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Geology of the Mount Brenner stock near Dawson City, Yukon Territory Lambert, Maurice Bernard 1966

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GEOLOGY OF THE MOUNT BRENNER STOCK NEAR DAWSON O I T Y YUKON  TERRITORY by  B.Sc,  A-  MAURICE BERNARD LAMBERT. University of B r i t i s h Columbia,  THESIS  1963  SUBMITTED I N P A R T I A L F U L F I L M E N T  THE REQUIREMENTS FOR THE D E G R E E OF MASTER OF  i n the  SCIENCE  Department of  GEOLOGY  We a c c e p t , t h i s t h e s i s a s c o n f o r m i n g t o required standard  THE U N I V E R S I T Y OF B R I T I S H April, 1966  the  COLUMBIA  OF  In presenting the  this thesis i n partial fulfilment  r e q u i r e m e n t s f o r an advanced degree at  of  the U n i v e r s i t y  of  B r i t i s h C o l u m b i a , I agree t h a t the L i b r a r y s h a l l make i t  freely  available  per-  f o r r e f e r e n c e and s t u d y .  I further  m i s s i o n f o r e x t e n s i v e c o p y i n g of t h i s purposes  may be g r a n t e d  his representatives„ cation  of t h i s  It  agree t h a t  thesis for  scholarly  by the Head of my Department i s understood  t h a t c o p y i n g or  t h e s i s f o r f i n a n c i a l g a i n s h a l l not be  w i t h o u t my w r i t t e n  or by publiallowed  permission.  Lambert)  Department  of  Geology  The U n i v e r s i t y of B r i t i s h C o l u m b i a , Vancouver 8, Canada. Date  i-lay 3. 1 9 6 6  •ti  11 ABSTRACT  The Mount B r e n n e r mentary  and m e t a s e d i m e n t a r y  east of  Dawson C i t y ,  of  major  four  rocka that  p o r p h y r i t l c hornblende  zone o f  coarse-grained pink the  outer  and the alignment primary  flow  of  modified  sharp.  internal  The r e g i o n a l the  forceful  injection.  erosion,  the  ation,  monzonite  porphyritlc  steeply  rocks a  outward  external  is  is  are  contact. concluded that  at  s t o c k was e m p l a c e d b y  The d i f f e r e n t  rock zones of  magma b y a c o m b i n a t i o n o f  v o l a t i l e and a l k a l i  central  structural trend  it  of  porphyry.  s t o c k can be a c c o u n t e d f o r by d i f f e r e n t i a t i o n o f biotite  a  (4)  All  intrusive  fine-  intermediate  s t o c k so t h a t b e d s  From s t r u c t u r a l e v i d e n c e , l e v e l of  of  gives the  contacts.  conformable w i t h the  the present  and  a l l rock types are  to  north-  a zone  an  monzonite  feldspar phenocrysts  i n the v i c i n i t y of  generally  quartz  s t r u c t u r e which conforms  dipping gradational contacts are  zone,  monzonite;  zone  (2)  (3)  sedi-  stock consists  an o u t e r  porphyry;  zone o f  for  The  a u s i t e - b l o t i t e monzonite;  coarse-grained monzonite  Except  (l)  folded  l i e AO m i l e s  Yukon T e r r i t o r y . •  concentric zones:  to medium-srained very  stock has i n t r u d e d  diffusion,  an  the augite-  crystal fraction-  and m u l t i p l e  intrusion.  Pag©  1  mmowGHion • LOO AT I OK AND AGO • i S O l B l i . I X ^ . . . . . . . . . . . . . . . . . . . . . . .  1  TOPOGRAPHY, DAAIiMGA AKD ALACIATION.»,.,«,..,,.• .  1  PREVIOUS GEOLOGICAL <*OHi£.  4  ACKNOWLEDGMENTS....  5  QAKAAAL  .........................  6"  OSOLOGY« «  THK MOUNT D^aLAKER OTOC A............................... GAKEfctAL DLSORIfTIOiS*. .»«..**..•..«•  8  .........  6  PETROGRAPHY.,,«*»**•..,•.,,,*...»,..,•...........  13  A u g l t e - b i o t l t e fen&onit®.................... 13 Monaoolte Porphyry  16  P o r p h y r t t l c Hornblende Monzonlte............  18  Pink Quartz Monzonlte Porphyry.»...*...*...«  21  Apllte  24  Inclusions.  ...»  24  VARIATION* »«.,»».».»#.»,  29  COAT •'•Of ;AArA^OAPiiI A«t, ,,.»,,.•,,.,..,.*...*.,•..,,  36  STA JGTURA, »,,.....  37  CA:A4IGAL  CO.A?OAlTIOft  AAL  ......,.«..»...,  I n t e r n a l Structure®  «...  37  F o l i a t i o n and L i neat Ion  37  Jointing,  40  I n t e r n a l Contacts.  *.... 44  i n t e r n a l Structures  I n t e r p r e t a t i o n of S t r u c t u r e s .and Mechanics of  Sfiplac6ffl@rit•••.•••••••«•••••••••••••»••  As s iiu i.  xoti  l) .11 X t i l ' . d i i t X  • ••••••••••••••••••••••• 1 Gli • • • • • • • • • • • • t  • •  Suttuaary of iSaplacement, C r y s t a l l i z a t i o n and l) i £ f c i' on 11 & t jL on • •••••»••••«••••••«#••••••• iii l i ^ l s JL 0 C? h A.t'jri  i t « i i i i i « t M « t i t * t * t * M i i « i i i i M « t i i * «  -A  V  i4»p'  Cm@lo%lml  map o f %b* Ht« 5 r » D n * r 3too*s,  Xufcon "territory  1 2  .....  led em ;sap »howtag ta® fjfcwaon a r e a Had l o c u t i o n o f t h e Mount Brenner s t o c k * , * * * lodes o f sp#ot;a&$3®  4  5  €  pao »t ;  2  f r o * th® Mount 3re»n®r  3 t O O T . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  3  In  11  V a r i a t i o n i n eooipa^itlon o f g r a n i t i c roo'*8 w i t h r e s p e c t t o d i s t a n c e f r o a the s&rgin of «rtcU rack t y p c » .  12  Variation dinpras, f o r ra.-'r *oo*s o f t h e 'ouot r» noer t»toe1«» TU« s.v@rag« %f«laht per cent o f O X U U B o f «mch rock tan® e"r« p l o t t e d ftualnst t o t a l silica,»*••*»*•,,«  32  Contoured dia^xaa-, of oxide® m » <3,lff«rc.-.ti . t I O J I index on v c i c a analysts© of roc <a f rasa th® ' .oa^t nr«srjn«r atoo'A have been *uperl3poa.-*<l••*•«»»»••••»«*•«****»«  33  TriJM)£ular ooaipos-ttloo dltigrfta on w h i c h the f o l l o w i n g a r e p l o t t e d ; rm'x.® o f t h e .'••'.aunt brmmv utooltj Daly*© awrsf-e  s a l t , andealt®, <Haoit«, S I K ! rhycHlt#$  aad  £ d t o f c o m p o s i t i o n v a r i a t i o n o f th®  a a j o r t t y o f ci«la-alic»ll voio«nlo a o £ p l u t o n l c etquences,«,.,,.,,»,.»«»...«•«•  ? fl  33  S t r u c t u r e Trend ,«ap o f ^t» Srenner atoeit, 'laXoa Territory*•*«••••••*•••••••*»»•••• 172 p o l e s t o f i l i a t i o n plottwd on & achaleSt  n«t  sad contoured*.  C o n t o u r s 2, 5, 15, »«a  o v e r l ' v Pfe-r 1,-i urea, .*.«.•«*••«.»•*.*• •  41  vi Page  Figure  9  10  213 p o l e s t o j o i n t s p l o t t e d on a Schmidt net and c o n t o u r e d . C o n t o u r s 2, 7, 14, 21, 28, and o v e r p e r 1^ a r e a , . . , , . . , . . .  42  Diagrammatic s k e t c h showing a h y p o t h e t i c a l development o f r o c k zones o f the Mount B r e n n e r stock*.•»•*••••«-••*»«•*••••••  55  Modal A n a l y s e s o f G r a n i t i c Rooks o f t h e Mount B r e n n e r stock.•«•«•»•.••«»«••••«••••»  X0  C a l c u l a t e d C h e m i c a l C o m p o s i t i o n o f Hooks Mount B r e n n e r s t o c k s .«••*.*««»•«»«»••***••» *  31  Tables  I II  following  Plates  Page A. Au?jtt@~blotit@ Monaonlt* w i t h rounded p l a t y a g g r e g a t e s o f b i o t i t e (X 1 } . . . . . . . . . . B. P o r p h y r i n i c Hornblende Manzonito w i t h d i s t i n c t alijmaeot o f feldspar c r y s t a l s  (-X. 1).•••.•••<»«..«  II.  . . . . .....««»•  A» Zoned, twinned p l a g i o e l a s e w i t h b r o a d a l t e r e d core i n b i o t i t e quartz d i o r i t e (Grossed n i c o l s X 1 0 ) . . . . . . . . . . . . . . . . . . . . . . B* Bent p l a ^ i o c l a s e w i t h s t r e s s t w i n n i n g i n a u g i t e - b l o t i t e monzonlte ( C r o s s e d n i c o l a  13  14  10)iMiMttfl«i«i«t*i.iiiaita«fi*«*4»***t«  14  Texture o f b i o t i t e (M.) I n a u g i t e - b l o t i t © a o n ? o n i t e (Grossed n i c o l s X 10) ...........  15  X  III.  13  hedral augite ( / i u j : J with rim o f hornb l e n d e (Hb) i n a u g i t e - b i o t i t e aonzonit©  (X 10)..•...*......•.....«....•«...........  15  A.  ".oaxonite p o r p h y r y  (X l/2)  E. Vein perthit® i n phenolryat o f m o n z o n i t e p o r p h y r y (Crossed* a i c o l s X 4 5 ) . A. F i l m p e r t h i t e i n ortho&l--*ne o f monzonite porphyry (Crossed n i c o l s X 4 5 ) , 11bite ' l e n s e s ' t r e n d p e r p e n d i c u l a r t o 151G) oleavag,© i n o r t h o e l a s e . ...*. B. N o n - p e r t h l t l c a r e a s around p e r i p h e r i c of plagloclas© g r a i n s i n o r t h o e l a s e o f s j o o z o n i t e p o r p h y r y ( O r o e e e d n i e o l s X 45) A#  Corroded, o r t h o e l a s e  crystal  core o f an orthosisBe phenocryst (Crossed,  nicols  (Or)  in  (Ph).  X 1 0 ) . . . . . . . . . . . . . . . . . . .  .  0. Pla.g.1 a c l a s © I n e l u a i o ; s i n o r t h o e l a s e o f t h e g l o m e r o p o r p h y r i t l c ph; ae o f t h e p o r p h y r i t l c hornblende nonzonite (Crossed n l c o l a X 1 0 ) . . . . . . . . . . . . . . . . . . . • 4.  Pink  quartz  monzonite  porphyry.  (.MI) .  13. f e r t a i t i c orthoelase uhenocryst w i t h I n c l u s i o n s o f euaedr;-!, u n s t r . i n e d quarts ( ») w r o s H e f ! n l c o l a X 10) , P. Zoning and allf/n*ae..rt o f p l a t l o o l j . ; ee Inclusions i n o r t h o e l a s e phenooryat• o f pi q a a r t i i :riooxonitv p o r p h y r y (';ro:-;..cc ; ;  ^, - a r t o f phenocryst »* ."J ' l o . v t i o n ( G r o s s e d  I n p i s t e A u t h i .J nicols X 10) ...... c  Xrie>;i,Ht I o n o f pli- ^ i o c l u o e i o a l u a i o n y i n o r t h o s i s s © p n f r a o c r y s t o f t a f c p i - ; ' < qufcr i p n s / j i i i t e p o r p h y r y ( G r o s s e d n l c o l a .•>. X } . . X. : l a . _ l o o l • a e c r ^ s t u l s w i t hi;; nnX p-..rtl i n c l u d e d i n t n e aargi.-j o f o r t a o e l e s © phen aryot p ^ r o a t i c P n i o l s X X) :  vili '•"ollowlaj?  Pac e  Plates  XI.  XII»  A. C o m b i n a t i o n twinned p l a g i o o l e B © i n c l u s i o n on o r t h o c l a s e p a e n o c r y s t ( C r o s s e d n i c o l s X 10}  22  B. rtyrmekite rial on p l « g \ o c l « s e s u r r o u n d e d by p o t a s h f e l d s p a r ( l i ) o f t h e pinfc q u a r t z r a o n z o n i t e p o r p h y r y . . . . . . . . .  22  A. O s c i l l a t o r y z o n i n g i n p l a g l o c l f s s e of pink q u a r t a m o n z o n i t e porphyry. ( G r o u s e d nicols X 1 0 ) . . . . . . . B. a l l n a l t e i n n i e t a c a i c t s t a t e {?) i n c l u d e d I n h o r n b l e n d e (Hfo). Kobe p l e o c h r o i c h:;lo end r a d i a t i n g f r a c t u r e s I n h o r n b l e n d e . (Pl*.«e l i g h t X 10) . . . . . . . . .  *3  A. I n c l u s i o n i n p i n k q u a r t z rsoozonit© porphyry. Potash feldspar phenecrysts • s t r e a m * e r o u n d t h e I n c l u s i o n ( X 1/3).*  23  ii. I n c l u s i o n i n p i n k q u a r t z raon.-.onite porphyry. ftote pheaocrysfcs In both q u a r t z ;flonzonlte and i n t h e i n c l u s i o n 1 1 ) « . . . . . . . . . . . . . . « . . . . . . . . . . . . . . . . . . XIII.  XIV,  A. Porphyroblast o f p e r t h l t e i n grsnob l a s t i c matrix, hate l i n e o f t i n y i n c l u s i o n s and I r r e g u l a r b o r d e r . (Grossed n i c o l s X 5).....  26  t>. O s c i l l a t o r y z o n i n g i n o r t h o c l u s e raegacryst •. , ( c r o s s e d n i c o l s X 3 ) . . . .  26  Photomicrograph  showing c o r r o d e d p l a g i o o l a s s p h e n o c r y s t s In an I n c l u s i o n , Block: b a c k g r o u n d i s p o t a s h f e l d s p a r . (Grossed n i c o l s X 10)  A,  1  B. Photomicrograph hornblende (Crossed (  showing p o i k i l i t i c n i c o l s ? A: 1 0 ) .  26  WJOAflOl M® AOOSaSZBZUTX The map-area lias l a west-central Yukon Territory about 40 allee a@rtli.aait o f Dawson City and 5 alias north of tha Tombstone liver (fig» 1 ) .  Th© Beateter Highway leaves  the Taylor Highway 20 ml%m. east of Dawson city and follows the Korth Klondike River*  Both of these gravel highways  are kept l a excellent condition for summer travel.  At  mile 4 § of the lesster Highway (in the vicinity of Morth Fork Pass) the north Klondike River swings southweatwari. along a broad valley.  fhis valley leads to a low, flat  pass into the valley of the $oohaton® River at about 1 1 miles from the Pewter Highway.  the map-area i s a two  day walk from the highway by way of these valleys.  The  area Is accessible by helicopter or, with dtff&tt&tar; by horses*  VOKXKtAJHX*  B8AIM®  Ut£  aLAOttXXGV  The region lies within th® physiographic unit known as the Ogilvie Mountains (Bostoek,  1948).  "The  range has a rugged and mountainous aspect and consists of long* branching, knife-edge crests, with sharp and often  F i g u r e I.  Index map showing the Dawson City the  Mount  Brenner  stock (lined  area and location  pattern).  of  p r e c i p i t o u s peaks* separated  by deeply out v a l l e y a » • ( C o e f c -  fa® m a p * * r e a Is e n t i r e l y a b o v e ti«fc-er-lls@  f i e l d , 1919}*  w h i c h l a a t as e l e v a t i o n o f approximately  4000 f e o t r  •Average r e l i e f I s 1500 f##t and peak® reach & aaxiaua e l e v a t i o n o f ?§GQ f e e t *  l a s r w s o f g r r * » l t i o r«ek» t h e  topography h a g a u n i q u e a h a m o t o r w h l o t i i»as boot* d o e o r i b o d by  -cConnell  (ly03)  i a th® f o l l o w i n g word®.  T h i s rock I s s t r o n g l y j o i n t e d v e r t i c a l l y «nd weat h e r e i n t o r n A n o a a , wed^e-en^eC ricVe--, e a r >o . : t e d &y 11 nen  of rh'-rp  > 1 . n i d e s ..•• *d  lofty  tower-shaped p e a k s * The p i l l a r e d c h a r a c t e r o f the r a t i o n i s so remarkable t h s t the p r o s p e c t o r s n e v e .riven i t t h e oaas© o f t h e t o A s t o n e country. The ares i s d raised by tfere© r l v o r syite.nst i n the west, the s o u t h f l o w i n g C h a o d l n d u S i v o r s y s t e m w i t h i t s i s a l n westward f l o w i n g t r i b u t a r i e s , t h e T a s b s t o a o a n d k i t t l e Twelve m i l e  R i v e r s j l a t h e e a s t , the north JClondike  liver  ay s t e a l and i n t h e s»o.rih. t h e Blackstoo® E l v e r ay s t o a t * t  The O g l i v i e w r a n i a l n s d i s p l a y o o r t a i d o r a b l e e v i d ence o f g i a e l a t i o n *  Main v n l l e y s have U-shaped c r o s s -  profile© and the u p - . - e r m o s t - p a r t s o f t r i b u t a r y geoerelly  hold  termineto  tut-nn.  co-xiifion f e ' i t a r c s ,  valleys  I n g l a c i a l c l r a . u a a, w h i ^ h c o s a a o n l y  -.rotes,  uor-'«, «: id a • • r t n - :  v.t i l e y c a r e  ,>a^ii I c e pa tones e t i l l occupy a few  o f the s i r ; a c a on t ;.e norto. nX< e s o f  , - a per ^e .ID: t h o f  '* S y e n i t e  Lakes'*,*  aouatalns  Valley g l a c i e r s  and occupied  gle. e l a t i o n * into  streams  older valley  only a short  (Green a n d R o d d l c k  Since  1962)..  a n d canyons  The work c o n s i s t e d  mm  In t h e T o m b s t o n e a r e a  mad® by M c C o n n e l i (1903) a n d C o e k f l e l d  were  (1916 a n d 1929).  o f r e c o n a o l t e r l n g th® c o u n t r y a n d  examining a l n e r a l d e p o s i t s * mapred th® area a t a s c a l e  Green and Roddick of 1 i n c h t o 4 a i l e s  known as O p e r a t i o n O g i l v t e .  (1961) s a p p e d  dlataoee  oottoms.  Early i n v e s t i g a t i o n s  project  #  have cut narrow t r e n c h e s  PREVIOUS QmimiQAL  a  f r o m tae  t h e v a l l e y s o f t h e North K l o n d i k e  a n d ' Chandindu R i v e r s b a t e x t e n d e d beyond t h e m o u n t a i n s  emerged  t h e a u r f l e t a l geology*  (1961)  a s p a r t of  Vernon a n d Hughes Teapelman-fflult  (1964 a n d 1965) c a r r i e d out m a p p i n g i n t h e T o m b s t o n e :uaparea  at a scale  o f 1 Inch t o 1/2 ; a i l e f o r e v e n t u a l  a t i o n at 1 I n c h to 1 a i l © *  The p r e s e n t  public-  f i e l d work w a s  carried  o u t by t h e w r i t e r d u r i n g t h e months o f J u l y a n d  August,  1964, while employed by the G e o l o g i c a l  Canada,  Survey o f  AQ.i * ) <LXD3 -11 .13  the w r i t e r was ably a s s i s t e d i n the f i e l d by l a t e Mr,  Beanie  the  Lastiuka.  Thanhs are dm encouragement and  to Mr. .D, J . Temp-el3«ar<- 1 u.lt f o r  s t i m u l a t i n g d i s c u s s i o n during the coars©  o f f i e l d wor* . and to Mr. E, Montgomery f o r preps r a t i o s o f t  t h i n sections* 1 wish to expresa a p w c l a l a p p r e c i a t i o n t o P r o f e s s o r S, 0 ,  MeTgtggart, o f the U n i v e r s i t y o f B r i t i s h , ,  Columbia, under whose supervision t h i s t h e s i s was  written*  Rocks o f S i l u r i a n &ge ooeupy the northwestern pooket)*  (Teapel.nnn-Al.ait,  1965}  c o m e r of the map-area (see map In  these rocks oonalst mainly of  interbedded  black cr.erto and a r g l l l l t e a and are here about 1 0 G 0 foet thick. In. the northeastern and southwestern p a r t s of th® mac-area, the black cherts and a r g i i i l t e s are o v e r l a i n confor-aauly by a s e r i e s c o n s i s t i n g of black shale, a r g i l l l t e , s l a t e , p h y l l i t e a^d t h i n bedded q u a r t s i t e of J u r a s s i c age. A oontlnous t h i n bed of limestone, which contains  'iddle  Permian f o s s i l s , marks the contact between these two  rook  units. Da rk blue-grey to grey qua rt;' i t e o f Lower Cretaceous  age  (Tetapelaan-Sluit, personal communication)«  occupies the southeastern part ©f th© map  area,.  *iusrtz-  i t e s > are massive and o b a r a c t e r l s t l o a l l y have t h i n partings of s l a t e and p h y l l i t e (commonly g r s p h i t i c ) between t h i c k e r beds*  The. maximus thickness of t h i s u n i t has been  est Lias ted to be 45,000 feet (Green and Roddick, 1962)* Resent work, however, suggests that the roc its are Isoo l i n a l l y folded with steeply dipping a x i a l plc.ies and that  1 the t r u e Va.iokne.sa i s l a t h e order o f 5000 f e e t , contact  The  between t h e q u a r t s i t e and th® u n d e r l y i n g rooka " l a  c o n s i d e r e d t o be a g e n t l y s o u t h e a s t - d i p p i n g t h r u s t  surface  upon which t h e quartzit® has aoved t o the northwest.* (Teajpeliaun-Kluit, 1965).  Two l a r g e s t o c k s t h a t i n t r u d e th® sedimentary and aetamorphia roc&e I n t h e Tombstone map-area range i n composition  froxn d l o r i t e t o s y e n i t e .  Textures o f these  rocks range f r o a f i n e - g r a i n e d end equlgranulor, grained  and p o r p h y r i t i c .  t o coarse-  f o l i a t i o n i n th© rock i a  shown by a l l l a m e n t o f t a b u l a r f e l d s p a r pbenocrysts. I n c l u s i o n s a r e re.ro.  Age of t h e g r a n i t i c  r o c k s I s not  known, though they a r e thought to be co;ate.tiparsneoue w i t h g r a n i t i c rooks f a r to the southeast which have been dated as Middle Cretaceous (Green and 'Roddick, 1962) .  1  Tempelman-Kluit,  personal  communication  DESCRIPTION  Mount Brenner i a the highest peak o f the moat northern g r a n i t i c body i n the To.&b stone asp-orea*  In  t h i s t h e s i s the g r a n i t i c body i s referred, t o as th© Mount Brenner stocs*  Th©  i n t r u s i o n o^eupies s roughly  e l l i p t i c a l area w i t h i t s lo/ig a x i s t r e n d i n g northeastward. I t crops out over an area o f about 1? square mllee and e x h i b i t s a more or less, concentric arrangement o f i t s various subdivisions* A segment of auglt#-*blotlte laonaonite l i e s around the western p a r t of the e t o e W  A orescent*  shaped* narrow band o f <aonsonlte porphyry l i e s a l o n g the inner contact of t h i s segtse-it*  Inward from t h i a band,  and f o r c i n g s broad i r r e g u l a r r i n g , i s a body of p o r p h y r i t l c hornblende aonzonite, composed a s l a l y of pink  t h e c e n t r a l p s r t o f the stocfc i s quarts aoBzo.-ilt* porphyry.  A  body of a p l l t e l i e s near the eastern side of the pioK quartz .wnzonite porphyry.  The cont e t a between each  of the rock 'types ere In aost places g r a d e t i o n a l , although the nonsonite porphyry l o c a l l y shows a © h a r p contact age i n s t a u g i t e * b i o t i t e moosonlte.  intrusive (4odes of  each rock type are shown 1 B f a b l e I* Dykes are not common w i t h i n the stock and very few of theia were found l a the adjacent country rocks,  j  s e r i e s of dykes i s found i n th© east c e n t r a l hart of th© pink quartz monzonite porphyry  near where the p o r p h y r i t l c  hornblende mon*oiite protrudes i n t o the core.  In t h i s  area a p l i t e dykes, that range frota a few inches to 20 fes-t t h i c k , follow steeply d i p p i n g north to northeast t r e n d i n g joints*  Dykes of pinfc quartz won&onlte porphyry  J o i n t s i n pink quarts aonmoatt© porphyry the same coaponiton and t e x t u r e .  of  fill  approximately  So dykes were traced  outward into the surrounding country rocks*  Sear the  southeastern contact of the stock, however* & few s i l l s of p o r p h y r i t l c hornblende s ode s i t e , that range from 0*5 feet wide intrude the shale.  t o 15  In the same ares a f e l d -  spar porphyry  dyke cuts across the f o l i a t i o n i n black  phyllite,  Euhadrsl, t h i n * t a b u l a r potash f e l d s p a r  pbenoorysts are a l i g n e d p a r a l l e l to walla of the dyke to give the rock a d i s t i n c t  foliation.  TABLE MODAL A N A L Y S E S  OF  GRANITIC  MOUNT B R E N N E R Rock  Type  Specimen  Auglte - B l o t l t e  Number  1  2  42.5  34.6  3.8  4uartz Biotito  5  41  38.8  42.6  27.1  20.2  25.5  23.6  26.4  7.2  1.2  2,6  27.2  17.5  16 4  15.6  5.1  0.7  4.2  15.3  13.5  11.5  10.7  Feldspar  Hornblende  tr.  Auglte  tr.  Muscovite(sericite)  tr.  Total  3  4  Plagloelase Potash  Accessories  12.7  0  1.0  0.9  2.0  1.6  2.7  0.8  0.9  tr.  0.2  1.1  Epidote  tr.  0.2  0.8  Allanite Sphene tr.  Garnet  tr.  0.1  tr.  Chlorite  tr.  Opaque  tr.  Average P e r c e n t a g e An I n p l a g i o c l a s e  43  •atlo o f -feiasp- r :  t  b j l  c  tr.  0.14  tr.  tr.  ——-  tr.  tr. tr.  1.6  tr.  tr.  0.3  tr.  1-1.2  ^ o d e s i r e p l o t t e r . 5; F i L .--e z o f s p e c i m e n s iz, s. . v*n lis * l . r e  tr.  quartz Monzonite Porphyry  Aplite  , .9  10  11  12  13  14  KornblendeBiotite Fyroxenite 15  39. S  50  24,1  35.7  29  39.2  34.9  34.3  38.0  2.9  43.0  31.7  56.8  43.6  50.6  36.7  41.9  48.6  30.1  3.4  1.4  2.3  1.0  5.3  10  13.8  13.7  11.3  27.5  —  1.0  1.2  0.2  0.5  12.2  7.9  9.3  0.3  0.2  0.5  —  7.6  11.9  14.5  6.5  1.0  0.4  .  1.7 tr.  .  0.3  2.1  0.3  0.4  0.2  0.3  0.3  0.2  0.1  0.2  tr.  0.5 tr.  tr.  tr.  tr. tr.  tr.  tr.  tr.  0.1  0.4  0.2 .  0.9  tr.  7.9  2.7  0.5  tr.  25.6  56.1  tr.  1.0  tr.  tr.  5.5  . 3.0  tr. tr.  6.7  0.1  tr.  2.0  tr,  tr.  0.4  1.1  ___  0.1  44  j 1:11 u.iasej  8  Pink  tr.  1.6 tr.  tr.  tr.  tr.  tr.  0.2  4.1 2.0  tr. tr.  tr.  ——  Calcite  1.5  7  6  tr.  0.1  Zircon  Porrhyrltic Hornblende Monzonite  Monzonite Porphyry  —  ROCKS  STOCK  1.0  Apatite  Myrmeklte  1  Monzonite  I  41  0.3  54  l-sl.5  1.0  tr.  0.14  '  1.4  1.5 50  lii.3  tr.  tr.  1.7  ' 0.7  1.2  0.3  0,3  0.7  0.4  tr.  tr.  tr.  tr.  2.7  1.0  tr.  tr.  tr.  1.5  0.3  33  31  45  27  32  1:0.9  1:1,6  1:0.4  1:0.8  1:0.5  0.3 28  1:1  0.5  0.3  0.3  0,8  1  0.3  tr.  26  30  1:0.8  1:0.7  0.2  .  28  1:1  3.o 45  1:2.8  QUARTZ  J4  '13 10  L  .12  POTASH FELDSPAR  PLAGIOCLASE LEGEND A  Aplite  •  Pink quartz  monzonite  +  Porphyritic  a  Monzonite porphyry  hornblende monzonite  o Augite biotite Figure 2.  porphyry  monzonite  Modes of specimens from Mount Brenner  Stock.  Numbers refer to specimens in Table I and In Figure  4.  12 •70 • 60 . 50 Ld H X LU t— W CC <  •40  o _]  5 °2 _  .30  ? i:  O  LU ^ .  • 20 - 10  a  . 0 -30  N H CC  <  -20  o t— o z . z 0  Q:  - 10  w  ce ui  - 0  0.  -90  •15  -80 -70  _i <  -60  cc ^  ?o  -50  oz -40 3* <  cc <T H  2»  UI  -30  a.  h-20 .6  10 13 14 Aplite  10 12  Pink quartz  Porphyritic  monzonite  hornblende  porphyry  monzonite  • 0 Monzonite  Augite biotite  porphyry  monzonite  APPROXIMATE AVERAGE DISTANCE FROM OF EACH ROCK T Y P E Figure 3.  BOUNDARY  Variation in composition of granitic rocks with respect to distance from the margin of each rock type. Location of specimens shown in Figure 7.  -4  13 ?*ETiiOGEAFHY  The a u g i t e - b i o t l t e ular  aegsnent a r o u n d the w e s t e r n  of the Iferoua lies  istociu  A zone  biotite- quartz  along the  grades  into  of  diorite  outer contest  that  alignment  of  h«s  forms hexagonal  1 to  end 200 f e e t f r o m t h e  grades from f i n e occurs  b o t h as  The p l a t y minute  (i) ,  crystals*  the  texture  (4)  fractures A zone  up t o 2 c * .  i n the  rock  The  of dark-green,  of  biotite  tale  rook,  biotite  aggregates*  In diameter, l i e  (pi-. I  zone  between  and t h e  h e x a g o n a l p l a t e s and a a p l a t y  wide,  trachytoid  2 mau i n d i a m e t e r *  to medium-grained  aggregates,  foot  monsBonlte  f o l i a t i o n shown by  contact  garnet-  This  sugite-biotite  irreg-  borders  fiae-graloed,  l e s s than 1  1  plaKlocla.se  plates  f o r m a an  with hornfels.  a distinct  euhedral  unit  and southwestern  darlr.-grey,  grey, f i n e - g r a i n e d  (2 a n d 3)  150  aonzonit©  along  A),  medium-grained,  hornolende-  1 Tables  Nuaibere I n p a r e - . i t h e s & s r e f e r t o a p e c i a e a s I a n d I I a n d i n r - l g u r e s 2, 3 , an<^ 7 .  in  Jrlate  1  biotit© pyr-oxenite  (15),  f e e t wide and at l e a s t  1^0  2000  f e e t l o n e , l i e s along the contact between t h e u u g i t e - b l o t i t e monzonite and the monzonite porphyry*  'The c o n t a c t s  between t h i s r o c k and the surrounding rocks a r e g r a d a t i o n a l over a d i s t a n c e o f about 5 f e e t * In t h i n s e c t i o n , p l a g l o c l a s e from near th® o u t e r oontuot  d i s p l a y s o s c i l l a t o r y zoning and broad, e u h e d r a l ,  sausaurltized  oores  ( p i , II/*) • whereas p l a g l o c l a s e from th©  main p e r t o f t h i s u n i t d i s p l a y s normal zoning and i s r e l a -  t i v e l y unaltered.  The s u b - t r a c h y t l c a l l y s l i c e d  gr&ias  commonly ar© bent ( p i , I I B ) and show s t r e s s twinning, (curved  poly s y n t h e t i c twin l a m e l l a e ) *  %rmeklte  occurs  along some c o n t a c t s between p l a g l o c l a s e sod o r t h o e l a s e . Weakly pe r t h i t l c  (27^* 52 degrees) i s I n t e r -  orttiools.se  s t i t i a l t o p l S j / i o c l s s e and a u g l t e , o r e n c l o s e s and p a r t l y broad zone o f randomly  replaces plaglocl'.se,  orient-  ated i n c l u s i o n s o f p l a g l o c l a s e , a p a t i t e , z l r o n , and b i o t l t e occurs i n the cores o f some o r t b o c l e s e g r a i n s * b i o t i t e ( p i . IIIA) has the f o l l o w i n g p l e o c h r o l c X, pale t a n t o y e l l o w i s h - t a n ;  i. rovin t o Cr.rk r e d d i s u - b r o w n , ilron  inclusions  :  ache-Tie i  Y, medium brown; Z, dark  Bent c l e .vyges are common,  produce c;-. rk brown p l e o c h r o l c h«los.  Green h o r n b l e n d e commonly  subhe'. m l g r a i n s  Brown  forms rima  o f fU-^lte ( p i . H I E ) .  round s^nhefral t o  d o r n b l v ode *md  rlate  11  Zoned, t w i n n e d p l & g i o c l a s e w i t h b r o a d a l t e r e d c o r e I n b i o t i t e q u a r t z d i o r i t e (Grossed a i c o l 3 X 10)  Bent p l a g l o c l a B e w i t h s t r e s s t w i n n i n g i n a u g l t e b i o t i t e m o n z o n i t e ( G r o s s e d n i c o l s X 10)  15 b i o t i t e f o r i s.aall patches turou^hout mottled texture,  augite g i v i n g I t a  Auglte h a s en o p t i c rngle (2V ) o f Z  60*2 degrees and an e x t i n c t i o n a n ^ l e (Z o) o f 46 decrees. A  4uartz occupies i r r e g u l a r areas between almost a l l other minerals.  Fractured g r a i n a and undulatory  ©re very c-xamon*  extinction  I n the gernetlferous b i o t i t e qu r t z  d l ^ r i t e , quartz commonly contains i n c l u s i o n s o f sub rounded plagioclase laths. Pale green, weakly p l e o c h r o i c d l o p s i d l c - a u g l t e o f the  hornblende-blotlte pyroxenlte has aa o p t i c aagle (2V ) Z  of obout 60 degrees and an e x t i n c t i o n angle ( Z c ) of 45 A  degrt j.©.  .rlaglooluee, potash f e l d s p a r &oc;  I n t e r s t i t i a l to the suhedral pyroxene, has bent clci-. vft&es*  i o t l t c are  b i o t i t e commonly  Green hornblec.de a .id b i o t i t e form  irreg.ilur patches t h a t have r e p l a c e d pyroxene.  A.  Texture of b i o t i t e (31) i n a u g i t e - b i o t l t e monzonite (Croeaed n l o o l e X. 10)  16 'Monzonite,.Porphyry  Monzonite porphyry  (Plate IV) forms an a r c u a t e  bond, 500 t o 1000 f e e t wide, which lie® between b t o t i t e euglte monsonlte and aediua-grained hornblende monzonite. Lenac-s of t i l l s rook also occur a t the c o n t a c t between t h e p o r p h y r i t l c monzonite and. t h e pink q u a r t z monzonite prophyry.  Pel© grey, eubedral, t a b u l a r phanocrysts o f  orthoelase  mtcroperthite,-. that avers g,« 6 cm, by 2 l/2 ca,  by 1 1/2 cm,, are s e t i n a medium- to coarse-grained trachytold monzonite m a t r i x .  L u s t r o u s , dark-greea  hornblende prisms are i n t e r s t i t i a l t o pla.^loeisse and orthoelase.  Streaks end elongated c l o t s of hornblende  c r y s t a l s l o c o l l y give the rook a weak l i n e a t l o n . lenses of monxonlt© porphyry  The  d i f f e r from the main band i n  th:;t they have a c o a r s e - g r a i n e d m a t r i x and the alignment of c r s t a l s i s i n d i s t i n c t . In t h i n s e c t i o n , the o r t h o e l a s e .aicropert.il te ph«nocrysts have a euhedral general form, though th© c r y s t a l boundaries ?<re minutely i r r e g u l a r and the outermost  portions  p a r t l y o r wholly enclose p l a g l o c l a s e , pyroxene and opaque minerals o f th© m a t r i x * tinguished  O s c i l l a t o r y zoning, i s d i s -  by v a r i a b l e concentrations  d i f f e r e n c e In e x t i n c t i o n angles,  o f pert.;ite  in  & by  A &rsat v a r i e t y o f  H.  Vein p e r t h l t e In phenocryst o f monzonite porphyry (Orooeed n l c o l e X 45)  17  nicroperthitle textures that rang® from very f i n e f i l m s or strings, to veins or networks of veins (braid perthlte) and  I r r e g u l a r patches, (Emmons, 1953), may  within a s i n g l e phenocryst.  be  displayed  film p e r t h l t e ( p i , V h)  i s made up of a .multitude  of lenaoid pl&bioclase  t i o n s , averaging 0 , 0 5 ma,  wide by 0 , 2 mm.  orientated perpendicular orthelase*  to the  segrega-  long, t h a t are  ( 0 1 0 ) cleavage i n  In some places, the lenc-es coalesce to form  long narrow " s t r i n g s " (pi* ¥ 3 ) ,  Pla. l o c i . ae i n vein  p e r t h l t e ( p i * IV B) occurs as long, narrow, l o c a l l y branching forms, that are generally elongated d i r e c t i o n aa f i l m and the veins, and  string perthlte,  i n the saae Orthelase  between  <••«round the edges of oquant plagiocl.-se grains  ( p i , V f,) , i s almost non-pertnitic,  The v.:. ry f i n e ,  regularly orientated perthitic Intergrowths, the absence of c r o s s - c u t t i n g r e l a t i o n s h i p s , and  the gradatiom.l nature of  the various types of p e r t h l t e , suggest an absolution o r i g i n . Inclusions In the orthelase phenocryats are mainly pi a;.?, loc la se but auglte, hornblende, b l o t l t e , apatite and  o c c a s i o n a l l y orthoelase are also present.  Included  p l a g l o c l a s e c r y s t a l s are oriented most commonly with  their  long di.uen - ions parallel to the ( 0 1 0 ) plane of orthocln Be but fllao l i e p a r a l l e l to ol er cryst I faces.  The  1 -rger c r y s t a l s , that range in length from 1 to 2 am.,  are  ilato V  Non-partnltlc a m i around perlpherlaa of clagloclaae gralne In orthoclaaa o f naonionita porphyry (Croaaed n l e o l a X 4 5 )  euhedral whereas s m a l l e r c r y s t a l s are subhe^ral t o anhedral and have irregul-.-r boundaries.  Alaaoat a l l p l a g l o c l a s e  Inclusions cisplay weak, normal z o n i n g and poly synthetic twinning.  SubUedral  Inclusions of weakly zoned, sparsely  p e r t h l t i c orthoelase occur Just within the b o r d e r s o f the p h e n o c r y s t and are o r i e n t a t e d p a r a l l e l t o a c r y s t a l face of t h e phenocryst*  The included orthoelase i s not i n  o p t i c a l c o n t i n u i t y with the orthoelase host* Orthoelase of the matrix occurs as randomly o r i e n t a t e d p e r t h l t i c patches, up to 2,3 tarn* In u l & j i e t e r , that are i n t e r s t i t i a l t o p l a g l o c l a s e .  C r y s t a l s of subheJr.-ii,  wee:<ly z o n e d , p l a g l o c l a s e , t h a t range In s i z e from 0.3 x 0*5 <B». to 2 x 4 , 5 twinning,  m*,  generally are bent and. show s t r e s s  My r .a© k i t e I s common along cont-.-ets between  plagiocluse oad o r t h o e l a s e ,  .-.ug.ite o c c u r s as subbedral  grains riai.ned by green hornblende,  o r s s s m a l l cores within  l a r g e anhedral g r a i n s o f h o r n b l e n d e ,  f o r p h y r l t 1 c H o r n b l e n d e ;tonaonlte  P o r p h y r i t l c h o r n b l e n d e .aonzonite forma an i r r e g u l a r r i n g with an e x t e n s i o n i n t o t h e c e n t r a l c o r e on the e a s t e r n s i d e . trashy t o l d t e x t u r e .  T h i s l i g h t g r e y roc'* has a d i s t i n c t Pole grey orthoelase for.as euheoral,  tabul r pnenocrysts that a v e r a g e 2 to 3 .aa* wide and 10 to  15 fflffl. l o n g , hornblende to  Light  orthoelase  contact  of this  texture  formed  p l a g l o c l a s e a n d c l a c k , euhedral  aediua-grftined m a t r i x a r e i n t e r s t i t i a l  o f the  the l a r g e r  grey  unit,  crystals.  the r o c k  by c l u s t e r s ,  Hear the i n n e r  has a g l o m e r o p o r p h y r i t i c  averaging  2 to  m e t e r , o f anhedral t o subhedral h o r n b l e n d e Orthoelase and  the  phenocrysts  here  have  very  5  orthoelase  crystals,  irregular outlines  thin  s e c t i o n , weakly  phenocrysts  perthltic,  subhedral  have sainutely I r r e g u l a r  that p a r t l y e n c l o s e minerals o f the matrix* shows weak o s c i l l a t o r y  trie  t h e zones  range  aonlng, a n d t h e o p t i c  from  50 t o 70  degrees  c r y s t a l s to 55 d e g r e e s in t h e r l a s *  borders  VI A),  class  O r t no angles  (2V ) X  i n the c e n t e r o f  Some pheno-  c r y s t s have c o r r o d e d orthoelase c r y s t a l s I n t h e i r (Plate  dia-  traehytold t e x t u r e i s i n d i s t i n c t . In  of  in  -MB,  cores  Included p l a g l o c l a s e l a t n s a r e o r i e n t -  ated w i t h l o n g d i m e n s i o n s p a r a l l e l t o t h e c r y s t a l boundhost,  o f the  zoned  pla, 1 j c l a s e o f t h e matrix b e a r s t i n a l b l t l o  where  in c o n t a c t  rare.  Euhedral  to subbedral normally  aries  ;  Bent  with  orthoelase,  plagloclase crystals,  yraekit©  mantles  i s not  that show polysyn-  t h e t i c s t r e s s twinning, are most common i n specimens f r o m near  tne o u t e r c o n t a c t o f t h i s u n i t ,  hi-e t h e f o l l o w i n g p i c o e h r o l e  scheme;  Zoned  hornblende  Plate  VI  C o r r o d e d o r t h o c l a e e c r y s t a l (Or) I n c o r e o f a n o r t h o c l a s e phenocryst (Pa). (Crossed n i c o l s X 10)  P l a g l o c l a s e i n c l u s i o n s i n o r t h o c l a s e of the g l o a e r o p o r p h y r i t i c phase o f t h e p o r p h y r l t l c hornblende monzonite ^Crossed n i c o l s X 10)  Gores  p a l e brown  aims  p a l e green- brownish-green i s n brown  gree•:.  tsh-brown  o l i v e green bluish-green  E x t i n c t i o n a n g l e s (?. e) a r e a u o u t 25 d e c r e e s i n th© core and A  30 d e g r e e s  i n th© r i m s .  P y r o x e n e occurs as rounded cores  w i t h i n hornblende o r a s subbedre1 g r a i n s w l t a r e a c t i o n rims of  hornblende,  -v.ua.rtz, showing u n d u l s t o r y  extinction,  f i l l s a n g u l a r s p a c e s between a l l o f t h e o t h e r m i n e r a l s . In of  t h i n s e c t i o n s o f t h e g l o m e r o p o r p h y r i t i c pnas©  t o i s u n i t , e l o n g a t e d , a n h e o r a l o r t h o c l a s e g r a i n s have a  "spongy" t e x t u r e t h a t I s due t o abundant i n c l u s i o n s o f p l a g i o c l a s e ( p i , VIA), see  csJ?  O r i e n t a t i o n o f the i n c l u s i o n s o f  t o be r e l a t e d to the s u b t r a c h y t i c t e x t u r e o f the r o c t  r a t h e r than to any s p e c i f i c c r y s t e n o g r a p h i c d i r e c t i o n i n orthoclase,  Subhedral p l a g i o c l a s e phenoeryats  (A-O^^Q)  have I r r e g u l a r , t h i n , a l b l t l c borders where i n c o n t a c t w i t h orthoclase.  P l a g i o c l a s e o f t h e m n t r i x ( A n ^ ) has a  s i m i l a r t e x t u r e , b u t where enclosed i n o r t h o c l a s e the g r a i n s a r e rounded and have i n d i s t i n c t  boundaries.  Hornclenrie has formed between t h e l o r g e r p l a g i o c l ' j s e c r y s t a l s and h a s p o l ' t l l l t l c a l l y  e n c l o s e d t h e ©mailer - p l a g i o -  c l a s e g r - i n s , a p a t i t e , m a g n e t i t e , and c l i n o p y r o x e n e .  21 Pink .Quartz J'ori&onlte Porphyry  Pink q u a r t z monzonite porphyry the gtook,  forms the core of  P r e f e r r e d o r i e n t a t i o n o f pinfc, t h i c k  t a b u l a r o r t h o e l a s e phenocrysts, t h a t range from 8 t o 12 l o n g and from 3 to 5 mm. foliation. blende and  wide, g i v e s the rock an  mm.  indistinct  F i n e - t o medium-grained pi a ..loolase, bornq u i r t a are i n t e r s t i t i a l t o the o r t h o e l a s e  phenocrysts*  Lustrous,  dark green  euhedral prisms up to 3mm. In  hornblende  forms  long.  thin section, mieroperthitlc ortnocl^se  phenocrysts ( p i * V I I I ) have a euhedral general haoit, though t h e i r boundaries are minutely i r r e g u l a r .  All  phenocrysts d i s p l a y d e l l c o t e but obvious o s c i l l a t o r y Boning. zoned.  " b i n outer margins of the c r y s t a l s are not Optic angles ( 2 V ) of adjacent zones may X  vary  as much n s 10 degrees, end the v a r i a t i o n o f angles measured w i t h i n a s i n g l e c r y s t a l i s from 52 degrees to 65 The lowest v a l u e ( ? V  x  ^  A  degrees.  ') was obtained In the unzoned  outer m r g l n of a c r y s t a l .  Euhedral t o subhedral  plagloclase i n c l u s i o n s , g e n e r a l l y l e s s than 1 ma. l o n g , are o r i ' -titted most commonly w i t h s i d e pinecolds to the (010)  plane o f o r t h o c l e s e but are a l s o  to other c r y s t a l faces ( p i . IX A ) .  parallel  parallel  3ome pla;.ioele»ee  Plate V i i  B.  l e r t n i t l o orthoclase phenocryst with inclusions of euhedral, unstrained quartz ('4) (Crossed nlcole X 10)  A«  •  booing and alignment of plagloclase inclusion* in orthoelase phenoeryst of pink q u a r t s acnzonit* porphyry ( C r o s s e d nlcola % 5)  I a r t o f phsnocryst i n plate A a t h i g h e r signification (Croaked nlcols A 1 0 )  22 c r y s t a l s are only p a r t l y  Included ( p i . IX ~) o r a r e l y i n g  a t t h e edge o f t h e o r t h o c l a s e phenocrysts ag i f they were ettac.-.ed t o t h e crystal face o f the phenocrysts ( p i . I X B}« Combination t w i n n i n g (Ross, 1957)# i n i n c l u d e d plagioclase c r y s t a l s i a common ( p i . X A ) .  Inclusions show weak  normal zoning with compositions ranging from Aa-^ i n cores to r.a . In' r i m s . ni  CO  Textures of o r t h o c l a s e phenocryste sug.-est growth i n a f l u i d magma. we£3  The o r i e n t a t i o n o f inclusions  probably e f f e c t e d by a process ( d e s c r i b e d by Hlbbard,  1955) o f c o n t i n u a l attachment o f p l a g i o c l a s e c r y s t a l s a t the sides o f growing .-rthoclase c r y s t a l s .  Orthoclase  formed early enough t o b© aligned by magmatlc f l o w , and the growth o f paenocry ats continued u n t i l t h e l a s t stages of s o l i d i f i c a t i o n o f the magma.  The irregular borders  on t a e generally euhedral crystals, and t h e presence o f l a t e forating minerals, such as quarts and hornblende. Included In the o u t e r margins of phenocryste, are evidence o f t h e l a t e r stages o f growth. P l a g i o c l a s e o f the m a t r i x where in contact w i t h p e r t . i l t o , has a well developed r i m of myraeklte ( p i , X E ) , Complex o s c i l l a t o r y zoning Is common with zones t e n d i n g t o be more a l b l t l c toward t h e periphery ( p i , XI h) *  Plate  A.  Orientation of plagloclase inclusions in orthoelase phenocryst of the pink quartz m o n z o n i t e p o r p h y r y ( C r o s s e d nicols X 5 )  B.  P l a g l o c l a s e c r y s t a l s w i t h i n and p a r t l y i n c l u d e d i n the margin o f orthoelase phenocryst (Grossed n i c o l s X 5)  Plate A  23 Composition ranges from An,,-* t o Aa»>,» two formsj  Quartz occurs i n  (l) i r r e g u l a r , i n t e r s t i t i a l ,  w i t h undulatory  f r a c t u r e d grains  e x t i n c t i o n ; and ( 2 ) e u h e d r a l , unfractured  grains, with even e x t i n c t i o n , t h a t are i n c l u d e d i n the unzoned margins o f p e r t h l t e phenocrysts ( p i . V I I 13), Zoned green hornblende t h a t has a pleochrolo scheme t h a t i s very s i m i l a r t o hornblende i n p o r p h y r i t l c hornblende monzonite (see p.20 )  f  p o l k i l l t l c a l l y encloses i n c l u s i o n s  o f p l a g l o c l a s e , a u g l t e , a p a t i t e , z i r c o n , and magnetite* Trace amounts o f auglte occurs e i t h e r as small grains rimmed by hornblende or a s t i n y grains p a l k i l t I c a l l y enclosed i n hornblende* h l l s n i t e , spheae and z i r c o n are very common accessory  miner-Is i n t h i s r o c k ,  A l l a n I t e f-<ro»B euhedral  prismatic c r y s t a l s t h a t are pleochrolo from dark brown t o 11..hit brown, some of which are twinned *  An i s o t r o p i c ,  yellow m i n e r a l , having very high r e l i e f , surrounded by H pleochrolo halo and r a d i a t i n g fractures i n hornblende, and w i t h a n- rrow rim of e p i d o t e , h/.s been t e n t a t i v e l y i d e n t i f i e BS a:.l- i t e In the me t a b l e t state ( p i , XI .-)» a c a r r a c t c r i o t l c acute rhombic h a b i t .  Sphene has  Zircon occurs as  minute grains that produce pleochrolo h^los i n hornblende*  A.  b.  O s c i l l a t o r y zoning i n p l a g l o c l a s e of pink q u a r t z monzonite porphyry (Grossed n i c o l s X 1 0 )  A l l a n l t e i n naetamict s t a t e ( ? ) i n c l u d e d In hornblende ( l i b ) . Note p l e o c h r o l o h a l o and r a d i a t i n g f r a c t u r e s In hornblende ( P l a n e l i g h t X 10)  24 Apllte  A b o d y o f aplit© occupies a s m a l l a r e a  eouthcenteral part  o f the stock.  fine-grained  rock  some p l a c e s ,  the rock  feldcper than  of  contains  plegioelaee,  orthoclase  i s  grey,  partly altered  In potash less  allotrlo-  interlocking  grains  Non-perthitic  quartz.  to  (generally  s e c t i o n , an  by  l a formed  orthoclase and  shreds throughout the  amount  In thin  texture  a pale  sparsely distributed  and a s m a l l  2/S) o f h o r n b l e n d e .  morphlc granular  is  monzonite composition.  o f quartz  phenocrysts  This  in the  sericite which  minute  forms  crystals.  Inclusions  Inclusions tinct  types  xenoliths once  a r e found;  and  evident.  inclusions  (2)  The  dar't-grey,  (l)plainly  stock.  Two  recognizable  second  type  dis-  wall  i s not  rock  at  makes u p t h e m a j o r i t y  of  in t h e stock..  o f country  uted in a u g l t e - b i o t i t e with  this  d a r k x e n o l i t h s whose o r i g i n  Xenoliths  unit  in  are rare  homfelrlc hor . f e l s i c  rock  monzonite  wall  roeKs.  roc<s  with  av . s p a r s e l y  near  r  the contact  The angular  distribof this  i n c l u s i o n s are to  subangul- r ,  25 bloekly  ant" y l a c b y  Inches  to  forms,  several  tbr.t range  Bedding In Inclusions  feet*  coaaonly p a r a l l e l  to bedding  clusions have obviously been elonga'.e i n c l u s i o n s l i e  of the  the  rock  bornfwis I n c l u s i o n s ' t o  little  that  these  rotated.  are  Inclusions  not In-  i n the  foliation  the  ie  Occasionally, •  between  sharp*  of  few  and t h e  inclusions  Proximity  similar hornfels  bedding, and forms  doubt  rocks,  Contacts  .mrrounding. host  relict  In wall  p a r a l l e l to  enclosing g r a n i t i c r o c k s . and  l u s l ^ e from a  wall  rocks,  Inclusions  are  leave local  xenollths of  wall rock. A large  inclusion of  l o n g a n d a maximum o f  375  eastern margin o f  stock*  side o f  the  quartzite was  not  --.ail  rotated  distributed  biotlte  rock,  or transported dark  inclusions  range  well  In s i z e  They  the  outline  3500  of  of  the  the  southern  adjacent  inclusion, therefore, i n the  vary  are  are  not  sparsely and i n  found  la  from subrounded,  roundJ* o v o i d and  probably  magma.- '  ( p i . XI1)  but  feet  w i t h i n the south-  monzonite p o r p h y r y  Hornblende m o n z o n i t e mjozjnite.  lies  about  The o u t l i n e  This  In p i n k q u a r t z  fragm-" ••.•ts t o that  feet w i d e ,  inclusion matches  The  phyritlc  the  quartzite,  about  augiteequant  fragments  c i g a r shaped  f r o m 1 o r 2 Inches t o  por-  1  foot.  phenocrysts in both in t h © inclusion (X 1)  i.ote  and  26  Elongated I n c l u s i o n s l i e p a r a l l e l t o primary i n pink quartz monzonite porphyry. generally were is  sharp but i n c l u s i o n s  occasionally  enveloped  Texture  found.  by a t h i n  varies  from  Boundaries a r e  with  diffuse  I n some  places  r i m of hornblende  sugary  to  foliation  boundaries the  crystals.  porphyroblastlc.  .Mega-  crysts  o f aubhedral t o euhedral potash f e l d s p a r  random  o r i e n t a t i o n I n a dark-green sugary In  thin  section  o f dark  (pi.  zoning,  and  of  Gores e x h i b i t  complex  XIII),  a v e r a g e 4iaa»  10 m.0. h a v e e u h e d r a l c o r e s a n d b r o a d , i r r e g u l a r , margins.  have  matrix.  inclusions  m i c r o p e r t n l t l c o r t h o c l a s e megacrysts, t h a t  blastie  inclusion  by  poifcilo-  oscillatory  contain euhedral p l a g i o c l a s e I n c l u s i o n s  some  w h i c h show c o m b i n a t i o n t w i n n i n g , t h a t ' r e o r i e n t a t e d  parallel  t o t h e (010) p l a n e  of orthoclase  host.  i r r e g u l a r un zoned m a r g i n s e n c l o s e r a n d o m l y  tro-->d  The orientated  ailo- r a l s o f t h e m a t r i x b u t do n o t e n c l o s e p l a g i o c l a s e  similar  to  zoned  t h a t found I n t h e c o r e ,  Aohedral o s o i l l a t o r i l y  p l a g i o c l a s e ph..nocrysts (•*'ng.. -^) t h a t a v e r a g e 1 am. by 3 airs,, 1-  are (pi.  c o r r o d e d a n d embayed by c o t e s h  XIV),  feldspar of the matrix  Subbedral t o enhedrel hornblende  ( p i . XIV c ) ,  t h a t r a > e s up t o 5 mm. i n l o n g e s t d i x e n s i n, c o l k l l i t i c a l l y • n c l o a e g r a i n s o f a u ^ i t e , plfigiocl*»se a n d p e r t h l t e . matrix  h-s a  The  <? r.\noble e t i c t e x t u r e made up o f i n t e r l o c k i n g  Plate X111  A.  P o r p h y r o b l a s t of p e r t h l t e matrix, bote l i n e o f t i n y I r r e g u l a r b o r d e r (Crossed  In granoblaatlo I n c l u s i o n s and n i c o l s X 5)  Plate  XIV  Photomicrograph, showing p o l k i l l t i c hornblende (Crossed nicols X 10)  grains Pale  green,  euhedral  rime  are generally  that  some o f  to ^nhedral crystals,  o f hornblende*  the area between, o f the  i n diameter.  are lees  almost  t h a n 0.2  that  o i l  which bear  reaction  latbllke cry stala  - Corroded,  ortb.oela.se,  aon-perthltic  am.  0»5  than  pleochroic dlopsidic-augite faras  weakly  plngioeleee (Angy)  cent  lag®  of  Anaadra-1,  mm* I o n s .  e n c l o s e s , c o r r o d e s , end  other ainerals makes  up  fills  45 p e r  matrix*  the  origin  o f th® d a r k  inclusions  I s not r e a d i l y  apparent because their textures »*vre developed after tuey were  enclosed i n the  reory s t a l l l z a t l u n  of the xenoliths.  megj'crysts (••'•licate oris..tailin  H o r n f e l a l e textures indicate  magma.  oscillatory  o f included  Textures  o f c o r e s of  stoning and preferred  plagioclase)  are identical,to  textures of phenoorysta i n otner rock zones of t h e ©took* fh«se large  crystals  probably are pnenocrysts that developed  pol*ilobla»tic margins a s  they  continued  t o grow d u r i n g  r e c r y s t a l l i z a t i o o f t h e inclusion, A changes that  aiagma enn re not  ^lth  whicn * i v s t n e a a z i i n e r d  o f the  Inclusions and ae>.". v b l a e  en :. oalng cry stallXsing .ma» ma,  t i  effect  similar to  Ho  11iUld,  saturated with a certain ;;»muer of a reaction series, c a n aelt  Inclusions  consist 1 i  g  of minerals fcelo ging to an  2c e a r l i e r s t ^ e of t.-.e re-.ction series (bowen, 192o,  p. 221) •  ?aus, augite would be oat of equilibrium In a ai^aw of quarts monzonite composition where bornble.-de Is the stable a-.flc mineral pb^se.  The presence of abundant  augite i n the Inclusions Indicates eltber non»equllibriu3» or, perhaps*, that the xenolith was relatively anhydrous ard impermeable, Pletributtoo of inclusions In the porphyrltio horn, lends ttonsohltc and in the pink quartz monaorjlte por'hyry but not in the au ..Ite b i o t i t e monzonite, the <;  presence af ahundnat <H »p»idio»auglte, and of otic Hintory zoning In cores of meifiorysts, auggeai  that  the inclusions  are cognote a n d w« r<* originally uuglts-biotit© monzonite, which h a  been engulfed and reoryst'-ilized by the  surround 1:.*^, .tttgao.  29  Chemical composition o f the main rock zones o f t h i s stock changes a y s t e s a t i o a l l y from the periphery o f th© stock to the core.  T h i s v a r i a t i o n i s i l l u s t r a t e d by  nodal a n a l y s e s (Table I ) , by c a l c u l a t e d cnej&lcal a n a l y s e s ( f a b l e I I ) , and by v a r i a t i o n diagrams ( f i g s . 4, 5, and 6 ) . y»odes (Table I) were calculated by the standard point-count method a v e r a g i n g 1800 p o i n t s per t h i n - s e c t i o n .  In coar-  sely p o r p h y r i t l c rocks the r a t i o o f phenocrysts to matrix was measured from hand specimens by t r a c i n g the r e s p e c t i v e areas on m i l l i m e t e r gridded t r a n s p a r e n t paper, and th© matrix composition WJS computed from thin sections. T h e o r e t i c a l chemical a n a l y s e s (Table I I ) o f the rocks warec a l c u l a t e d from, the modes.  Compositions o f f e l d s p a r s ,  quart*, a p a t i t e , sphan© and magnetite were c a l c u l a t e d from standard f o r m i l a e .  The average p l a g l o c l a s e compositions  were estimated from the ran.e of zoning and from tae r-iU-tive widths o f respective zones. blende, b l o t l t e , M u s c o v i t e , u l l a n i t e  Compositions o f hornand  dlopsldic-augite  were taken from tables of chemical c o a l y s e s , given i n Deer, iowie and Passman (1952), f o r m i n e r a l s frsm s i m i l a r rock types and with s i m i l a r o p t i c a l p r o p e r t i e s to tuose o f the aount hrenner stock.  Average values of the chealcal  30 analyses  from  diagrams  shown I n F i g u r e s : . , 4  each, r o c k  zone are  In' ices*, that  are -plotted  Figures  oh,  Ja  and  which represent These  analyses  are  5000  used  5,  t  6.  and  analyses the  the  variation  Differentiation  a g a i n s t various,  superimposed  Include  to plot  on  oxides  frequency  In W a s h i n g t o n ^  compostitiona  of  In diagrams  tables.  a l l igneous  rocks.  Go^position atically  from  following  tne  of  the  periphery  zones  rock  o f the  stock  to  varies th©  system-  core  In  the  manner.  1.  Total Iron  2.  mafic and  3.  minerals,  Anorthite c o o t e n t  «mount (fig*  4,  and  alkalies  and  9 clearly  between  rocks  Illustrate  quartz +  generally  increases  the  increase  (fig. 5 ) .  d e g r e e of  of the Houat rrenner stock and  _ ^Differentiation 'Puttie (19*0) a s t h e sum o f + kalellite.  4),  5)  Differentiation i n d i c e s  6  oxides  ( f i g . 3 and  of plegloclase  4. figures  • ue-tly  conse  ( f i g , 3).  of quartz 3,  and  magnesium, d e c r e a s e  decreases  normative  main  most  difference rocks,  I n d e x i s d e f i n e d by T h o r n t o n and t h e weight p e r c e n t a g e s o f o r t h o e l a s e + a l b i t e + nephcline + leoeltfc  31 TABLE I I  CALCULATED CHEMICAL COMPOSITION OF ROCKS Mt. B r e n n e r S t o c k  310  2  2  2°3  Fe 0 2  FeO  2  3  62.BT  57.30  53.76  .92  T10' A 1  1  3  17.11 1.29 4,45  HnO 2.50  CaO  4.18  Na 0 2  K 0 2  2  .75  17.61  18.00 -  1,62  1.74  4.64  5.32  .04  MgO  P O  .73  5  2.77  3.27  4 . 53.43 .64 19.02 1.56 5.35  5  Average 2 to 5  6  53.96  54.61  59.42  .47 18.83 1.64 4.92  .05  1.06  2.76  3.05  .15  .51 18.36  18.57  1.54 5.05  7 60.77 .15 19.92  8  9  Average  7 to 9  10  11  12  13  Average 10 t o 1 3  59.39  61.53  61,25  65.38  64.91  .38  62.15  66.22  .12  64.79  .14  .10  .16  .42  .14  .20  17.53  1 8 ,.72  17,94  16.83  19.73  16.15  17.99  18.12  14  72.38  15  43.05 1.51  16.03  7.69  .75  1.33  1.75  .72  1.02  .84  .73  2,47  2,45  ,43  .32  2.71  1.77  2.11  1.42  1.31  1.06  .03  .55  .03  .01  .02  .02  .01  1.63  1.28  1.43  .89  .95  .75  .57  .79  .13  8.14  .28  1.10  2.96  1,37  .1.58  . 5o 1.08  .27  6.81  .36  13,69  .01  .15  5.63  7.92  7.46  8.21  7.35  4.75  5.62  4.15  3.88  4.75 •  3.01  3.59  6.38  2.75  2.35  3.49  2.55  3.33  2.95  4.24  4.86  2.97  3.98  3.10  2. 32  13.20  4.23  4.54  3.69  4.25  2.16  3.02  6.16  3.22  3.65  4.48  5.80  4.60  4.03  3.53  4.71  7.91  3.96  6.04  .68  5.32  7.18  4,90  5.91  .35  .29  .16  6.61  6.15  .31  4.27  .08  3.03  .38  .25  .12  .08  .04  .19  .11  .08  .78  .06  .06  '  .43  * C h e m i c a l c o m p o s i t i o n s a r e C a l c u l a t e d f r o m modes, a n d numbers r e f e r t o s p e c i m e n s i n T a b l e I a n d i n F i g u r e s 3 a n d 4.  —  -  Figure 4 .  V a r i a t i o n diagram for.rook zones of the Mount Brenner stock. The average weight per cent of oxides of each rock zone are p l o t t e d agalnat t o t a l s i l i c a : A= a u g i t e - b i o t i t e monzonite: B = monzonite porphyry; C.= p o r p h y r l t i c hornblende monzonite; D=pink quartz monzonite porphyry; E = apllte; and F = hornblende-biotite pyroxenlte.  ro  33  100  90  80  70  60  50  40  30  20  10  0  DIFFERENTIATION INDEX Figure 5a Figure 5 . C o n t o u r e d diagrams o f o x i d e s vs. d i f f e r e n t i a t i o n index on which a n a l y s e s o f r o c k s from t h e Mount B r e n n e r s t o c k have been superimposed. Contour diagrams, which r e p r e s e n t 5 0 0 0 a n a l y s e s i n Washington's T a b l e s , a r e m o d i f i e d a f t e r T h o r n t o n and T u t t l e , I 9 6 0 , p. 6 7 ^ - 6 7 9 . A= a u g i t e - b l o t i t e monzonite, B= monzonite p o r p h y r y , C=* p o r p h y r i t l c h o r n b l e n d e monzonite, D - p i n k q u a r t z monzonite p o r p h y r y , E = a p l i t e , and F= h o r n b l e n d e - b i o t i t e p y r o x e n l t e .  100  90  80  70  60  50  40  30  DIFFERENTIATION INDEX Figure 5b  20  10  0  35  Fe " +• F e ' "  Figure 6 . T r i a n g u l a r c o m p o s i t i o n d i a g r a m on w h i c h the f o l l o w i n g data are p l o t t e d } 1. Rocks o f t h e Mount B r e n n e r s t o c k ? A = a u g i t e h l o t l t e monzonite, B = monzonite p o r p h y r y , G = p o r p h y r i t l c hornblende monzonite, D = pink q u a r t z monzonite p o r p h y r y , E = a p l l t e , F = h o r n blende-biotite pyroxenlte. 2. D a l y ' a a v e r a g e b a s a l t ( 1 ) , a n d e s l t e (2), d a o i t e (3) and r h y o l i t e (A). 3. B e l t o f c o m p o s i t i o n v a r i a t i o n o f t h e m a j o r i t y o f c a l c - a l k a l i v o l c a n i c and p l u t o n l c sequences ( a f t e r T l l l e y , 1950, p . 4 8 ) ; t h e shaded a r e a .  36 th® smooth trend towards "Petrageny's residua system"-, and the degree o f s i l i c a o v e r s a t u r a t l o n or u n d e r s a t u r a t l o n o f  each rock zone,  j  J  r.  /gr,»  JT  Contact e f f e c t s i n s l a t e s and s h a l e s on weatern and n o r t h e r n b o u n d a r i e s o f the stock extend up t o 200 f e e t from the igneous contact* contact  Immediately adjacent to the  h o r o f e l s i e t e x t u r e i a well developed and b e d d i n g in  the rock has been almost c o m p l e t e l y o b l i t e r a t e d . from the  c :<ntaot t h e h o r n f ^ l e i e  texture  to the o r i g i n a l t e x t u r e o f s h a l e s and In t h i n s e c t i o n , t e x t u r e p o r p h y r o b l a s 11 c, w i t h spongy a granoblaatio  -matrix.  gradually  'Farther ;;  lves  way  slf-te.  of the h o r n f e l s i s  p o r p h y r o b l a s t s of o o r d l e r i t e i n  Pl^ochrolc,  red-brown  olotlte  occur© along bedding planes o r as t i n y v e i n l e t a f i l l i n g fractures in homfela.  The  t y p i c a l m i n e r a l assemblage  i s quartz, plaglooiese, c o r d i e r i t e , s e r l c i t e , b i o t i t e ,  E x p e r i m e n t a l s t u d i e s by- Bo wen  (19 ;7), and  and  by  Thornton and T u t t i e (1950), demonstrate t h a t the system S I O 2 - Unf, l o i O j - ::AISIO4 I s the -:oal toward which a l l :uaga n t l c l i q u i d a move on f r a c t i o n a l c r y s t a l l i z a t i o n . Bowen (1937) has named t h i s system "Petrogeny's r e s i d u a system". ?  37  minor amounts o f p y r l t e loclase  and a e r t c i t e  Igneous  and z l r o n .  •  i n the h o m f e l s  The amount o f increases  plag-  toward  the  contact.  Contact f o r not  effects  m o r e t h a n 150  feet  Metaaorphlsm has had a immediately  adjacent  whereas q u a r t z l t e  throughout  contact  Within the  the  rock*  igneous  l a pale  contact, quart"it®  grey  contact  bleached  alony- b e d d i n g p l a n e s  recognizable  effect;  f u r t h e r f r o a the  seminated throughout zone o f  from the  "bleaching"  the  dark b l u i s h - g r e y . concentrated  in. q u a r t z Iter, a r e  Is  aureole  white,  medium t o  .--one, b l o t i t e  and f i n e l y  Fine-grained pyrlte the  to  is  dispersed Is  dis-  and was n o t e d b e y o n d  the  bleaching,  STRUCTURE  Internal  Foliation planar tion la  ia  flow  of  (010)  almost  t o 75  o  and Lineati,pn»-  r  structure, faces of  everywhere  is  Structures.  Foliation,  here  considered to  marked by t h e p r e f e r r e d  tabular feldspars.  orienta-  Its  attitude  v e r t i c a l but l o c a l v a r i a t i o n s  BO d e g r e e s a r e  common ( f i g .  present i n a l l r o c k z o n e s o f t h e  6  ).  stock  The and I t  be  of  dip  ©tructure ia  39 generally  most d i s t i n c t  The o v e r a l l ture  are  continuity  Illustrated  zonite  zonite light  biotlte  porphyry,  pattern  monzonite  i s everywhere  pistes,  the  m a t r i x has  plane of  foliation,  contact  oresent  band e x c e p t  s o u t h e r n t i p s where and f o l l o w s  hornblende monzonite the  u n i t but  generally  conforms  streaks  matrix give hornblende  it  cuts  th© t r e n d  across  i n the  i s w e l l developed  to  the  outer  structures  the mon-  i n the  the  a l i n e a r element prisma w i t h i n the  show a r a n d o m o r i e n t a t i o n .  near  rare.  clots  to the streaks  of  monand th© the  northern  gradational  the  i n the of  by  p a r a l l e l to  adjacent  contact  are  and e l o n g a t e d  the  giving  porphyritlc  F o l i a t i o n i n the p i n k  i a i n d l a t i n o t or abseat  Linear porphyry  shaped  monzonite. porphyry  por-  p a r a l l e l to  segregations of dark  Foliation is  contact  and I n  I n some a r e a s o f  structure,  and  struc-  augite-blotlte  rock a gnelssic this  the  i n d i s t i n c t f o l i a t i o n l a accentuated  minerals w i t h i n tae  of  of  zones.  7,  In f i n e - g r a i n e d ,  generally  subparailel  and c o n c e n t r i c  mrnzonite  contact.  the  and i n t e r m e d i a t e  In a u g i t e - b l o t l t e  hornblende  intrusive  outer  In Figure  Foliation phyritlc  In the  quartz  periphery  core. this In  It zone.  monzonite  hornblende  rock. . nd c l o t s  Where e l o n g a t e d  of  in  the  Individual commonly Inclusions  40 ar® present, they l i e w i t h i n the plane of  f o l i a t i o n with  their longest dimensions p i t c h i n g at very steep angles.  •Jointlo»f.-  Two prominent sets of s t e e p l y dipping  arc- w e l l devsloped throughout the stock;  joints  one s e t i s  p a r a l l e l o r aubparallel to p l a n a r s t r u c t u r e ; the o t h e r set Is normal to the p l a n a r s t r u c t u r e o r outs a c r o s s I t a t large angles.  Along the western aergla- of the stock,  j o i n t * that ar® normal t o f o l i a t i o n out a c r o s s the igneous contact and continue i n t o the country r o c k s .  Joints  t h a t are parallel to f o l i a t i o n occur i n a l l ro«H zones* Put they are most well developed i n th* monzonite porphyry* Poles to Joints are plotted on a lower hemisphere Schmidt net p r o j e c t i o n in /igure 9 .  They form two amin con-  centration* ; the c o n c e n t r a t i o n In the south*"stem quadrant corresponds t o Joints t h a t are parallel to p l a n a r s t r u c t u r e ; c o n c e n t r a t i o n s i n northeastern and. southwestern quadrants correspond to j o i n t s that cut p l a n a r structures at large Angles, spaced, and  @Rch  The j o i n t s are a l l very closely a t t i t u d e on the wp  j o i n t s of the same a t t i t u d e .  represents 8 to 10  Generally, the j o i n t s are  saooth, remarkably even and p e r s i s t e n t , »llc<c sides.  Son® shows  A few j o i n t s i n pink q u a r t z monzonite  porphyry are f i l l e d w i t h s p l i t s and pink q u a r t z monzor.it©  Figure  8.  172 p o l e s t o f o l i a t i o n p l o t t e d on a Schmidt n e t and contoured,, Contours 2 , 6, 1 6 , a n d o v s r 16% p e r 1% a r e a 0  N  1  W  Figure  9.  231 p o l e s t o J o i n t s p l o t t e d on a S c h m i d t n e t and c o n t o u r e d , Contours 2 T 14 2 1 , 2 8 , a n d o v e r 28# p e r 1% a r e a . ' t  43 porphyry have  dykes.  been  feature  hinor fracture  " h e a l e d " by  i s seen  hornblende  the  about  35  degrees  closely  giving  spaced  the v i c i n i t y viewed  from  apparent joints  ness  a  from  a  that  joints  are nearly joints the  are  a  feet  The  fractures  has  fractures prisms  stoc't.  These  also  that  ran e  either  i t a t low  parallel  angles.  primary  end  and  foliation of the  join  toick-  southern  occur within  country  Intersection  not  in  f c  well  of  dipping  branch  are  In the a d j a c e n t a o r n f e l s  are  that  fractures  c o n t a c t and  and  the appearance  steeply  curved  rock  hocks i n  t o 3 inches occur at the western  intrusive  for  horizontal.  structure  of  area,  are deeply weathered,  network o f l e n t i c u l a r  the  In a n o t h e r  s t o c k where t h e  tight  parts  t o the  joints  Very  In o n l y  dipping  .'Shallow d i p p i n g  developed.  1/2  joints  or parallel  sheeted e f f e c t .  well  feet  few  those  i n the lower  valley,  subparallel  bedding,  o f the  across  are  a distance  borders of  even  This  were o b s e r v e d  area,  glecial  In areas o f the  are  form  I n one  of these  sedimentary  to  dipping, j o i n t s  o f a U-shaped  surfaces,  hornblende.  areas  poor.  looalities.  walls  green  i n a l l rock phases,  Shallow three  dark  s u r f a c e s i n many  5  developed rock. or  cut  curved  44 fracture surfaces forms a l l n e a t i o n that•plunges  steeply i n  the d i r e c t i o n o f d i p o f the I n t r u s i v e c o n t a c t .  "racture  s u r f a c e s are coated w i t h reddish-brown l l m o n l t e ,  This  type of f r a c t u r i n g has been given the term " s l i p - c l e a v a g e " by Gompton (1955).  I n t e r n a l Contacts.-  Almos-t a l l c o n t a c t s between  igneous  rock zones ere g r a d a t l o n s l and thus are somewhat approximate (See map. i n p o c k e t ) ,  T h e i r s t e e p l y dipping  a t t i t u d e s a r e shown by t h e i r lack o f d e f l e c t i o n i n c r o s s i n g ridges and v a l l e y s .  The boundary between e u g i t e - b i o t i t e  monzonite and the monzonite porphyry, which c-aj be defined within distances r a n g i n g from 5 to 20 f e e t , i s th© l e a s t g r a d a t l o n a l o f a l l the I n t e r n a l c o n t a c t s ,  .a aharp  contact between these two rock types occurs a t one l o c a l i t y near t h e northern end o f the monzonite porphyry band. The boundary between a u g i t e - b i o t l t e monzonite and p o r p h y r i t l c hornblende monzonite l a accurate to within 150 f e e t . Contact between monzonite porphyry and p o r p h y r i t l c hornblende monzonite i s gradatlonol over a dlstance o f about 100 feet.  Glomerophyritie  hornblende monzonite f o r us a  t r a n s i t i o n zone between p o r p h y r i t l c h o r n a l c d e monzo.-.lte and the pink quartz monzonite porpnyry,  T h i s boundary Is  very d i f f i c u l t to locate more a c c u r a t e l y t h a n within 200 feet.  45  Contact between a p l l t e and the s u r r o u n d i n g rock zones was not observed.  External Structures  In plan t h e s«!ount Brenner s t o c k i s r o u g h l y e l l i p t i c .1 w i t h i t s l o n g a x i s t r e n d i n g northeastward. The n o r t h e a s t r e g i o n a l t r e n d of toe country rock s t r u c t u r e s Is g e n e r a l l y concordant w i t h the o u t l i n e o f the stock (fig* 4),  A l o n g the southwestern and n o r t h e a s t e r n  margins o f the s t o c k , the bedding makes on abrupt swing to conform w i t h the f l a n k s o f the i n t r u s i o n .  Along the  southern boundsry o f the stock, s h a l e beds t h a t d i p north©-'-.stward t o e a s t w a r d , about 1000 f e e t from t h e contact, dip steeply contact.  southward Immediately a d j a c e n t  igneous to the  The q u a r t s i t e shows t h i n p l a t y cleavage i n a  zone 500 t o 800 f e e t wide a l o n g t h e s o u t h e a s t e r n boundary of the stock. massive.  beyond  t h i 3  d i s t a n c e the q u a r t z i t e i s  Discordant r e l a t i o n s h i p s occur i n the n o r t h -  western c o r n e r o f the map-area, where the I n t r u s i o n a c r o s s the l i m e s t o n e  and  chert  u n i t s and  i n the  cuts  eastern  part- o f the map a r e s , where the s t o c k c u t s a c r o s s t h e c o n t a c t between q u a r t z i t e , C o n t a c t w i t h the country  and  s h a l e and p h y l l l t e .  rocks i s I n v a r i a b l y  sharp.  o  46  Intense b r e c e i a t l o n o r m y l o n i t i z a t l o n l a found i n contact zonea.  HI 3?OhY OF T H i PLUTO&  I n t e r p r e t a t i o n o f Structures and i-iecha n i c a o f Emplacement  Foliation mechanisms;  (1)  can be produced by three d i f f e r e n t  dimensional o r i e n t a t i o n of phenocrysts by  l a m i n a r f l o w i n an uprising, magma * ( primary flow structure) j (2) t e c t o n i c o r i e n t a t i o n of phenocrysts metamorphismj and  during  regional  (3) mimetic o r i e n t a t i o n of phenocrysts  by growth around m i n e r a l s that already have a p r e f e r r e d o r i e n t a t i o n ( G e l l a t l y , 19-4). the rock, and  The non-layered nature of  the lnck of ublqultus  cataclasm i n pheno-  c r y s t s and i n trie m a t r i x , suggest that the f o l i a t i o n  l a not  I'll met 1c o r i e n t a t i o n  the r e s u l t of t e c t o n i c o r e i n t a t l o n ,  snould be c h a r a c t e r i z e d by phenocrysts having,cores of. e a r l i e r formed p l a g l o c l a s e c r y s t a l s whose o r i e n t a t i o n  c r y s t a l s i n \ '1 ^ * °f d i m e n s i o n a l o r i e n t a t i o n 7fv^ X l u l d are discussed by ~ v ( i g x \ i S l i 3 8  (1901), Hutchinson  (1956), and K i l l s  8 l  (1963).  7  of  47  r e f l e c t s the alignment o f plugloclace  These f e a t u r e s ere not common i n the  In the matrix. Blount  rocks.  Brenner  The o v e r a l l conformity o f f o l i a t i o n w i t h rock zone boundaries and the presence o f elongated I n c l u s i o n s o r i e n t a t e d p a r a l l e l t o f o l i a t i o n suggests that the  f o l i a t i o n Is a primary flow s t r u c t u r e . outward d l p o l n g a t t i t u d e s  The steep  of flow structures  throughout  the stock i o d i c - t e that v e r t i c a l movement has been prominent  end  that  the stock i s not domed at t h e present  l e v e l of erosion, .'lost o f the Joints were measured In the c e n t r a l and northwestern parts o f th© stock.  The p e r p e n d i c u l a r  r e l a t i o n s h i p between the two sets of s t e e p l y d i p p i n g J o i n t s shown i n Figure 9 , t h e r e f o r e , i s s t a t i s t i c a l l y  valid  only f o r t h i s region and not f o r the e n t i r e stock.  The  presents of a p l l t e and pink quartz aonzonlte porphyry dykes f i l l i n g a case of the s t e e p l y dipping j o i n t s and the occurrence o f hornblende and quartz along the fractures,  strongly  suggests that the j o i n t s are primary.  Fractures  normal t o p l a n a r s t r u c t u r e are i n t e r p r e t e d as t e n s i o n J o i n t s related, t o a a l i g h t arching of the r i g i d upper p a r t s of the stock due to upward pressure fro i n the magma below. Xuznetsov  (1951)  suggests that as cooling, proceeds from  above, & succession of the t e n s i o n - j o i n t s can be forced  48 p r o g r e s s i v e l y at depth.  The  J o i n t s are analogous to  the c r o s s - j o i n t s ' d e s c r i b e d by Balk (193?) and to the j o i n t s described by : a r t i n ( 1 9 5 1 ) . 1  to f o l i a t i o n are poaaibly due cooling;  B-G  Joints parallel  to c o n t r a c t i o n  during  the f r a c t u r e s tend to develop a l o n g planes  of r e l a t i v e mechanical weakness, suggest t h a t the stock was were folded and  faulted.  structural relations  emplaced a f t e r the country rocks The  j o i n t s , t h e r e f o r e , probaoly  are not r e l a t e d to r e g i o n a l compressive s t r e s s i n the enclosing  rocks. The  shallow d i p p i n g j o i n t s probably are not  primary o r i g i n .  of  T h e i r broadly undulatory nature and  t h e i r s a b p a r a l l e l l a m to the topography I n d i c a t e e x f o l i a t i o n phenomena.  Jahne (1943) a t t r i b u t e s such l a r g e - s c a l e  e x f o l i a t i o n s t r u c t u r e s to r e l i e f through removal of superincumbent l o a d , b l i p cleavages and the l l n e a t i o n formed by t h e i r I n t e r s e c t i o n " I n d i c a t e c i r c u m f e r e n t i a l stretchier-, of  the  Balk (193?) d e f i n e s c r o s s - J o i n t s as tnose j o i n t s that are normal to flow l i n e s . .--artIn (1951) d e s c r i b e s B-C j o i n t s t h a t are normal to the planar s t r u c t u r e of a g r a n i t i c d l a p l r that bus no l i n e a r s t r u c t u r e s .  •a  49  b a t h o l i t h i o walls, combined wit a coiapression and flattening normal t o the walls,*' ( h i l l s , 1963, P» 357), That the stock was eaplaced i n a fluid or partly f l u i d state i s suggested by the primary flow structures, the presence o f r o t a t e d wall rock i n c l u s i o n s without d i s r u p t i o n of t h e i r i n t e r n a l structure, the l a c k of raylooltlzatlon o r b r o c c i a t i o n a l o n g the o u t e r c o n t a c t , and the presence of a hornfela c o n t a c t aureole around the stock,  Structural  data I n d i c a t e that a t the l e v e l now exposed by e r o s i o n the stock was e&placed by f o r c e f u l I n j e c t i o n , lines o f evidence a r e ;  The main  ( l ) near v e r t i c a l p l a n a r and l i n e a r  flow .structures; (2) o v e r a l l c o n t i n u i t y and c o n c e n t r i c p a t t e r n o f primary flow s t r u c t u r e s ; (3) s l i p cleavages and platy cleavages i n the outer contact zone; ( 4 ) warping o f country rock s t r u c t u r e s ...round the stock; and (5) bedding i n shales upturned t o confona w i t h the contact of th© stock. The foregoing structural evidence seeas t o I n d i c a t e that the stock was emplaced i n the f o l l o w i n g ai&nner, ^ risln  e  f l u i d magma aaade rooaa f o r i t s t l f i&alnly b y  should*.ring aside the regionally folded sedimentary and me ta sedimentary rocks.  In response to the upward and.  outward pressure exerted by tne upsurging magma, bedding in the country rocks deflected upwards and outward3 to  -*4  50 conform w i t h the I n t r u s i v e c o n t a c t , and the flanking, rocks f r a c t u r e d to form s l i p cleavages and plat/ cleavages. The magus engulfed a large segment o f quartzite j u s t within the souther-stern margin of the stock, and and  i t a l s o engulfed  r o t a t e d a few much s m a l l e r b l o c k s of country rock along  the o u t e r margin.  isfter p a r t i a l  c o n s o l i d a t i o n o f the  upper end outer portions of the magma, to form the finegrained auglte-biotlte  monzonite, the magma continued to That  surge upward, and i t Intruded this o u t e r " s h e l l " . the upward movement of the  f l u i d core continued u n t i l  the  lust stages of consolidation i s suggested by the v e r t i c a l flow structures in almost a l l p a r t s of the pink quartz monzonite porphyry.  fracturing took place before the  lower p a r t o f the stock was  completely c o n s o l i d a t e d , and  j o i n t s formed to accommodate s t r e s s e s due to magmatic pressure from below and t o - c o o l i n g and c o n t r a c t i o n of the rock.  Assimilation  Uoota ainatlon of the magma has occurred along the eastern m. rgln of the stock, where a u g l t e - b i o t l t e monzjolte i s i n contact w i t h p e l i t i c h o r n f e i s .  The  r e s u l t of c i.'ita&liv-ition i s to change the & u ^ l t e - b l o t l t e  51  monzonite Into a g a r n e t l f e r o u s quartz d i o r i t e , I n a zona which I s l e s a than one f o o t wide,  Ro evidence f o r con-  tarain - t i o n occurs where g r a n i t i c rocks a r e i n contact w i t h I f assimilation  quartzite.  had taken piece i n s i g n i f y  i o a n t amounts the f o l l o w i n g evidence would be expected; ( l ) numerous i n c l u s i o n s of country rock I n a l l stages o f r e s o r p t i o n ; ( 2 ) the number o f I n c l u s i o n s shoul-" i n c r e a s e toward the p e r i p h e r y ; ( 3 ) h e t e r o g e n e i t y o f o u t e r . ones showing v a r i o u s degrees o f c o n v e r s i o n t o the present rock types; and (4) t h e o u t e r zones should become i n c r e a s i n g l y more s i l i c e o u s toward t h e contact s i n c e h i g h l y s i l i c e o u s country rocks would, be e s s i ml l a t e d , v i r t u a l l y aoaent i n t h i s stock, e r o s i o n , t o a r e f o r e , contamination  Such evidence i s A t the present l e v e l o f  o f th> magma by a s s i m i -  l a t i o n of country rock has not been a f a c t o r In causing c o m p o s i t i o n a l v a r i a t i o n o f the stock.  Differentiation  The systematic c o m p o s i t i o n a l v a r i a t i o n w i t h i n t h e stock (See p.30 ' the f o l l o w i n g f i g u r e s ) l a compatible :n (  w i t h the concept  of d i f f e r e n t i a t i o n - of a ma:: ma as i t  c r y s t a l l i z e d inward from the margin. monzonite i s conride red  The a u g i t e - b i o t i t e  t o represent th© ^ i m p o s i t i o n o f t h e  52  parent f l u i d because I t was the f i r s t rock zone to be eopl cad and because i t has & composition from which a l l other rock types can be formed by r e l a t i v e l y  simple  processes of d i f f e r e n t i a t i o n . That the main rock types of the stock pass from under-saturated, through s a t u r a t e d t o oversaturated  types  (fl&. 5ai seems t o suggest t h a t the thermal b t r ^ l e r of the  system b l 0  crossed. thermal  2  - nahlblC^ -  SAI0IO4  (Bowen 1937)  has been  T l l l e y (1957) suggests, however, t h a t the b a r r i e r which prevents the t r a n s i t i o n from  syenite  t o granite i n toe experimental melts '"say be "Inoperative i n a n a t u r a l melt where i t s composition Is modified by the presence of nafic components, and f u r t h e r , notable In v o l a t i l e content".  disparlt  The trend from undersaturated to  ovsrseturatec" magaa can ce accompli oh ed by v a r i o u s processe of e l i t e ; t i o n of undersaturated  map-mas,  louring,  c r y s t a l l i z a t i o n o f trie magma, the r e s i d u a l l i q u i d can be enriched i n s i l i c a by r e a c t i o n or inc.* of r e a c t i o n of  th©  l i q u i d w i t h minerals t h a t have c h a r a c t e r i s t i c s of lneongruent melting,,  Tllley  (1957) suggests that  "precipita-  t i o n of mwgnetlte In the place of aenerlna w;-uld tnrow excess s i l i c a i n t o the l i q u i d i f t h i s oxide pnase f a i l e d to resorb".  The formation of hornblende by r e a c t i o n of  l i q u i d w i t h pyroxene w i l l a l s o r e s u l t i n s e t t i n g f r e e of  53  q u a r t z , arid thus the p o s s i b l e q u a n t i t y of a .auartzose d i f f e r e n t i a t e w o u l d be a u g m e n t e d The change o f t h e SlOg c u r v e saturoted  ( f i g . 5a) from tae u n d e r -  Into the s a t u r a t e d areas of the diagram  corresponds w i t h the a p p e a r a n c e major  (Bowes, 1923, p, 9 0 ) ,  mafic m i n e r a l .  The 5 l 0  o f hornblende as the content  o  Increases  toward t h e ovc-rsaturated area w i t h ino easing amojot o f hornblende (compare p o i n t s 3 end C w i t h the corresponding, modes  i n Table  Thus, the formation o f hornblende  I).  may h o v e played a signifies.*at r o l e I n s i l l c a t l o n o f t h e residual  The f o r e g o i n g processes w i l l  liquid.  t h e compostlon o f the f i n a l r e s i d u a l corresponding  l i q u i d s and t h e  rocks, b u t can only cause d i f f e r t - n t i a 1 1 on i f  r e l a t i v e movement b e t w e e n c r y s t a l s and r e s i d u a l brought about by p r o c e s s e s such a a r u v l t y c  "filter  curves  separation o r  oxides  of  the h o r n b l e n d e - c l o t I t e  pyrox-  show s i g n i f i c a n t d e p a r t u r e s from t h e emooth v a r i a t i o n ( f i g s , 4 and 6 ) .  a t t r i b u t e d by  This sort  of departure  is  bowen i l 9 2 a ) t o c r y s t a l a c c u m u l a t i o n .  If  hornblende monzonite i s t o b e f o r m e d from  the  porphyrltic  the  a uglte-blotite  of  liquid i s  prcsslrg". Certain  enlte  affeot  m o n z o n i t e by t h e  blotite-hornblende  pyroxeulte  separation  of components  alone, the r a t i o of  volumes of the p y r o x e n l t e to components of t h e m o n z o n i t e  54 s h o u l d be approximately 1 to 3 ( c a l c u l a t e d f r o a f i g . 4 ) . M i g r a t i o n o f water and a l k a l i s can a l s o c o n t r i bute to d i f f e r e n t i a t i o n o f a. -magma.  Water w i l l  diffuse  and d i s t r i b u t e i t s e l f i n a magiaa so t h a t the chemical potential of t h e water I s the sane t h r o u g h o u t t h e magma chamber.  A l k a l i s and c e r t a i n m e t a l s w i l l  co-ordinate  with the w a t e r and w i l l be concentrated In the regions o f lowest pressure and temperature  (Kennedy, 1355)#  A h y p o t h e t i c a l sequence of events t h a t i s proposed to e x p l a i n the development of rock zones, i s I l l u s t r a t e d d l a g r a a a t l c a l l y In F i g u r e 10, o f v o l a t i l e and a l k a l i d i f f u s i o n and  The  processes  of f r a c t i o n a l  crystal-  l i z a t i o n , which e f f e c t e d d i f f e r e n t i a t i o n o f the parent magma, were Interrupted by During the f i r s t  Intrusion. ©tag© of Intrusion ( f i g ,  10a)  the upper and outer p a r t s o f the magsaa which formed a cupola o f a l a r g e r zaagiaa chamber, cooled very r a p i d l y t o form the fine-grained a u g l t e - b l o t i t e monzonite.  .Effects  o f the hot magma on the r e l a t i v e l y c o o l country rocks were t o produce a zone of c o r d l e r l t e h o r n f e l s In c e l l t i c g a r n e t i f e r o u s quartz d l o r i t e at the contact w i t h  r o c k s , end a "bleached" zone In q u a r t z i t e .  roc'<s,  pelitio  V>ith t h e  Feldspar  Pyroxene  F i g u r e 10a  Figure  10b  LEGEND Aplits Pink quartz porphyry  monzonite  P o r p h y r i t l c hornblende monzonite Monzonite porphyry Hornblende-blotite pyroxenlte Augite-biotlte monzonite Country F i g u r e 10c  rocks  Present l e v e l o f erosion  F i g u r e 10. Diagrammatic s k e t c h showing a h y p o t h e t i c a l development o f rock zones o f t h e Mount Brenner 3tock„  I n i t i a l emplacement o f the aagaia, temperature and pressure g r a d i e n t s were e s t a b l i s h e d i n t h e magma chamber* and  Water  alkali® tended t o d i f f u s e I n the magma and they g r a d -  ually collected  and became c o n c e n t r a t e d I n the upper and  o u t e r regions of the cupola, the regions of l o w e s t p r e s s u r e and temperature. crystallised  L a r g e potassium feldspars  from t h e h i g a l y f l u i d s i k a l i - r l e h magma thus  produced.  Early  formed pyroxene c r y s t a l s sank I n t h e  f l u i d l i q u i d and tended to accumulate oa the more v i s c o u s magma Immediately"below this region*  The magma below  t h i s r e g i o n was a l s o b e g i n n i n g to c r y s t a l l i z e , and processes e f f e c t i n g d i f f e r e n t i a t i o n o f the magma were c o n s t a n t l y t a k i n g p l a c e , that l e , d i f f u s i o n of volatlles and  o f a l k a l i s , and s i l i c a t i o n of r e s i d u a l fluids by the  mechanisms p r e v i o u s l y mentioned (See page 52 ), and g r a v i t y se-pa ration, Following c o n s o l i d a t i o n o f the outer find up e r parts of t h e cupola, t h e saagiaa continued t o surge upward, and  i t intruded the a u g i t e - b l o t l t e monzonite (fig*  The  accumulated  pryoxene and the large o r t h o c l a s e crystals,  that l i e immediately and  10b),  below tue outar zone, were dragged up  spread along the wall of the chamber by the upward, and  outward f o r c e of t h e i n t r u d i n g magiaa.  This cryatal  57  aiush l a t e r c o o l e d to form toe hornblende-biotite p y r o x e n It® and t h e monzonite porphyry*  The newly  Intruded  magma c o n t i n u e d to c r y s t a l l i z e inward i n a zone which l a now t h e porphyritlc h o r n b l e n d e  zone),  a o a z o n l t s (th© i n t e r m e d i a t e  'Che medium- to c o a r s e - g r a i n e d t e x t u r e of this  zone s u g g e s t s t h a t t h e magma c o o l e d ©ore s l o w l y than d i d  the auglte-blotite monzonite, p o s s i b l y because t h e c o u n t r y r o o k s were p r e h e a t e d by t h e e a r l i e r stag© o f i n t r u s i o n , The  mobile c o r e c o n t i n u e d t o surge upward,  C r y s t a l s suspended I n t h e f l u i d were aligned by magma-tic flow.  The magma r e a c t e d w i t h t h e p a r t i a l l y c o n s o l i d a t e d  I n n e r margin of th© I n t e r m e d i a t e zone t o form t h e t r a n s i t i o n zone o f glomeroporphyrltio h o r n b l e n d e  monzonite,  Th©  l e n s e s o f monzonite p o r p h y r y , which o c c u r In some p l a c e s a t t h e c o n t a c t between t h e c o r e and t h e porp~.y r i t i c blende  horn-  monzonite, may have formed by p r o c e s s e s s i m i l a r t o  those which formed t h e main band o f monzonite porph..<ry» The c o r e c o o l e d slowly t o produce t h e c o a r s e - g r a i c e d p i n k q u a r t s monzonite porphyry  (fig. lOo),  Fracturing o c c u r r e d b e f o r e t h e core was completely c o n s o l i d a t e d .  I t e f f e c t e d a sudden release o f  p r e s s u r e , and a sudden escape ing l i q u i d .  o f v o l a t l l e s from t h e remain-  These changes would b r i n g about a  58  s i g n i f i c a n t r i s e of the temperature o f c r y s t a l l i z a t i o n of the remaining l i q u i d , and possibly would ease a . "sweeping-out" a c t i o n of i n t e r s t i t i a l f l u i d s *  Consequently,  tne p a r t i a l l y f l u i d core crystallized r a p i d l y t o form the zone of f i n e - g r a i n e d leuco-quartz monaonlte, and the f l u i d s which f i l l e d f r a c t u r e s cooled r a p i d l y t o form aplite dykes (fig* 1 0 c ) .  Some of the c r y s t a l mush of the core was  swept i n t o f r a c t u r e s , where I t consolidated  to form pink  quartz asonzonite porphyry dykes*  ouafftar?/  of emplacement. Crystallization  n  and D i f f e r e n t i a t i o n  The Mount Brenner stock has i n t r u d e d f o l d e d rocks of S i l u r i a n to bower Cretaceous age.  The r e g i o n a l  structural trend i a modified In the v i c i n i t y o f the atock so t h a t bed a are g e n e r a l l y conformable with the lntrunlve contact.  That the stock was eatplaced by f o r c e f u l  i n j e c t i o n of a f l u i d magma i a indicated by ( 1 ) near v e r t i c a l planar and linear primary flow structures that have an o v e r a l l c o n t i n u i t y and c o n c e n t r i c pattern} ( 2 ) s t r u c t u r e s in the country rock that warp around the stock; (3) bedding i n shales t h a t turns upward to conform w i t h the contact of the a t o c k j and ( 4 )  s l i p cleavages and piety cleavages t h a t  59  occur along the outer contact z 5ne.  Piecemeal atoping  was o f only minor Importance during, emplacement of the stock. Chemical sad atlneraloglcal v a r i a t i o n s of t h e rock zones have a systematic t r e n d toward *'petro .e?jy *• v  r e s i d u a system* , from th® periphery of the stock t o the 1  core,  T h i s v a r i a t i o n i s c o n s i s t e n t w i t h the concept  of d i f f e r e n t i a t i o n o f an auglte-blotlte monzonite parent magma as It crystallized from t h e w a l l s of the magma chamber inward.  The following, h y p o t h e t i c a l sequence  of events i s proposed t o e x p l a i n the development of t h e rock zones,  A r i s i n g f l u i d magma, p o s s i b l y guided by  the r e g i o n a l s t r u c t u r e , made room f o r I t s e l f mainly by s h o u l d e r i n g a s i d e the country r o c k s .  The upper and outer  p o r t i o n s of the magma c h i l l e d t o form a " s h e l l " of f i n e grained auglte-biotlte monzonite.  D i f f e r e n t i a t i o n of  the magma was c o n t r o l l e d by g r a v i t y separation o f mafic minerals and by d i f f u s i o n oi' volatile®, ahd a l k a l i e s . The magma continued to surge upward, and intruded the p a r t i a l l y c o n s o l i d a t e d outer " s h e l l " .  I n c l u s i o n s of  the " s h e l l " were engulfed and carried, along with the r i s i n g soag.aa.  Pyroxene c r y s t a l s and l a r g e orthoelase  c r y s t a l s that l i e in the magma j u s t below the a u g i t e - b l o t i t e  50  monzonite, were dragged up and spread a g a i n s t the side o f the chamber by the upward and outward force of the I n t r u d i n g magma*  The c r y s t a l muah l a t e r cooled, to  form the horoblende-biotite pyroxenlte and the monzonite porphyry.  The magma proceded to c r y s t a l l i s e inward  In the i n t e r m e d i a t e zone. to surge upward.  The mobile core continued  The saagma reacted w i t h the p a r t i a l l y  consolidated Intermediate zone and formed the gloioeroporpayrltic  hornblende  monzonite.  The core cooled  slowly to produce pink quartz monzonite porphyry. Fracturing took place before the lower p a r t of the stock wan completely consolidated.  J o i n t s formed to  accommodate s t r e s s e s due to magmatio pressure form below and to cooling-and c o n t r a c t i o n of the rock.  The  fracturing e f f e c t e d a sudden release of pressure and of volatiles from th© remaining l i q u i d .  As. a result of  these changes, the p a r t i a l l y fluid core crystallized r a p i d l y to form the zone of fine-groined lauco quartz monzonite, and the fluids that f i l l e d f r a c t u r e s cooled rapidly to form a p l l t e dykes.  The e f f e c t s of wall rock  contamination are insignificant compared to the overall ©volution of rock zones by the processes of magmatie d i f f e r e n t ! a. tlon.  51 TBLlOGRaPHLY  Balk, R. 1937s Balk,  S t r u c t u r e s In Igneous :ioc -r; G&ol. Soc, Aiaer., •.'lem. 5.  3, and G r o u t , ? . F .  1934;  S t r u c t u r a l Study o f The Snowbank "took; Geoi, Soc, Amer, B u l l , , V. 45, p, 621*635,  Bostock, H.S, 1957; "'ukon T e r r i t o r y ; Geol* Surv,- Canada, '%m» 284, 1948J  o f The Canadian C o r d i l l e r a , w i t h S p e c i a l Reference t o Th® Area K o r t h o f the f i f t y - F i f t h P a r a l l e l ; . Geol. Surv. Canada, tte-n. 247,  Physiography  Bowen, h.b. 1922s  Sahavlour o f I n c l u s i o n s i n Igneous ha .aa; Jour, Gaol, v. 30, p, 5 1 3 - 5 7 0 .  1928:  The .Evolution o f t h e Igneous Hocks; P r i n c e t o n University Press.  ~1937;  r  Recent  and  high-temperature  r e s e a r c h on  silicates  I t s s i g n i f i c a n c e i n Igneous e o l o g g j c  Amer, -Jour. S c i . , v. 3 3 , p. 1-21,  Buddlngton, A . F . 1959$ G r a n i t e ampl. cement w i t h S p e c i a l R e f e r e n c e t o >.-orth -merica| G e o l , Soc. Amer,, B u l l . , v* 70, p. 671-747. C o c k f l e l d , W.ia, lilh: S i l v e r - l e a d D e p o s i t s of" t h e L i t t l e T w e l v e m i l e A r e a ; G e o l , Surv, Canada, hum. n e p t . , lj>lu, pt. a, p , 1 and p t , B , p , 1-17. 1913:  Explorations In the O g i l v i e iiangej Geol. Surv,, Canada, Sum, R e p t . , 1 9 1 9 , f t . A, :.. 1, and pt. a, P.  1-7.  a»a«  Co a p t a n ,  C a l i f o r n i a ; G e o l . .3sc. 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