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The Caulfield "Quartz-diorite": a modal determination and petrographic observations 1942

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L £b ft-) THE CAULFEILD "QUARTZ-DIORITE": MODAL DETERMINATION and PETROGRAPHIC OBSERVATIONS by Magnus Lunde -0- Thesis submitted i n P a r t i a l Fulfilment the Requirements for the Degree of MASTER OF ARTS i n the Department of GEOLOGY -0- The University of B r i t i s h Columbia April,1942 CONTENTS Introduction 1 Acknowledgements 1 Summary of Geology 2 Selection of Specimens . 2 Megascopic Description 3 Microscopic Description 3 (a) Texture 4- (b) Grain Size 4 (c) A l t e r a t i o n 4 (d) Esse n t i a l Minerals 5 (i) Plagioclase 5 ( i i ) Quartz 6 ( i i i ) Orthoclase . 6 (e) Secondary and Accessory Minerals . ' 7 (i) B i o t i t e , Chlorite 7 ( i i ) Epidote 8 ( i i i ) Titanite,, Rutile,Leucoxene 8 (iv) Magnetite, Apatite . 9 (v) Hornblende 9 ( v i ) ' S e r i c i t e , K aolinite 9 ( v i i ) Zircon 1 0 ( v i i i ) Caleite, Pyrite, Hematite, Limonite . . . 1 0 Modal Determination ' . . 1 0 Table of Slide Analyses 12 C l a s s i f i c a t i o n 13 Observations and Conclusions 13 (a) General Nature of the Rock 1 3 (b) E f f e c t s of assimilation 1 5 (c) History "of the Magma 1 5 (d) Temperature Conditions i n the Magma 1 9 (e) Pressure Conditions i n the Magma 1 9 Summary 19 THE CAULFEILD "QUARTZ-DIORITE": MODAL DETERMINATION AND PETROGRAPHIC OBSERVATIONS INTRODUCTION In 194-0, at the suggestion of Dr. C. 0. Swanson, the writer began a petrographic study of the so-called quartz - d i o r i t e that underlies the C a u l f e i l d area, a r e s i d e n t i a l d i s t r i c t situated about one mile and a half east of Point Atkinson on the north shore of Burrard I n l e t . The work,baaed, mainly on thi n sections made from twenty-three c a r e f u l l y selected specimens, v/as carried out i n the geological lab- oratories of the University of B r i t i s h Columbia under the supervision of Dr. Swanson. ACKNOWLEDGEMENT S The v/riter i s g r a t e f u l to Dr. Swanson for guid- ance and advice given during the preparation of the material for t h i s thesis. The enclosed map i s a tracing made by Dr. Gunning and students who carried out a geological survey of the area i n 194-0. -2- SUB-MARY OF GEOLOGY The geology, which needs only b r i e f mention, i s summarized from a report by one3" of the students who examined the area with Dr. tunning. The area i s underlain by the south- ern extension of the Coast Range bath o l i t h which here surrounds a sma.ll, highly schistose roof-pendant known as the C a u l f e i l d formation. The age of the ba t h o l i t h i s possibly Upper Jurassic and that of the roof-pendant T r i a s s i c or late Paleozoic.These rocks are cut by a p l i t e dikes and s i l l s which grade into peg- matitic phases here and there. Post-Eocene(?) basic dikes comprise the f i n a l intrusive phase; they intrude the a p l i t e . The reader i s referred to the map for a more comprehensive picture regarding d e t a i l s . SELECTION OF SPECIMENS Twenty-three surface samples were chosen from an area of roughly 500 by 100 yards of the batholith; the locations are shown by numbers on the map and these corresp- ond to those on the handspecimens and thin sections examined. Choice of material was guided largely by features not essent- >̂ i a l l y c h a r a c t e r i s t i c of the rock. Thus no rock i n contact with dikes, inclusions or shears was collected; strongly weathered samples were avoided. In a word, the material picked i s as far as practicable representative of the rock as a whole. 7 M.S.Lougheed > MEGASCOPIC DESCRIPTION The rock i s medium-grained, l i g h t grey to l i g h t tan where weathered and has a g r a n i t i c texture. The minerals seen are plagioclase, quartz, orthoclase, b i o t i t e , c h l o r i t e , hornblende,, magnetite and pyrite. The plagioclase occurs as rectangular crystals v/hich show considerable v a r i a b i l i t y i n size, some exceeding 5mm. i n length. Striated cleavage planes are common. A few of the c r y s t a l s are altered at the centre to a white opaque substance. Both quartz and orthoclase are seen as i r r e g u l a r patches which appear to be i n t e r s t i t i a l to to the plagioclase grains. The colorless quartz reaches dimen- sions over 5mm.; the l i g h t pink orthoclase i s about the same size at a maximum. B i o t i t e , which i s conspicuously p l e n t i f u l , occurs as lustrous black plates that occasionally exceed 2mm. In general i t i s fresh but i n places i t has altered to green c h l o r i t e . Hornblende occurs sparingly as long black c r y s t a l s . Examination under the hand-lens reveals the presence of a number of small magnetite grains. Pyrite i s seen as occasional small grains and veinlets. MICROSCOPIC DESCRIPTION A b r i e f description of the texture,. grain size, a l t e r a t i o n , minerals and the r e l a t i o n s between them w i l l be given. The statements made v / i l l have only general application to a l l the slides examined. Marked deviations from the gen- e r a l trend v / i l l be noted accordingly. (a) Texture. With the exception of the plagio- -4- -clase which i s generally subhedral to euhedral, the esse n t i a l minerals are anhedral. The texture i s therefore to he described as hypidiomorphic. (b) G-rain size. For the purpose of comparison, measurements were taken along the greatest dimension of plag- ioclase, quartz, orthoclase and b i o t i t e grains. Dimensions taken i n t h i s manner show that the average plagioclase c r y s t a l i n each of the s l i d e s f a l l s between 2.2mm. and 1.3mm, the av- erage of a l l the sl i d e s being 1.7 mm. S i m i l a r i l y for quartz these l i m i t s are between 2.4 mm. and 1.4 mm., the average being 1 .9 mm.; for b i o t i t e , between 1.6 and 0 . 8 , the average 1.1. The orthoclase was not averaged but i t has a d e f i n i t e tendency towards a size larger than the other minerals. Often i t exceeds 7mm. and in sl i d e #23 i t reaches nearly 12 mm. The conclusion, then, i s that the rock as a whole shows a rough uniformity i n the size of i t s i n d i v i d u a l minerals. The size of the plagioclase crystals varies from tiny grains to those that commonly exceed 4 mm. i n length; no one c r y s t a l was observed to go beyond 6 mm., however. Likewise quartz and orthoclase show a wide range i n dimensions. B i o t i t e tends to be more uniform, i n general not deviating as greatly from i t s average dimensions. (c) Alteration. The rock as a whole i s f a i r l y fresh. Where a l t e r a t i o n has taken place i t i s confined mainly to the plagioclase. Here i t i s frequently r e s t r i c t e d to the core of the feldspar i n dense masses but more often i t has affected the entire feldspar i n varying degrees . In a. single s l i d e only a few of the plagioclase c r y s t a l s may be altered while the remainder may be comparatively fresh. The a l t e r a t i o n products comprise epidote c h i e f l y , s e r i c i t e , kaolin, and'-.-.Cal- c i t e r a r e l y . These minerals occur singly or together i n vary- ing proportions i n the plagioclase. Quartz i s p r a c t i c a l l y un- altered. Orthoclase has been lent a brownish t u r b i d i t y by the presence of very fine-grained material, probably kaolin. Bio- t i t e has been changed i n varying degrees to c h l o r i t e but the t o t a l extent of a l t e r a t i o n i s quite small. T i t a n i t e shows more or less a l t e r a t i o n to leucoxene. (d) Ess e n t i a l minerals. (i) Plagioclase. This mineral occurs as elorragated to equidimensional c r y s t a l s which commonly show corroded outlines, e s p e c i a l l y i n contact with orthoclase. Reaction rims are common and several may be present i n a single feldspar. Zoned crystals with andesine cores and oligoclase rims are frequently seen but much of the plagioclase i s also not zoned. Albite twins.are by far the most abundant;Carlsbad- a l b i t e and p e r i c l i n e types are observed occasionally. The composition of the plagioclase was deter- mined largely from the extinction angles on al b i t e twins. Unfortunately/some of the s l i d e s , p a r t i c u l a r i l y #14,15,17,20 and 21, do not contain enough suitable a l b i t e twins to permit the i d e n t i f i c a t i o n of the plagioclase with certainty. In these cases the determination of the indices of r e f r a c t i o n by- means of o i l s of known r e f r a c t i v e indices was resorted to -6- and -the' i d e n t i t y of the feldspar deduced from Tsuboi's chart?"* The plagioclase composition for a l l the slides averages out to Ab^An^i and individual c r y s t a l s f a l l within the l i m i t s Ab 74An 26 a n d Ab64An36- ( i i ) Quartz. Occurs as i r r e g u l a r , equi- dimen-sional patches to elongated forms which act as a ground- mass for the other minerals. Now and then i t contains i n c l u s - ions of corroded plagioclase as well as other minerals. But for rare s e r i c i t i z a t i o n and some dust-like inclusions , i t i s clear and fresh. Undulating extinction i s quite common. ( i i i ) Orthoclase. Occurs'much i n the same way as quartz i n that i t occupies the spaces between the other minerals. In slides #11 and 1 5 i t forms tiny v e i n l e t s which follow cleavage planes and fractures in the plagioclase. Often i t exhibits a p o i k i l i t i c r e l a t i o n s h i p to the other min- erals; s l i d e #23 i l l u s t r a t e s t h i s structure exceptionally well, the orthoclase serving as host to scattered c r y s t a l s of plagioclase, magnetite, apatite and. b i o t i t e . Occasionally the plagioclase has been strongly corroded by the orthoclase mass and i n some instances i s almost completely dissolved.. In s l i d e s #5,6,14, 2 1 , where orthoclase i s in contact with the plagioclase the periphery of the l a t t e r shows under high mag- n i f i c a t i o n what appears to be fine myrmekitic texture. What has up to t h i s point been c a l l e d orthoclase shows in part properties that are foreign to t h i s mineral. For instance, in slides #1,3,5,7,8,14,16,19,20, t h i s feldspar ^ Rogers and Kerr, Thin Section Mineralogy, p.213,1933. exhibits twinning i n the form of a single set of fine l i n e s which may be long and straight or short and scattered. More- over, 2V i s often too large or small for orthoclase; i n sections #2,7,13, i t i s i n some cases about 50 degrees and i n other sections i t appears to be d e f i n i t e l y greater than 72 degrees, the maximum for orthoclase. From t h i s data i t i s evident that i n addition to, microcline and anorthoclase are also present i n the rock i n proportions that the writer did not try to determine. In" t h i s paper the term orthoclase w i l l always include anorthoclase and microcline. Another very common feature of the i s the presence of small,irregular patches whose properties are not r e a d i l y determinable. Rough p a r a l l e l orientation of the more elongated grains in some cases suggests i n c i p i e n t micro- perthite. (e) Secondary and accessory minerals (i ) B i o t i t e and Chlorite. The b i o t i t e i s a dark brown normal variety which usually shows subhedral outlines. Often i t i s altered to c h l o r i t e which forms long wedges that penetrate the cleavage; or i t may be altered at i t s boundaries with the development of fringes of c h l o r i t e . Only i n a few cases -has complete replacement taken place. P r a c t i c a l l y a l l the c h l o r i t e i n the rock i s pseudomorphous after b i o t i t e . The c h l o r i t e , which i s a rather dark green, i s always length fast and has anomalous interference colors of very low order. It i s probably the variety penninite.Both b i o t i t e and c h l o r i t e commonly contain inclusions of apatite -8- and t i t a n i t e . ( i i ) Epidote. This mineral occurs i n two ways: one as as fine-grained a l t e r a t i o n product of plag- ioclase, and the other as comparatively large grains which may occur v/ith any of the minerals in the rock. In the f i r s t type of occurrence, where i t i s always associated v/ith plagio- clase,. i t i s seen as small, more or less clear, i r r e g u l a r grains scattered throughout the plagioclase and as dark,dense masses concentrated at the cores of the plagioclase c r y s t a l s ; i n t h i s l a t t e r case i t frequently shov/s a c h a r a c t e r i s t i c non- descript greyish interference color of low order. In general there i s a certain amount of s e r i c i t e and kaolin(?) associat- ed with these masses. In the second, coarser type the epidote occurs as equidimensional to elongated forms that are usually v/ell under 5 mm. The mineral i s colorless to lemon yellow, r a r e l y orange, and shows s l i g h t pleochroism. As i n the case of orthoclase, the term epidote very l i k e l y includes related minerals, namely, z o i s i t e and c l i n o z o i s i t e . Small grains of these minerals are d i f f i c u l t to distinguish from one another i n thin section. ( i i i ) Titanite,rutile,leucoxene.These minerals, almost invariably associated, are largely confined to b i o t i t e . The t i t a n i t e , which i s more or less altered to leucoxene, often displays a marked tendency to p a r a l l e l the b i o t i t e cleavage as long i r r e g u l a r grains; i t i s also seen to follow along b i o t i t e peripheries i n the form of t h i n ragged rims. Rutile could not be i d e n t i f i e d v/ith certainty - 9 - except i n one or two instances where the c h a r a c t e r i s t i c twin- ning, color and r e l i e f could be more c l e a r l y discerned under high magnification. The mineral occurs very sparingly as tiny needles and plates. (iv) Magnetite and apatite. Occur sparingly throughout the sl i d e s . Magnetite occurs as anhedral masses to well-formed cr y s t a l s that seldom exceed 0.5 mm. As a rule i t i s fresh but occasionally i t i s completely or part- i a l l y altered to hematite. The apatite i s of the ordinary w e l l - c r y s t a l l i z e d variety; c r y s t a l s are usually well under 0.'3 mm. (v) Hornblende. This mineral was noted i n ten of the sl i d e s but only i n #5,21"\22, does i t att a i n proportion of any importance. I t i s dark green and has a maximum extinction of about 30 degrees. The crys t a l s are subhedral to splendidly euhedral and range over 5'5mm. (vi) S e r i c i t e and k a o l i n i t e . While these minerals are common to most of the plagioclase, the average amount i s very small. Only i n a few cases, as i n slid e s #10,13, does the feldspar display marked/sof th i s type; i n these instances epidote,. the dominant a l t e r a t i o n mineral i n the rock, i s either absent or present i n small quantities only. Jiaolin tends to be a l i t t l e coarser grained than the s e r i c i t e . The fine-grained brownish material so frequently observed i n the orthoclase i s whitish and opaque i n trans- mitted light,, and may well be kaolin. ( v i i ) Zircon. This mineral i s d i s t - ributed scantily throughout the sl i d e s . The crystals are stout and rounded, and are usually under 0.5 rnm. When i n b i o t i t e they are sometimes surrounded by oleochroic haloes. ( v i i i ) Caleite, p y r i t e , hematite and limonite. A l l of these are comparatively rare in the rock. Caleite has probably originated through the a l t e r a t i o n of plagioclase with which i t i s associated. In slide #23,the: .; only place' i s at a l l conspicuous,, i t occurs as numerous small, i r r e g u l a r grains i n some of the plagioclase. One exceptionally large grain here reaches 0.4 mm., but the average i s much smaller. Pyrite i s seen as the odd grain or two i n some of the thin sections. It may or may not be rim- med by hematite which also replaces a l i t t l e of the magnetite. Weathering of the mafic minerals has produced occasional limonite stains. MODAL DETERMINATION The percentages of indi v i d u a l minerals i n each of the sli d e s was determined by means of the Rosiwal method. No one slide has an area one hundred times as great as the area of the largest grains that are present to the extent of one per cent of the rock, a condition required for accuracy 1 within one per cent i n the modal determination of rocks. This rule has special bearing on the orthoclase, which often occupies unusually large areas. It i s to be expected, there- ^ Larsen,E.S. and Miller,F.S.,"The Rosiwal method and the modal determination of rocks",Amer.Min.,XX,1935 -11- -fore, that the mineral compositions of in d i v i d u a l s l i d e s 'will vary considerably and that a single s l i d e cannot be taken as representative of the rock as a whole. This point i s best i l l u s t r a t e d by the analyses (see below) of s l i d e s #21', 21", 21"',.. a l l of which v/ere taken from handspecimen #21. However, i f a l l the s l i d e s are pictured as combined into a single large area, then t h i s area has the proportion(in r e l - ation to the largest grains that comprise one percent of a slide) required for one per cent accuracy, provided the rock i s uniform. Nov; i f i n d i v i d u a l s l i d e compositions are consid- ered as parts of t h i s imaginary s l i d e , t o t a l l e d and averaged, the r e s u l t i n g composition should represent the rock, as a whole. The analyses below have thus been treated, the average of each constituenttaken to be representative of the entire rock. This data was then used as a basis for the c l a s s i f i c a t - ion of the rock.Because of the fine-grained nature of some of constituents and uneven d i s t r i b u t i o n of others, most of the small percentages given i n the table below are approximations rather than exact calculations. Daahed l i n e s i n the columns si g n i f y either absence of the mineral or i t s presence toan extent of less than 0.1 percent. -12- , TABLE OF SLIDE ANALYSES % S e r i c i t e Kaolin Pyrite Hematite Calcite Limonite Hn- bld. Epi. Rut. T i t a . Leuc. Slide Plag. Q,tz:. Or. Bio Ohio. Mag. Ap. \ 1 . ' 5 8 . 9 ' 1 7 . 2 14 . 5 ' 6.6 * \ 0 . 2 ' 0 . 4 0 . 2 0 . 8 0 . 1 0 . 3 0 . 8 2 . 54 . 7 28 .7 6 . 5 1 .4 2 . 8 1 .3 1 0 . 7 - 3- 48 . 9 2 9 . 0 9 . 1 1 7 - 9 ----- 1 . 9 0 . 5 0 . 9 1 . 0 0 . 4 0 . 4 "4 . 44.0 31.1 19 . 6 3.0 0 . 3 0 . 9 0 . 8 0 . 3 r - 0- 3 4 . 5 39 . 3 12. 2 7.<j 0 . 5 0 . 8 0 . 2 3 . 0 0 . 6 0 . 5 0 . 5 6. 53 . 8 24 .7 13 . 5 5.4 0 . 1 1 . 4 0 . 1 0 . 5 0 . 1 0 . 1 0 . 3 7 • 51 .7 28 .1 1 1 . 4 5-f 0 . 3 1 . 1 0 . 4 0 . 8 0 . 1 0 . 6 n o . 6 0 . 9 2 5 . 8 7 . 7 2 . ] 0 . 3 0 . 4 0 . 1 1 . 8 0 . 1 0 . 8 9- 49 -0 26 .7 1 0 . 7 8.q 0 . 5 2 . 9 0' .5 0 . 7 0 . 3 0 . 7 10. 36 . 3 26 .3 2 3 . 3 i 3 . 9 1 . 4 0 . 4 1-3 2 . 5 1 . 0 1*7 1 1 . 39-8 26 .9 14 . 4 8 . 1 5 - 3 'l.O 0.1 3 . 0 1 . 0 0 . 4 12. 45 . 5 31 . 2 15 -6 i.e) 2 . 9 0 . 3 0 . 3 0 . 9 1 . 4 0 . 3 13. 43 . 0 2 7 . 2 20 .7 0 . 7 0..4 4 . 2 0 . 8 1 .5 1 4 . 46 . 0 29 . 0 1 3 . 5 8.e) 1 . 0 0 . 6 0 . 2 0 . 4 0 . 4 0 . 3 15. 48 . 8 34 . 6 10 . 9 2.6 1 . 1 1 . 6 0 . 3 0 . 2 0 . 3 0 . 2 1 $ . 68 . 0 1 5 . 6 1 1 . 9 i.4 0 . 7 0 . 3 0 . 1 1 . 3 0 . 3 0 . 4 17. 52 . 9 32 . 4 •8 .5 2 . 2 1 i 1 . 3 1 . 0 0 . 1 0 . 2 1 . 0 0 . 4 1 8 , 51 . 5 31-3 1 1 . 0 1 3 . 8 I 1 .1 0 . 2 0 . 1 0 . 5 0 . 2 0 . 3 1.9, 25-3 49 . 7 19 . 2 r 3-1 0 . 6 0 . 7 0 . 1 - - - - 0 . 4 0 . 2 0 . 1 : .2.0 . 4 4 . 9 34 .7 13 . 9 4 . 6 1 0 . 3 0 . 2 0 . 1 0 . 7 0 . 5 0 . 1 21'•. 42 . 1 20.1 30 . 3 1 6.3 0 . 2 0 . 2 0 . 2 0 . 6 J2L'U . 48 . 0 3 1 . 1 16 . 2 3 ' 1 0 . 1 0 . 6 0 . 1 0 . 1 0 . 1 21;" '. 4 4 . 6 3 1 . 7 13 . 2 5'. 5 i 0 . 1 0 . 4 0.1 3 . 6 0 . 4 0 . 4 22 . . 36.7j 38 .3 18.1, i 3-^ 0 . 4 0 . 1 2 . 2 , 0 . 4 ( 0 . 2 2 3 - 48 . 1 24.l' 18. \ 6 . 4 0 . 1 0 . 8 2 . 2 — - | 0 . 4 J 0 . 4 1 . 9 Av. 47.1 29-4 14.6 4.6 0.'9 "0.7 "'o. 2 ' 0.5 1.0 6.5 0.5 - 1 3 - CLASSIFICATION According to Lindgren 1, :I. The t r u l y character- i s t i c feature of the granodiorites i s that the soda-lime feldspar, which i s always a calcareous oligoclase or an andesine, : i s at least equal to double the amount of the a l k a l i feldspar. The l a t t e r may be taken to vary from 8 to 2 0 per cent" i n a rock with an assumed feldspar content of 6 0 per cent. The C a u l f e i l d plutonic, which i s about 6 0 per cent feldspar, has about three times as much andesine(Abg^Anyj ) as orthoclase, as well as an abundance of quartz, 'i'he rock may therefore be classed as a granodiorite, i n Lindgren 1s o c sense of the word.. In the c l a s s i f i c a t i o n by Johannsen which includes the monzonites, i t i s intermediate.between quartz monzonite and- t o n a l i t e ( q u a r t z - d i o r i t e ) and i s hence given the name monzotonalite. The two terms are synonymous since they refer to the same rock, and either may be applied correctly to the C a u l f e i l d Intrusive. OBSERVATIONS AND CONCLUSIONS (a) General nature of the rock. In most respects the granodiorite i s a normal variety with a l i t t l e more quartz and plagioclase and less mafic minerals than the average. Its average grain size i s somewhat, smaller than that common to 3 most granodiorites. A comparison between Lindgren's normal •^Waldemar Lindgren^"Granodiorite and other intermediate rocks" Amer.Jour.Sci.,IX,1900,p.277- "-Albert Johannsen: "A descriptive petrography of igneous rocks" University of Chicago Press,Chicago,111 . , . 1 9 3 2 , I I , p . 3 2 0 -'Lindgren, op. c i t . , p. 2 7 ^ - 2 7 5 & 2 8 2 . -14- . granodiorite and that of the C a u l f e i l d area i s shown below.. Lindgren's granodiorite _ C a u l f e i l d granodiorite (i ) light-colored, granular rocks Ditto . . ( i i ) composed of quartz, oligoclase or andesine,biotie and/or hornblende,orth- Ditto, titanite,magnetite, apatite, zircon ( i i i ) decomposed rock never sets free Ditto i ' ' e 2 ( - ! 3 impart red color to the rock (iv) hypidiomorphic texture Ditto (v) b i o t i t e & hornblende are sometimes, plagioclase nearly always partly i d i o - • Ditto morphic (vi) these minerals usually cemented to- Ditto gether by l a t e r mass of orthoclase qtz. ( v i i ) orthoclase may assume form of micro- cli n e ; p e r t h i t i c growth may be present in Ditto small amounts ( v i i i ) plagioclase range AbyQ to Ab.̂ g Aby^ to Abg^ (ix) biotite,dark brown normal variety Ditto (x) hornblende,green or brownish green; green;maximum maximum extinction about 1 8 - 2 0 degrees • extinct.about 3 0 deg. Mode % Flag. Qtz. Or. fr.aflcr Lindgren 44.0 2 3 . 0 14.0 14.0 C a u l f e i l d 47.1 2 9.4 1 4 . 6 6.0 -15- (b) E f f e c t s of assimilation. As well as enclos- ing the C a u l f e i l d roof-pendant, the granodiorite contains many other much smaller inclusions. These have been greatly metamorphosed and i n some cases only vague outlines or "ghosts" of them now remain in the olutonic. I t i s evident, then,that assimilation taken place to some degree. For thi s reason i t might well be suspected that absorption of foreign material by the o r i g i n a l magma may have been considerable, and that as a consquence l o c a l e f f e cts and variations should be seen i n the rock. However, the microscope show the rock to be uniform in most respects throughout 'the area. The conclusion, there- fore, i s that neither the roof-pendant nor the inclusions have been assimilated to such an extent as to have exerted an important effect on the composition of the magma. (c) History of the magma. At i t s beginning,prior to the onset of c r y s t a l l i z a t i o n , the state of the magma was one of high temperature and pressure. The f i e l d evidence - p a r t i a l absorption of inclusions and possibly the highly altered character of the roof-pendant - points c l e a r l y to such a condition. The magma was presumably produced by pro- cesses of 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 from a, more basic source-magma, according to the theory of magmatic d i f f e r e n t - i a t i o n . The course of c r y s t a l l i z a t i o n and ultimate s o l i d i f i c a t i o n of the granodiorite magma as outlined below has been interpreted from the nature of c r y s t a l boundaries of indiv i d u a l minerals and other features i n the sl i d e s . -16- The magma began c r y s t a l l i z a t i o n with the precip- i t a t i o n of small amounts of widely-scattered c r y t a l s of zircon, apatite, magnetite and t i t a n i t e . The early formation of these minerals i s proven by the fact that they occur as inclusions i n l a t e r minerals. Of them, a p a t i t e and zircon were e a r l i e r than magnetite which i s occasionally observed to enclose them. A f t e r the magma"had cooled somewhat, hornblende began to sett l e out, probably i n small quantity, from the l i q u i d u n t i l equilibrium between the two was established. A t t h i s stage began the production of plagioclase i n the form of a few small c r y s t a l s scattered t h i n l y throughout the magma. Further cool- ing and adjustment i n equilibrium brought about reaction between the l i q u i d magma and hornblende and the resultant formation of b i o t i t e , some of which began to envelope the odd small c r y s t a l of plagioclase as well as apatite etc. as i t grew. Evidence for these l a t t e r developments i s seen i n the sli d e s . In slid e # 5 , for example, a c r y s t a l of hornblende i s rimmed by b i o t i t e and i n other s l i d e s i t i s intergrown with the b i o t i t e . Not infrequently do small crystals., of plagioclase occur as inclusions i n the b i o t i t e . The magma now began the simultaneous production of b i o t i t e and plagioclase slowly but on a large scale. The contemporaneity of these two minerals i s shown by the fact that t h e i r c r y s t a l outlines often exhibit mutual interference i n development, one forming indentations i n the other. Contin- c r y s t a l i i z a t i o n f i n a l l y exhausted the magma of these two constituents. -17- At t h i s point i n the process of consolidation the magma had become a hot mush of crystals which were separated by i n t e r s t i t i a l l i q u i d of highly s i l i c i o u s , g r a n i t i c composition. F a l l i n g temperature then caused the s o l i d i f i c - ation of thi s l i q u i d into orthoclase and quartz masses whose shapes were largely governed by the d i s p o s i t i o n of the prev- iously formed cr y s t a l s . Under the microscope these masses are seen as ir r e g u l a r patches or anhedra. In the ordinary course of c r y s t a l l i z a t i o n i n g r a n i t i c magmas-, the formation of orthoclase precedes that of quartz. Here, however, to a considerable extent the reverse i s true. In some of the slides( # 6 , 9 , 1 1 , 1 6 , 1 9 , 2 2 , 2 3 ) ortv>oc I a*>e the ^tta^FcrS extends tongues or ve i n l e t s into the quartz, or p a r t i a l l y enclose*or completely engulfs quartz grains. Slide # 2 3 affords a p a r t i c u l a r i l y good i l l u s t r a t i o n of the l a s t r e lationship. The quartz, then, i s i n no small measure younger than' the orthoclase i n i t s formation. Moreover,there i s a general tendency for the quartz'- to approach euhedral form than i n the case of orthoclase; t h i s i s to be taken as additional evidence of the e a r l i e r C r y s t a l l i z a t i o n of quartz. There is,nevertheless, evidence similar to that i n the case of orthoclase that some of the quartz i s l a t e r than the, orthoclase. It i s p l a i n that there was a large overlap between the c r y s t a l l i z a t i o n periods of quartz and orthoclase end that the two were precipitated simultaneously for a time. -18- By now a l l the major constituents of the magma had been successively precipitated and s o l i d i f i e d i n accord- ance with changing physical and chemical conditions i n the cooling melt. The magma had become a body of hot, s o l i d rock. I t remained only for the ef f e c t s of gases and solution - the end products of c r y s t a l l i z a t i o n trapped i n the hot rock - to bring the history of the magma to a close. Such emanations soaked through the body of the rock, attacking and p a r t i a l l y replacing plagioclase and b i o t i t e c h i e f l y . This action Is responsible for the formation of a l l the c h l o r i t e , epidote and s e r i c i t e , some of the t i t a n i t e and possibly some of the magnetite in the.rock. Thus a good deal of the accessory minerals i n the rock owes., i t s . o r i g i n to deuteric or hydro thermal action rather than to di r e c t p r e c i p i t a t i o n from the magma. The c r i t e r i a supporting this'conclusion are far from abundant. They are as follows: (l) some of the epidote, magnetite and t i t a n i t e occurs sa ve i n - l i k e forms and i n irreg u l a r shapes which replace other minerals {: ' (2) the confinement of a p a r t i c u l a r type of a l t e r a t i o n to a p a r t i c u l a r mineral,e.g. c h l o r i t e to b i o t i t e (3) these minerals are not products of weathering by surface solutions.or other s u p e r f i c i a l agencies. In connection v/ith the l a s t point, the most noteworthy feature i s that the amount of c h l o r i t e , epidote and s e r i c i t e i s not proportional to the degree of weathering i n the rock. ~ " " - 1 9 - Very fresh specimens, for example, often contain more of these minerals than do those that are r e l a t i v e l y altered. (d) Temperature conditions i n the magma,. The general uniformity i n texture and composition in the rock t e s t i f y to uniformity i n temperature conditions during the cooling of the magma. The f a l l i n temperature was undoubtedly slow and gradual throughout. Evidence of th i s i s seen in the following (1) the common occurrence of zoning and reaction rims i n plagioclase c r y s t a l s (2) the absorption and corrosion of plagioclase borders by orthoclase and quartz. These features are hardly to be expected under any conditions other than slow cooling. (e) Pressure conditions i n the magma. The slow consolidation took place under s t a t i c pressure. Except for occasional", s l i g h t l y warped c r y s t a l s of plagioclase and b i o t i t e , strained quartz and tiny fractures, there i s no evidence whatever i n the rock of important movement having taken place i n the magma during i t s s o l i d i f i c a t i o n . SUMMARY The C a u l f e i l d plutonic i s not a quartz'-diorite but normal variety of granodiorite. Throughout the area sampled i t i s uniform i n both texture and mineral composition. There i s no evidence of i t being t r a n s i t i o n a l to either quartz - d i o r i t e or granite. I t does not seem to have been influenced greatly by assimilation of the numerous inclusions i t contains. Its genesis i s normal. With the exception of orthoclase, which c r y s t a l l i z e d abnormally late,the paragenesis i s regular. - 2 0 - BISLIOGRAPHY Rogers,A.F. and Kerr,?.F.,"Thin-section mineralogy", McGraw-Hill Book Company,Inc., New York, 1 9 3 3 Lindgren,Waldemar,"Granodiorite and other intermediate rocks", Amer.Jour.Sci.,IX,1900 Johannsen,Albert,"A descriptive petrography of igneous rocks", University of Chicago Press,Chicago, I l l i n o i s , 1 1 , 1 9 3 2 Larsen,E.S. and Miller,F.S.,"The Rosiwahl method and the modal determination of rocks",Amer. Min.,XX,1935 a* LEGEN D Po3t E.occ ne i 7 Basic DyKes U p p e r J u r a 551 c B a s i c Porphyrite DyK<fS T r i a s s i c o r P a l a e o z o i c Quartz- - di or i t« 1 Caulfe iU f orrnatiorx GEOLOGY after S H O W I N G S P E C IM E N L O C A T I O N S Sco le \ © o' = (••' r*' IP if* 0 1 1 1_ Traced. b\


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