, Quebec Ronsberg, Bohemia (Berry and Thompson, 1962, p.191) O-Radna, Romania (Berry and Thompson, 1962, p.193)  78  T i t a n i f e r o u s magnetite commonly exsolves or u l v o s p i n e l , o r both (Ramdohr, 1953).  l a t h s of i l m e n i t e  Weak l i n e s on the magnetite  f i l m corresponding t o the strong l i n e s of i l m e n i t e are absent.  The  only l i n e s not i n accord with the s p i n e l s t r u c t u r e (three on the h e r c y n i t e f i l m , one on the magnetite f i l m ) can a l l be indexed as the strong l i n e s o f perovskite.  I f u l v o s p i n e l were present  i n appreciable  quantity i n the magnetite, a broadening o r doubling of the d-spacings would be apparent, as u l v o s p i n e l has a l a r g e r c e l l edge than magnetite. T h i s i s not the case f o r the magnetite under c o n s i d e r a t i o n . P o l i s h e d s e c t i o n s were made o f specimens o f pyroxene o k a i t e , o k a i t e , m e l i l i t e o k a i t e , and pegmatitic  m e l i l i t e - c a l c i t e rock, as  w e l l as of one specimen of a l t e r e d nepheline o k a i t e that shows an unduly l a r g e amount (2$) o f s p i n e l i n t h i n s e c t i o n . a rock contains present  a f r a c t i o n o f one percent  spinel.  Normally such The opaque  minerals  i n these rocks are magnetite, p y r r h o t i t e , c h a l c o p y r i t e ,  p e r o v s k i t e , and hercynite s p i n e l . Microscopic  examination reveals the immediately n o t i c a b l e  f e a t u r e , even with medium power, that the magnetite i n each p o l i s h e d s e c t i o n i s crowded with a network of t i n y , o r i e n t e d , i n c l u s i o n s , o r lamellae.  The s i z e o f these lamellae,  lath-like although  v a r i a b l e w i t h i n any one magnetite g r a i n , i n general increases with the s i z e of the host.  The lamellae are darker grey than magnetite,  the l a t t e r mineral having a p i n k i s h cast beside them.  They are  o r i e n t e d i n three d i r e c t i o n s only, thus o c c u r r i n g as p l a t e s on the (1G0) one  planes of magnetite, which i s t y p i c a l of the e x s o l u t i o n o f  s p i n e l from another.  Ilmenite i s commonly exsolved  from  magnetite, but i s o r i e n t e d along the ( l l l ) p l a n e s o f i t s host; i t would thus show f o u r l a m e l l a r d i r e c t i o n s i n p o l i s h e d s e c t i o n .  The  lamellae, which taper at the edges, vary i n breadth from l e s s than one micron t o 150 microns, averaging between 10 and 50 microns.  In  s e c t i o n s cut p a r a l l e l to one of the cubic planes o f magnetite, i n which are seen two sets o f lamellae at r i g h t angles, the greatest l a m e l l a r thickness i s about -J- micron.  The lamellae are n e i t h e r  p l e o c h r o i c nor a n i s o t r o p i c ; i l m e n i t e lamellae are commonly a n i s o tropic i n polished section. magnetite with exsolved  Figure 38 shows a t y p i c a l s e c t i o n of  hercynite  lamellae.  Figure 38t  E x s o l u t i o n lamellae of h e r c y n i t e on the ( 1 0 0 ) planes of magnetite ( r e f l e c t e d l i g h t , x260)  Figure 39» E x s o l u t i o n blebs and lamellae of h e r c y n i t e i n magnetite. Note the decrease i n s i z e of the lamellae i n the v i c i n t y of the blebs (reflected l i g h t , x260)  In a d d i t i o n to the lamellae, many magnetite g r a i n s show e x s o l u t i o n of hercynite as small-rounded blebs that are noto r i e n t e d with respect to the c r y s t a l l o g r a p h i c d i r e c t i o n s of magnetite.  These may  be s c a t t e r e d throughout small areas w i t h i n a  magnetite g r a i n , but are more commonly present  at the edges of  host g r a i n s , or at the boundary between magnetite and some i n c l u s i o n , such as a p a t i t e .  I n d i v i d u a l blebs may  be as l a r g e as  5 0 microns i n diameter, and they are then large enough to show the green i n t e r n a l r e f l e c t i o n t y p i c a l of h e r c y n i t e .  Such l a r g e  bodies of h e r c y n i t e are the same as those that are v i s i b l e i n t h i n s e c t i o n as green s p i n e l i n c l u s i o n s i n magnetite.  Exsolution  lamellae are absent i n the immediate v i c i n i t y of the rounded b l e b s , are increase i n s i z e and q u a n t i t y i n p r o p o r t i o n to-the distance away from them ( f i g .  39)•  Perovskite i s d i s t i n g u i s h e d from magnetite by i t s smoother appearance, i t s deep orange i n t e r n a l r e f l e c t i o n , and i t s b l u i s h grey c o l o u r .  Magnetite i n contact with p e r o v s k i t e appears  l i g h t creamy grey, and perovskite appears d i s t i n c t l y blue. o v s k i t e i s normally f r e e of any i n c l u d e d m a t e r i a l .  An  Per-  interesting  f e a t u r e i s a t h i n rim of p e r o v s k i t e that- commonly borders  grains  of magnetite and the i n c l u s i o n s w i t h i n magnetite ( f i g . 4 0 ) . rim i s u s u a l l y of the order of 5 to 2 0 microns t h i c k . continuous  It i s  round most magnetite g r a i n s i n pyroxene o k a i t e .  places i t may  This  In  swell to form i r r e g u l a r masses of p e r o v s k i t e  adjacent to magnetite.  In m e l i l i t e o k a i t e and pegmatite phases  i t i s not so c o n s i s t e n t l y present., and may  be absent but f o r  small patches o f perovskite at the edges of magnetite g r a i n s . Rims of pale coloured perovskite have been described i n t h i n s e c t i o n on pages 3 4 and The migratory  36. e x s o l u t i o n of hercynite to magnetite  g r a i n boundaries has not l e d to i t s i n v a s i o n of the p e r o v s k i t e rims; h e r c y n i t e forms blebs w i t h i n magnetite so that the combined hercynite-magnetite  to p e r o v s k i t e boundary i s smooth.  rounded i n c l u s i o n s may  Small  be present i n magnetite, and the same  r e l a t i o n s h i p to exsolved h e r c y n i t e i s shown.  In a few  places  Figure 4 0 ; Perovskite rim around a p a t i t e i n c l u s i o n i n magnetite with exsolved h e r c y n i t e ; note the blebs of h e r c y n i t e exsolved from magnetite at the perovskite-magnetite border (reflected light, x260)  Figure 4 1 t Magnetite ( l i g h t grey) r e p l a c i n g the core of and rimming a euhedral h e r c y n i t e c r y s t a l ( r e f l e c t e d l i g h t , x 9 0 )  h e r c y n i t e exsolved at the boundaries of magnetite g r a i n s to form a t h i n , discontinuous  rim around i t s former host, e s p e c i a l l y where the  p e r o v s k i t e rim i s absent. The specimen that contains more h e r c y n i t e than i s u s u a l shows some i n t e r e s t i n g f e a t u r e s not seen i n the other s e c t i o n s . Here h e r c y n i t e forms d i s c r e t e octahedral c r y s t a l s , some of which may  have magnetite at the core.  a l s o , as may  occur  more complex examples, such as a compound g r a i n with a  magnetite core, a surrounding ( f i g . 41)•  The reverse s i t u a t i o n may  hercynite zone, and a magnetite rim  Magnetite, as i n the other s e c t i o n s , i n v a r i a b l y c a r r i e s  s p i n e l lamellae.  Hercynite i n t h i s s e c t i o n contains exsolved mag-  n e t i t e , commonly present  as small, round, unoriented blebs: more  r a r e l y i t i s crudely a l i g n e d as s t r i n g s of blebs on the  (100)  planes of the s p i n e l host ( f i g . 4 2 ) . A f e a t u r e n o t i c a b l e under high m a g n i f i c a t i o n i s the presence of bright, blue-white h e r c y n i t e lamellae ( f i g . 4 3 ) .  patches i n magnetite s i t u a t e d between These are e s p e c i a l l y prevalent  adjacent to cracks o r v e i n l e t s i n magnetite, and probably  represent  a l t e r a t i o n to maghemite or hematite. P y r r h o t i t e i s normally  l a t e r than magnetite, but i n a  few cases i s included? w i t h i n magnetite, normally at the outer edges of magnetite g r a i n s .  I t u s u a l l y forms d i s c r e t e g r a i n s w i t h i n the  rock mass, or may  be i n or at the boundaries of rims of perovskite  around magnetite.  P o l i s h e d t h i n s e c t i o n examination r e v e a l s t h a t  the many small opaque i n c l u s i o n s i n hauyne seen i n t h i n s e c t i o n are mostly p y r r h o t i t e .  Ghalcopyrite i s uncommon, and only  occurs  i n d i r e c t a s s o c i a t i o n with p y r r h o t i t e ; i t i s u s u a l l y found as small g r a i n s at the edges of larger, p y r r h o t i t e g r a i n s . Examination of magnetite g r a i n s at a m a g n i f i c a t i o n of xl80G under o i l r e v e a l s i n some g r a i n s an extremely f i n e network of s p i n e l lamellae between the l a r g e r lamellae already s t u d i e d .  These  f i n e lamellae are so small that they cannot be p r o p e r l y resolved at t h i s m a g n i f i c a t i o n , although i t can be seen that they have the same o r i e n t a t i o n as the l a r g e r hercynite lamellae. a cloud of t i n y needles  concentrated  They appear as  at the centres of the gaps  Figure 42:  Magnetite ( l i g h t grey) exsolved from h e r c y n i t e (medium grey f i e l d ) ( r e f l e c t e d l i g h t , x260)  F i g u r e 43: A l t e r a t i o n of magnetite ( l i g h t grey) t o maghemite o r hematite (very l i g h t grey) between exsolved h e r c y n i t e lamellae ( r e f l e c t e d l i g h t , x260)  84  between hercynite lamellae near the edges of magnetite g r a i n s ; they are only seen i n a few of the l a r g e r magnetite g r a i n s , more especi a l l y i n pyroxene o k a i t e .  It i s p o s s i b l e that they represent  the  e x s o l u t i o n of a d i f f e r e n t s p i n e l , perhaps u l v o s p i n e l ( F e 2 T i 0 4 ) , they may  e q u a l l y well be hercynite exsolved  l a r g e r lamellae.  although  at a l a t e r time than tbe  I t i s of i n t e r e s t to note that the magnetite of  t u r j a i t e from Kola Peninsula contains only 0.98$ TiC>2, and t h a t , l i k e the magnetite of o k a i t e , i t does not contain exsolved i l m e n i t e (Johannsen, 1938, that okaite magnetite may content  p. 323).  The  lamellae of  l a t t e r s i m i l a r i t y suggests  also contain very l i t t l e TiC>2.  The  of the o r i g i n a l magma appears to have been deposited  titanium exclus-  i v e l y as perovskite, and combined i n . t i t a n a u g i t e i n pyroxene okaite; t h i s i s probably  a f u n c t i o n of the o x i d a t i o n s t a t e of the i r o n ,  i n d i c a t e s a predominance of f e r r i c i r o n (see page  115).  Polished s e c t i o n s were made of each of the two tioned on page 10 as p o s s i b l y being limestone or septa w i t h i n the okaite i n t r u s i o n . of magnetite.  and  rocks men-  or dolomite i n c l u s i o n s  Both rocks contain small amounts  They were examined under o i l at xl800 m a g n i f i c a t i o n .  Ko v i s i b l e e x s o l u t i o n lamellae were seen i n the magnetite of e i t h e r of these rocks; perovskite i s absent from both a l s o . The  s p i n e l s are d i v i d e d i n t o three s e r i e s , comprising  whose t r i v a l e n t ions are A l , Fe, or Cr (Palache  et a l . , 1944> P-«  those 687).  D i f f e r e n t members of the same s e r i e s e x h i b i t more or l e s s complete solid solution. considered,  Members of d i f f e r e n t s e r i e s , as i n the case being  show l i m i t e d s o l i d s o l u t i o n .  I t may  greater the d i f f e r e n c e i n the c e l l edges of two  be suspected  that, the  s p i n e l s of d i f f e r e n t  s e r i e s , the more r e s t r i c t e d w i l l be the s o l i d s o l u t i o n between them. The  s o l i d s o l u t i o n of hercynite i n magnetite has not been  studied i n d e t a i l to date, as f a r as can be ascertained.  It i s  known that hercynite exsolves from magnetite (Ramdohr, 1955* P« proving that the two minerals  708),  form a l i m i t e d s o l i d s o l u t i o n s e r i e s .  The percentage of hercynite that can be contained  i n magnetite  presumably increases with temperature, as has been shown f o r other s p i n e l s o l i d s o l u t i o n s , such as u l v o s p i n e l i n magnetite ( N i c k e l , 1958).  An estimate  of the temperature of formation  of magnetite  might therefore be deduced by studying the r e - s o l u t i o n of h e r c y n i t e i n magnetite by annealing at elevated temperatures, provided that: r e - s o l u t i o n takes place i n a reasonably Allowing that there was  short, p e r i o d of time.  enough f e r r o u s i r o n and aluminum present  to allow maximum s o l i d s o l u t i o n of h e r c y n i t e i n magnetite at the time of formation, may  the amount of hercynite exsolved from magnetite  i n d i c a t e a range of formation temperatures f o r magnetite.  There  does not seem to be any r a d i c a l d i f f e r e n c e i n the amount of h e r c y n i t e exsolved from magnetite i n the various p o l i s h e d s e c t i o n s examined; n e i t h e r are there any apparent d i f f e r e n c e s i n the t e x t u r e s of the e x s o l u t i o n intergrowths  i n the d i f f e r e n t rock types.  I t i s there-  f o r e suggested that the magnetite of the various members of the okaite s e r i e s , from pyroxene o k a i t e to m e l i l i t e - c a l c i t e  rock,  c r y s t a l l i z e d under s i m i l a r temperature c o n d i t i o n s . The the two  absence of e x s o l u t i o n lamellae i n the magnetite of  p e r i c l a s e bearing marbles i s considered to i n d i c a t e a  d i f f e r e n t o r i g i n f o r these rocks than f o r the surrounding the t e x t u r e s and mineralogies  okaites;  of these rocks lead to the same  conclusion. Bate of c o o l i n g governs the amount and s i z e of h e r c y n i t e lamellae exsolved from magnetite; quick c o o l i n g prevents e x s o l u t i o n t a k i n g place.  I f quick c o o l i n g had occurred, an X-ray powder  photograph of magnetite would give a c e l l edge smaller than that of pure magnetite.  The  c e l l edge c a l c u l a t e d f o r magnetite from  okaite. agrees c l o s e l y with that of pure magnetite.  Allowing that  there are no other i m p u r i t i e s s t i l l i n s o l i d s o l u t i o n In o k a i t e magnetite, t h i s agreement suggests that a l l the h e r c y n i t e i s exsolved; t h i s i s i n d i c a t i v e of a r e l a t i v e l y slow rate of c o o l i n g . The  same conclusion i s supported by the normally  coarse g r a i n s i z e  of the rocks of the okaite s e r i e s . The p o l i s h e d s e c t i o n of the rock containing both magnetite and h e r c y n i t e i n appreciable amounts reveals that h e r c y n i t e c r y s t a l l i z e d before magnetite.  Magnetite was  then deposited as d i s c r e t e  g r a i n s , or as wide rims on h e r c y n i t e c r y s t a l s .  Some of the g r a i n s  i n d i c a t e that magnetite has replaced the core of h e r c y n i t e  crystals.  The temperature at which h e r c y n i t e and magnetite were deposited was t h e r e f o r e below that at which h e r c y n i t e and magnetite become m i s c i b l e i n appreciable p r o p o r t i o n .  Both minerals  are thought  to be the e a r l y products of c r y s t a l l i z a t i o n o f the o k a i t e s e r i e s magma.  There i s no evidence f o r secondary o r i g i n o f e i t h e r  h e r c y n i t e or magnetite.  THE ALTERATION OP ROCKS OP THE OKAITE SERIES  The Nature of A l t e r a t i o n A l t e r e d o k a i t e has been described i n hand specimen on pages 18 and 22.  I t has been noted that, a l t e r a t i o n i s very i r r e g u l a r i n  i t s d i s t r i b u t i o n , and that: one part of an average s i z e d hand specimen may be completely a l t e r e d at one end and q u i t e f r e s h at the other. Specimens of t h i s nature are found to be e s p e c i a l l y u s e f u l f o r t r a c i n g the courses of a l t e r a t i o n of s p e c i f i c minerals.  Thin s e c t i o n s were  made of t h i r t y a l t e r e d specimens of various members of the o k a i t e s e r i e s , the u n a l t e r e d counterparts of which have already been described.  I t i s found that, a l t e r a t i o n i s due almost e n t i r e l y to the  breakdown of m e l i l i t e and hauyne, and to the c r y s t a l l i z a t i o n of  new  minerals i n place of them. For the most part a l t e r a t i o n i s hydrothermal i n nature. Small f r a c t u r e s i n many rocks have allowed access to hydrothermal s o l u t i o n s , and a l t e r a t i o n has taken place on e i t h e r side of these fractures.  Completely a l t e r e d rocks commonly show that a c l o s e net-  work of f r a c t u r e s has allowed a l t e r a t i o n to take place throughout rock.  In rocks where no t r a c e of f r a c t u r i n g i s found,  f l u i d s may  hydrothermal  have soaked through the rock, g a i n i n g access along g r a i n  boundaries and mineral cleavage, or they may  have caused d e p o s i t i o n  of secondary minerals to s e a l such f r a c t u r e s as may existed.  the  have p r e v i o u s l y  There i s evidence f o r both s i t u a t i o n s . A l t e r a t i o n v a r i e s i n c h a r a c t e r a great d e a l , not only i n  the k i n d of a l t e r a t i o n minerals formed, but also i n the  independent  p r o p o r t i o n and extent of formation of a l t e r a t i o n minerals i n any rock.  one  Before d i s c u s s i n g i n d e t a i l the r e l a t i o n s between the various  types of a l t e r a t i o n , i t i s necessary to d e s c r i b e the a l t e r a t i o n minerals themselves.  M e l i l i t e , which forms the bulk of the rocks  under c o n s i d e r a t i o n , shows e s s e n t i a l l y f o u r types of a l t e r a t i o n , each of which i s g r a d a t i o n a l to the next.  Hauyne a l t e r s  to a g r a d a t i o n a l s u i t e of d i f f e r e n t a l t e r a t i o n minerals.  similarly It w i l l  be seen that i t i s an easy matter to d i s t i n g u i s h between the products  of a l t e r a t i o n of m e l i l i t e and hauyne. products,  The d i f f e r e n t , a l t e r a t i o n  e s p e c i a l l y i n the case o f m e l i l i t e , are presumably  governed mainly by the temperature of the invading hydrothermal fluids.  I t w i l l be shown that there i s not a great deal of chemical  change i n v o l v e d i n the production o f a l t e r a t i o n minerals,  except  f o r the a d d i t i o n of water and minor carbon d i o x i d e .  The A l t e r a t i o n of M e l i l i t e M e l i l i t e a l t e r a t i o n can be d i v i d e d i n t o f o u r main types; there i s a complete gradation between each type. of the f o u r types predominates i n any one rock.  In general, one In a few rocks a l l  f o u r types occur together with u n a l t e r e d m e l i l i t e .  The various  types o f a l t e r a t i o n are t a b u l a t e d below.  Table XII Types of A l t e r a t i o n of M e l i l i t e  I.  U n i d e n t i f i e d brownish to c o l o u r l e s s a l t e r a t i o n minerals along f r a c t u r e s and cleavage; k e r n e l s of u n a l t e r e d m e l i l i t e may be present  II  Intergrown v e s u v i a n i t e and f i b r o u s c e b o l l i t e , with minor c a l c i t e i n places  III.  Intergrown v e s u v i a n i t e and diopside l a t h s ; minor c a l c i t e and yellow andradite, the l a t t e r around magnetite only  I?  Coarse v e s u v i a n i t e , andradite, pale green b i o t i t e , and. c a l c i t e ; minor pale yellow augite and m o n t i c e l l i t e  The f i r s t  signs of m e l i l i t e a l t e r a t i o n are seen along-  cleavage planes or minute f r a c t u r e s . minerals  This i s type I a l t e r a t i o n .  Two  can be d i s t i n g u i s h e d , although n e i t h e r can be p o s i t i v e l y  identified.  They normally  i s not c o n s i s t e n t .  occur together, although t h i s a s s o c i a t i o n  One mineral has low b i r e f r i n g e n c e (about  0.008),  moderate p o s i t i v e r e l i e f , p a r a l l e l e x t i n c t i o n , and p o s i t i v e elongation.  I t forms t i n y f a n shaped aggregates of t h i n p l a t e s p r o j e c t i n g  i n t o m e l i l i t e from e i t h e r side o f small f r a c t u r e s , and i n places  completely replaces m e l i l i t e between f r a c t u r e s ( f i g .  44)•  I t Is  very s i m i l a r to j u a n i t e as described by Larsen and Goranson i n a l t e r e d m e l i l i t e from Iron H i l l , Colorado.  I t i s rare at  (1932) Oka,  having been found i n small amounts i n only a few specimens of p a r t l y altered okaite. to  I t s i d e n t i t y i s not c e r t a i n , and i t w i l l be  referred  as mineral l a , since i t i s one of two minerals found i n the type I  a l t e r a t i o n of m e l i l i t e . The other a l t e r a t i o n mineral found with mineral l a forms c l e a r patches with broad brown edges i n m e l i l i t e . remains completely u n i d e n t i f i e d , although i t may  This mineral be the same as an  u n i d e n t i f i e d mineral mentioned by Larsen and Goranson i n a l t e r e d m e l i l i t e from Iron H i l l .  I t w i l l be r e f e r r e d to as mineral l b .  It  has a d i s t i n c t l y higher b i r e f r i n g e n c e (about 0 . 0 2 0 ) than t h a t of mineral l a , but has s i m i l a r r e l i e f , and also has p o s i t i v e elongation and p a r a l l e l e x t i n c t i o n .  I t forms sheaves of p a r a l l e l p l a t y f i b r e s  whose long axes tend to be disposed p e r p e n d i c u l a r l y to m e l i l i t e cleavage planes.  In some rocks, mineral l b i s observed to have  p a r t l y replaced mineral l a . form a brownish,  In many a l t e r e d rocks these two  minerals  f u n g u s - l i k e growth i n m e l i l i t e , l e a v i n g small k e r n e l  of u n a l t e r e d m e l i l i t e between the more widely spaced f r a c t u r e s along which these minerals have formed ( f i g .  45)*  A much more common secondary mineral i n o k a i t i c rocks i s contained i n type I I a l t e r a t i o n , and i s i d e n t i f i e d as c e b o l l i t e . It  forms pale brown to c o l o u r l e s s f i b r e s , u s u a l l y present as curved,  r a d i a t i n g groups.  C e b o l l i t e may  form along f r a c t u r e s and  cleavage  i n m e l i l i t e , and i n some rocks i s separated from m e l i l i t e by a border of minerals l a and l b , though normally c e b o l l i t e r e p l a c e s melilite directly.  C e b o l l i t e commonly forms a mass of f i b r o u s  sheaves that have completely replaced m e l i l i t e .  Where m e l i l i t e  o r i g i n a l l y formed euhedral c r y s t a l s with i n t e r s t i t i a l c a l c i t e , the l a t t e r has remained q u i t e f r e s h , and c e b o l l i t e i s confined w i t h i n the o u t l i n e s of the m e l i l i t e c r y s t a l s , forming pseudomorphs ( f i g . In  46)  t h i n edges of some t h i n s e c t i o n s , the spaces between c e b o l l i t e  f i b r e s are f i l l e d with an almost i s o t r o p i c mineral of high p o s i t i v e r e l i e f , i d e n t i f i e d as v e s u v i a n i t e .  90  Figure 45t Type I a l t e r a t i o n of m e l i l i t e c r y s t a l s i n c a l c i t e (c); note the kernels of unaltered m e l i l i t e (il) (crossed M c o l s , x20)  Figure 46* The c r y s t a l o u t l i n e s o f m e l i l i t e i n c a l c i t e (c) are p r e served by type I I a l t e r a t i o n t o c e b o l l i t e (z) (crossed N i c o l s , x20)  The p r o p e r t i e s of c e b o l l i t e from Oka agree c l o s e l y with those described f o r the type m a t e r i a l from I r o n H i l l , Colorado, by Larsen and S c h a l l e r (1914).  I"t shows high b i r e f r i n g e n c e i n t h i n  s e c t i o n (about 0.035)> although i n many s e c t i o n s the c r y s t a l s are so f i n e that overlapping f i b r e s of d i f f e r e n t o r i e n t a t i o n obscure the true i n t e r f e r e n c e c o l o u r s . parallel extinction.  C e b o l l i t e has p o s i t i v e e l o n g a t i o n and  Attempts  to measure the r e f r a c t i v e i n d i c e s  were u n s u c c e s s f u l u s i n g o i l s on powdered m a t e r i a l .  O i l s were  a p p l i e d t o c e b o l l i t e i n a t h i n s e c t i o n without a cover g l a s s ; an approximate  range o f 1.60 ( N ) to 1.63 ( N ) obtained by t h i s method x  z  i s i n good agreement with the values JS • 1.595 and N X  z  • I.628  given by Larsen and S c h a l l e r . A t h i n s e c t i o n was made of c e b o l l i t e from the type l o c a l i t y i n order t o compare i t to c e b o l l i t e from Oka. 47 and 48 show photomicrographs  of the two.  Figures  The cover g l a s s e s were  removed from t h i n s e c t i o n s of c e b o l l i t e from both l o c a l i t i e s . C e b o l l i t e was removed from both sections with a needle a f t e r areas of almost pure m a t e r i a l had been i d e n t i f i e d with the petrographic microscope.  The X-ray powder data f o r the m a t e r i a l from both  Figure 47i  C e b o l l i t e replacement of m e l i l i t e i n a l t e r e d okaite from Oka (crossed N i c o l e , 148)  F i g u r e 48*. C e b o l l i t e replacement of m e l i l i t e i n a l t e r e d uncompahgrite from the type l o c a l i t y , Iron H i l l , Colorado. In both photographs the semi-opaque mineral at lower r i g h t i s perovskite (crossed N i c o l s , 148)  93  l o c a l i t i e s are given i n Table X I I l . from Oka  The data given f o r m a t e r i a l  are averaged from three almost i d e n t i c a l powder photographs  of m a t e r i a l from d i f f e r e n t places on Husereau H i l l . of m a t e r i a l from Oka of  and Iron H i l l are compared with the X-ray data  v e s u v i a n i t e , and i t i s seen that the l a t t e r mineral accounts f o r  a l i t t l e more than h a l f the d-spacings, lines. of  The X-ray data  i n c l u d i n g most of the strong  Seventeen l i n e s of the Oka m a t e r i a l photograph, and s i x t e e n  the Iron H i l l photograph, however, are unaccounted f o r by vesuv-  ianite.  Moreover, two  strong l i n e s (dm - 3.282 and 3*062) c o i n c i d e  with v e s u v i a n i t e l i n e s , but have much higher i n t e n s i t i e s . of  Several  the l i n e s not explained by v e s u v i a n i t e show p r e f e r r e d o r i e n t a t i o n ,  a feature t y p i c a l of powder photographs of f i b r o u s minerals. i s an almost exact d u p l i c a t i o n i n the Oka  photograph of the  due to c e b o l l i t e i n the Iron H i l l photograph.  There lines  This i n i t s e l f i s  considered to be adequate proof of the i d e n t i t y of c e b o l l i t e i n a l t e r e d m e l i l i t e at  Oka.  As f a r as i s known, X-ray powder data f o r c e b o l l i t e have not been obtained before.  The d-spacings  and i n t e n s i t i e s f o r  c e b o l l i t e are given i n columns 1 and 2 of Table XIV as they appear i n columns 2 and 3 of Table XIII, except have been subtracted. d-spacings  that v e s u v i a n i t e l i n e s  Column 3 of Table XIV g i v e s the average  f o r c e b o l l i t e , with adjusted i n t e n s i t i e s .  A powder  photograph of pure v e s u v i a n i t e and powder photographs of the c e b o l l i t e - v e s u v i a n i t e mixtures  from Iron H i l l and from Oka  are  compared i n f i g u r e 49« In many t h i n s e c t i o n s c e b o l l i t e surrounds areas o r v e i n l e t s of coarser grained, c o l o u r l e s s m a t e r i a l composed of h i g h l y b i r e f r i n g e n t needles or l a t h s , i d e n t i f i e d as d i o p s i d e , i n a groundmass of v e s u v i a n i t e and a l i t t l e c a l c i t e ; t h i s i s type I I I a l t e r ation. ite of  As with c e b o l l i t e - v e s u v i a n i t e a l t e r a t i o n ,  diopside-vesuvian-  a l t e r a t i o n i s a l s o confined to m e l i l i t e , the c r y s t a l o u t l i n e s which are preserved where they occur ( f i g .  50)*  Thin s e c t i o n s  r e v e a l a complete t r a n s i t i o n between types II and I I I a l t e r a t i o n s .  Table  XIII  Comparison of X-ray Powder Data f o r Vesuvianite and f o r Mixtures of Vesuvianite and C e b o l l i t e  1 I  i 2  d  5-90  3  4-69 4.03  2 1 2  2 1  3.469 3.244 3.054 2.999 2.946  10 8  2.752 2.593  5  2.452  IB  2.332  i  1  2.194  3  2.122  IB  1.997 1.960  £  I  3 m  I  dm  7-13 5-93 5.47  3 1 2 3B 1 i£ 3 1 2  7.09 5.97 5-47 4-79  d  11.0  1  2  1 2 3  2  1  1.882  £2  1.793 1.762  £ 3 6  1.679 ) 1.662 ) 1.621  4 2 3 1 3 2* 2 6 7  4.03 3.84 3.65 3-47 3.28 3.07  2 2 10 8 1 5  2.94 2.89 2.75 2.60 2.52 2.45 2.4O  * 4 1 1 2B 1 2  2.28 2.20 2.16 2.12 2.08 1.998  1* 1* 1 £ 2 2 1  1.926 1.886 1.828 1.795 1.764 1.741 I.715  3B  1.672  6  I.625  4B 6  IB 1 10 8 £ 5 £ 1 3 1 £ 2 £ 2  £1 1 1  * 2 1  £  4.6I  4.O4 3.88 3.63 3.48 3.27 3.05 2.95 2.88 2.76 2.60 2.53 2.46 2.38 2.33 2.28 2.20 2.17 2.13 2.08 2.006 1.963 1.931 1.881 1.833 1.797/ 1.768 1.746 1.720  3B  1.673  6  1.628  Vesuvianite; A.S.T.M. Card No. 11-145 Vesuvianite plus c e b o l l i t e ; Iron H i l l , Colorado Vesuvianite plus c e b o l l i t e ; Oka, Quebec  Table XIV X-ray Powder Data f o r C e b o l l i t e  1  I  d  4  7-13 5-47  3 2  3.84  3  5 6 2 1  *  4 1 1  1* 1 2 1  3  2  I  d  7.09 5.47  6 4  7.11 5-47  3.88 3.63 3.27 3.05  5  3 8 10  3.86 3.64 3.28  1  2.88  3  * * 3  2.53 2.38 2.28 2.17  I  m  3  2 (3B  3 1  3.65 3.28 3.07 2.89 2.52 2.40 2.28 2.16 2.08 1.926 1.828 1.741 I.715  4 5  * * 1 1 1, *  4.7.9)  2.08  1.931 1.833 1.746 1.720  1 1 6 1 1 2 2 3 1  3.06 2.89 2.53 2.39 2.28 2.17  2.08 1.929 1.831 1.744 I.718  1  C e b o l l i t e : Iron H i l l , Colorado. from column 2, t a b l e XIII  2  C e b o l l i t e ; Oka, Quebec. column 3 , t a b l e XIII  3  Average d-spacings f o r c e b o l l i t e ; i n t e n s i t i e s are adjusted t o g i v e a maximum o f 10  Vesuvianite l i n e s  subtracted  Vesuvianite l i n e s subtracted from  The g r a i n s i z e o f v e s u v i a n i t e i s coarse enough f o r i t . t o be i d e n t i f i e d by i t s high p o s i t i v e r e l i e f , i t s anomalous i n t e r f e r e n c e colours (deep B e r l i n blue t o khaki brown), and i t s u n i a x i a l negative character.  Where the diopside l a t h s contained i n v e s u v i a n i t e are  f i n e grained they appear s i m i l a r t o c e b o l l i t e except that they show inclined extinction.  In s e c t i o n s where t h i s intergrowth i s coarser  grained diopside tends t o l o s e i t s l a t h form and assumes a more i r r e g u l a r o u t l i n e ( f i g . 5l)«  I t s b i r e f r i n g e n c e i s about 0.030, i t s  r e l i e f i s s l i g h t l y lower than that, of v e s u v i a n i t e , and g r a i n s with low b i r e f r i n g e n c e commonly show two perpendicular cleavages.  Some  96  F i g u r e 49* Comparison o f X - r a y powder photographs o f ( l ) » v e s u v i a n i t e from Salmo, B . C . , ( 2 ) : c e b o l l i t e - v e s u v i a n i t e mixture from I r o n H i l l , C o l o r a d o , (3). c e b o l l i t e - v e s u v i a n i t e mixture from Husereau H i l l , near Oka, Quebec.  F i g u r e $0t anisotropic  Type III a l t e r a t i o n o f m e l i l i t e c r y s t a l s d i o p s i d e l a t h s are set i n a groundmass o f v e s u v i a n i t e ( c r o s s e d N i c o l s , x20)  i n c a l c i t e (c); nearly isotropic  diopside g r a i n s are large enough to give i n t e r f e r e n c e and has a value of about 6 0 ° .  positive, Z  2V i s  figures;  The average e x t i n c t i o n  angle  C i s about 4 0 ° ; t h i s measurement cannot be made a c c u r a t e l y because  of the u n s u i t a b i l i t y of the g r a i n s f o r o b t a i n i n g i n t e r f e r e n c e  figures.  X-ray powder photographs of type I I I a l t e r a t i o n intergrowth show a mixture of v e s u v i a n i t e and pyroxene l i n e s , confirming the i d e n t i f i c a t i o n made from o p t i c a l p r o p e r t i e s . The  extent of type I I I a l t e r a t i o n i s v a r i a b l e .  I t may form  small s p h e r u l i t i c growths i n m e l i l i t e that i s otherwise u n a l t e r e d except along f r a c t u r e s  and cleavage ( f i g . 52). These  radiating  bursts have a t h i n outer rim of c e b o l l i t e , and are g e n e r a l l y astride  a t h i n a l t e r a t i o n v e i n l e t , o r at the i n t e r s e c t i o n  fractures.  situated  of two  A t r a n s i t i o n may e x i s t w i t h i n a s i n g l e t h i n s e c t i o n  from  u n a l t e r e d m e l i l i t e , through a zone where s p h e r u l i t i c growths of d i o p s i d e - v e s u v i a n i t e i n c r e a s e i n s i z e and number to where they coalesce to replace m e l i l i t e completely.  Alteration veinlets i n  m e l i l i t e may be zoned, with type I I I a l t e r a t i o n i n the c e n t r a l and  part,  with successive rims of types I I and I a l t e r a t i o n . As  the g r a i n  s i z e of type I I I a l t e r a t i o n  increases,  regardless of whether or not the m e l i l i t e i s completely small patches of yellow andradite occur, e s p e c i a l l y  altered,  at the edges  of magnetite grains i n contact with the o r i g i n a l m e l i l i t e ( f i g . 53). The  most extreme stage of t h i s i s type IV a l t e r a t i o n , where large  patches of andradite and v e s u v i a n i t e occur separately, surrounded by  d i o p s i d e - v e s u v i a n i t e intergrowth i n p a r t l y a l t e r e d m e l i l i t e .  Small patches of m o n t i c e l l i t e  occur i n two t h i n s e c t i o n s of t h i s  nature; pale sage-green b i o t i t e appears as new c r y s t a l s or as rims on p r e - e x i s t i n g  brown-green b i o t i t e .  Diopside has a f a i n t y e l l o w  c o l o u r i n the coarsest g r a i n s observed.  X-ray powder photographs  were used to confirm the i d e n t i t i e s of m o n t i c e l l i t e the  and andradite;  l a t t e r has a c e l l edge of 12.061. The  Alteration  of Hauyne  A rough c o r r e l a t i o n  can be made between the a l t e r a t i o n of  m e l i l i t e and hauyne, as shown In Table XV.  T h i s c o r r e l a t i o n must  Figure 51* Type III a l t e r a t i o n of m e l i l i t e . White and grey l a t h s and i r r e g u l a r patches are diopside; black groundmass i s mainly v e s u v i a n i t e ; A i s a p a t i t e , C c a l c i t e (crossed H i c o l s , x48)  F i g u r e 52: S p h e r u l i t i c a l t e r a t i o n of m e l i l i t e (M) showing zones of a l t e r a t i o n types (I, II, and III); a p a t i t e (A) and c a l c i t e (c) are u n a l t e r e d (crossed N i c o l s , x48)  Figure 53* Replacement of m e l i l i t e (M) by andradite garnet (G), e s p e c i a l l y at contact to magnetite ( b l a c k ) ; c a l c i t e (c) surrounds m e l i l i t e c r y s t a l s (plane l i g h t , x20)  be considered t o represent only the most commonly observed s i t u a t i o n ; i t does not appear to c o n s t i t u t e a general r u l e as there are many exceptions to i t .  Table XV C o r r e l a t i o n of the Types o f M e l i l i t e and Hauyne A l t e r a t i o n s Melilite  Hauyne  Type I  Z e o l i t e s with minor c a l c i t e and embryonic c r y s t a l s of g r o s s u l a r i t e ; may be almost f r e s h , with network of o r i e n t e d i n c l u s i o n s v i s i b l e  Type I I  V a r i a b l e z e o l i t e s plus c a l c i t e and w e l l c r y s t a l l i z e d g r o s s u l a r i t e ; v e s u v i a n i t e c r y s t a l s at contact to altered melilite  Type I I I  P a r t i a l z e o l i t e c r y s t a l l i z a t i o n with minor c a l c i t e ; may show minor development of embryonic g r o s s u l a r i t e  Type IV  Almost u n a l t e r e d , o r p a r t i a l z e o l i t e c r y s t a l l i z a t i o n ; o r i g i n a l o r i e n t e d i n c l u s i o n network may be v i s i b l e  Some specimens of o k a i t i c rocks i n which m e l i l i t e i s una l t e r e d show that hauyne i s minutely r e c r y s t a l l i z e d to f i b r o u s with very low b i r e f r i n g e n c e .  Where t h i s a l t e r a t i o n has not  zeolite  proceeded  f a r , the commonly observed o r i e n t e d network of opaque i n c l u s i o n s i n hauyne i s preserved. The  same type of hauyne a l t e r a t i o n i s also found i n o k a i t e s  i n which m e l i l i t e shows i n c i p i e n t a l t e r a t i o n along f r a c t u r e s  and  cleavage; i n a d d i t i o n to zeolilte, which commonly has weak b i r e f r i n g ence ( 0 . 0 0 3 ) ,  low negative r e l i e f , p o s i t i v e elongation:, and  e x t i n c t i o n , s t r i n g s of t i n y embryonic c r y s t a l s with p o s i t i v e have formed along f r a c t u r e s .  relief  Small blebs of c a l c i t e g i v e the hauyne  a peppered appearance i n t h i n s e c t i o n between crossed N i c o l ( f i g . 54 and  parallel,  prisms  55).  Hauyne shows i t s greatest degree of a l t e r a t i o n i n rocks i n which m e l i l i t e i s changed to c e b o l l i t e - v e s u v i a n i t e intergrowth, or type I I a l t e r a t i o n .  Z e o l i t e and c a l c i t e form the main groundmass.  The" z e o l i t e i s v a r i e d i n nature; i t i s commonly composed of "columnar f i b r e s , but may  be i n the form of sheaves of r a d i a t i n g blades.  e x t i n c t i o n i s p a r a l l e l or s u b - p a r a l l e l to e l o n g a t i o n , which may p o s i t i v e , negative, or both.  The be  B i r e f r i n g e n c e v a r i e s between O.OO4 and  0 . 0 1 0 , and r e l i e f i s i n v a r i a b l y negative.  P r i s m a t i c cleavage i s  p a r a l l e l to elongation, as i s l a m e l l a r twinning.  The  are g e n e r a l l y too small, or top. c l o s e l y interwoven  with each'other  c a l c i t e to g i v e comprehensible  interference figures.  z e o l i t e grains  X-ray powder  photographs g i v e p a t t e r n s comparable to those of thomsonite natrolite.  I t i s suspected that these two  the present  and  z e o l i t e s form the bulk of  z e o l i t e a l t e r a t i o n i n hauyne, but that other undetermined are present a l s o .  and  zeolites  Mo f u r t h e r work has been done on the z e o l i t e s i n  study.  Within the c a l c i t e - z e o l l i . e groundmass, c o l o u r l e s s equant c r y s t a l s with square or hexagonal cross s e c t i o n s are observed. areas of t h i n s e c t i o n s that are t h i c k e r than normal these are seen to be small dodecahedra ( f i g . 5 6 ) .  The  crystals  c r y s t a l s have high  p o s i t i v e r e l i e f and show low b i r e f r i n g e n c e (< 0 . 0 0 4 ) . commonly twinned  In  They are  i n d i a g o n a l l y opposed t r i a n g u l a r segments.  The  e x t i n c t i o n i s p a r a l l e l to the outer, edge of each c r y s t a l segment.  Figure 54* Altered hauyne i n pyroxene okaite. Small c r y s t a l s of g r o s s u l a r i t e garnet are set i n a groundmass of z e o l i t e and c a l c i t e . Px i s t i t a n a u g i t e , X i s m e l i l i t e a l t e r a t i o n type I, and the opaque mineral i s magnetite (plane l i g h t , x20)  Figure 55: The same as f i g u r e 54 (crossed M c o l s , x 2 0 ) . Compare the appearance of f r e s h hauyne i n f i g u r e 1 2 , page 3 7  Figure 56t A l t e r e d hauyne i n o k a i t e . G r o s s u l a r i t e garnet (G) and c a l c i t e (C), and v e s u v i a n i t e (v) i n contact with m e l i l i t e a l t e r a t i o n type I I ( z ) , have replaced hauyne. A i s a p a t i t e , B i s b i o t i t e , and P i s perovskite (plane l i g h t , X48)  Each segment i s separated  from the next by a t h i n band of t i n y  p a r t i c l e s of opaque i m p u r i t i e s , s i m i l a r i n appearance to those seen i n the c h i a s t o l i t e v a r i e t y of a n d a l u s i t e .  Several dozen c r y s t a l s  were extracted with a needle from a s p e c i a l l y prepared uncovered t h i n s e c t i o n with greater thickness than normal.  An X-ray powder  photograph of t h i s m a t e r i a l proved i t to be g r o s s u l a r i t e with a c e l l edge of 11.88A*.  I t i s p o s s i b l e to f i n d a l l stages i n the  development of g r o s s u l a r i t e i n a l t e r e d hauyne; t i n y embryonic c r y s t a l l i t e s develop through a stage of increased s i z e , where the c r y s t a l form i s s t i l l i r r e g u l a r but where twinning may  be seen, to  the f i n a l well c r y s t a l l i z e d stage shown i n f i g u r e 56. Where a l t e r e d hauyne i s i n contact with type II a l t e r a t i o n of m e l i l i t e , v e s u v i a n i t e forms a rim of pale yellow p r i s m a t i c c r y s t a l s that p r o j e c t i n t o the c a l c i t e - z e o l i t e - g r o s s u l a r i t e groundmass ( f i g . 56).  T h i s v e s u v i a n i t e shows s t r i k i n g l y anomalous  i n t e r f e r e n c e c o l o u r s ranging from deep p u r p l i s h blue through r e d d i s h brown t o pale o l i v e brown. In rocks showing types I I I and IV a l t e r a t i o n of m e l i l i t e , hauyne may  appear almost f r e s h .  and c a l c i t e may  Minor c r y s t a l l i z a t i o n of z e o l i t e  be present, and hauyne then has the same appearance  as that described i n o k a i t e where m e l i l i t e i s a l t e r e d only t o type I minerals along f r a c t u r e s and cleavage. i s rare.  The development of g r o s s u l a r i t e  The o r i g i n a l o r i e n t e d network of opaque i n c l u s i o n s may  observed i n some t h i n s e c t i o n s where m e l i l i t e i s completely to type I I I d i o p s i d e - v e s u v i a n i t e intergrowth.  be  altered  I t i s noted that most,  o k a i t i c rocks with type I I I m e l i l i t e a l t e r a t i o n do not c o n t a i n hauyne or i t s a l t e r e d equivalent; some c o n t a i n nepheline which shows only minor a l t e r a t i o n along g r a i n boundaries to f i b r o u s z e o l i t e  and  rare c a n c r i n i t e .  The A l t e r a t i o n of Other Eock Forming Minerals In rocks i n which m e l i l i t e i s a l t e r e d along f r a c t u r e s  and  cleavage only, none of the other minerals except hauyne and perhaps t i t a n a u g i t e i s a l t e r e d i n any way.  I t i s noted that pyroxene o k a i t e  has not been found with m e l i l i t e replaced by any other than types I or I I a l t e r a t i o n minerals.  T i t a n a u g i t e i n such a rock may  show minor  a l t e r a t i o n to b i o t i t e ; i n one t h i n s e c t i o n a small amount of u r a l i t i c hornblende  was  seen at t i t a n a u g i t e g r a i n boundaries.  a p a t i t e , c a l c i t e , and magnetite  F i g u r e 57 shows  i n o k a i t e i n which m e l i l i t e shows  advanced type I a l t e r a t i o n along f r a c t u r e s . In o k a i t i c rocks with type I I m e l i l i t e a l t e r a t i o n , only magnetite  shows any form of a l t e r a t i o n o r replacement;  i n c i p i e n t a l t e r a t i o n to yellow andradite garnet may magnetite  here minor  be seen where  i s i n contact with m e l i l i t e a l t e r a t i o n products.  In rocks  with type I I I m e l i l i t e a l t e r a t i o n , the development.- of andradite around magnetite  i s more pronounced, and may  ragged p e r o v s k i t e grains ( f i g . 58)•  also be seen around  Here also b i o t i t e commonly has  ragged o u t l i n e s , and has a pronounced bird's-eye s t r u c t u r e that i s not seen to the same extent i n the b i o t i t e of u n a l t e r e d rocks. some rocks b i o t i t e i s p a r t l y a l t e r e d to a c o l o u r l e s s micaceous  In  Figure 57« A l t e r e d o k a i t e , with m e l i l i t e (M) showing almost complete a l t e r a t i o n of types I and I I (z) along f r a c t u r e s , and s p h e r u l i t i c type I I I d i o p s i d e - v e s u v i a n i t e a l t e r a t i o n (D). Hauyne (H) i s a l t e r e d to z e o l i t e and c a l c i t e . A p a t i t e (A), b i o t i t e (B), c a l c i t e ( c ) , perovskite ( P ) , and magnetite (black) are t o t a l l y u n a l t e r e d (plane l i g h t , x20)  Figure 58t Andradite garnet (G) a s s o c i a t e d with ragged magnetite (Mt) i n okaite with type I I I m e l i l i t e a l t e r a t i o n (D). A i s a p a t i t e , B i s b i o t i t e , and P i s perovskite (plane l i g h t , x20)  mineral which forms i n : the same o r i e n t a t i o n as the b i o t i t e i t r e p l a c e s , and has low f i r s t order b i r e f r i n g e n c e ; i t i s probably a type of c h l o r i t e . In rocks i n which m e l i l i t e shows type IV a l t e r a t i o n , magnetite i s considerably replaced by andradite.  The  original  green-brown b i o t i t e has an overgrowth of pale sage green b i o t i t e ; the i n n e r o r i g i n a l b i o t i t e i s ragged and i r r e g u l a r i n o u t l i n e , but the second generation b i o t i t e has r e s t o r e d smooth c r y s t a l edges to i t .  A p a t i t e and primary c a l c i t e are not a l t e r e d .  Where o k a i t i c rocks are a l t e r e d on e i t h e r side of f r a c t u r e s the a l t e r a t i o n products  i n m e l i l i t e are zoned with respect to the  f r a c t u r e s as described on page 97•  C e b o l l i t e f i b r e s and  diopside  l a t h s are g e n e r a l l y o r i e n t e d at r i g h t angles to f r a c t u r e s . i d u a l bands of a l t e r a t i o n along f r a c t u r e s may 1 mm  and 10 mm  t h i c k , although normally  Indiv-  be anywhere between  s e v e r a l f r a c t u r e s occur  together, and t h e i r associated a l t e r a t i o n bands coalesce.  Band  thickness i s governed by the a l t e r a t i o n of m e l i l i t e ; a l l other minerals, except hauyne, that are w i t h i n such a band remain u n a l t ered.  A large b i o t i t e g r a i n , f o r i n s t a n c e , may  be s i t u a t e d at the  centre of an a l t e r a t i o n band so that, i t p r o j e c t s i n t o f r e s h m e l i l i t e on e i t h e r s i d e ; the small f r a c t u r e , along which m e l i l i t e  alteration  has taken p l a c e , passes through the b i o t i t e with no v i s i b l e  effect.  The f r a c t u r e s r e s p o n s i b l e f o r even the thickest, bands of a l t e r a t i o n are very narrow.  In some rocks the f r a c t u r e s themselves  are f i l l e d with minerals that have been deposited d i r e c t l y from hydrothermal s o l u t i o n , r a t h e r than by a l t e r a t i o n of the w a l l minerals.  These minerals were deposited evenly along the length of  f r a c t u r e s regardless of the type of w a l l mineral.  Pine grained  c a l c i t e i s the commonest mineral deposited i n t h i s manner, and i s the only one that can be p o s i t i v e l y i d e n t i f i e d .  The other  minerals  take the form of t h i n bands of brownish or c o l o u r l e s s m a t e r i a l that forms a coxcomb s t r u c t u r e w i t h i n the f r a c t u r e s . Some a l t e r a t i o n zones have d i f f u s e bands of very f i n e grained, pale blue m a t e r i a l bordering the c e n t r a l f r a c t u r e s .  This  m a t e r i a l seems at f i r s t glance to be an a l t e r a t i o n of m e l i l i t e ,  as  b i o t i t e , a p a t i t e , magnetite, and p e r o v s k i t e are enclosed and una f f e c t e d by i t .  C a r e f u l examination, however, reveals that i t i s  a l a t e r replacement of m e l i l i t e a l t e r a t i o n minerals.  I t forms  sharp boundaries with d i o p s i d e — v e s u v i a n i t e a l t e r a t i o n of m e l i l i t e , which i t also cuts as small o f f s h o o t s of the main v e i n l e t s .  This  pale b l u i s h a l t e r a t i o n has been described f o r hand specimen on page 22.  An X-ray powder photograph o f t h i s m a t e r i a l g i v e s a  mixture of the patterns o f c a l c i t e and o f another mineral whose d-spacings are midway between those o f a n a l c i t e and w a i r a k i t e ; the l a t t e r mineral i s the calcium analogue of a n a l c i t e .  Ko f u r t h e r  work has been done on t h i s m a t e r i a l .  The A l t e r a t i o n o f M e l i l i t e - C a l c i t e Rock I t was noted on page 103 that, pyroxene  o k a i t e shows only  types I and I I m e l i l i t e a l t e r a t i o n , and that type I I I a l t e r a t i o n i s more commonly found i n m e l i l i t e o k a i t e than i n okaite proper. S i m i l a r l y , type IV a l t e r a t i o n i s found mainly i n m e l i l i t e o k a i t e and pegmatite phases.  There seems, t h e r e f o r e , t o be a crude  c o r r e l a t i o n between rock type and type of m e l i l i t e a l t e r a t i o n .  In  pegmatite phases o f m e l i l i t e o k a i t e and i n m e l i l i t e - c a l c i t e rock type IV a l t e r a t i o n i s c a r r i e d a stage f u r t h e r .  Alteration i s  i n v a r i a b l y patchy; the products o f r e a c t i o n are u s u a l l y coarse grained, and d i f f e r e n t a l t e r a t i o n minerals are i r r e g u l a r l y tributed.  dis-  I t appears that a l t e r a t i o n has become more a matter of  r e a c t i o n with r e s i d u a l magmatic f l u i d s than with m i g r a t i n g hydrothermal f l u i d s . Several- t h i n s e c t i o n s o f m e l i l i t e - c a l c i t e rock and o f pegmatite phases o f m e l i l i t e o k a i t e contain l a t e - f o r m i n g pale yellow pyroxene, i n many rocks apparently grown on p a r t l y a l t e r e d melilite crystals.  T h i s pyroxene  appears to have been deposited  at about the same time as f i b r o u s a p a t i t e and euhedral magnetite and p e r o v s k i t e .  I t i s c e r t a i n l y formed e a r l i e r than the e n c l o s i n g  calcite. A t h i n s e c t i o n was made from a specimen of m e l i l i t e o k a i t e pegmatite i n which bands and patches o f pyroxene  occur.  Microscopic examination  r e v e a l s that one such patch i s due t o  a l t e r a t i o n centred about a s e r i e s o f c l o s e l y spaced f r a c t u r e s i n m e l i l i t e c r y s t a l s i n a c o a r s e l y c r y s t a l l i n e c a l c i t e groundmass. In the p a r t s o f the t h i n s e c t i o n f u r t h e s t removed from the pyroxene patch, m e l i l i t e i s r e l a t i v e l y u n a l t e r e d , except f o r minor development o f c e b o l l i t e along cleavage and f r a c t u r e s .  At the edge o f  the patch m e l i l i t e i s a l t e r e d t o a band of c e b o l l i t e - v e s u v i a n i t e intergrowth (type I I a l t e r a t i o n ) .  Separating t h i s from the yellow  pyroxene c r y s t a l s i s an i r r e g u l a r zone o f d i o p s i d e - v e s u v i a n i t e intergrowth (type I I I a l t e r a t i o n ) .  Towards the c e n t r a l part o f  the patch v e s u v i a n i t e disappears, diopside i n c r e a s e s i n g r a i n s i z e and assumes a yellow c o l o u r , and orange-yellow appears.  andradite garnet  At the centre of the patch there i s no evidence o f  m e l i l i t e at a l l , i t s place being e n t i r e l y taken by yellow pyroxene and orange andradite? present throughout,  small amounts o f pale green b i o t i t e are  and are unusual i n that they show a p i n k i s h  c o l o u r when s e c t i o n s perpendicular to C are o r i e n t e d east-west on the microscope stage.  There are p a r t s of the t h i n s e c t i o n where  types I I and I I I a l t e r a t i o n s are missing; here m e l i l i t e i s separated from yellow pyroxene only by a t h i n zone where the two are intergrown. The rock contains a small percentage i n pegmatite  phases.  of hauyne, unusual  Within the a l t e r a t i o n patch hauyne i s changed  to z e o l i t e , although the o r i g i n a l opaque i n c l u s i o n network i s preserved i n some g r a i n s . The presence o f m o n t i c e l l i t e i n two s e c t i o n s of a l t e r e d m e l i l i t e o k a i t e pegmatite  r a i s e s the question whether o r not the  m o n t i c e l l i t e - c a l c i t e - m e l i l i t e intergrowth described on page 47 may i t s e l f be a product o f a l t e r a t i o n of m e l i l i t e .  Small amounts  of yellow pyroxene are present i n f r e s h m o n t i c e l l i t e - b e a r i n g m e l i l i t e o k a i t e s ; b i o t i t e i s sage-green, not the t y p i c a l green-brown b i o t i t e o f o k a i t e .  darker  Yellow andradite i s a l s o present  i n one o f the two specimens, and here occurs along an i r r e g u l a r fracture. I f yellow pyroxene, m o n t i c e l l i t e , coarse grained andradit and sage-green b i o t i t e are i n f a c t a form o f a l t e r a t i o n o f m e l i l i t e  as has "been i n d i c a t e d , such a l t e r a t i o n d i f f e r s i n nature from the more t y p i c a l l y hydrothermal types I, I I , and I I I a l t e r a t i o n s .  It  may w e l l represent h i g h temperature a l t e r a t i o n caused by r e a c t i o n between m e l i l i t e c r y s t a l s and i n t e r s t i t i a l l i q u i d during the f i n a l stages of c r y s t a l l i z a t i o n .  In places the intergrowth between  m o n t i c e l l i t e and m e l i l i t e appears somewhat l i k e a e u t e c t i c i n t e r growth.  The a s s o c i a t e d c a l c i t e , however, suggests that an i n c r e a s -  i n g C0~2 content i n the remaining f l u i d caused m o n t i c e l l i t e and c a l c i t e to. c r y s t a l l i z e i n place of m e l i l i t e .  Yellow pyroxene i s  more d e f i n i t e l y a product of r e a c t i o n , and there appears to be a gradation between r e a c t i o n of t h i s k i n d and a l t e r a t i o n o f a more s p e c i f i c a l l y hydrothermal nature, the l a t t e r being e f f e c t e d at, lower temperatures by m i g r a t i n g f l u i d s .  Sequence of A l t e r a t i o n M e l i l i t e has been shown to d i s p l a y a complete sequence of a l t e r a t i o n from the stage where m o n t i c e l l i t e or pyroxene,  calcite,  and andradite are formed to the stage where type I minerals replace it.  I t i s b e l i e v e d that t h i s sequence r e f l e c t s a change i n the  nature of the f l u i d s that caused a l t e r a t i o n .  The r e a c t i v e f l u i d s  are thought to be derived d i r e c t l y from the okaite s e r i e s magma. The formation of b i o t i t e and primary c a l c i t e as l a t e products of c r y s t a l l i z a t i o n t e s t i f i e s to the c o n c e n t r a t i o n of water and carbon d i o x i d e i n the r e s i d u a l magmatic f l u i d .  Secondary c a l c i t e and the  hydrous a l t e r a t i o n products of m e l i l i t e ( v e s u v i a n i t e , c e b o l l i t e , etc.) and hauyne ( z e o l i t e s ) i n d i c a t e formation from watery  fluids  charged with carbon dioxide. M o n t i c e l l i t e probably s t a r t e d t o form i n p l a c e of m e l i l i t e during the l a s t stages of c r y s t a l l i z a t i o n , and thus i s a product of r e s i d u a l magmatic f l u i d r e a c t i o n .  The formation o f y e l l o w pyroxene  and andradite garnet at the expense of m e l i l i t e represents a cond i t i o n somewhere i n between r e s i d u a l magmatic f l u i d r e a c t i o n and t r u e hydrothermal a l t e r a t i o n .  Successive stages of a l t e r a t i o n then  took p l a c e with i n c r e a s i n g l o c a l i z a t i o n and m i g r a t i o n of the r e s i d u a l f l u i d s , now p r o p e r l y termed hydrothermal f l u i d s .  I t i s thought that  the change i n the nature of the r e a c t i v e f l u i d s , and hence i n the type of a l t e r a t i o n e f f e c t e d , was change.  governed p r i m a r i l y hy  temperature  Figure 59 shows the r e l a t i o n between the v a r i o u s a l t e r a t i o n  minerals of m e l i l i t e and decreasing temperature,  as deduced from  t e x t u r a l evidence seen i n t h i n s e c t i o n and from c o n s i d e r a t i o n of the nature of the a l t e r a t i o n minerals  themselves.  M e l i l i t e A l t e r a t i o n Types IVa  IVb  Melilite Primary c a l c i t e Monticellite Sage-green b i o t i t e Andradite Yellow pyroxene Colourless diopside Vesuvianite Secondary c a l c i t e Cebollite Mineral l b Mineral l a  II  I  \  Decreasing  Figure 59*  III  temperature  Diagrammatic r e p r e s e n t a t i o n of the change i n formationi of m e l i l i t e a l t e r a t i o n ; minerals with temperature  M e l i l i t e and primary c a l c i t e are i n c l u d e d i n . f i g u r e as guides.  59  Type IV a l t e r a t i o n i s d i v i d e d i n t o two p a r t s , IVa and  IVb; these represent, m o n t i c e l l i t e - c a l c i t e - m e l i l i t e intergrowth and coarse grained yellow pyroxene-andradite-vesuvianite a l t e r a t i o n respectively.  Types I I I , I I , and I are the hydrothermal  alteration  types d i o p s i d e - v e s u v i a n i t e , c e b o l l i t e - v e s u v i a n i t e , and minerals l a and l b r e s p e c t i v e l y . The continued c r y s t a l l i z a t i o n of primary c a l c i t e type IV a l t e r a t i o n i s warranted  through  by the f a c t t h a t i t i s seen to  enclose y e l l o w pyroxene c r y s t a l s i n some specimens of a l t e r e d m e l i l i t e - c a l c i t e rock.  The term secondary c a l c i t e i s used to  d i s t i n g u i s h the c a l c i t e that i s i n t i m a t e l y i n c l u d e d i n the hydrothermal a l t e r a t i o n types.  The dashed l i n e r e p r e s e n t i n g sage-green  b i o t i t e i n d i c a t e s that i t Is not c o n s i s t e n t l y present.  The dashed  part of the andradite l i n e i n d i c a t e s i t s formation i n small amounts around magnetite g r a i n s . are  Yellow pyroxene and c o l o u r l e s s diopside  joined; between a l t e r a t i o n types IVb and I I I because they are  t r a n s i t i o n a l to one  another.  Prom l e f t to r i g h t i n t h i s diagram there i s a p r o g r e s s i v e t r e n d from i n t e r s t i t i a l r e a c t i o n with r e s i d u a l f l u i d s to hydrothermal. a l t e r a t i o n s p e c i f i c a l l y c o n t r o l l e d by f r a c t u r e s .  As t h i s trend i s  developed, the degree of p e n e t r a t i o n o f such f l u i d s i n t o the body of the rock decreases. although i t may  Thus type I I I d i o p s i d e - v e s u v i a n i t e a l t e r a t i o n ,  be found along s p e c i f i c f r a c t u r e s , i s more commonly  found t o have r e p l a c e d m e l i l i t e i n a manner suggesting that the rock had been soaked i n the a l t e r i n g medium. a l t e r a t i o n , at the low temperature  In c o n t r a s t , type I  end of the s c a l e , i s i n v a r i a b l y  a s s o c i a t e d with f r a c t u r e s and cleavage i n m e l i l i t e , and  rarely  penetrates m e l i l i t e f o r more than one or two m i l l i m e t r e s on e i t h e r side o f f r a c t u r e s .  Commonly o c c u r r i n g zones of lower  around higher temperature  temperature  a l t e r a t i o n minerals, such as bands of  c e b o l l i t e b o r d e r i n g d i o p s i d e - v e s u v i a n i t e v e i n l e t s i n m e l i l i t e , are evidence that d i f f e r e n t a l t e r a t i o n minerals were produced by the same hydrothermal  fluid.  Chemical Changes i n A l t e r a t i o n I t i s not f e a s i b l e t o t r y to r e l a t e the r e a c t i o n s i n volved i n a l t e r a t i o n by chemical equations because most of the minerals i n v o l v e d have complex and v a r i a b l e chemical compositions. The parent m e l i l i t e has already been shown to have a type molecule formula of approximately ^356010^555 the equivalent of t h i s expressed as a chemical formula i s s 7JSTaCaAlSi 0Y. 2 C a A l S i O Y . l l C a M g S i O Y j 2  2  2  2  2  t h i s i s approximately equivalent to N a ^ a ^ l ^ A l ^ S l i i O ^ Q .  The  approximate chemical formulae o f m e l i l i t e and of the important a l t e r a t i o n minerals are l i s t e d below i n Table XVI. ,  Table XVI Approximate Chemical Formulae of M e l i l i t e and i t s A l t e r a t i o n Minerals Mineral  S u b s t i t u t i n g Ions  Chemical Formula  Melilite  Na2Cai4Mg3Al3Si.ii0 o  Fe", Fe"'  Monticellite  CaMgSiO^  Pyroxene  CaMgSi206 (.= diopside)  Andradite  Ca3Fe Si30 2  Vesuvianite  Ca Mg2 4 9 36' 2°  Cebollite  Ca5Al2Si.3Oi4.2H2O?  ?  Juanite (= l a ? )  CaioMg4Al2Siii03 .4H2G ?  •j  4  2  (Fe")  S i  1  ....  1  A l  Na, Fe", Fe" , A l , e t c . Na, Fe", Fe'", etc.  2 H  G  (Mg)  1 G  9  Comparison o f the above chemical formulae shows that the only a d d i t i o n i n a l t e r a t i o n was H2C  The formation of c a l c i t e  i n d i c a t e s an a d d i t i o n of CO2, and explains the r e l a t i v e increase o f A l plus Mg to Ca i n the a l t e r a t i o n minerals.  The Fe i n andradite  i s probably derived mainly from magnetite* these two minerals have been shown to be i n c l o s e a s s o c i a t i o n .  The o r i g i n a l sodium and  i r o n content of m e l i l i t e (Na20 t 4.26$, Fe^s  0.50$, FeO : 1.97$)  can be e a s i l y accomodated i n pyroxene and v e s u v i a n i t e , and probably i n c e b o l l i t e and minerals l a and l b .  I t i s p o s s i b l e , however, that  some of the sodium was removed i n t o nearby country rocks t o form nepheline, a e g i r i n e , and soda-hornblende as products of f e n i t i z a t i o n . I f the chemical content of the hydrous f l u i d s was not the prime f a c t o r governing the formation of d i f f e r e n t a l t e r a t i o n mineral assemblages, then changes i n the p h y s i c a l environment must have been responsible.  Such a change would be e f f e c t e d by the passage o f hot'  hydrous f l u i d s i n t o c o o l e r environments. rocks can be described as automorphic.  The a l t e r a t i o n of o k a i t i c The s i g n i f i c a n c e of the  degree of automorphism i s discussed i n a l a t e r s e c t i o n .  112  COMPARISON OF OKAITE AND SIMILAR ROCKS  Introduction Three l o c a l i t i e s , other than Oka, where m e l i l i t e - r i c h i n t r u s i v e rocks occur have already been mentioned. Hill,  Colorado, where uncompahgrite was f i r s t  These are: Iron  described by Larsen  and Hunter (1914)$ Kusnavolok, T u r j a , K o l a Peninsula, where Ramsay (1921) f i r s t  described t u r j a i t e ; Gwasi, Kenya (McCall, 1959)? where  both t u r j a i t e and uncompahgrite occur. of  A more d e t a i l e d account of each  these l o c a l i t i e s i s given i n the f o l l o w i n g paragraphs.  Table XVII  gives the average modes f o r the m e l i l i t e rocks from Oka and from each of  the three l o c a l i t i e s described below.  The mode of j a c u p i r a n g i t e  from Husereau H i l l , Oka, i s included i n t h i s t a b l e because i t shows a considerable chemical s i m i l a r i t y to okaite (see Table X V I I l ) . Iron H i l l , Colorado The complex a l k a l i c stock at Iron H i l l  intrudes Precambrian  g r a n i t e ; i t i s exposed w i t h i n a roughly t r i a n g u l a r area o f a l i t t l e more than twelve square miles.  I t i s a composite i n t r u s i o n whose rock  types are uncompahgrite, pyroxenite, i j o l i t e ,  soda s y e n i t e , nepheline  gabbro, and quartz gabbro, intruded i n that order.  These rocks are  arranged i n a crudely sub-concentric p a t t e r n around a c e n t r a l block of d o l o m i t i c marble.  The marble contains a l i t t l e a p a t i t e throughout, and  near i t s contacts with uncompahgrite o r pyroxenite i t c a r r i e s a small percentage of b i o t i t e and soda-pyroxene.  The marble i s exposed w i t h i n  a two and a h a l f square mile area, and Larsen considers that i t i s hydrothermal i n o r i g i n , perhaps deposited by "... a hydrothermal s o l u t i o n i n the open throat of an o l d volcano, ..." (1942,  p. 9)«  Larsen  favours d e r i v a t i o n of the i n t r u s i v e rocks from b a s a l t i c magma by limestone a s s i m i l a t i o n , probably at depth (1942,  p. 3 9 ) '  The average o f two modes given f o r uncompahgrite i s l i s t e d i n column 7 of Table XVII.  Uncompahgrite i s s i m i l a r to m e l i l i t e  okaite (Table XVII, column 5)» with the exception that i t contains a considerable percentage o f pyroxene; t h i s pyroxene i s not the t i t a n a u g i t e of pyroxene okaite, but i s similar, to the yellow pyroxene  Table XVII Comparison of the Average Modes o f Okaite and S i m i l a r Books  Titanaugite (Nepheline (Hauyne Melilite Augite Biotite Calotte Apatite Magnetite Perovskite Andradite  1  2  3  75  35 — 18 22  _  15  -  -'1  8 1 3 9 4 —  1 1 6 1 —  16  -  42  -  18 2 4 7 4 tr  4  5  6  7  — 23 55  —  — — 73  — -— 68 13 2 2 1 10 3 2  -7  2 3 7 3  -  -  70 tr 5 5 5 10 5 tr  -1  1 1 14 9 tr  1 - j a c u p i r a n g i t e , Oka; 2 - pyroxene o k a i t e , Oka; 3 - t u r j a i t e , K o l a Peninsula; 4 - o k a i t e , Oka; 5 ~ m e l i l i t e o k a i t e , Oka; 6 - uncompahg r i t e , Gwasi, Kenya; 7 - uncompahgrite, Iron H i l l , Colorado^  found i n m e l i l i t e - c a l c i t e rock at Oka.  Larsen (1942) and Larsen and  Goranson (1932) have described a sequence of m e l i l i t e a l t e r a t i o n i n uncompahgrite that i s very s i m i l a r to that given f o r m e l i l i t e from Oka i n f i g u r e 59, page 109. Kusnavolok, T u r j a , K o l a Peninsula At Kusnavolok t u r j a i t e occurs as a small body i n t r u s i v e i n t o Precambrian  porphyritic granite.  I t i s a s s o c i a t e d with i j o l i t e , -  and also with "... considerable bodies of pyroxene c a r b o n a t i t e and c a r b o n a t i t e . . . " (Shand, 1945, led  P»506).  A s s o c i a t e d c a r b o n a t i t e dikes  Kranck (1928) to conclude that an outside source of calcium  required to e x p l a i n the unusual nature of t u r j a i t e .  was  Column 3 i m  Table XVII shows that t u r j a i t e i s very s i m i l a r to o k a i t e (column 4) except that i t contains nepheline i n s t e a d of hauyne; b i o t i t e i s more abundant than i n the average o k a i t e , although i n some specimens of o k a i t e b i o t i t e comprises  20$ of the t o t a l , mineral content.  Melilite  i n t u r j a i t e i s a l t e r e d i n a s i m i l a r manner to the m e l i l i t e at Oka and at Iron H i l l .  Gwasi, Kenya In  the neighbourhood of Gwasi, Kenya, McCall (1959) has  described a sub-volcanic i n t r u s i v e complex of a l k a l i c and carbonate rocks.  The country rock i s Precambrian  granite.  The i n t r u s i v e  sequence i s as f o l l o w s : i n t r u s i o n of an i j o l i t e stock, and metasomatic-formation of a s s o c i a t e d nepheline s y e n i t e ; i n t r u s i o n o f a massive cone sheet of uncompahgrite, one and a h a l f miles long and 700 feet maximum t h i c k n e s s ; explosive rupture of the c e n t r a l part of  the stock with the formation of a b r e c c i a pipe, and accompanied  by the e x t r u s i o n of n e p h e l i n i t e lavas; explosive i n t r u s i o n o f carbo n a t i t e cone sheets and dikes. . McCall c l a s s i f i e s the m e l i l i t e rock as uncompahgrite, although i t r a r e l y contains any pyroxene.  I t s mode i s given i n  column 6 of Table XVII, and i t appears to be almost i d e n t i c a l to m e l i l i t e o k a i t e (column 5)» in  T u r j a i t e i s present i n the Gwasi complex  subordinate amounts t o uncompahgrite.  M c C a l l s d e s c r i p t i o n of 1  these rocks i n t h i n s e c t i o n i n d i c a t e s that t h e i r mineralogies and textures are the same as those of o k a i t e and m e l i l i t e o k a i t e .  ,  The Gwasi complex i s one o f seven c a r b o n a t i t e bearing plugs o r vents s i t u a t e d along a r i f t  fault.  Limestone of sedimentary  o r i g i n i s not known i n the country rocks of the area.  Similarities The s i m i l a r i t i e s between these three complexes and the Oka complex are obvious. Precambrian  A l l are s m a l l , composite  gneiss o r g r a n i t e .  intrusions i n  The rock a s s o c i a t i o n s are s i m i l a r ;  the m e l i l i t e bearing rocks are a l l a s s o c i a t e d with i j o l i t i c and also with e i t h e r c a r b o n a t i t e or 'limestone . 1  of  rocks  The mineralogies  the m e l i l i t e rocks are d i r e c t l y comparable; the accessory minerals  are e s p e c i a l l y s i m i l a r i n a l l cases.  I t has been shown that m e l i l i t e  has a s i m i l a r composition at three of the f o u r l o c a l i t i e s , and data for  m e l i l i t e from the f o u r t h l o c a l i t y (Gwasi) i n d i c a t e the same  compositional s i m i l a r i t y .  The same a l t e r a t i o n products of m e l i l i t e  ( c e b o l l i t e , v e s u v i a n i t e , diopside) are recorded at each complex,  as are the same magmatic r e a c t i o n minerals (pyroxene, a n d r a d i t e ) . Primary c a l c i t e i s an e s s e n t i a l minor c o n s t i t u e n t i n a l l th.e m e l i l i t e bearing rocks.  Comparison of Chemical Chemical  compositions of o k a i t e and s i m i l a r rocks are  compiled i n Table XVIII. analyses.  Compositions  Most of them are averages of two or three  They are arranged approximately i n the order of the  okaite s e r i e s . i s included f o r  An a n a l y s i s of j a c u p i r a n g i t e from Husereau H i l l comparison.  The mineralogies of the various rock types are r e f l e c t e d i n t h e i r chemical compositions.  Thus the higher content of aluminum  and a l k a l i s i n o k a i t e and t u r j a i t e i s due to the presence i n these rocks of hauyne and nepheline r e s p e c t i v e l y . of  The higher p r o p o r t i o n  calcium i n uncompahgrite and m e l i l i t e o k a i t e i s accounted f o r by  the l a r g e percentage of m e l i l i t e . j a c u p i r a n g i t e , and i s around 1.35 ratio;, i n magnetite  i s 2.22.  The Pe203/PeO r a t i o i s 2.6 i n i n a l l the other: rocks5 the same  In j a c u p i r a n g i t e , f e r r i c i r o n i s  accomodated i n t i t a n a u g i t e , as w e l l as i n magnetite;  i n the other  rocks, f e r r o u s i r o n s u b s t i t u t e s f o r magnesium i n m e l i l i t e , approximately i n the r a t i o of lt4« (1.5  to 1.9)  The t o t a l Pe/Mg i s about the same  i n a l l - of the analyses i n Table XVIII; any f l u c t u a t i o n  i n t h i s r a t i o i s explained by a v a r i a b l e magnetite  content.  The  percentages of p e r o v s k i t e , a p a t i t e , and c a l c i t e are r e s p o n s i b l e f o r the Ti02, P 0 5 , and CO2 2  contents r e s p e c t i v e l y .  It i s unfortunate that chemical analyses are not for  e i t h e r pyroxene o k a i t e or m e l i l i t e o k a i t e .  available  Columns 1 and 2 i n  Table XVIII show that there i s a considerable s i m i l a r i t y between the j a c u p i r a n g i t e from Husereau H i l l and the average of f o u r analyses of  okaite.  MgO,  By comparison to j a c u p i r a n g i t e , o k a i t e has l e s s Si02,  and t o t a l Pe, and more AI2Q3 and a l k a l i s .  I t i s thought that  an a n a l y s i s of pyroxene o k a i t e would show a composition intermediate to  j a c u p i r a n g i t e and o k a i t e , and that i t s Pe G3/PeO r a t i o would be  about  2  2.2  116  Table XVIII Comparison of Chemical Analyses of Okaite and S i m i l a r Rocks  sio Ti0  2  2  AI2O3  2°3 FeO MnO MgO CaO Na 0 K 0' H 0 BaO p o co S Cl, F  3  4  5  33-53  30.87  38.26  2.39 14.12 5.93 4.12 0.51 4.99 24.45 -.4.52  34.72 3-31 12.19  34.20  3.95  3.52 0.25  8.38 22.61 1.90 . O.54 1.14  2  2  2  -  2.33  5  0.09  2  1. 2. 3. 4. 5. 6.  2  11.30 9.18  F e  2  1  -  -  6.44 4.82 0.28 5.84  19.08 5.11  1.28 1.20  3.05  0.42 1.59  -  2.30  6 32.10  4.40  I.85  6.94 8.21  6.01 6.93  7.03  4.62  0.16 8.13  0.20 8.13 28.96  6.20 30.30  1.68 0.30 0.92 0.03 1.30  2.25 0.27  2.84 O.57  0.83 0.28  0..26  0.54 0.79  2.16 2.20  28.50  2.32  1.88 0.81  0.91  0.17  O.58 O.17  0.02  -  0.03  0.03 —  2.95  7-76 7.10 5.10  -  -  J a c u p i r a n g i t e , Husereau H i l l , Oka; one a n a l y s i s (Gold, 1962) Okaite, Oka; average o f f o u r analyses ( S t a n s f i e l d , 1923, 1926; Gold) T u r j a i t e , K o l a Peninsula; average of three analyses (Larsen, 1942) Uncompahgrite, Gwasi, Kenya; average of two analyses ( M c C a l l , 1959) Uncompahgrite, Iron H i l l ; average of two analyses (Larsen, 1942) M e l i l i t e o k a i t e , Oka; c a l c u l a t e d chemical composition  Column 6 In Table XVIII i s the c a l c u l a t e d chemical compo s i t i o n of m e l i l i t e o k a i t e .  The average mode given i n column 5 °f  Table XVII was used, t a k i n g a m e l i l i t e composition o f Na^Ge^oAk^, and allowing f o r minor s u b s t i t u t i o n o f Fe" for, A l and Fe" f o r Mg. 1  The c a l c u l a t e d composition shows that the S i 0  2  content: would be  about the same as i n o k a i t e ; there would be l e s s A l 0 3 and a l k a l i s , 2  and a l i t t l e more MgO and CaO. are apparent Iron H i l l .  Except f o r S i 0  2 j  the same d i f f e r e n c e s  i n the analyses of uncompahgrite from both Gwasi and The c a l c u l a t e d chemical composition of average o k a i t e  i s d i r e c t l y comparable t o the average of f o u r analyses of o k a i t e given i n column 2 of Table XVIII.  I t i s noted that the average  a n a l y s i s o f o k a i t e contains s e v e r a l percent l e s s s i l i c a the other analysed rocks.  than any o f  117  M e l i l i t e Rocks at Scawt H i i l l , Northern Ireland; At Scawt H i l l , Northern I r e l a n d , T i l l e y (1929,  1931)  has described: a small d o l e r i t e boss that i n t r u d e s chalk, and, d i s p l a y s a remarkable  h y b r i d zone at i t s contact:.  The s u r f a c e  of the i n t r u s i o n are about 600 f e e t by 900 f e e t ,  dimensions  the f l a t - l y i n g chalk formation i s approximately 100 feet  and  thick.  The age of the chalk i s l a t e Cretaceous; the d o l e r i t e i n t r u s i o n : i s Miocene i n age, and i s a s s o c i a t e d with the Plateau B a s a l t s of Northern I r e l a n d (Charlesworth, 1953).  Throughout the i n -  t r u s i o n , except w i t h i n a few yards of the contact:, the d o l e r i t e i s f i n e grained and uniform.  The contact: i s sharply defined;  the chalk, which i s almost pure calcium carbonate except f o r f l i n t nodules, has been metamorphosed w i t h i n a few f e e t of the contact to an unusual assemblage of rare c a l c - s i l i c a t e s . Uncontaminated d o l e r i t e at Scawt H i l l has the chemical composition of a normal gabbro o r b a s a l t ; i t s mode i s * a u g i t e 30$, o l i v i n e 12$,  p l a g i o c l a s e 50$, magnetite  a p a t i t e 1$.  In the contaminated  and i l m e n i t e 7$>  zone, the f i r s t  and:  s i g n of change  i n the d o l e r i t e i s an i n c r e a s e i n pyroxene content, u n t i l the rock can be termed pyroxenite; o l i v i n e i s absent, and the pyroxene i s r i c h e r i n AI2O3 than i t i s i n the d o l e r i t e , and has rims ot. titanaugite.  T h i s rock i s t r a n s i t i o n a l to pyroxenite i n which  the pyroxene i s h i g h l y p l e o c h r o i c t i t a n a u g i t e .  Plagioclase i n  these rocks i s converted to nepheline and z e o l i t e s .  The  next  stage of contamination i s the development of t i t a n a u g i t e - m e l i l i t e rock which contains small amounts of nepheline.  M e l i l i t e shows  a r e a c t i o n r e l a t i o n to t i t a n a u g i t e ; i t i s formed by the r e a c t i o n of  t i t a n a u g i t e with the surrounding l i q u i d .  The f i n a l stage of  contamination at Scawt H i l l i s the production of m e l i l i t e rock i n small pockets next to the contact.  T h i s rock i s composed  mainly of m e l i l i t e , with minor amounts of t i t a n a u g i t e  and  nepheline, and accessory a p a t i t e , magnetite, p e r o v s k i t e , and calcite.  Here m e l i l i t e has c r y s t a l l i z e d d i r e c t l y from the  contaminated  d o l e r i t e magma.  In some places i t : has reacted  with i n t e r s t i t i a l l i q u i d to> form garnet and green pyroxene; i n  other places i t i s hydrothermally a l t e r e d t o c e b o l l i t e . A l l the rocks b r i e f l y described here are i n s i d e the contact, and have c r y s t a l l i z e d from magma.  The a d d i t i o n o f CO2  by the absorption o f lime from the chalk increased the f l u i d i t y of the magma, a l l o w i n g coarse c r y s t a l l i z a t i o n i n the h y b r i d zone. This i s a w e l l exposed example o f what happens t o a b a s i c magma where i t i s able t o a s s i m i l a t e calcium carbonate.  The small  s c a l e o f the production o f a l k a l i c rocks l e d T i l l e y t o s t a t e that "... t h i s i s an example o f the r e s t r i c t e d p o t e n t i a l i t y o f igneous magma t o generate a l k a l i types by a s s i m i l a t i o n . " (1931, p. 467). I"t must be remembered, however, that the d o l e r i t e i n t r u s i o n i s very s m a l l , and that the limestone formations i s only about 100 feet t h i c k .  I t i s probable also that the chalk was  very near t o the surface at the time o f i n t r u s i o n , being o v e r l a i n only by the Miocene b a s a l t flows.  The f i n e g r a i n s i z e o f the bulk:  of the d o l e r i t e p l u g i n d i c a t e s f a i r l y quick c o o l i n g , supporting the i d e a o f near surface c o n d i t i o n s .  I t i s thought  that the small,  s c a l e o f the production o f a l k a l i c rocks at Scawt H i l l , was governed more by the environmental c o n d i t i o n s than by the suggested  limited  p o t e n t i a l i t y o f b a s a l t i c magma t o a s s i m i l a t e limestone. The marked s i m i l a r i t y between the Scawt H i l l h y b r i d rocks and the j a c u p i r a n g i t e - pyroxene o k a i t e - o k a i t e s e r i e s at Oka merits discussion.  Table XIX compares chemical analyses o f t i t a n a u g i t e -  m e l i l i t e rock and m e l i l i t e rock from Scawt H i l l , and j a c u p i r a n g i t e and o k a i t e from Oka.  The s i m i l a r i t i e s i n chemical composition and  i n petrography are r e a d i l y apparent.  The major d i f f e r e n c e between  the rocks from the two l o c a l i t i e s i s the t o t a l Fe/^ffg r a t i o .  The  development o f the rock s e r i e s from d o l e r i t e t o m e l i l i t e rock at Scawt H i l l i n v o l v e d an i n c r e a s e i n t h i s r a t i o from 1.6 too 19.0. The same trend i s not found i n the o k a i t e s e r i e s .  I f a parent  b a s i c magma i s p o s t u l a t e d t o have developed the o k a i t e s e r i e s by means of limestone a s s i m i l a t i o n , then i t must have been much r i c h e r i n magnesium i n the f i r s t p l a c e , o r , more l i k e l y , the a s s i m i l a t e d limestone must have been decidedly d o l o m i t i c .  Table  XIX  Comparison of Chemical Analyses of M e l i l i t e Rocks from Scawt H i l l and Oka  Si0 Ti0 AI2O3 2  2  33.53 3.95 11.30 9.18 3.52 O.25 8.38 22>61 1.90 0.54 1.14  2  2  F  30.87 2.39 14.12 5-93 4.12 0.51 4-99 24.45 4.52 1.28 1.20 0.42 1-59  29.14 2.17 10.46  2.33 0.09  -  -  0.30  -  5  4  0.64 0.20  -  2.36 21.25 1.50 0.38 5-76  2  3  8.54 7.85 0.34 1.06 28.40 2.18 0.19 5.58 0.07 0.68 3.02  11.95  2  S Gl,  37-54 2.04 3.60  2  C0  2  13-39  Fe 03 ' FeO MnO MgO CaO Na 0 K0 H0 BaO P 0. 2  1  o  -  -  2.32 0.91 0.02  0.08;  1. T i t a n a u g i t e - m e l i l i t e rock, Scawt H i l l ( T i l l e y and Harwood, 2. J a c u p i r a n g i t e , Husereau H i l l , Oka (Gold, 1962) 3 . M e l i l i t e rock, Scawt H i l l ( T i l l e y and Harwood, 1931) 4. Average o k a i t e , Oka ( S t a n s f i e l d , 1923, 1926; Gold, 1962)  There i s a d i r e c t analogy between the pyroxenite - m e l i l i t e rock s e r i e s at Scawt H i l l okaite  s e r i e s at Oka,  although the  the s e r i e s at Scawt H i l l . to give m e l i l i t e , and  The  is  "...  development of and  the  jacupirangite  r e a c t i o n of t i t a n a u g i t e with l i q u i d  the r e a c t i o n of m e l i l i t e to give garnet T i l l e y and  same sense that  and  Harwood  series titanaugite - m e l i l i t e - grossular  a r e a c t i o n s e r i e s i n the  the  l a t t e r i s c a r r i e d f u r t h e r than  d i o p s i d i c augite are noted at both l o c a l i t i e s . state that the  1931)  garnet  augite - hornblende -  b i o t i t e forms a r e a c t i o n s e r i e s i n normal d i f f e r e n t i a t i o n . " (1931, p. 4 6 6 ) .  I t i s noted that the garnet formed from m e l i l i t e at  i s andradite, not be  g r o s s u l a r i t e ; t h i s d i f f e r e n c e i s not  s i g n i f i c a n t with respect Not  to the  Oka  thought to  reaction series.  only the mineralogies, but the  main rock forming minerals, m e l i l i t e and  compositions of  the  t i t a n a u g i t e , are s i m i l a r at  120  Scawt H i l l melilite  and Oka.  Table XX compares the chemical analyses of  and t i t a n a u g i t e from both l o c a l i t i e s . -  Table XX Comparison of Chemical Analyses of M e l i l i t e Titanaug ;iiie from Scawt H i l l . and Oka  Si0 Ti0 A1 0 2  1  2  3  4  38.35  38.68  40.28 3.85  39.89  13.06  10.14  10.30  14.94  1.15 5-75  0.45  5.35 7-92  5.62 2.01  -  2  2  Fe 0 FeO MnO MgO CaO Na 0 K 0 H 0 2  3  3  1.81.  -  2  2  and  6.38 34-28  3.98 0.25  3.92 tr  —  2  -  0.35  3.16 33.64  2.30  0.01.  0.21  7.78  10.14  23-57  24.33  0.36  0..10  tr 0.1.9  O.O3 0.14  1. M e l i l i t e , N a ^ G e i g A k ^ ; Scawt H i l l ( T i l l e y , 1929) 2. M e l i l i t e , N a 7 G e A k ; Oka (Gold, 1962) 3  11  52  3 . T i t a n a u g i t e , C a M g F e , 21$ A l f o r S i ; Scawt H i l l ( T i l l e y and Harwood, 1931) 4 . T i t a n a u g i t e , CaccMgopFe,.., 2 4 - 4 $ A l f o r S i ; Oka (Gold, 1.96a) 5 3  2 4  2 3  M e l i l i t e from Scawt H i l l , contains more i r o n than m e l i l i i i e from Oka.  I t s type molecule formula, which allows f o r the s u b s t i t r -  u t i o n of f e r r i c i r o n f o r aluminum i n g e h l e n i t e and of f e r r o u s i r o n f o r magnesium i n akermanite, i s Na^Ge^qAk^g. melilite,  This i s s i m i l a r to Oka  whose type molecule formula i s Na^Ge^A-ct^? the only s i g n i f -  i c a n t d i f f e r e n c e i s that, more of the g e h l e n i t e type molecule i s present; in. Scawt H i l l m e l i l i t e . Scawt H i l l m e l i l i t e  The r e f r a c t i v e i n d i c e s and b i r e f r i n g e n c e of  ( N : I..647, N s I . 6 3 6 , N - N 0  e  1929) are higher than those of Oka m e l i l i t e N - N « - 0 . 0 0 6 ; see page 6 3 ) . e  Q  e  0  s -0.011;  (No * 1 . 6 3 4 ,  N  e  Tilley, : I.628,  The high values f o r Scawt H i l l  melilitre  r e f l e c t the h i g h p r o p o r t i o n of i r o n s u b s t i t u t i o n . The main s i m i l a r i t i e s i n the t i t a n a u g i t e from both l o c a l i t i e s are the abnormally h i g h percentages of aluminum s u b s t i t -  u t i n g f o r s i l i c o n ] , and the somewhat: higher percentage o f calcium than i s present i n normal augite.  The percentage of. f e r r o u s i r o m  i n Scawt H i l l t i t a n a u g i t e i s much g r e a t e r than i n Oka t i t a n a u g i t e , although the percentage  of f e r r i c i r o n i n each i s almost the same.  The compositions o f m e l i l i t e and t i t a n a u g i t e r e f l e c t the i r o n r i c h nature of the contaminated  d o l e r i t e magma at Scawt H i l l .  T i l l e y and Harwood (1931) have shown that c o n t i n u i n g contamination of  the c r y s t a l l i z i n g magma l e d t o an increase i n the t o t a l - Fe / Mg  r a t i o , and a decrease i n the content o f s i l i c a ;  the i n c r e a s e i n  calcium content was e f f e c t e d by the simple a d d i t i v e process o f assimilation.  The a l k a l i c rocks at Scawt H i l l have unquestionably  been produced, by the a s s i m i l a t i o n o f calcium carbonate by/ b a s a l t i c magma. at  The o v e r a l l s i m i l a r i t y , between the products of a s s i m i l a t i o n  Scawt H i l l and the o k a i t e s e r i e s rocks at Oka has been shown.  Can i t ; t h e r e f o r e be p o s t u l a t e d that a s i m i l a r process, though on a much l a r g e r s c a l e and i n a d i f f e r e n t ; environment, was r e s p o n s i b l e for  the m e l i l i t e bearing rocks o f the Oka complex?  T h i s question  i s discussed i n the section; on petrogenesis.  Chemical Nature o f M o n t i c e l l i t e A l n o i t e at Husereau H i l l M o n t i c e l l i t e a l n o i t e i s the l a t e s t phase o f i n t r u s i o n : i n the Oka complex.  I t forms narrow dikes and small, plugs within; the  complex i t s e l f and a l s o i n - t h e surrounding Precambrian of  rocks.  Many  the m o n t i c e l l i t e a l n o i t e bodies are b r e c c i a t e d , and seem t o be  r e l a t e d t o the l a t e formed diatreme b r e c c i a s that are s c a t t e r e d throughout  the bordering country rocks.  The m o n t i c e l l i t e a l n o i t e  body at Husereau H i l l i s a small, plug that cuts rocks o f the okaite s e r i e s , and incorporates fragments o f these rocks at i t s margins. Table XXI compares the chemical compositions o f m o n t i c e l l i t e a l n o i t e from Husereau H i l l and a l n o i t e from the type l o c a l i t y , Alno I s l a n d , Sweden (johannsen,  1  1937, P> 383). These two; analyses are  e s s e n t i a l l y d i f f e r e n t only i n that there i s more alumina and c o r r e s pondingly l e s s s i l i c a i n m o n t i c e l l i t e a l n o i t e .  There i s a d e f i n i t e  s i m i l a r i t y between the analyses o f m o n t i c e l l i t e a l n o i t e , j a c u p i r a n g ite,  and, t o a l e s s e r extent, average okaite (see Table X V T I l ) .  1322  Table XXI Comparison o f Chemical Analyses o f M o n t i l c e l l i t e A l n o i t e from Husereau H i l l , Oka, and A l n o i t e from Aln& I s l a n d , Sweden  Si0 Ti0 A1 0 Fe 0 FeO MnO MgO CaO Ua 0 K 0 H 0 BaO SrO Li 0 Cr 03 Zr0 2  2  2  3  2  3  2  2  2  2  2  2  p S o 2  5  Cl, F C0 2  1  2  30.13 2.38 11.02 7.34 4.17 O.52 12.01 22.48 1.43 2.07 0.95 0.89  33.37 2.56 7.56 5.33 6.39 0.20 12.50 20.25 2.03 3.06 3.16 0.32 0.14 0.03 0.08 0.03 I.50 0.31 0.23 I.56  - •  ' -  0.05 -  2.88 0.20 tr 1.17  1. M o n t i c e l l i t e a l n o i t e , Husereau H i l l , Oka ( S t a n s f i e l d , 1923, p. 440) 2.  The  A l n o i t e , Stornaset,  Alnfl I s l a n d , Sweden (Johannsen, 1937> P* 383)  only important d i f f e r e n c e i s that m o n t i c e l l i t e a l n o i t e  contains  a g r e a t e r percentage o f magnesium than any analysed member of the okaite s e r i e s . The unstable nature o f m o n t i c e l l i t e a l n o i t e magma at the time o f s o l i d i f i c a t i o n has been demonstrated (see page 5 6 ) . The v a r i e d and r e a c t i v e mineral assemblage o f t h i s rock makes i t quite u n l i k e any member o f the okaite s e r i e s , even though chemical s i m i l a r i t y i s shown.  The amount o f contamination by i n c l u d e d fragments makes i t ;  d i f f i c u l t t o accept the a n a l y s i s as representing t h i s rock c r y s t a l l i z e d .  the magma from which  T e x t u r a l d i f f e r e n c e s between the o k a i t i c  rocks and m o n t i c e l l i t e a l n o i t e r e f l e c t the d i f f e r e n t environments i m which these rocks were formed.  Bowen, who s t u d i e d the a l n o i t i c rocks at I s l e  Cadieux,  Quebec (1922), argues t h a t , i n a d d i t i o n t o a s s i m i l a t i o n o f limestone, there are two ways o f o b t a i n i n g a l n o i t e .  One i s "... by r e a c t i o n  between a l k a l i n e l i q u i d and already c r y s t a l l i z e d pyroxene . . . "s the other i s by p r e c i p i t a t i o n "... from, a l i q u i d r i c h in. f e l d s p a t h o i d a l molecules and pyroxene but at the same time not r i c h enough i n lime to  contain m e l i l i t e i n the s t o i c h i o m e t r i c sense." (1928, p. 268).  The mineral r e a c t i o n r e l a t i o n s preserved i n the q u i c k l y cooled m o n t i c e l l i t e a l n o i t e at Husereau H i l l i n d i c a t e the f i r s t o f these two a l t e r n a t i v e s .  However, the extreme preponderance o f lime and  the notable CO2 content, not only i n m o n t i c e l l i t e a l n o i t e , but i m n e a r l y a l l o f the analysed rocks o f the Oka complex, add much weights to  the theory that the formation o f these rocks was u l t i m a t e l y  dependent on the a s s i m i l a t i o n o f limestone, as was f i r s t  suggested  by S t a n s f i e l d (1923, p.433) f o r the rocks o f the Oka complex.  PETKQGENESIS  The Okaite S e r i e s I t has been shown that there i s a modal gradation: fromi j a c u p i r a n g i t e through pyroxene o k a i t e , o k a i t e , and m e l i l i t e o k a i t e to m e l i l i t e - c a l c i t e rock; t h i s i s the okaite s e r i e s .  Development!;  o f t h i s g r a d a t i o n a l s e r i e s i s based on the f o l l o w i n g modal changes: decrease of the percentage  of t i t a n a u g i t e , with corresponding  increase of m e l i l i t e content, from j a c u p i r a n g i t e through pyroxene o k a i t e t o o k a i t e ; decrease i n the percentage  of hauyne ( o r nepheline)  from o k a i t e t o m e l i l i t e o k a i t e ; i n c r e a s e i n the percentage of primary c a l c i t e from m e l i l i t e o k a i t e to m e l i l i t e - c a l c i t e rock. A cursory examination of both hand specimens and thinx sections of some of the c a l c i t e - r i c h rocks of the Oka complex r e v e a l s that t h e i r s i l i c a t e mineralogy i s c l o s e l y r e l a t e d i n some aspects to the mineralogy of type IV m e l i l i t e a l t e r a t i o n .  M e l i l i t e - c a l c i t e rock,  i n which e s s e n t i a l l y the only s i l i c a t e s are m e l i l i t e and  biotite,  grades i n t o c a l c i t e - r i c h rocks i n which the m e l i l i t e content  decreases  as the content of m o n t i c e l l i t e ( o r f o r s t e r i t e i n some rocks) and yellow augite i n c r e a s e s .  These rocks i n t u r n grade i n t o  rocks i n which the m o n t i c e l l i t e ( o r f o r s t e r i t e ) content  calcite-rich decreases  with i n c r e a s i n g content of yellow pyroxene and orange andradite. Throughout t h i s sequence the content of c a l c i t e i n c r e a s e s from about 1'5$ to 70$  or more, and such rocks are r e f e r r e d to as c a l c - s i l i c a t e  carbonatites.  B i o t i t e and the accessory minerals magnetite,  perov-  s k i t e , and a p a t i t e are the same as i n the o k a i t i c rocks, only i n increased amounts; a p a t i t e e s p e c i a l l y i s concentrated i n these rocks. Perovskite becomes the n i o b i a n v a r i e t y dysanalyte, and pyrochlore and n i o c a l i t e may  occur as a c c e s s o r i e s a l s o .  Continuing the s e r i e s ,  the c a l c - s i l i c a t e content decreases u n t i l the rock can be c a l l e d s o v i t e , as described by von Eckermann  (1948,  p:.  13).  I t should be  noted that s o v i t e and c a l c - s i l i c a t e c a r b o n a t i t e comprise 40$  of the.exposed  at l e a s t  rocks of the Oka complex. O k a i t i c rocks make up  perhaps 10$ of the complex.  D e t a i l i n the present work i s r e s t r i c t e d to the o k a i t i c rocks, i n which general term i s i n c l u d e d a l l the m e l i l i t e - b e a r i n g rocks of the okaite s e r i e s .  The v a l i d i t y of the development of t h i s  s e r i e s has been demonstrated by the mineral a s s o c i a t i o n s and textures seen i n t h i n s e c t i o n , and by the apparently l i n e a r modal g r a d a t i o n . Figure 60 i s a diagram that shows the modal v a r i a t i o n of the rocks of the o k a i t e s e r i e s . mineral percentages  The diagram was  constructed by p l o t t i n g the  i n the orders b i o t i t e , t i t a n a u g i t e , hauyne (and  nepheline), m e l i l i t e , m o n t i c e l l i t e , yellow pyroxene ( a u g i t e ) , and a c c e s s o r i e s . rocks.  calcite,  Some of the modes are averaged from s e v e r a l s i m i l a r  Modes are p l o t t e d i n the order of the o k a i t e s e r i e s from  j a c u p i r a n g i t e to m e l i l i t e - c a l c i t e rock, and are a r b i t r a r i l y so that the combined hauyne-nepheline  spaced  f i e l d i s a s t r a i g h t band.  Figure 60 has no other use than that of d i s p l a y i n g the v a r i a t i o n i n mineral composition through the o k a i t e s e r i e s ; no time sequence can be d e r i v e d from i t . that temperature  certainly  I t w i l l be shown, however,  of c r y s t a l l i z a t i o n i s probably lower toward the  carbonatite end of the s e r i e s .  E x t r a p o l a t i o n of the s e r i e s from  m e l i l i t e - c a l c i t e rock t o s o v i t e i s based on a few t h i n s e c t i o n s , and f u r t h e r work must be done before i t can be considered to have much s i g n i f i c a n c e .  Nevertheless, i t was  considered worthwhile  to  i n c l u d e t h i s p r o j e c t i o n of the o k a i t e s e r i e s because, i f i t I s found to be v a l i d , i t presents a clue to the method of formation of c a r b o n a t i t e magma w i t h i n the Oka  complex.  Considerations of Temperature and Pressure There are two types of chemical r e a c t i o n i n v o l v i n g m e l i l i t e that have taken place w i t h i n the o k a i t e s e r i e s .  One i s the formation  of m e l i l i t e by the break-down of t i t a n a u g i t e , as noted i n pyroxene okaite.  The other i s the break-down of m e l i l i t e to form m o n t i c e l l i t e  and/or pyroxene and c a l c i t e , as noted i n m e l i l i t e - c a l c i t e rock. r e a c t i o n s are probably f u n c t i o n s of the upper and lower  Such  temperature  l i m i t s of s t a b i l i t y of m e l i l i t e , these depending on other conditions such as pressure and chemical content. The c o n d i t i o n s under which t i t a n a u g i t e breaks down to give m e l i l i t e are not known, but i f t h i s r e a c t i o n was  dependent on a  (1) \ J a c u p i r a n g i t e (1) (3) (1)  Pyroxene Okaite  (5) (1) (1) (5) . (2) > Okaite  (2) (3)  M e l i l i t e Okaite  (2) ' M e l i l i t e - C a l c i t e fiock ^tONTICEjLLITE)  y C a l c - s i l i c a t e Carbonatite  \ fy /  CALCITE  Sovite 0  20  40  60  80  100$  Figure 60: Diagram showing the modal v a r i a t i o n w i t h i n the okaite s e r i e s , based on p l o t t e d modes. Figures In brackets are the number o f rocks from which each p a r t i c u l a r mode was averaged. The carbonatite extension o f the okaite s e r i e s i s speculated on the b a s i s o f examination o f a few t h i n s e c t i o n s  chemical changes i n the magma, such changes were not pronounced; analyses o f j a c u p i r a n g i t e and o k a i t e (see Table X V I I l ) are not r a d i c a l l y d i f f erent,. s u g g e s t i n g that: the magmas from which these rocks c r y s t a l l i z e d were o f s i m i l a r composition.  A slight increase  i n the magmatic content o f c a l c i u m may have encouraged the f o r m a t i o n of m e l i l i t e i n p l a c e o f t i t a n a u g i t e .  The a d d i t i o n o f CaO t o any  t y p i c a l pyroxene molecule may produce a m e l i l i t e , as, f o r i n s t a n c e : CaMgSi 0g + CaO = Ca MgSi 07. 2  2  2  Kranck ( 1 9 2 8 ) suggests t h a t m e l i l i t e o r i g i n a t e d by a s t r o n g r i s e i n gas pressure i n the magma, e s p e c i a l l y C0 , i n d u c i n g a break-down o f 2  s i l i c a t e m i n e r a l s and r e l e a s i n g s u f f i c i e n t c a l c i u m f o r the f o r m a t i o n of m e l i l i t e .  I f such a p o s t u l a t e d r i s e i n . C 0 p r e s s u r e was d e r i v e d 2  from the d i s s o c i a t i o n o f c a l c i t e by a s s i m i l a t i o n o f l i m e s t o n e , then a source f o r an i n c r e a s e i n CaO i n the magma i s I m p l i e d , and i t need not be d e r i v e d from the s i l i c a t e s themselves.  I t has been shown,  however, t h a t t h e chemical natures o f t h e j a c u p i r a n g i t e and o k a i t e magmas were probably s i m i l a r , and r t i s thought, t h a t the t i t a n a u g i t e m e l i l i t e r e a c t i o n I s more l i k e l y t o have been dependent on temperature than on gross chemical change.  High C 0 pressure would have 2  probably i n h i b i t e d t h i s r e a c t i o n , as i s shown below. The lower temperature known ( B a r k e r and T u t t l e , I956).  l i m i t o f s t a b i l i t y o f akermanite I s At a temperature  a l i t t l e above  700°C. akermanite breaks down t o g i v e m o n t i c e l l i t e and w o l l a s t o n i t e , a c c o r d i n g t o t h e equation: C a l g S i 0 7 = CaMgSi0 2  2  4  + CaSi.03.  The temperature o f t h i s r e a c t i o n i s not a p p r e c i a b l y a f f e c t e d by pressure.  Although the m e l i l i t e i n o k a i t i c rocks i s not pure  akermanite, rocks have been d e s c r i b e d i n which m e l i l i t e and montic e l l i t e (and c a l c i t e ) a r e i n t e r g r o w n , i n d i c a t i n g t h a t both m e l i l i t e and m o n t i c e l l i t e were s t a b l e products o f c r y s t a l l i z a t i o n at t h i s stage.  W o l l a s t o n i t e i s not present i n these r o c k s , so although  t h i s i n t e r g r o w t h i s r e m i n i s c e n t o f t h e above r e a c t i o n , w o l l a s t o n i t e was probably not formed w i t h m o n t i c e l l i t e because o f a l a c k o f s i l i c a i n the magma, c a l c i t e forming i n i t s p l a c e .  T h i s can be I l l u s t r a t e d  by the f o l l o w i n g e q u a t i o n , but i t i s understood t h a t t h e s u b t r a c t i o n  128  of s i l i c a represents a d e f i c i e n c y i n the magma, and d i d not: occur at the time o f c r y s t a l l i z a t i o n : Ca MgSi 0<jr - S i 0 2  2  At  2  + C0  2  - CaMgSiO^ + CaCO^.  low G 0 pressures akermanite breaks down below 800°C. t o 2  give d i o p s i d e and c a l c i t e , according t o the equation: Ca ffigSi20>7 + C 0 2  2  = CaMgSi 06 + CaCC^. 2  T h i s temperature i n c r e a s e s with Increase o f pressure, being about 950OC.  at 1,000 atmospheres,  and w e l l over 1,000°G. at 2,000  atmospheres.  These f i g u r e s are probably not d i r e c t l y a p p l i c a b l e at Oka, as m e l l l i t i e i s not pure akermanite.  However, evidence that t h i s r e a c t i o n occurred  i s apparent i n the type IV a l t e r a t i o n transition  from m e l i l i t e - c a l c i t e  o f m e l i l i t e , and a l s o i n the  rock t o c a l c - s i l i c a t e c a r b o n a t i t e  c o n t a i n i n g pyroxene. The formation o f m e l i l i t e from t i t a n a u g i t e i s , i n a sense, the  reverse o f the r e a c t i o n discussed above.  The r e a c t i o n o f t i t a n -  augite w i t h l i q u i d t o g i v e m e l i l i t e i n rocks t r a n s i t i o n a l  from  j a c u p i r a n g i t e t o o k a i t e appears t o have occurred at a h i g h e r temperature than the r e a c t i o n o f m e l i l i t e and C 0 t o give pyroxene and 2  calcite.  I t has been shown that akermanite i s unstable at h i g h  pressures; the same i s probably t r u e o f n a t u r a l m e l i l i t e s , those found i n o k a i t i c  rocks.  such as  Thus the r e a c t i o n o f t i t a n a u g i t e t o  g i v e m e l i l i t e must have occurred at low C 0 pressure, probably i n the 2  presence o f excess CaO.  The reverse r e a c t i o n from m e l i l i t e t o  pyroxene, o c c u r r i n g at a lower temperature, i s compatible with a r i s e i n C 0 content i n the magma, as evidenced by the i n c r e a s e i n 2  the  primary c a l c i t e content i n the okaite s e r i e s from j a c u p i r a n g i t e  to m e l i l i t e - c a l c i t e  rock (see f i g u r e 6 0 ) .  Natural m e l i l i t e probably represents a s t a b l e compoundwith a composition near that o f the lowest temperature m e l t i n g mixture i n the system sodium m e l i l i t e - g e h l e n i t e - akermanite.  Such  a compound may be s t a b l e at lower temperatures than pure akermanite. With t h i s i n mind, a c o n s i d e r a t i o n o f the v a r i o u s pressure-temperature s t a b i l i t y curves i n v o l v i n g akermanite, m o n t i c e l l i t e , diopside, and c a l c i t e ( T u t t l e and Harker, 1956; Turner and Verhoogen, i 9 6 0 ) leads t o p o s s i b l e temperature and pressure ranges f o r the c r y s t -  a l l i z a t i o n of the okaitiic rocks.  C r y s t a l l i z a t i o n of m e l i l i t e i n  place of t i t a n a u g i t e probably occurred at a temperature of about' 800°C. at l e s s than 400 bars t o t a l pressure, with a low p a r t i a l pressure of CO2. or  Break-down of m e l i l i t e t o give c a l c i t e and pyroxene  m o n t i c e l l i t e probably took place between 700°C. and 750°C. a t  about 25O bars t o t a l pressure, also with low CO2 p a r t i a l pressure. The okaite s e r i e s i s thought to have developed f i r s t of  by an increase  CaO i n a b a s i c magma, followed by a r i s e i n C 0 content.  An i n -  2  crease i n CO2 content with decreasing temperature and pressure does not  n e c e s s i t a t e an increase i n the p a r t i a l pressure of C 0 . 2  It  i s p o s s i b l e that an estimate o f the i n i t i a l  crystalliz-  a t i o n temperature of o k a i t i c magma could be obtained by l a b o r a t o r y study of the v a r i a t i o n with temperature of the l i m i t s of s o l i d s o l u t i o n between h e r c y n i t e and magnetite. taken i n the present work.  Such a study was not under-  Presence o f exsolved hercynite i n magnetite  probably i n d i c a t e s high temperature c r y s t a l l i z a t i o n . almost complete, suggesting f a i r l y slow c o o l i n g .  Exsolution i s  Absence of exsolved  i l m e n i t e o r u l v o s p i n e l i n okaite. magnetite i s probably due to the high o x i d a t i o n s t a t e of the magmatic i r o n content, and does not t h e r e f o r e suggest a low c r y s t a l l i z a t i o n  temperature.  There i s some evidence that can be used f o r estimating temperatures at which hydrothermal a l t e r a t i o n took place i n o k a i t i c rocks.  Type IV a l t e r a t i o n probably occurred at temperatures close to  that of the f i n a l stage of magmatic c r y s t a l l i z a t i o n , perhaps 600°C.  around  Under these circumstances i t i s hard t o draw the l i n e between  hydrothermal a l t e r a t i o n and magmatic r e a c t i o n .  In the type I I a l t e r -  a t i o n of hauyne, g r o s s u l a r i t e and z e o l i t e are associated. i s stable between 300<>C. and 750°C. (Yoder,  1950).  Grossularite  Experiment has  shown that most z e o l i t e s are formed.between 100°C. and 300°C. (Barth,  1962), but that some, such as a n a l c i t e (Turner and Verhoogen, I960), may  be s t a b l e above 500°C. under c e r t a i n c o n d i t i o n s .  On t h i s basis  it  i s estimated that type I I a l t e r a t i o n took place i n the lower part,  of  the s t a b i l i t y range of g r o s s u l a r i t e , o r at temperatures  400°C.  around  A probable range of temperature f o r the a l t e r a t i o n of o k a i t i c  rocks by r e s i d u a l hydrothermal f l u i d s i s below 300°C. f o r type I a l t e r a t i o n t o 600°C. o r above f o r type IV a l t e r a t i o n .  Sequence of I n t r u s i o n Gold (1962, personal communication), who has made an extensive study o f the Oka complex, b e l i e v e s the general  sequence  of i n t r u s i o n to be as f o l l o w s : emplacement o f most o f the s o v i t e s and c a l c - s i l i c a t e c a r b o n a t i t e s , followed by the rocks of the o k a i t e s e r i e s from m e l i l i t e - c a l c i t e rock to j a c u p i r a n g i t e ; i n t r u s i o n o f the ijolite-urtite formation  s e r i e s ; i n t r u s i o n o f a l n o i t e and lamprophyre, and  o f diatreme b r e c c i a s .  The author has not made a compre-  hensive study o f the f i e l d r e l a t i o n s of the various rock  types,  p a r t l y because o f poor surface exposure, but mainly because he was unable to study the vast amount o f d r i l l - c o r e data that has been c o l l e c t e d during e x p l o r a t i o n f o r niobium w i t h i n the Oka complex. However, s e v e r a l e x p l o r a t i o n trenches  expose s i t u a t i o n s where the  sequence proposed by Gold does not hold. In many places w i t h i n the Oka complex i j o l i t i c rocks are b r e c c i a t e d and are cut by c a l c - s i l i c a t e carbonatite dikes, from a few inches t o s e v e r a l f e e t wide.  I n one place, near the north-east  contact o f the complex, a large d i k e - l i k e body o f f i n e  grained  i j o l i t e , bounded on e i t h e r side by c a r b o n a t i t e , grades i n t o a rock that has the appearance and composition  o f f e n i t i z e d gneiss; i n thin;  s e c t i o n small patches o f f i n e l y c r y s t a l l i z e d nepheline are seen, and may represent  and c a l c i t e  replaced o r i g i n a l f e l d s p a r g r a i n s .  The  o f n e p h e l i n i z a t i o n has been invoked by von Eckermann (1948)  process  i n order to e x p l a i n the a l k a l i s y e n i t e s at Alnfl I s l a n d , and also; b y McCall  (1959) t o account f o r the syenites and p o s s i b l y even the  i j o l i t e s o f the Gwasi complex.  I t I s p o s s i b l e that the i j o l i t e s  w i t h i n the Oka complex are i n f a c t o f f e n i t i c o r i g i n ; these may have been rheomorphosed and intruded during formation complex, although some may have acquired t h e i r p r e s e n t by metasornatic  processes  rocks  of the  mineralogies  without rheomorphism.  Tabular o r l e n t i c u l a r bodies of pyroxene o k a i t e , m e l i l i t e okaite pegmatite, and m e l i l i t e - c a l c i t e rock are seen w i t h i n the main body o f okaite at Husereau H i l l .  Horizons of pyroxene okaite  that are conformable t o the general arcuate s t r u c t u r e o f the whole complex may be t r a c e d f o r s e v e r a l hundred f e e t along s t r i k e , and are normally  l e s s than a hundred f e e t wide.  Bodies of m e l i l i t e okaite  and m e l i l i t e - c a l c i t e rock are l e s s r e g u l a r In d i s t r i b u t i o n and be traced i n the same way,  although t h i s may  be due  cannot  to poor exposure  and to the d i f f i c u l t y of d i s t i n g u i s h i n g between these rocks and i n the f i e l d .  The  okaite  contacts between members of the okaite s e r i e s appear  to be g r a d a t i o n a l wherever they are seen, so i t i s impracticable to conclude a sequence of i n t r u s i o n f o r them. It Oka  would seem, t h e r e f o r e , that the sequence of i n t r u s i o n at  i s not a c l e a r - c u t s i t u a t i o n where d i f f e r e n t , rock types are  i a t e d with a p a r t i c u l a r time of i n t r u s i o n .  assoc-  Such can be s a i d only of  the p e r i p h e r a l a l n o i t e s , lamprophyres, and diatreme b r e c c i a s that i n v a r i a b l y show c r o s s - c u t t i n g r e l a t i o n s to the rocks that them.  contain  The f i r s t part of the i n t r u s i v e sequence advocated by Gold i s  contrary to the p o s t u l a t e d trend of development of the okaite s e r i e s . I f a l l the i n t r u s i v e rocks of the Oka  complex are comagmatic, i t would  seem l o g i c a l to suggest that i n t r u s i o n stemmed from a complex magma chamber system whose magma v a r i e d i n composition at  any one time.  Thus where one  rock type i s seen to have been  emplaced e a r l i e r than another i n one sequence may  have occurred  i n different: parts  elsewhere.  part of the complex, the  reverse  Before s p e c u l a t i n g as to; how  such a s i t u a t i o n might a r i s e , i t i s necessary to consider the  petro-  l o g i c r e l a t i o n s h i p between the okaite s e r i e s and the other rock types present w i t h i n the complex.  R e l a t i o n of the Okaite S e r i e s to Other Rocks of the Oka It  Complex  i s b e l i e v e d that the demonstrated r e l a t i o n s h i p ; between the  various members of the o k a i t e s e r i e s implies a common genetic Much of the Oka  complex, however, i s composed of other rock  namely carbonatites of v a r i e d m i n e r a l o g i c a l composition,  origin.  types,  ijolite,  u r t i t e , m e l t e i g i t e , a l n o i t e , and small q u a n t i t i e s of many unusual types that are not named.  A r e l a t i o n s h i p has been suggested f o r the  o k a i t e s e r i e s and s o v i t e , c a l c - s i l i c a t e carbonatites being the connecting  the two.  The  link  chemical, s i m i l a r i t y of the m o n t i c e l l i t e  a l n o i t e of Husereau H i l l to j a c u p i r a n g i t e and okaite has been shown (page  121).  I j o l i t e and urtit-e are i n t i m a t e l y a s s o c i a t e d at Okas they are f i n e grained f o r the most p a r t , u n l i k e most other rock types of the complex.  Thin s e c t i o n examination reveals that many of .them have  r e c r y s t a l l i z e d from o r i g i n a l l y coarser grained rocks. mentioned, many of the i j o l i t e s are b r e c c i a t e d .  The  As has been cementing m a t e r i a l  i s normally c a l c i t e ; b i o t i t e has formed at the edges of l a r g e fragments, and may  e n t i r e l y replace smaller fragments.  There does not  appear to  be a d i r e c t r e l a t i o n between the i j o l i t e - u r t i t e s e r i e s and the s e r i e s w i t h i n the Oka  complex.  The  okaite  jacupirangite-ijolite-urtite  a s s o c i a t i o n i s well known at s e v e r a l l o c a l i t i e s ,  one  of which i s the  a l k a l i n e complex at Ice River, B r i t i s h Columbia ( A l l a n , 1914)'  Allan  suggests that these rocks o r i g i n a t e d by a process of d i f f e r e n t i a t i o n , from j a c u p i r a n g i t e to u r t i t e . Oka  suggests a connection  urtite series. i n calcium,  The  The  occurrence of j a c u p i r a n g i t e at  between the okaite s e r i e s and the i j o l l c t e -  pyroxene of j a c u p i r a n g i t e at Oka,  however, i s r i c h  aluminum and titanium, whereas the pyroxene of I j o l i t e i s  aegerine or aegerine-augite,  r i c h i n sodium.  a pyroxene of composition intermediate  A pyroxenite  to these two,  containing  such as might be  expected i n a b a s i c d i f f e r e n t i a t e of the i j o l i t e - u r t i t e s e r i e s , i s not found at Oka,  as f a r as i s known.  F i e l d a s s o c i a t i o n w i t h i n the  complex does not s u s t a i n a r e l a t i o n s h i p between j a c u p i r a n g i t e  Oka  and  ijolite. The m e l t e i g i t e s are strange rocks composed mainly of aegerine,  brown andradite,  wollastonite.  a p a t i t e , c a l c i t e , and,  The m e l t e i g i t e s show a close  r e l a t i o n s h i p ; to the i j o l i t e - u r t i t e s e r i e s . m e l t e i g i t e s at Oka  "...  The  wollastonite  bearing  appear to be i d e n t i c a l i n many respects to the  w o l l a s t o n i t e bearing Von  i n many rocks,  With i n c r e a s i n g c a l c i t e content they grade into; s o v i t e s  containing minor aegerine and a p a t i t e .  Island.  nepheline,  rocks described by von Eckermann (1948) at Alno  Eckermann s t a t e s that the occurrence of w o l l a s t o n i t e  i s very c h a r a c t e r i s t i c of the t r a n s i t i o n - z o n e between the  f e n i t e boundary and the rheomorphic rocks." (p. 44)• the w o l l a s t o n i t e m e l t e i g i t e s at Oka  inner  I"t i s noted that  occur at or near the contacts  the complex, and are a s s o c i a t e d with f i n e grained u r t i t e and  ijolite.  I j o l i t e as such i s rare at Alnd I s l a n d , but i s found i n the same' association.  of  Hypothetical O r i g i n of the Oka Complex The Monteregian petrographic b a s i c and a l k a l i c rocks.  province  I s c h a r a c t e r i z e d by  These are intruded as stocks, plugs, and  dikes along a l i n e a r b e l t extending f o r about 140 miles east of the Oka  complex, which i s the most westerly  of these i n t r u s i o n s . The  most common a l k a l i c rock i s nepheline syenite; of the b a s i c gabbro and e s s e x i t e predominate.  rocks,  Hone of these rocks, except f o r  small amounts of nepheline s y e n i t e , i s found w i t h i n the Oka complex. Nonetheless, the widespread occurrence of gabbro and e s s e x i t e p l a u s i b i l i t y t o the i d e a that the i n t r u s i v e rocks  lends  at Oka were derived  from a b a s i c parent magma. It has been shown that the a l k a l i c rocks at Scawt H i l l , which are s i m i l a r i n nature t o those o f the okaite s e r i e s , have been derived from a gabbroic  magma by limestone s y n t e x i s .  rocks i n the Oka area are f o r the most part: gneisses although one discontinuous stone i s present.  The country of various  kinds,  band of what i s probably G r e n v i l l e lime-  A few miles t o the north, c r y s t a l l i n e limestone  i s abundant i n the G r e n v i l l e rocks that u n d e r l i e the P a l e o z o i c sediments of the St. Lawrence Lowlands.  I t i s therefore  feasible  to propose that limestone may be present  at depth beneath the Oka  region. It i s suggested that: the okaite s e r i e s of the Oka complex was  derived from a b a s i c magma, such as gabbro o r e s s e x i t e , by a  process of progressive  limestone a s s i m i l a t i o n .  I t i s considered  p o s s i b l e that b a s i c magma from deep i n the earth's  c r u s t , o r from  the upper mantle, intruded t o a higher l e v e l , there forming a magma chamber w i t h i n limestone.  There i t was able t o a s s i m i l a t e lime, i t s  composition becoming changed i n such a way that i t g r a d u a l l y assumed more and more the nature of a, carbonatite l i q u i d .  Successive  in-  t r u s i o n s of magma at various stages of contamination from various p a r t s of t h i s h y p o t h e t i c a l magma chamber may be supposed to account f o r the d i v e r s i t y of the i n t r u s i v e rocks of the complex. I t i s p o s s i b l e that the a d d i t i o n of lime t o a b a s i c magma would cause c r y s t a l l i z a t i o n of a b a s i c , c a l c i u m - r i c h pyroxenite,  such  as j a c u p i r a n g i t e , l e a v i n g a l i q u i d p o r t i o n that might d i f f e r e n t i a t e  i n the d i r e c t i o n o f the i j o l l t e - u r t i t e s e r i e s .  However, c o n t i n u i n g  a d d i t i o n o f lime t o t h i s l i q u i d would cause continued c r y s t a l l i z a t i o n of the  l i m e - r i c h s i l i c a t e s , and the c r y s t a l l i z i n g rock would change i n d i r e c t i o n o f okaite and m e l i l i t e o k a i t e .  E v e n t u a l l y , as the  o r i g i n a l s i l i c a content was used up by the c r y s t a l l i z a t i o n pf c a l c s i l i c a t e s , the magma would assume the nature o f a c a r b o n a t i t e  liquid.  Daly ( 1 9 3 3 ) has been a major advocate o f the theory of the production o f a l k a l i c magmas by limestone s y n t e x i s .  I t i s well known,  hov/ever, that the break-down o f calcium carbonate below i t s d i s s o c i a t i o n temperature i s endothermlc. the  This f a c t has been used t o deny  p o s s i b i l i t y of limestone a s s i m i l a t i o n as a major f a c t o r i n the  d e s i l i c a t i o n of a common parent magma.  The argument i s probably v a l i d  i n the case o f a c i d i c magmas, the temperatures o f which may be too low to  overcome t h i s problem.  B a s a l t i c magma i s l i q u i d at low pressure at  a temperature o f about 1 , 1 0 0 ° C .  T h i s temperature increases with  pressure so that at a depth of f i f t y kilometers i t i s about 1 , 2 0 0 ° C . It has been shown that o k a i t e probably c r y s t a l l i z e d at a temperature i n the neighbourhood o f 800°C., o r between 300°C. and 4 0 0 ° C . below the temperature of the h y p o t h e t i c a l b a s i c parent magma.  I t i s thought  that such a d i f f e r e n c e i n temperature could provide enough heat t o overcome the problem o f the endothermic break-down o f calcium carbonate by r e a c t i v e a s s i m i l a t i o n . W y l l i e and T u t t l e , working with s i l i c e o u s melts, have made the  suggestion that  "... the low s o l u b i l i t y o f carbon d i o x i d e i n  g r a n i t i c and f e l d s p a t h i c l i q u i d s i s due t o the f a c t that they probably contain only a small p r o p o r t i o n o f cations which are not s t r u c t u r a l l y bound to the a l u m i n o - s i l i c a t e networks o f the l i q u i d s , i . e . , there i s only a small, p r o p o r t i o n o f cations a v a i l a b l e f o r carbonate formation w i t h i n the l i q u i d s . " ( 1 9 5 9 a , p. 6 5 4 ) .  Prom t h i s suggestion they i n f e r  that carbonates are more s o l u b l e i n a l k a l i c than i n s i l i c e o u s magmas. Gabbro contains l e s s t o t a l alumina plus s i l i c a than g r a n i t e , so i t i s probable that gabbroie magma can d i s s o l v e a l i t t l e more carbonate than can g r a n i t i c magma.  I f a gabbroie magma becomes d e s i l i c a t e d by the  process o f r e a c t i v e limestone a s s i m i l a t i o n , i t s temperature being lowered i n the process, then the f a c i l i t y limestone might be increased.  with which i t could d i s s o l v e  This, mechanism i s t e n t a t i v e l y proposed  for  the derivation of the okaite Wyllie  that  b e l o w 700°C.  (1959b and c, 1960a and b) have shown  and T u t t l e  carbonate l i q u i d s  s e r i e s r o c k s f r o m a g a b b r o i c magma.  can exist  at low p r e s s u r e at temperatures  A d d i t i o n t o these l o w - v i s c o s i t y  liquids of s i l i c a  o t h e r o x i d e s , s u c h as m a g n e s i a and a l u m i n a , c a n cause of  basic  silicates,  s u c h as o l i v i n e  of Ca3(P04) ,  PgOtj c a u s e s c r y s t a l l i z a t i o n counterpart  and m e l i l i t e . 2  i s the mineral apatite.  crystallization  The'additionof  a compound whose n a t u r a l  The s u g g e s t e d development  okaite series i s the reverse of t h i s  with  of the  a d d i t i v e p r o c e s s , and c a n be  c o n c e i v e d as t a k i n g p l a c e by t h e c o n t i n u e d a d d i t i o n o f c a l c i u m c a r b onate t o a s i l i c a t e  melt.  Intrusion of alnoite  and,, lamprophyre  and t h e f o r m a t i o n o f  d i a t r e m e b r e c c i a s p e r i p h e r a l t o t h e complex were t h e l a s t Monteregian  activity  at  Hill  Husereau  at. Oka.  magma at d e p t h . is the  The n a t u r e o f t h e m o n t i c e l l i t e  has been d i s c u s s e d .  been d e r i v e d f r o m t h e l a s t  liquid  presence o f resorbed o l i v i n e  that  Most o f t h e l a m p r o p h y r e s bearing pyroxenites; t h i s  stemmed f r o m  p o r t i o n s o f a contaminated, b a s i c  Perhaps  i s suggestive of derivation  from  C02.  especially  a r e t i t a n i f e r o u s pyroxenit.es o r o l i v i n e again i s suggestive of d e r i v a t i o n  I t i s possible that  less  series.  charged with v o l a t i l e s ,  contaminated  diatreme b r e c c i a s are thought residual  t o have  I t s e x p l o s i v e n a t u r e at t h e time o f i n t r u s i o n  i t was h i g h l y  a b a s i c magma.  alnoite  A l t h o u g h c o n t a i n i n g a h i g h p r o p o r t i o n , o f CaO, i t  a g a b b r o i c magma.  of  T h i s rock i s thought  r i c h e r i n m a f i c oxides than rocks o f the okaite  reflects  stages of  carbonate l i q u i d s .  these l a s t  phases  from;  of intrusion,  p a r t s o f a b a s i c magma chamber. t o have been formed These  liquids,  The  by t h e e x p l o s i o n !  h i g h l y charged  with  CO2, moved t o h i g h l e v e l s where t h e c o n f i n i n g p r e s s u r e was u n a b l e to  c o n t a i n them; e x p l o s i o n r e s u l t e d ,  and t h e f r a g m e n t s  shattering the country rocks,  were cemented by c a r b o n a t e s f r o m t h e i n t r u d e d  liquid. Detailed  study o f the i j o l i t e s ,  a s s o c i a t e d r o c k s i s o u t s i d e t h e scope evidence, apparent  from both f i e l d  u r t i t e s , m e l t e i g i t e s , and  o f t h e p r e s e n t work.  and l a b o r a t o r y work, t h a t  There i s supports  the  theory that  t h e s e r o c k s were d e r i v e d from t h e c o u n t r y r o c k s by  the  metasomatic  p r o c e s s o f f e n i t i z a t i o n which,  I n i t s most  advanced  stages, was accompanied by rh.eomorph.ism.  The mechanism of t h i s  process i s . not discussed, but i t i s thought to have been s i m i l a r t o that proposed by von Eckermann ( 1 9 4 8 ) f o r the f e n i t e s and rheomorphic f e n i t e s of the Alno I s l a n d a l k a l i n e complex. C0  2  A d d i t i o n o f lime and  to a melting i j o l i t i c f e n i t e during rheomorphism,  e i t h e r by  a s s i m i l a t i v e s o l u t i o n of limestone o r by mixing with a c a r b o n a t i t e l i q u i d , could lead t o the formation of the carbonate-rich m e l t e i g i t e s and other unusual rocks found w i t h i n the Oka complex. Figure 6 1 i s a t r i a n g u l a r diagram on which are p l o t t e d compositions of various rock types pertinent t o the a s s o c i a t i o n s found w i t h i n the Oka complex.  The average compositions used i n t h i s  diagram are taken from Barth ( 1 9 6 2 ) ; three analyses of rocks from Scawt H i l l  ( T i l l e y and Harwood, 1 9 3 1 ) are included because they i l l u s -  t r a t e w e l l the trend from gabbro to m e l i l i t e rock. analyses are of rocks from the Oka complex. apparent i n the diagram.  The remaining  Two trends are r e a d i l y  The trend on the l e f t  side of the diagram,  from average gabbro towards the calcium-magnesium-iron apex, includes the at  okaite s e r i e s , m o n t i c e l l i t e a l n o i t e , and the contaminated rocks Scawt H i l l ;  i t r e f l e c t s p r i m a r i l y an increase i n CaO and a c o r r e s -  ponding decrease i n S i 0 2 -  The trend on the r i g h t side of the diagram  i s more h y p o t h e t i c a l ; i t i s the p o s t u l a t e d trend o f g r a n i t i c rocks through f e n i t i z a t i o n .  At the point on t h i s trend where  rheomorphism  occurs, a d d i t i o n of lime would move the trend i n the d i r e c t i o n of the calcium-magnesium-iron apex.  Thus both l i n e s of development  trend  towards carbonatite as an end product. The Oka complex has the s t r u c t u r e of two sub-concentric r i n g complexes.  I t i s s i t u a t e d a s t r i d e a dome-like bulge on the  a n t i c l i n a l Beauharnois a x i s . Oka^ Precambrian i n l i e r  I t i s p o s s i b l e that t h i s dome of the  was formed at the time of Monteregian a c t i v i t y  by the emplacement of a large body of magma at depth.  Various con-  taminated magmas were able to penetrate to; higher c r u s t a l along l i n e s of weakness formed by t e n s i o n during doming.  levels  Figure 61; 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.  T r i a n g u l a r composition diagram on which the f o l l o w i n g ro are p l o t t e d :  Average essexite (Bartih, 1962) Average gabbro (Barth, 1962) Nepheline d o l e r i t e , Scawt H i l l ( T i l l e y and Harwood, 1931) T i t a n a u g i t e - m e l i l i t e rock, Scawt H i l l ( T i l l e y and Harwood, J a c u p i r a n g i t e , Oka Average o k a i t e , Oka M e l i l i t e rock, Scawt H i l l ( T i l l e y and.Harwood, 1931) M o n t i c e l l i t e a l n o i t e , Oka ( S t a n s f i e l d , 1923) Average g r a n i t e (Barth, 1962) Average nepheline syenite (Barth, 1962) Average u r t i t e (Barth, 1962) I j o l i t e , Oka Average i j o l i t e (Barth, 1962) Wollastonite m e l t e i g i t e , Oka  1931)  SUMMARY  The Oka complex c o n s i s t s of two overlapping, sub-concentric r i n g complexes that i n t r u d e the Precambrian  rocks of the Oka  inlier.  Contained w i t h i n the complex are the i n t r u s i v e rocks of the o k a i t e s e r i e s , v a r i o u s types of c a r b o n a t i t e s , i j o l i t e , u r t i t e , m e l t e i g i t e , a l n o i t e , lamprophyre, and carbonate-cemented diatreme b r e c c i a .  The  carbonate rocks of the complex have been considered by some to be m i n e r a l i z e d G r e n v i l l e limestone; evidence has-been produced that i n a l l p r o b a b i l i t y they are I n t r u s i v e c a r b o n a t i t e s .  to show  Recent  experimental work has confirmed the p o s s i b i l i t y of the existence of carbonatite magmas. Okaite and r e l a t e d rocks are coarse grained, are grey i n colour, and have a g r a n i t i c t e x t u r e .  They form s t e e p l y d i p p i n g or  v e r t i c a l arcuate dikes, as do most other rocks of the Oka  complex.  M e l i l i t e i s the predominant mineral; hauyne, nepheline, t i t a n a u g i t e , b i o t i t e , and c a l c i t e are present i n v a r y i n g amounts. magnetite,  Apatite,  and p e r o v s k i t e are e s s e n t i a l accessory minerals.  The  o k a i t i c rocks are d i v i d e d i n t o f o u r main types, according to t h e i r mineralogies.  Pyroxene o k a i t e contains t i t a n a u g i t e and hauyne.  Okaite contains hauyne and/or nepheline, but no; t i t a n a u g i t e . okaite contains no t i t a n a u g i t e , hauyne, or nepheline. rock contains more than 10$ c a l c i t e , and grades i n t o carbonatite.  Melilite  Melilite-calcite calc-silicate  These rock types, together with j a c u p i r a n g i t e ,  comprise  the okaite s e r i e s . Microscopic examination  r e v e a l s that pyroxene o k a i t e i s  r e l a t e d to okaite by the r e a c t i o n of t i t a n a u g i t e with magmatic l i q u i d to  form m e l i l i t e .  There i s a complete compositional g r a d a t i o n from  j a c u p i r a n g i t e , through pyroxene o k a i t e , o k a i t e , and m e l i l i t e o k a i t e , to  m e l i l i t e - c a l c i t e rock.  There Is no> d i r e c t g r a d a t i o n between any  of  the rocks that are not adjacent i n t h i s sequence; f o r i n s t a n c e ,  pyroxene o k a i t e i s not found to grade d i r e c t l y i n t o m e l i l i t e o k a i t e . The accessory minerals were the f i r s t to c r y s t a l l i z e ; primary was  calcite  i n v a r i a b l y l a s t , and i t i s subordinate i n amount i n a l l rocks  except m e l i l i t e - c a l c i t e rock.  The composition of n a t u r a l m e l i l i t e s can he expressed i n terms o f the three type molecules akermanite.  sodium m e l i l i t e , g e h l e n i t e , and  M e l i l i t e from rocks o f the okaite s e r i e s i s found t o  he very s i m i l a r i n composition t o m e l i l i t e from other a l k a l i c complexes.  These n a t u r a l m e l i l i t e s a l l have compositions o f about  Na^Ge-^QAkc^.  A d e t a i l e d study o f the t i t a n a u g i t e from pyroxene  okaite r e v e a l s i t t o be a h i g h l y c a l c i c pyroxene with an abnormal amount o f aluminum s u b s t i t u t i n g f o r s i l i c o n .  A study o f the opaque  minerals i n p o l i s h e d s e c t i o n shows that the magnetite  of o k a i t i c  rocks i n v a r i a b l y contains lamellae o f exsolved h e r c y n i t e , and that small amounts o f h e r c y n i t e with exsolved magnetite some rocks.  are present i n  Neither i l m e n i t e nor u l v o s p i n e l were seen t o have been  exsolved from magnetite,  suggesting that the magnetite  i s a low-  titanium variety. In many places i n . the Oka complex, rocks o f the o k a i t e s e r i e s are a l t e r e d to a dense, f i n e grained, cream to. pale grey coloured rock i n which the accessory minerals are u n a l t e r e d . A l t e r a t i o n was accomplished by hydrothermal from r e s i d u a l magmatic f l u i d s .  f l u i d s that o r i g i n a t e d  A l t e r a t i o n mainly a f f e c t e d m e l i l i t e  and hauyne; a v a r i e t y o f a l t e r a t i o n mineral assemblages was produced, the p a r t i c u l a r assemblage depending on the temperature o f the hydrothermal  fluids.  The mineral c e b o l l i t e i s i d e n t i f i e d as an  a l t e r a t i o n product o f m e l i l i t e . type may be determined  I t i s shown that the o r i g i n a l rock  by t h i n s e c t i o n examination o f a l t e r e d rocks.  Four stages o f a l t e r a t i o n are d i s t i n g u i s h e d . r e l a t i o n s h i p between the high temperature  There appears t o be a  a l t e r a t i o n o f m e l i l i t e by  r e s i d u a l magmatic f l u i d s and the products o f the r e a c t i o n o f m e l i l i t e with magma i n m e l i l i t e - c a l c i t e rock.  The type of a l t e r -  a t i o n v a r i e s i n such a way as t o suggest that the amount o f r e s i d u a l f l u i d increased towards the c a r b o n a t i t e end o f the okaite s e r i e s . Several other a l k a l i c complexes c o n t a i n rocks that are extremely s i m i l a r t o rocks o f the o k a i t e s e r i e s , both chemically and i n mode o f occurrence.  At Scawt H i l l a gabbroie magma has been  shown t o produce rocks s i m i l a r t o those o f the okaite s e r i e s by limestone a s s i m i l a t i o n .  Consideration o f experimentally known data concerning the s t a b i l i t y o f akermanite  leads t o estimates o f temperature  and pressure  at the time o f c r y s t a l l i z a t i o n o f the okaite s e r i e s magma. of temperature  f o r hydrothermal  Estimates  a l t e r a t i o n are made, based on the  s t a b i l i t y l i m i t s of the a l t e r a t i o n minerals.  The sequence o f  i n t r u s i o n i s u n c e r t a i n , and does not appear t o be r e l a t e d t o rock type.  I n t r u s i o n of a l n o i t e , lamprophyre, and diatreme b r e c c i a took  place l a t e r than the formation o f any of the other rock types the Oka complex.  withim  A t r i a n g u l a r composition diagram shows the r e l -  ationship- between the rocks of the okaite s e r i e s , and suggests d e r i v a t i o n from a b a s i c magma by the a d d i t i o n of calcium; the diagram also shows that i j o l i t e , u r t i t e , and m e l t e i g i t e are notd i r e c t l y r e l a t e d t o the o k a i t e s e r i e s .  CONCLUSIONS  There I s a g e n e t i c r e l a t i o n s h i p between the rocks o f the okaite series.  They probably owe t h e i r o r i g i n t o the a s s i m i l a t i o n  of limestone by a b a s i c magma. continuous  I t i s p o s s i b l e that a process of  a d d i t i o n o f limestone t o a basic magma, such as gabbro  or e s s e x i t e , l e d t o the development o f the o k a i t e s e r i e s , t h i s process  culminating i n the formation o f a carbonate magma that  c r y s t a l l i z e d as c a l c - s i l i c a t e carbonatite and s o v l t e .  Crystalliz-  a t i o n o f the okaite s e r i e s magma occurred near the s u r f a c e , at  a depth of l e s s than two k i l o m e t e r s .  probably  The process o f a s s i m i l a t i o n  of limestone may have taken place at considerably g r e a t e r depth. Temperatures of c r y s t a l l i z a t i o n are thought t o have been of the order of 800°C. f o r pyroxene o k a i t e , and nearer t o 700°C. f o r m e l i l i t e - c a l c i t e rock and c a r b o n a t i t e . Most of the other a l k a l i c rocks of the Oka complex are not d i r e c t l y r e l a t e d t o the okaite s e r i e s .  I j o l i t e and u r t i t e are  thought t o have been derived from the Precambrian country rocks by a process o f advanced metasomatic f e n i t l z a t i o n accompanied by rheomorphism.  M e l t e i g i t e and soda-pyroxene c a r b o n a t i t e may have  been produced by the a d d i t i o n of calcium carbonate t o p a r t l y o r wholly molten f e n i t e s during rheomorphism.  On t h i s b a s i s , many of  the b r e c c i a t e d and a l t e r e d i j o l i t e bodies w i t h i n the complex may represent: septa between dikes of i n t r u s i v e rocks.  I n t r o d u c t i o n of  a l n o i t e and lamprophyre represents the l a s t phase of i n t r u s i o n from a contaminated magma chamber below. A l t e r a t i o n of the okaite s e r i e s rocks was automorphic i n nature.  The f a c t that a l t e r a t i o n i s found t o be more widespread  i n rocks towards the carbonatite end of the o k a i t e s e r i e s i s i n d i c a t i v e that there was a g r e a t e r tendency f o r the r e s i d u a l concent r a t i o n of v o l a t i l e s as the s e r i e s developed.  Thus c r y s t a l l i z i n g  m e l i l i t e okaite may have produced, considerable a l t e r a t i o n w i t h i n i t s e l f at temperatures c l o s e t o that of the l a s t stages of c r y s t allization.  As the r e s i d u a l hydrothermal f l u i d s c o l l e c t e d and  moved t o the c o o l e r environment o f already c r y s t a l l i z e d  okaitic  142  rocks,  they  stages  of  formed  a l t e r a t i o n It  i s  successfully  by. a  basic  given  second  of  to  dissolve  two basic  enriched  contaminated have  are  magma.  by  process  to  that  represents  about  magma  magma.  a  i s  magma i n  from  a  limestone  dissolved.  At  had i n  dissolved, composition.  stage and  at  at  and  of  of  the a  the  t h i s  calcium  the  okait-e  of  other  i n  series that one  end,  was  basic  process  i s  contaminated,  by  of  approaching  The  the thought been  of  the  which  m e l i l i t e - c a l c i t e amount  magma be  a  had  at  be  may  are  end  300°C.  okaite  s i l i c o n .  carbonate  last  environ-  The  f i r s t  contamination  considerable magma  c e r t a i n  producing  At  may  t r a n s i t i o n  The  magma  limestone.  stage  which  which  of  below  between  impoverished  contaminated  amount the  that  limestone,  s o l u t i o n rocks  stage  the  limestone  understood.  processes.  with  calcium  represents  the  poorly  thought  overlapping,  jacupirangite had  yet  magma,  t h e - t r a n s i t i o n a l  The  certain  as  It  involves  c r y s t a l l i z e d  represents  temperatures  accomplished  carbonatite  basic  at  of  magma  reaction  occurred  assemblages;  assimilation  series  the  probably  mineral  that  conditions  brought  temperature  concluded  mental  and  lower  able series, l i t t l e rock  limestone carbonatite  ADDENDUM  At, the time of preparation, of t h i s t h e s i s , l i t t l e o:r no work had been: done on the s o l i d s o l u t i o n between magnetite hercynite.  and  It-was suggested on pages 84 and 85 that the c r y s t - .  a l l i z a t i o n temperature  of the magnetite  of o k a i t i c rocks might be  estimated i f the magnetite - h e r c y n i t e e x s o l u t i o n intergrowths were to be studied by annealing experiments  at elevated  temperatures.  A recent paper by Turnock and Eugster (1962) concerns the phase r e l a t i o n s of i r o n and aluminum oxides below 1 , 0 0 0 ° C , and deals i n . d e t a i l with s o l i d s o l u t i o n between magnetite  and h e r c y n i t e .  The  solvus f o r these two minerals i s given, and Is shown to be i n s e n s i t i v e to v a r i a t i o n of pressure up to 4,000 bars.  Turnock and  Eugster s t a t e that "... the consolute temperature  i s 860° + 15°C."  (p« 543).  Magnetite  and h e r c y n i t e occur together as i n d i v i d u a l  grains i n some o k a i t i c rocks, i n d i c a t i n g that they crystallized', below the solvus; each shows e x s o l u t i o n of the other;  Maximum  extent of s o l i d s o l u t i o n In both minerals i s estimated to be between 25$ and 30$ by weight.  A p p l i c a t i o n of t h i s percentage  range to the  solvus given by Turnock and Eugster suggests a c r y s t a l l i z a t i o n temperature  very c l o s e to 800°C. T h i s temperature  with temperatures  i s In accord,  estimated by other means.  REFERENCE  Turnock, A . C , and Eugster, H.P., 1962, F e - A l oxides: phase r e l a t i o n s h i p s below 1,000°C; Jour. Petrology:, v. 3, p. 533-565-  SELECTED BIBLIOGRAPHY  Ahrens, L.H., 1952, The use of I o n i z a t i o n potentials-, part; I; i o n i c r a d i i of the elements* Geochim. et Cosmochim. Acta, v. 2, p. 155-169. A l l a n , J.A., 1914, Geology of the F i e l d map-area, B r i t i s h Columbia and A l b e r t a : Canada Geol. Survey Mem. 35, 312 p. Backlund, H.G., 1932, On the mode of i n t r u s i o n of deep seated a l k a l i n e bodies: Geol. I n s t . Upsala B u l l . , v. 24, p. l-24« Barth, T.F.W., 1932, The chemical composition of n o s e l i t e hauyne: Am. 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