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The Caulfield "Quartz-diorite": a modal determination and petrographic observations Lunde, Magnus 1942-12-31

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L £b ft-) THE CAULFEILD "QUARTZ-DIORITE": MODAL DETERMINATION and PETROGRAPHIC OBSERVATIONS by Magnus Lunde -0-Thesis submitted in Partial Fulfilment the Requirements for the Degree of MASTER OF ARTS in the Department of GEOLOGY -0-The University of British Columbia April,1942 CONTENTS Introduction 1 AcknowledgementsSummary of Geology 2 Selection of Specimens .Megascopic Description 3 Microscopic Description(a) Texture 4-(b) Grain Size 4 (c) Alteration(d) Essential Minerals 5 (i) Plagioclase(ii) Quartz 6 (iii) Orthoclase . 6 (e) Secondary and Accessory Minerals . ' 7 (i) Biotite, Chlorite 7 (ii) Epidote 8 (iii) Titanite,, Rutile,Leucoxene 8 (iv) Magnetite, Apatite . 9 (v) Hornblende 9 (vi)'Sericite, Kaolinite 9 (vii) Zircon 10 (viii) Caleite, Pyrite, Hematite, Limonite ... 10 Modal Determination ' . . 10 Table of Slide Analyses 12 Classification 13 Observations and Conclusions 1(a) General Nature of the Rock 13 (b) Effects of assimilation 5 (c) History "of the Magma 1(d) Temperature Conditions in the Magma 19 (e) Pressure Conditions in the Magma 1Summary 1THE 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 -diorite that underlies the Caulfeild area, a residential district situated about one mile and a half east of Point Atkinson on the north shore of Burrard Inlet. The work,baaed, mainly on thin sections made from twenty-three carefully selected specimens, v/as carried out in the geological lab oratories of the University of British Columbia under the supervision of Dr. Swanson. ACKNOWLEDGEMENT S The v/riter is grateful to Dr. Swanson for guid ance and advice given during the preparation of the material for this thesis. The enclosed map is a tracing made by Dr. Gunning and students who carried out a geological survey of the area in 194-0. -2-SUB-MARY OF GEOLOGY The geology, which needs only brief mention, is summarized from a report by one3" of the students who examined the area with Dr. tunning. The area is underlain by the south ern extension of the Coast Range batholith which here surrounds a sma.ll, highly schistose roof-pendant known as the Caulfeild formation. The age of the batholith is possibly Upper Jurassic and that of the roof-pendant Triassic or late Paleozoic.These rocks are cut by aplite dikes and sills which grade into peg-matitic phases here and there. Post-Eocene(?) basic dikes comprise the final intrusive phase; they intrude the aplite. The reader is referred to the map for a more comprehensive picture regarding details. 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- ^> ially characteristic of the rock. Thus no rock in contact with dikes, inclusions or shears was collected; strongly weathered samples were avoided. In a word, the material picked is as far as practicable representative of the rock as a whole. 7 M.S.Lougheed > MEGASCOPIC DESCRIPTION The rock is medium-grained, light grey to light tan where weathered and has a granitic texture. The minerals seen are plagioclase, quartz, orthoclase, biotite, chlorite, hornblende,, magnetite and pyrite. The plagioclase occurs as rectangular crystals v/hich show considerable variability in size, some exceeding 5mm. in length. Striated cleavage planes are common. A few of the crystals are altered at the centre to a white opaque substance. Both quartz and orthoclase are seen as irregular patches which appear to be interstitial to to the plagioclase grains. The colorless quartz reaches dimen sions over 5mm.; the light pink orthoclase is about the same size at a maximum. Biotite, which is conspicuously plentiful, occurs as lustrous black plates that occasionally exceed 2mm. In general it is fresh but in places it has altered to green chlorite. Hornblende occurs sparingly as long black crystals. Examination under the hand-lens reveals the presence of a number of small magnetite grains. Pyrite is seen as occasional small grains and veinlets. MICROSCOPIC DESCRIPTION A brief description of the texture,. grain size, alteration, minerals and the relations between them will be given. The statements made v/ill have only general application to all the slides examined. Marked deviations from the gen eral trend v/ill be noted accordingly. (a) Texture. With the exception of the plagio--4--clase which is generally subhedral to euhedral, the essential minerals are anhedral. The texture is 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 biotite grains. Dimensions taken in this manner show that the average plagioclase crystal in each of the slides falls between 2.2mm. and 1.3mm, the av erage of all the slides being 1.7 mm. Similarily for quartz these limits are between 2.4 mm. and 1.4 mm., the average being 1.9 mm.; for biotite, between 1.6 and 0.8, the average 1.1. The orthoclase was not averaged but it has a definite tendency towards a size larger than the other minerals. Often it exceeds 7mm. and in slide #23 it reaches nearly 12 mm. The conclusion, then, is that the rock as a whole shows a rough uniformity in the size of its individual minerals. The size of the plagioclase crystals varies from tiny grains to those that commonly exceed 4 mm. in length; no one crystal was observed to go beyond 6 mm., however. Likewise quartz and orthoclase show a wide range in dimensions. Biotite tends to be more uniform, in general not deviating as greatly from its average dimensions. (c) Alteration. The rock as a whole is fairly fresh. Where alteration has taken place it is confined mainly to the plagioclase. Here it is frequently restricted to the core of the feldspar in dense masses but more often it has affected the entire feldspar in varying degrees . In a. single slide only a few of the plagioclase crystals may be altered while the remainder may be comparatively fresh. The alteration products comprise epidote chiefly, sericite, kaolin, and'-.-.Cal cite rarely. These minerals occur singly or together in vary ing proportions in the plagioclase. Quartz is practically un altered. Orthoclase has been lent a brownish turbidity by the presence of very fine-grained material, probably kaolin. Bio tite has been changed in varying degrees to chlorite but the total extent of alteration is quite small. Titanite shows more or less alteration to leucoxene. (d) Essential minerals. (i) Plagioclase. This mineral occurs as elorragated to equidimensional crystals which commonly show corroded outlines, especially in contact with orthoclase. Reaction rims are common and several may be present in a single feldspar. Zoned crystals with andesine cores and oligoclase rims are frequently seen but much of the plagioclase is also not zoned. Albite twins.are by far the most abundant;Carlsbad-albite and pericline types are observed occasionally. The composition of the plagioclase was deter mined largely from the extinction angles on albite twins. Unfortunately/some of the slides, particularily #14,15,17,20 and 21, do not contain enough suitable albite twins to permit the identification of the plagioclase with certainty. In these cases the determination of the indices of refraction by-means of oils of known refractive indices was resorted to -6-and -the' identity of the feldspar deduced from Tsuboi's chart?"* The plagioclase composition for all the slides averages out to Ab^An^i and individual crystals fall within the limits Ab74An26 and Ab64An36-(ii) Quartz. Occurs as irregular, equi-dimen-sional patches to elongated forms which act as a ground-mass for the other minerals. Now and then it contains inclus ions of corroded plagioclase as well as other minerals. But for rare sericitization and some dust-like inclusions ,it is clear and fresh. Undulating extinction is quite common. (iii) Orthoclase. Occurs'much in the same way as quartz in that it occupies the spaces between the other minerals. In slides #11 and 15 it forms tiny veinlets which follow cleavage planes and fractures in the plagioclase. Often it exhibits a poikilitic relationship to the other min erals; slide #23 illustrates this structure exceptionally well, the orthoclase serving as host to scattered crystals of plagioclase, magnetite, apatite and. biotite. Occasionally the plagioclase has been strongly corroded by the orthoclase mass and in some instances is almost completely dissolved.. In slides #5,6,14, 21, where orthoclase is in contact with the plagioclase the periphery of the latter shows under high mag nification what appears to be fine myrmekitic texture. What has up to this point been called orthoclase shows in part properties that are foreign to this mineral. For instance, in slides #1,3,5,7,8,14,16,19,20, this feldspar ^ Rogers and Kerr, Thin Section Mineralogy, p.213,1933. exhibits twinning in the form of a single set of fine lines which may be long and straight or short and scattered. More over, 2V is often too large or small for orthoclase; in sections #2,7,13, it is in some cases about 50 degrees and in other sections it appears to be definitely greater than 72 degrees, the maximum for orthoclase. From this data it is evident that in addition to, microcline and anorthoclase are also present in the rock in proportions that the writer did not try to determine. In" this paper the term orthoclase will always include anorthoclase and microcline. Another very common feature of the is the presence of small,irregular patches whose properties are not readily determinable. Rough parallel orientation of the more elongated grains in some cases suggests incipient micro-perthite. (e) Secondary and accessory minerals (i) Biotite and Chlorite. The biotite is a dark brown normal variety which usually shows subhedral outlines. Often it is altered to chlorite which forms long wedges that penetrate the cleavage; or it may be altered at its boundaries with the development of fringes of chlorite. Only in a few cases -has complete replacement taken place. Practically all the chlorite in the rock is pseudomorphous after biotite. The chlorite, which is a rather dark green, is always length fast and has anomalous interference colors of very low order. It is probably the variety penninite.Both biotite and chlorite commonly contain inclusions of apatite -8-and titanite. (ii) Epidote. This mineral occurs in two ways: one as as fine-grained alteration 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 first type of occurrence, where it is always associated v/ith plagio clase,. it is seen as small, more or less clear, irregular grains scattered throughout the plagioclase and as dark,dense masses concentrated at the cores of the plagioclase crystals; in this latter case it frequently shov/s a characteristic non descript greyish interference color of low order. In general there is a certain amount of sericite 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 is colorless to lemon yellow, rarely orange, and shows slight pleochroism. As in the case of orthoclase, the term epidote very likely includes related minerals, namely, zoisite and clinozoisite. Small grains of these minerals are difficult to distinguish from one another in thin section. (iii) Titanite,rutile,leucoxene.These minerals, almost invariably associated, are largely confined to biotite. The titanite, which is more or less altered to leucoxene, often displays a marked tendency to parallel the biotite cleavage as long irregular grains; it is also seen to follow along biotite peripheries in the form of thin ragged rims. Rutile could not be identified v/ith certainty -9-except in one or two instances where the characteristic twin ning, color and relief could be more clearly discerned under high magnification. The mineral occurs very sparingly as tiny needles and plates. (iv) Magnetite and apatite. Occur sparingly throughout the slides. Magnetite occurs as anhedral masses to well-formed crystals that seldom exceed 0.5 mm. As a rule it is fresh but occasionally it is completely or part ially altered to hematite. The apatite is of the ordinary well-crystallized variety; crystals are usually well under 0.'3 mm. (v) Hornblende. This mineral was noted in ten of the slides but only in #5,21"\22, does it attain proportion of any importance. It is dark green and has a maximum extinction of about 30 degrees. The crystals are subhedral to splendidly euhedral and range over 5'5mm. (vi) Sericite and kaolinite. While these minerals are common to most of the plagioclase, the average amount is very small. Only in a few cases, as in slides #10,13, does the feldspar display marked/sof this type; in these instances epidote,. the dominant alteration mineral in the rock, is either absent or present in small quantities only. Jiaolin tends to be a little coarser grained than the sericite. The fine-grained brownish material so frequently observed in the orthoclase is whitish and opaque in trans mitted light,, and may well be kaolin. (vii) Zircon. This mineral is dist ributed scantily throughout the slides. The crystals are stout and rounded, and are usually under 0.5 rnm. When in biotite they are sometimes surrounded by oleochroic haloes. (viii) Caleite, pyrite, hematite and limonite. All of these are comparatively rare in the rock. Caleite has probably originated through the alteration of plagioclase with which it is associated. In slide #23,the: .; only place' is at all conspicuous,, it occurs as numerous small, irregular grains in some of the plagioclase. One exceptionally large grain here reaches 0.4 mm., but the average is much smaller. Pyrite is seen as the odd grain or two in some of the thin sections. It may or may not be rim med by hematite which also replaces a little of the magnetite. Weathering of the mafic minerals has produced occasional limonite stains. MODAL DETERMINATION The percentages of individual minerals in each of the slides 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 in the modal determination of rocks. This rule has special bearing on the orthoclase, which often occupies unusually large areas. It is 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 individual slides 'will vary considerably and that a single slide cannot be taken as representative of the rock as a whole. This point is best illustrated by the analyses (see below) of slides #21', 21", 21"',.. all of which v/ere taken from handspecimen #21. However, if all the slides are pictured as combined into a single large area, then this area has the proportion(in rel ation to the largest grains that comprise one percent of a slide) required for one per cent accuracy, provided the rock is uniform. Nov; if individual slide compositions are consid ered as parts of this imaginary slide, totalled and averaged, the resulting 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 classificat ion of the rock.Because of the fine-grained nature of some of constituents and uneven distribution of others, most of the small percentages given in the table below are approximations rather than exact calculations. Daahed lines in the columns signify either absence of the mineral or its presence toan extent of less than 0.1 percent. -12- , TABLE OF SLIDE ANALYSES % Sericite Kaolin Pyrite Hematite Calcite Limonite Hn-bld. Epi. Rut. Tita. Leuc. Slide Plag. Q,tz:. Or. Bio Ohio. Mag. Ap. \ 1. ' 58.9' 17. 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 29.0 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- 34.5 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 11.4 5-f 0.3 1.1 0.4 0. 8 0.1 0.6 n o. 60.9 25. 8 7.7 2.] 0.3 0.4 0.1 1.8 0.1 0. 8 9- 49-0 26.7 10.7 8.q 0.5 2.9 0'.5 0.7 0.3 0.7 10. 36.3 26.3 23.3 i 3.9 1.4 0.4 1-3 2.5 1.0 1*7 11. 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 27. 2 20.7 0.7 0..4 4.2 0.8 1.5 14. 46.0 29.0 13.5 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 15. 6 11.9 i.4 0.7 0.3 0.1 1.3 0.3 0.4 17. 52.9 32.4 •8.5 2.21 i 1.3 1.0 0.1 0. 2 1.0 0.4 18, 51.5 31-3 11.0 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 . 44.9 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 31.1 16.2 3'1 0.1 0. 6 0.1 0.1 0.1 21;" '. 44.6 31.7 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 23- 48.1 24.l' 18. \ 6.4 0.1 0. 8 2. 2 — -| 0.4J 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 -13-CLASSIFICATION According to Lindgren1, :I. The truly character istic feature of the granodiorites is that the soda-lime feldspar, which is always a calcareous oligoclase or an andesine,: is at least equal to double the amount of the alkali feldspar. The latter may be taken to vary from 8 to 20 per cent" in a rock with an assumed feldspar content of 60 per cent. The Caulfeild plutonic, which is about 60 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, in Lindgren1s o c sense of the word.. In the classification by Johannsen which includes the monzonites, it is intermediate.between quartz monzonite and- tonalite(quartz-diorite) and is 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 Caulfeild Intrusive. OBSERVATIONS AND CONCLUSIONS (a) General nature of the rock. In most respects the granodiorite is a normal variety with a little more quartz and plagioclase and less mafic minerals than the average. Its average grain size is 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.,. 1932,II,p.320 -'Lindgren, op. cit., p. 27^-275 & 282. -14- . granodiorite and that of the Caulfeild area is shown below.. Lindgren's granodiorite _ Caulfeild granodiorite (i) light-colored, granular rocks Ditto . . (ii) composed of quartz, oligoclase or andesine,biotie and/or hornblende,orth- Ditto, titanite,magnetite, apatite, zircon (iii) decomposed rock never sets free Ditto i''e2(-!3 impart red color to the rock (iv) hypidiomorphic texture Ditto (v) biotite & hornblende are sometimes, plagioclase nearly always partly idio- • Ditto morphic (vi) these minerals usually cemented to-Ditto gether by later mass of orthoclase qtz. (vii) orthoclase may assume form of micro cline; perthitic growth may be present in Ditto small amounts (viii) 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 18-20 degrees • extinct.about 30 deg. Mode % Flag. Qtz. Or. fr.aflcr Lindgren 44.0 23.0 14.0 14.0 Caulfeild 47.1 29.4 14.6 6.0 -15-(b) Effects of assimilation. As well as enclos ing the Caulfeild roof-pendant, the granodiorite contains many other much smaller inclusions. These have been greatly metamorphosed and in some cases only vague outlines or "ghosts" of them now remain in the olutonic. It is evident, then,that assimilation taken place to some degree. For this reason it might well be suspected that absorption of foreign material by the original magma may have been considerable, and that as a consquence local effects and variations should be seen in the rock. However, the microscope show the rock to be uniform in most respects throughout 'the area. The conclusion, there fore, is 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 its beginning,prior to the onset of crystallization, the state of the magma was one of high temperature and pressure. The field evidence -partial absorption of inclusions and possibly the highly altered character of the roof-pendant - points clearly to such a condition. The magma was presumably produced by pro cesses of fractional crystallization from a, more basic source-magma, according to the theory of magmatic different iation. The course of crystallization and ultimate solidification of the granodiorite magma as outlined below has been interpreted from the nature of crystal boundaries of individual minerals and other features in the slides. -16-The magma began crystallization with the precip itation of small amounts of widely-scattered crytals of zircon, apatite, magnetite and titanite. The early formation of these minerals is proven by the fact that they occur as inclusions in later minerals. Of them, apatite and zircon were earlier than magnetite which is occasionally observed to enclose them. After the magma"had cooled somewhat, hornblende began to settle out, probably in small quantity, from the liquid until equilibrium between the two was established. At this stage began the production of plagioclase in the form of a few small crystals scattered thinly throughout the magma. Further cool ing and adjustment in equilibrium brought about reaction between the liquid magma and hornblende and the resultant formation of biotite, some of which began to envelope the odd small crystal of plagioclase as well as apatite etc. as it grew. Evidence for these latter developments is seen in the slides. In slide #5, for example, a crystal of hornblende is rimmed by biotite and in other slides it is intergrown with the biotite. Not infrequently do small crystals., of plagioclase occur as inclusions in the biotite. The magma now began the simultaneous production of biotite and plagioclase slowly but on a large scale. The contemporaneity of these two minerals is shown by the fact that their crystal outlines often exhibit mutual interference in development, one forming indentations in the other. Contin-crystaliization finally exhausted the magma of these two constituents. -17-At this point in the process of consolidation the magma had become a hot mush of crystals which were separated by interstitial liquid of highly silicious,granitic composition. Falling temperature then caused the solidific ation of this liquid into orthoclase and quartz masses whose shapes were largely governed by the disposition of the prev iously formed crystals . Under the microscope these masses are seen as irregular patches or anhedra. In the ordinary course of crystallization in granitic magmas-, the formation of orthoclase precedes that of quartz. Here, however, to a considerable extent the reverse is true. In some of the slides( #6,9,11,16,19,22,23) ortv>oc I a*>e the ^tta^FcrS extends tongues or veinlets into the quartz, or partially enclose*or completely engulfs quartz grains. Slide #23 affords a particularily good illustration of the last relationship. The quartz, then, is in no small measure younger than' the orthoclase in its formation. Moreover,there is a general tendency for the quartz'- to approach euhedral form than in the case of orthoclase; this is to be taken as additional evidence of the earlier Crystallization of quartz. There is,nevertheless, evidence similar to that in the case of orthoclase that some of the quartz is later than the, orthoclase. It is plain that there was a large overlap between the crystallization periods of quartz and orthoclase end that the two were precipitated simultaneously for a time. -18-By now all the major constituents of the magma had been successively precipitated and solidified in accord ance with changing physical and chemical conditions in the cooling melt. The magma had become a body of hot, solid rock. It remained only for the effects of gases and solution - the end products of crystallization trapped in the hot rock - to bring the history of the magma to a close. Such emanations soaked through the body of the rock, attacking and partially replacing plagioclase and biotite chiefly. This action Is responsible for the formation of all the chlorite, epidote and sericite, some of the titanite and possibly some of the magnetite in the.rock. Thus a good deal of the accessory minerals in the rock owes., its. origin to deuteric or hydro thermal action rather than to direct precipitation from the magma. The criteria supporting this'conclusion are far from abundant. They are as follows: (l) some of the epidote, magnetite and titanite occurs sa vein-like forms and in irregular shapes which replace other minerals {: '' (2) the confinement of a particular type of alteration to a particular mineral,e.g. chlorite to biotite (3) these minerals are not products of weathering by surface solutions.or other superficial agencies. In connection v/ith the last point, the most noteworthy feature is that the amount of chlorite, epidote and sericite is not proportional to the degree of weathering in the rock. ~ ""-19-Very fresh specimens, for example, often contain more of these minerals than do those that are relatively altered. (d) Temperature conditions in the magma,. The general uniformity in texture and composition in the rock testify to uniformity in temperature conditions during the cooling of the magma. The fall in temperature was undoubtedly slow and gradual throughout. Evidence of this is seen in the following (1) the common occurrence of zoning and reaction rims in plagioclase crystals (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 in the magma. The slow consolidation took place under static pressure. Except for occasional", slightly warped crystals of plagioclase and biotite, strained quartz and tiny fractures, there is no evidence whatever in the rock of important movement having taken place in the magma during its solidification. SUMMARY The Caulfeild plutonic is not a quartz'-diorite but normal variety of granodiorite. Throughout the area sampled it is uniform in both texture and mineral composition. There is no evidence of it being transitional to either quartz -diorite or granite. It does not seem to have been influenced greatly by assimilation of the numerous inclusions it contains. Its genesis is normal. With the exception of orthoclase, which crystallized abnormally late,the paragenesis is regular. -20-BISLIOGRAPHY Rogers,A.F. and Kerr,?.F.,"Thin-section mineralogy", McGraw-Hill Book Company,Inc., New York, 1933 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, Illinois, 11,1932 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 i7 Basic DyKes Upper J ura 551 c Basic Porphyrite DyK<fS Triassic or Palaeozoic Quartz- - di or it« 1 CaulfeiU f orrnatiorx GEOLOGY after S H OWING S P EC IM E N LOCATIONS Sco le \ © o' = (••' r*' IP if* 0 1 1 1_ Traced. b\ 


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