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A study of some volcanic rocks from Harrison Mills, British Columbia Burley, Brian John 1954

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A STUDY OF SOME VOLCANIC ROCKS FROM HARRISON MILLS BRITISH COLUMBIA by BRIAN JOHN BURLEY A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in the Department of GEOLOGY AND GEOGRAPHY We accept this thesis as conforming to the standard required from candidates for the degree of MASTER OF SCIENCE Members of the Department of GEOLOGY AND GEOGRAPHY THE UNIVERSITY OF BRITISH COLUMBIA APRIL, 1954 1 ABSTRACT Large bodies of keratophyric t u f f s and keratophyric flows from Harrison M i l l s , B r i t i s h Colombia are described s t r u c t u r a l l y and p e t r o l o g i c a l l y . The o r i g i n of the a l b i t i c feldspar i n these rocks i s discussed and concluded to be primary. P e t r o l o g i c a l comparisons are made between the t u f f s and the flows. Both types of rocks were a r t i f i c i a l l y fused to glasses, and the r e f r a c t i v e indices of these glasses are compared. From these studies the t u f f s and the lavas are concluded to be consanguineous. The keratophyric flows were analysed chemically and found s i m i l a r to keratophyric rocks from other l o c a l i t i e s . The o r i g i n of the lavas i s discussed. It i s con-cluded that these rocks were probably formed by d i f f e r e n -t i a t i o n of a trondjhemitic magma possibly contaminated by soda r i c h sediments. TABLE OF CONTENTS PAGE ABSTRACT i INTRODUCTION II Previous Work i i Field Work i i Acknowledgments i i i PHYSIOGRAPHY 1 GENERAL GEOLOGY 3 STRUCTURAL GEOLOGY 7 Regional Discussion 7 Local Structures 7 (a) Folds 7 (b) Faults 8 (c) Joints 9 Tensional Joints 9 Shearing Joints 12 PETROGRAPHY . . 14 Pyroclastic rocks 14 Classification 14 Macroscopic Description 14 Microscopic Description 15 Origin of the fragments in the tuff . . 17 Description of Flows 20 Macroscopic Description . . . . . 20 TABLE OF CONTENTS CONTINUED PAGE Microscopic Description 20 Chemical Analysis 25 Origin of Textures 27 Origin of Feldspar 29 Relationship between Flows and Pyroclastics . . . 30 Fusion of rocks into glasses 32 PETROGENESIS 34 Comparison of Analyses 4-0 CONCLUSIONS 4-3 BIBLIOGRAPHY 44 APPENDIX 46 LIST OF ILLUSTRATIONS PLATE PAGE I, Enlarged part of Hope Map Sheet (737A) . . . . 3a II. Map showing structural environment of Harrison Mills Section 7a III. Differential thickness of columnar jointing. . 12a IV. Thinly bedded tuff in volcanic breccia . . . . 15a V. (a) Photomicrograph showing crystal fragments 16a (b) Photomicrograph showing l i t h i c fragments 16b VI. Photomicrograph showing small veinlets of quartz . . 17a VII. Photomicrograph Spherulitic Keratophyre . . . . 21a Vila Sketch showing major features of a spherulitic keratophyre . . 21b Vllb Diagramatic sketch of an amygdaloidal keratophyre 21c VIII. Reaction rims around corroded quartz grains . . 22a i i INTRODUCTION The area studied i s approximately four miles to the east of Harrison M i l l s , B r i t i s h Columbia on the Agassiz Road. The general area has been mentioned i n previous reports by a number of geologists including H. Bauerman (1885), G.O. Smith and F.C. Calkins (1904), R.A. Daly (1912), N.L. Bowan (1914 and C.H. Crickmay (1929-30). Most recently the Hope Map Sheet (737A) has been published by Geological Survey of Canada. The writer has obtained most of his Information from the publications of Crickmay and the explanatory notes on the Hope Map Sheet. To the best of the writer*s knowledge there has been no previous detailed p e t r o l o g i c a l descriptions of the Harrison Lake lavas. In the explanatory notes accompanying the Hope Sheet (737A) these lavas have been described as "commonly porphyritic lavas, of varying composition, assoc-iated with coarse volcanic breccias and agglomerates.*1 The area has been sparsely mineralized. The commonest mineral i s p y r i t e accompanied l o c a l l y by galena and sp h a l e r i t e . The Hell's Angel prospect i s situated approximately i n the middle of the studied outcrops. This prospect i s now abandoned. i i i The area was v i s i t e d by the w r i t e r three times during the winter season of 1953-54. The writer would l i k e to express his gratitude to Dr. K.C.McTaggert, Professor of Petrology i n the University of B r i t i s h Columbia, for his advice and constructive c r i t i c i s m s and also for the use of his personal equipment. The writer would l i k e a l so to thank Dr. H.C. Gunning, Dean of Applied Science i n the University of B r i t i s h Columbia, for his h e l p f u l suggestions on the structure of the area. Thanks are also extended to Mr. A.C .N , de Voogd for assistance i n the f i e l d , Mr. J.A. Donnan for the preparation of t h i n sections and Mrs. J.A. Donnan for the laborious task of typing the manuscript. o PHYSIOGRAPHY The area i s i n the southern t i p of the Coast Mountains. The r e l i e f of the mountains i s about 4000 feet. The Fraser River i n this area i s about seventy miles from i t s d e l t a . The r i v e r i s slow running, and forms large meanders on a correspondingly large flood p l a i n . The studied outcrops are exposed i n road cuts up to f i f t y feet high on the northern side of the Agassiz road. The southern side of the road i s a precipitous c l i f f . At the foot of the c l i f f are the sand f l a t s of the Fraser River. The walls of the section are sheer, and r a i n water runs o f f without obstruction. Two small creeks cross under the road, and these seem to have eroded along the traces of f a u l t surfaces. The three major rock types i n the area are: volcanic breccias, t u f f s and lavas. These three rock types weather i n a d i f f e r e n t manner. Hard blocky volcanic breccias form structureless c l i f f s . The matrix of the breccias weathers to a l i g h t e r green than the included fragments. The bedded tu f f s are less well cemented than the volcanic breccias and are consequently permeable, and therefore more re a d i l y altered. D i f f e r e n t i a l erosion accentuates the s t r a t i f i c a t i o n . The lavas weather much the same as the volcanic breccias, except where the i r cooling structures, such as columnar j o i n t i n g are evident. 2 The rock cuts are separated by eight talus zones. Growing upon these zones i s a dense underbrush and a medium sized growth of trees. The talus zones generally occur either on t u f f s or at junctions between contrasting rocks. This may be: f o r -tuitous, or, because most of the junctions are faulted. The slopes on either side of the f a u l t have a d i f f e r e n t angle of repose. The lesser angle i s more e a s i l y able to support vegetation. GENERAL GEOLOGY The po s i t i o n of the studied section i s shown on Plate I, an enlarged part of the Hope Sheet (737A). Bed rock units recognized i n the general area are Palaeozoic to T e r t i a r y i n age. The Palaeozoic rocks are Carboniferous i n age and consist of limestones, sandstones and a r g i l l i t e s with i n t e r -calated greenstones. These rocks are known l o c a l l y as the Chilliwack Group. Mesozoic rocks are a r e a l l y the most important within the area. During Jurassic-Cretaceous times a geosyncline (?) was f i l l e d p a r t l y by f o s s i l i f e r o u s sediments and p a r t l y by contemporaneous lavas and agglomerates. The rocks of the section studied by the writer were reported by C.H. Crickmay to be of Middle Jurassic age. This age i s based on f o s s i l s from the overlying and pre-sumably underlying beds. Most of the f o s s i l l o c a l i t i e s are along the borders of Harrison Lake and Harrison River. According to explanatory notes on the Hope Sheet, the Jurassic rocks are as follows: "The basal member i s a thick assemblage of volcanic rocks (hori z o n t a l l y line d on Plate I ) . These rocks consist c h i e f l y of massive, commonly porphyritic lavas, of d i f f e r e n t compositions, associated with coarse volcanic breccias and agglomerates. These rocks <v T PLATE I 3 Alluvium O O P Upper Jurassic • Agassiz Sp. Kent Middle Jurassic? = Billhook Mysterious Creek Middle Jurassic Lavas - Echo Island Harrison Lake Carboniferous Chilliwack Gp. Granite Jurassic 4 are overlain, conformably, by a succession of s t r a t a (diagonally l i n e d on Plate I) whose f o s s i l content indicate the s t r a t a to be of l a t e Middle Jurassic or early Upper Jurassic age. On the mountain southwest of Harrison Hot Springs, s t r a t a of early Upper Jurassic age are succeeded, uncon-formably, by the Agassiz group, which consists of a basal conglomerate about 5000 feet thick overlain by 5000 feet of sandstone and black a r g l l l i t e . No i d e n t i f i a b l e f o s s i l s have been found at this l o c a l i t y , but a narrow fringe of supposedly the same argillaceous beds on the western edge of Cascade Peninsula i n Harrison Lake yielded an Upper Jurassic ammonite." The stratigraphy of the Mesozoie volcanic rocks and sediments within the area i s itemized below: (after C.H. Crickmay). Lower Cretaceous Upper Jurassic Middle Jurassic T r i a s s i c Formation Brokenback H i l l Peninsula Agassiz P r a i r i e Kent Billhook Mysterious Creek Echo Island Harrison Lake Unnamed rocks at Camp Cove Llthology Agglomerates Sandstone A r g i l l i t e Conglomerate Tuff A r g i l l i t e Tuffs Sandstone Agglomerates Lavas Various Thickness 3,600' 1,200« 5,000' 3,000' 1,800' 2,500' 2,700' 9,200' 3,000' F o s s i l s Aucella C r a s s i o l l i s Aucella Canadiana Anacardinoceras p e r r i n i Cadoceras sp Cadoceras Schmidt L i l l o e t t i a L i l l o e t e n s i s Gylindroteuthis Themis Total thickness 32,000' Mesozoic i n t r u s i v e rocks are reported from the area. A belt of serpentine outcrops about eight miles east of the Fraser Canyon, north of Hope (see Hope Sheet). These ser-pentines are not shown on Plate I. A series of acid intrusions cut the sedimentary rocks and the serpentines. These acid rocks have been noted on the Hope Sheet as gran-i t e s , granodiorites, and quartz d i o r i t e s . The in t r u s i v e rocks are c o l l e c t i v e l y dated as Jurassic to Lower Cretaceous. Flows of Cretaceous to T e r t i a r y age are reported from the area, and are c a l l e d the Skagit Formation (see Hope Map Sheet 7 3 7 A ) . These flows are basalt, andesite and r h y o l i t e . The Skagit Formation i s not shown on Plate I. 7 STRUCTURAL GEOLOGY Regional Discussion: The described area i s about s i x miles from the eastern contact of the "Coast Range B a t h o l i t h i c complex". The structure of the area has been described by C.H. Crickmay. The folding i n the area i s considered by Crickmay to be p a r a l l e l to the f o l d i n g associated with the Laramlde orogeny (see Plate I I ) . Crickmay postulates thrusting of the formations i n the area, from the east towards the west. He suggests a steep easterly dip for the thrust planes and believes that displacements along the thrust-planes were of the order of ten miles and that a thrust f a u l t i s present along the length of Harrison Lake. In places the Harrison Lake thrust i s described as a simple break, e.g. at the southern end of the lake, but elsewhere i t i s divided into several smaller thrusts. The smaller thrusts are separated from one another by schuppen. The schuppen have been determined mainly by f o s s i l evidence. This interpretation of the structure i s not univer-s a l l y accepted. Local Structures: (a) Folds: Folds have been indicated on the map i n the PLATE II 1 Map showing structural environment of Harrison Mills Section (After C.H. Crickmay) International boundary shown. The Coast Range including plutonics and areas of dominant Jurasside structures is shown as an area of small black crosses. The distribution and trends of dominant Laramide structures are shown by dashes. The glacial plain which li e s upon the "protected salient" is shown by stippling. The heavy black line marks the southern boundary of the salient. 8 folder, by means of strike and dip symbols. The dips of the formations in the eastern part of the area are gentle and to the southwest. In the vi c i n i t y of the Hell's Angel Prospect the formations are nearly vertica l . Immediately to the west of this prospect the dips are about 40° to the north-east. This 40° dip is found throughout the western portion of the section with some steepening towards the western extremity. A syncline with a steep eastern and a gentle western limb therefore l i e s between the Hell's Angel Prospect and the western end of the section. An interesting point emerges from this structural analysis. The "prospect" is situated at a position of marked change in dip of the formations and is close to the axis of the syncline. The folds in this road section have the general trend postulated by Crickmay for the whole region. The trend of the folds in this section is parallel to the east contact of the "Coast Range Batholithic Complex" and could have been formed by the crumpling of these formations against the massif. The dating of this folding is d i f f i c u l t within the area, although i t Is post middle Jurassic. (b) Faults: The faults in the area are parallel to one another, but a few minor dislocations are exceptions to this rule. The prevalent dip is 60° S 35°W. The direction 9 of the movements on the faults was d i f f i c u l t to determine. Most of the faults appear normal i f judged by the criterion of slickensiding. The faults occur mainly in the position of maximum folding and are more common in the volcanic breccia rather than in the tuffs. This is possibly caused by the greater f l e x i b i l i t y of the tuffs compared to the volcanic breccias. The formations in the vi c i n i t y of the prospect are highly folded, and i t is believed therefore that the pros-pect l i e s on a fault. A fault has been indicated on the cross section in broken line (see folder at back). The faults are concordant with the strike of the folds and are therefore possibly a result of the adjustment to stress of the rocks during folding. Conversely the faults may be later than the folds and have occurred at the weakest parts of the formations, v i z . the crest of the syncline. Joints: Two major types of jointing are formed in the rocks. (a) Tensional joints: These joints are formed by cooling stresses. (b) Shearing joints: Joints formed possibly by deformation of the rocks. Tension Joints or Joints caused by cooling: Columnar jointing is formed at many places in the section in keratophyric flows, but i t is not inevitably 10 present, however, for reasons to be discussed later. The writer found the jointing to be best developed in the thickest bodies of keratophyres. The largest devel-opment of columnar jointing in the section is about 600 feet east of the prospect. The joints in this place are 20 feet long and 2 feet thick. Two characteristic structures commonly identified in columnar Jointing v i z : b a l l and socket structures, and Dutch-cheese structure are not found i n this area. The factors and methods of formation of columnar . ? jointing have been discussed in a paper by James. The shape, attitude and size of columns are said to depend chiefly upon (a) viscosity of lava (b) temperature of lave (c) rate of cooling (d) regularity of cooling and the homogeneity of the lava. I f the above factors are not at their optimum conditions then the formation of columns may be inhibited or very markedly retarded. James states that only the thicker flows tend to form any columnar structure because shallow flows are lnhomogeneous. The columns form i n a downward direction from the flow surface. The actual surface is generally too disturbed, and therefore the columns start forming a small distance below the surface. The length and breadth of the columns depends upon the temperature and the rate of cooling. The thinner 11 the columns, the more rapid was the cooling. James con-sidered columnar structure to be due to slow rather than rapid cooling for the following reasons: (1) Rocks have low thermal conductivity and therefore thick masses w i l l cool slowly, and i t is a fact that columns are found in thick flows rather than thin. (2) Rocks are able to resist cracking for a longer period when the tension is very slowly increased by slow cooling. If tension is more rapidly increased, then the flow w i l l break into thin columns or fragments. (3) It is frequently recorded that the top of a flow is not columnar, but platy or scoriaceous. (4) In thin sections of columnar rocks there is very l i t t l e glass indicating slow cooling. The f i r s t three of the above observations seem to be logical and upheld in the present examination. The fourth, however, Is open to query. Spherulitic structure Is not uncommon in these rocks although there is very l i t t l e glass. The origin of spherulites is uncertain, but i t is believed due to quick cooling, though not so quick as to form glass. Since cooling takes place not only from the top surface but also from any other cooling surface, columns are free to form two opposite surfaces - such an occurrence is indicated in (Plate ITTi Plate HI shows a set of thick 12 columns in between two sets of thin columns. This is interpreted as being caused by cooling from two surfaces. The columns close to the cooling surfaces tend to be long, thin and irregular (see Plate III), whilst the columns further away from the cooling surfaces are short, thick, and more perfect in shape owing to the more homo-geneous state of the lava. The upper surface of the lava tends to be more heterogeneous than the lower, because (a) gases from the lower layers are irregularly distributed near the top (b) the upper surface is churned to some extent by movement. The thin and the massive sets of columns approach and fi n a l l y merge into one another, and there is then a plane junction, which simulates the junction between two lava flows (see Plate IIIV hut the absence of vesicles and scoraceous material, the exact similarity in density, texture, and of the actual blending of the columns, in places, show them to belong to the same flow. Mallet has made an estimate of the temperature at which cracking in lavas takes place. He puts the temperature of cracking for basic silicates at between 315° C - 500° C. This evidence is based on observations of metallurgical slags and the temperatures at which lava ejections crystallise and crack. Mallet considers that the temperature of cracking may be slightly higher for acid s i l i c a t e s . Joints due to deformation of the rocks: Three joint sets were noted in the section. In the 12a PLATE III Differential thickness of columnar jointing 13 eastern part of the section the general attitude of these jo i n t sets are 7 0 ° S 4 0 ° W, 30° N 35° W and 8 0 ° N 4 0 ° W. These join t s are best formed i n a s p h e r u l i t i c keratophyre flow. The joints are not well defined i n the western end of the section. One set of joints ( 7 0 ° S 4 0 ° W) i s approximately at right angles to the a x i a l planes of the fo l d s . The second and t h i r d set of joints v i z : 30° N 35° W and 8 0 ° N 4 0 ° W are approximately p a r a l l e l to the s t r i k e of the formation and the f o l d s . D i f f i c u l t y was experienced i n determining whether the joints were due to shearing or tensional forces. No s l i c k e n s i d i n g was noticed on the j o i n t s , but t h i s i s not in e v i t a b l y present on shear j o i n t s . It i s often supposed that tension fractures break around the pebbles of a con-glomerate, or the fragments i n a breccia, and that only shear fractures cut indiscriminately across pebbles and matrix. The joints i n the area cut through the breccias as i f they were homogenous, and this evidence would there-fore point to an o r i g i n by shearing. This c r i t e r i o n i s of no great use, however, since the rocks are massive and strongly cemented. 14 PETROGRAPHY The rocks in the studied section are either pyro-clastics or fine grained igneous rocks and w i l l be dis-cussed in that order. Pyroclastic Rocks: Pirsson in 1915 described a classification of pyroclastic rocks. The writer has used this classification in the following descriptions. Pirsson recognized two main divisions of the pyroclastic rocks, tuffs and volcanic breccias. He described tuffs as having a grain size ranging from the size of a small pea to the finest dust, and volcanic breccias as having a grain size ranging from the size of a nut to the size of an apple. Furthermore following Pirsson three main types of tuffs are recognized: (1) Y i t r i c tuffs are composed mainly of glassy fragments. (2) Crystal tuffs are composed mainly of crystal fragments. (3) Lithic tuffs are composed mainly of rock fragments. Macroscopic description: Both volcanic breccias and tuffs are present in the section. The fragments in the volcanic breccias are angular. At the Hell's Angel prospect, the volcanic breccia is very coarse, with blocks ranging from 1 inch to 1 foot in diameter. 15 The volcanic breccias are a deep olive green in colour. She fragments are slightly darker than the matrix. The tuffs are a lighter green than the volcanic breccias possibly because of the preponderance of finer material. The weathered surfaces of a l l the pyroclastics have a brownish tinge. The fragments in the volcanic breccias locally contain amygdules. The amygdules range from 1/2 inch to 2 inches in diameter and are drawn out into pencil-thin pipes, or they may be e l l i p t i c a l . The amygdules are f i l l e d by calcite, chlorite, pyrite and quartz. The volcanic breccias are either massive or are only poorly s t r a t i f i e d . They show this poor str a t i f i c a t i o n at the western end of the old road cut. The thickness of the individual beds ranges from 4 inches to 6 feet. Most of the beds, however, are between 4 inches to 1 foot in thickness. The tuffs are in places very well bedded (see Plate I V ) . The contacts between the tuffs and the volcanic breccias are very sharp. There is no evidence of apophyses, and the series appears conformable. The fine tuffs perhaps by reason of their relative permeability are carbonatised. The average specific gravity of the pyroclastics is 2.68. Microscopic description: Most of the tuffs in the area are l i t h i c tuffs containing fragments of 4 mm. in size or less. The remainder l?a PLATE IV Thinly bedded tuff in Volcanic Breccia 16 are c r y s t a l t u f f s , (Plate V). There i s no microscopic evid-ence of s t r a t i f i c a t i o n . The matrix of both the l i t h i c and c r y s t a l t u f f s i s composed of broken c r y s t a l s , small rock fragments, and a l i t t l e glass. The glass fragments form shapes i n d i c a t i v e of the glassy parent material. In p a r t i c -ular they show the shape of previous s p h e r i c a l glass bubbles. The glass i s not noticeably brown, and i t i s therefore con-cluded not to be "palagonite" i . e . hydrated b a s a l t i c glass. The index of the glass i s s l i g h t l y higher than Canada Balsam. Minerals present i n the specimens are feldspar, quartz, apatite, epidote, c a l c i t e , white mica, c h l o r i t e and p y r i t e . Feldspar and quartz make up 95 per cent of the unaltered rocks. Feldspar forms about 80 per cent of the rock. In the altered rocks, carbonate and c h l o r i t e form between 30 and 50 per cent, and carbonate i s generally the more abundant of the two minerals. Most of the larger fragments of feldspar are plagioclase between Ab<^ An^ and Ab^^-An^, but a few are sodic a l i g o c l a s e Abo,© -A^ io* T h e smaller fragments of feldspar are plagioclase with a low negative r e l i e f , but because of t h e i r small size t h e i r composition was not determined. In some specimens the plagioclase i s c l e a r , but i n others i t i s turbid. The plagioclase may show a spongy texture and c r y s t a l inclusions. The commonest twinning i s on the a l b i t e law, but Mannebach and Baveno 16a PLATE V Photomicrograph showing Crystal Fragments Photomicrograph showing Lithic Fragments 17 twins are not rare. The plagioclase has been altered by white mica, c h l o r i t e , epidote and carbonate* There are two generations of quartz i n the specimens. F i r s t there are fragments of quartz up to one eighth of an inch i n diameter. These fragments are frequently corroded with large embayments and contain m i c r o l i t e s . Second the quartz i s i n small stringers (see p l a t e V I ) . The stringers of quartz are about half an inch long and about one sixteenth of an inch wide. The quartz grains i n the stringers form a mosaic, and are clear and unstained. The quartz fragments may show a wavy extinction. White mica, epidote, c a l c i t e and c h l o r i t e are found i n the specimens as a l t e r a t i o n products of feldspar, and possibly other minerals. C a l c i t e i s the commonest a l t e r -ation mineral, but i n a few specimens c h l o r i t e i s more important. Some of the specimens are almost completely carbonatised. The rock fragments are altered more than the groundmass. Chlorite p a r t i c u l a r l y appears to a l t e r the rock fragments more than the matrix. C a l c i t e i s not quite so s e l e c t i v e . Small granules of apatite and small cubes of py r i t e are present i n some specimens. The o r i g i n of the t u f f s w i l l now be discussed. It has been stated above that l i t h i c and c r y s t a l t u f f s are the commonest pyroclastics present i n the area. On the possible s i g n i f i c a n c e of this feature, Pirrson has this to say (p.lS5) PLATE VI Photomicrograph showing small veinlets of quartz "While there are many exceptions to the r u l e , i t i s true, that the most frequent examples and largest masses of v i t r i c t u f f s , are found i n those derived from r h y o l i t i c and d a c i t i c magmas, less frequently from trachytes or andesites, and s t i l l less so from basalts." From this elementary c r i t e r i o n , i t may therefore be expected that the tu f f s did not have a r h y o l i t i c or d a c i t i c o r i g i n . In considering the o r i g i n of the broken cr y s t a l s i n a t u f f there are two p o s s i b i l i t i e s . (1) The crys t a l s may be blown from an exploding p a r t l y c r y s t a l l i z e d magma. These are c a l l e d primary c r y s t a l s . (2) They may be torn from the country rock surrounding the volcanic vent. These are cal l e d secondary c r y s t a l s . To determine the o r i g i n of the c r y s t a l s i n the t u f f s , the following c r i t e r i a were used (after Pirsson). (1) The c r y s t a l i s probably secondary, i f i t i s markedly d i f f e r e n t from the remainder of the minerals i n the,rock, e.g. an o l i v i n e c r y s t a l i n a quartz-feldspar t u f f . (2) Primary c r y s t a l s , produced i n a l i q u i d magma, may be regarded as prematurely born phenocrysts, and l i k e the phenocrysts of the lavas, they may be spongy, f i l l e d with inclusions consisting of other minerals, indeterminate micr o l i t e s or blebs of glass, or contain c a v i t i e s f i l l e d 19 with liquid or gas. They may also be corroded, with deep embayments; and in addition they may have, especially with feldspars, the clear glassy appearance associated with sanidine, in contrast with the ordinary appearance of the feldspars of the granular rocks. On the other hand, secondary crystals lack these distinctive properties. These features, however, are not an invariable receipt for making v distinctions between the two types of crystal. For the secondary crystal may have been derived from older lavas. In this case, Pirsson states " i f the crystals show corrosive embayments f i l l e d with a crystallised groundmass, i t may be safely inferred that they have been derived from older lavas." Furthermore i t is the author's opinion that i f a crystal is blown out of a liquid magma, then i t w i l l have less time to react with the surrounding magma and form reaction rims, than i f i t had been allowed to remain in the magma. It seems significant therefore that the quartz keratophyres which w i l l be discussed later, commonly have broad reaction rims around the quartz. These reaction rims are absent from the quartz crystals in the tuffs. Furthermore follow-ing Pirsson's c r i t e r i a the embayments in the quartz crystals are f i l l e d with material which does not differ from the matrix. The quartz crystals as mentioned above have corrosive embayments and contain microlites. The feldspars 20 are spongy. There are no minerals present that are i n -compatible with the general mineralogical composition of the rock. The feldspars are too altered to preserve any possible glassy appearance. It is therefore suggested that the material which composes the tuff is primary and formed as a result of the explosion of lavas. The mineralogical composition of the tuffs, namely a l b i t i c feldspar and quartz, suggests that they might well be termed keratophyric or quartz keratophyric tuffs. Descriptions of the Flows: The writer intends to discuss, f i r s t l y the minerals and textures of the rocks and secondly questions arising from the interpretation of these textures and minerals. The flows compose about one third of the total rocks in the area. The fresh rock is dark green in colour, weathering to a dark grey. Occasionally because of possible hematite staining the rocks have a pinkish tinge. The rocks have white phenocrysts of feldspar with small dark green areas of chlorite. The rocks are hard, compact and d i f f i c u l t to break. They give no exterior hint as to their possible spherulitic nature. The contacts between the tuffs and the flows are d i f f i c u l t to see, except where the flows have a chilled margin. The flows are locally distinct when they exhibit columnar jointing. Amygdules are not rare and are f i l l e d mainly with quartz and carbonate. The average specific gravity of the flows is 2.65. 21 Microscopic description: There are two major divisions of flows present. In one class quartz phenocrysts are present, and in the other they are absent. Both types have porphyritic f e l d -spars. The complete l i s t of minerals present in the rocks is as follows: quartz, feldspar, white mica, chlorite, epidote, calcite, apatite, sphene, hematitic grains, sphalerite, pyrite. There are three groundmass textures namely (1) Spherulitic, (2) Intersertal, (3) Trachytic. These groundmass textures are found in both types of lavas. Both intersertal and trachytic textures are common and i t is not proposed to discuss them. The spherulitic texture, however, is uncommon and its features are therefore more fu l l y described. The Spherulitic Flows: The spherulitic texture is best seen in plane polarised light (see plate V l l ) . The size of the spherulites ranges from 2 mm. down to a size barely visible under a high power lens. The spherulites under plane polarised light are clear, widely spaced, and are set in a clear groundmass with a few turbid patches. The groundmass occupies a very small proportion of the volume of the rock. The groundmass appears to consist of i l l defined brownish granules. The rock is divided into polygonal spaces when several spherulites are developed in close juxtaposition. 21a PLATE VII Photomicrograph Spherulitic Keratophyre Plain Polarised Light Cross NIcols PLATE VIIA S k e t c h s h o w i n g m a j o r f e a t u r e s o f a S p h e r u l i t i c K e r a t o p h y r e PLATE VIIB D i a g r a m a t i c s k e t c h o f a n A m y g d a l o i d a l K e r a t o p h y r e , s h o w i n g i t s c h a r a c t e r i s t i c t e x t u r e . The g r o u n d m a s s i s a f e l t e d a g g r e g a t e o f c a r b o n a t e , c h l o r i t e , w h i t e m i c a , a n d some e p i d o t e . I n s e t i n t h i s m a t r i x a r e f e l d s p a r l a t h s and a m y g d u l e s f i l l e d w i t h c o l l o f o r m q u a r t z . T h e o p a q u e m i n e r a l i s p y r i t e . 22a PLATE VIII Reaction rims around corroded quartz grains Plain Polarised Light Crossed Nicols 2 2 The r e l i e f of the spherulites is about that of quartz. Some of the spherulites are comprised of radiating fibres. Others are seen to be an aggregation of yet smaller spherulites. Certain of the spherulites have no radial structure, and they seem to extinguish in a wavy line or show feldspar twinning. A nucleus without radial structure is seen in places where the plane of the thin section passes through the centre of a spherulite. Because of the small fibrous nature of the spherulites i t was d i f f i c u l t to determine their mineralogical composition. But as mentioned above; in places plagioclase twinning was seen. The plagioclase was too small to be determined except that i t had a low negative r e l i e f . Quartz was seen intergrown with the plagioclase. It is therefore the writer's opinion, that the spherulites are an intergrowth of sodic plagioclase and quartz. On this point, Johannson (p. 1 7 ) states "Most spherulites are found in the acid glasses, and here they usually consist of radiating intergrowths of orthoclase and quartz needles with occasional intergrown trichites of dark minerals, perhaps magnetite and augite." The quartz phenocrysts occur as rounded and corroded crystals (see Plate V i l l i A reaction rim is present around every quartz phenocryst. The rims are about 1 / 2 mm. thick and consist of a fibrous radiating aggregate of minerals. The rims appear to be of the same composition as the: spherulites, and in places are continuous with them/ The 23 rims are therefore thought to be composed of sodic f e l d -spar and quartz. ' The rims are seldom more than two layers thick. The second layer i s only formed on the smaller quartz phenocrysts. Feldspars are found both as phenocrysts and i n the groundmass. The phenocrystic feldspars are about 4 mm. long. The crystals commonly have an interpenetrant habit and tend to form together i n small c l o t s , possibly because of d r i f t i n g i n currents i n the magma? The feldspars occasionally but not inv a r i a b l y have rims around them sim i l a r to those described around the quartz. The feldspars were determined as ranging from Ab^Q An^ G to Ab-^ QQ AnQ, by means of the Michel-Levy curves for low temperature f e l d -spars, i f the high temperature curves are used then the feldspar i s s l i g h t l y more sodic. ( — r e l i e f , x'~ OlCaa -12° - A b 9 2 Aug andLOOliX = 14° = A b 1 0 Q An Q). The feldspars were further determined by means of a 4-axis Universal Stage. The method used was described by Turner. The results of the determinations are that the feldspars are close to a l b i t e i n composition and that results do not f i t Van der Kanden's curves for high temperature a l b i t e . The results are included i n the appendix. The twinning of the feldspars was determined i n the process of determining their composition on the Universal Stage. The twinning i s very commonly Mannebach, or Baveno, or a complex-Mannenbach-Albite or possibly Ala type. 24 There are no r e l i c t textures of e a r l i e r c a l c i c feldspars. The feldspars are commonly cloudy or mottled by a l t e r a t i o n . The a l t e r a t i o n minerals are c a l c i t e and white mica. There are no normal coloured minerals present. Epidote i s sparingly di s t r i b u t e d i n the specimens. In some specimens there i s about two per cent sphene. Apatite was s l i g h t l y more abundant than sphene. A small c r y s t a l of sphalerite was seen i n one specimen. This specimen was taken close to a f a u l t , and the sphalerite was probably produced by aqueous solutions passing along the f a u l t . The rocks commonly contained amygdules. The amyg-dules are f i l l e d by quartz and carbonate commonly with centres of c h l o r i t e . The colloform banding of the quartz i s well brought out by greenish-brown bands of possibly hematitic and c h l o r i t i c i n c l usions. Occasionally p y r o c l a s t i c fragments were seen i n an igneous groundmass. They are thought possibly to be ash fragments that have f a l l e n into a lava. A l t e r n a t i v e l y the fragments may represent the auto-brecclated surface of a flow. Most of the rocks are therefore considered to be extrusive, though there i s a p o s s i b i l i t y of some being shallow i n t r u s i v e s . The rocks are composed mainly of highly sodic plagioclase and quartz. It i s proposed therefore to name them quartz keratophyre and keratophyres. A specimen of quartz keratophyre was analysed by Dr. H.B. Wiik of H e l s i n k i . His results are l i s t e d below. The norm has been calculated. Oxides Wt% S i 0 2 7 4 . 6 3 A 1 2 0 3 1 2 . 4 7 Fe 0 0 . 9 2 2 3 FeO 1.28 MnO 0 . 0 5 MgO 1 . 8 2 CaO 0 . 6 8 Na 20 4 . 5 9 K 2 0 1 . 4 7 TiO, 0 . 2 5 P 2 ° 5 0 . 1 0 H 2 0 " 1.38 H 2 0 " 0 . 0 7 co 2 0 . 2 5 BaO 0 . 0 2 FeS^ 2 0 . 0 4 1 0 0 . 0 2 Mol No. Mt. 1.238 0.122 0.006 .006 0.018 .006 0.007 0.045 0.012 0.074 0.016 .016 0.003 Or. Ab. An. 9. 11 .096 .444 .024 .051 .623 .016 .074 .012 .020 .009 .045 .003 . 0 1 2 .074 .003 0.077 0 . 0 0 4 0 . 0 0 6 0 . 0 0 1 Chemical composition of a quartz keratophyre from Harrison M i l l s , B.C. along with the normative minerals recast from the oxides. ro 26 . 6 2 3 mol .016 » .074 " .068 " .021 " .006 " .003 " Q Or Ab An C Hy Mt I I 38.5% Quartz 9.37$ Orthoclase 38.15$ A l b i t e 50.95 3.45$ A n o r t h i t e 2.04$ Corundum 5.46$ Hypesthene 1.34$ Magnetite .47$ Ilmenite .04$ P y r i t e S a l 91.49$ fem. 7. T o t a l 97.82 Oxides not cast i n t o the norm are:-co 2 .25$ H 20 1.45$ P 2 O 5 0.10$ T o t a l 1.80 Grand T o t a l = 97.82 - 1.80 = 99.62$ The discrepancy between t h i s f i g u r e and 100.02$ f o r the a n a l y s i s occurs because of s i m p l i f i c a t i o n of the arithmetic i n the c a l c u l a t i o n of the normative m i n e r a l s . 27 Comments on the textures and p e c u l i a r i t i e s of the minerals: S p h e r u l i t i c structure: Johannson p. 17 states "Spherulites represent rapid growths i n a quickly cooling magma, consequently they are usually of the same composition as the rocks i n which they were formed and contain a l l the minerals which could have c r y s t a l l i s e d from that magma." This theory f i t s very well the facts observed i n the rocks from the studied area. Hany theories have been advanced to account for the o r i g i n of spherulites. It has been suggested by Cross that the magma may s p l i t into hydrous s i l i c a and feldspar. The former forming into a globule of c o l l o i d a l S i 0 2 to serve as a nucleus for separate secretions from the magma. The c o l l o i d a l sub-stance may have been e n t i r e l y removed by l a t e c r y s t a l l i s a t i o n . Origin of the quartz phenocrysts: There are two p o s s i b i l i t i e s to explain the o r i g i n of quartz phenocrysts i n the flows. In the f i r s t case the quartz "phenocryst" may be r e a l l y a quartz xenocryst. In the second case the quartz may be primary and a product of early c r y s t a l l i s a t i o n . In the f i r s t case, i f a xenolith i s caught up i n a lava then i t w i l l be gradually corroded because of i t s lack of equilibrium with the surrounding magma. The least r e-fractory minerals w i l l be dissolved f i r s t , i . e . the f e l d -28 spars and coloured minerals. The most refractory mineral, i . e . quartz would be the l a s t to react. The remaining quartz grains would be expected to have corroded lobate borders with possibly strong reaction rims. On the other hand, i f a lava composed c h i e f l y of quartz and sodic feldspar were outpoured, then quartz phenocrysts of early formation would be s l i g h t l y out of equilibrium with the magma and have corroded lobate borders. This phenomenon of corrosion w i l l be discussed more f u l l y when the o r i g i n of the reaction rims i s discussed. A xenocrystic quartz c r y s t a l might be expect-ed to have foreign inclusions i n i t , but these were not seen i n the specimens. The feldspar phenocrysts have also been attacked by the magma, proving the phenocrysts generally were out of equilibrium i n the magma. The quartz phenocrysts are thought to be the product of ordinary c r y s t a l l i s a t i o n from a magma. Origin of the reaction rims about the quartz and small plagioclase phenocrysts: The texture of a l b i t i c feldspar rims around quartz phenocrysts i n keratophyric flows has been c i t e d by G i l l u l y (p. 229) as being probably caused by sodic metasomatism. It seems to the author that there i s another possible explan-ation. From Bowens reaction s e r i e s , a l b i t e must necessarily c r y s t a l l i s e out at si m i l a r temperatures to quartz. The reaction rimmed minerals are found i n those rocks which have a s p h e r u l i t i c structure. It has already been stated that s p h e r u l i t i c structure i s an example of complex i n t e r -growth of feldspar and quartz, proving that these two minerals were c r y s t a l l i s i n g out at s i m i l a r temperatures. Furthermore i t has been stated above that the reaction rims around the phenocrysts were of the same composition as the spherulites. This evidence therefore proves that the rims are the res u l t of reaction within a magma, rather than l a t e r metasomatism. Furthermore there i s i n these rocks no proof of the existence of a l b i t i s i n g solutions. There are numer-ous amygdules and veins, but both of these are f i l l e d with quartz, c a l c i t e , with or without c h l o r i t e . These amygdules also, are not r e s t r i c t e d to those rocks i n which the reaction rims are found, and therefore have no connection with the production of the reaction rims. The presence of a double-layered reaction rim around the smaller quartz grain i s explained as follows. The smaller a grain, the greater w i l l u be the r a t i o of i t s surface area to i t s volume. Therefore the depth to which reaction can take place i s greater in a smaller grain. Hence two layered rims may form on smaller grains. Discussion on composition and twinning of the feldspar: The feldspar i s mainly a l b i t e or sodic o l i g o c l a s e . The feldspars show no r e l i c t textures of more c a l c i c f e l d -spars. They are altered considerably by c a l c i t e , white mica, and some c h l o r i t e , but there i s l i t t l e epidote. It i s there-fore the writer's opinion that the feldspars owe t h e i r a l b i t i c nature to primary c r y s t a l l i s a t i o n rather than to a l b i t e metasomatism or a a u s s u r i t i z a t i o n . It has been stated above that the twinning of the feldspars i s commonly Mannebach, Baveno, or a complex Mannebach-Albite or possibly Ala type. In a paper by Gorai, i t i s stated that Mannebach twinning i s rare i n lavas. This i s a re s u l t of a study of about two thousand specimens. The studied specimens appear to be an exception to Gorai's r u l e . There are no normal coloured minerals i n the rocks. It w i l l be seen from the chemical analysis that the rocks are high i n alumina and low i n f e r r i c and ferrous i r o n . It has been stated by Wells that this i s the reason for the absence of coloured minerals i n keratophyric rocks. Genetic relationship between the flows and t u f f s : It i s obvious from the above descriptions that there are great mineralogical s i m i l a r i t i e s between the flows and t u f f s . It i s believed by the author that the t u f f s and the flows are genetically r e l a t e d . It was decided, however, to try to f i n d some other c r i t e r i o n , d i s t i n c t from mineralog-i c a l examination, by which a genetic r e l a t i o n s h i p between the flows and t u f f s may be demonstrated. Accordingly a technique described by W.H. Mathews 31 was used. A short abstract with the s i g n i f i c a n t conclus-ions of Dr. Mathews1 paper follows: The Refractive Indices of glasses by a r t i f i c i a l fusion of samples from selected suites of igneous rocks show a close co r r e l a t i o n with the chemical composition. Each of three suites studied has i t s own c h a r a c t e r i s t i c curves r e l a t i n g Refractive Index to the chemical compos-i t i o n of analysed samples. These curves could be used to es t a b l i s h the approximate composition of other rocks from the same suites for which no chemical analysis are a v a i l a b l e . Similar relationships can be expected for the igneous rocks of many other suites and petrographic provinces. There i s therefore no reason why non-glassy rocks should not be fused into a glass, as only v o l a t i l e s would be l o s t and a change i n the valence of the i r o n . It i s to be noted that the amount of H 20 present can appreciably a f f e c t the Befractive Indices. ( T i l l e y 1922). Mathews states, however, "By a r t i f i c i a l l y eliminating water and bringing i r o n to a uniform state of oxidation, however, a comparison of Indices, with composition i n the suite may thus be aided." It was thought since the t u f f s and flows were proved i n close relationship that they probably are of the same su i t e . It was therefore to be expected that the glass of a fused t u f f would have the same Refractive Index as the glass of i t s fused parent, i . e . probably the flows. Accordingly both the lavas and the t u f f s were fused and the Refractive Indices of t h e i r glasses compared. The actual technique followed may be read i n Mathews* publication, some of the d i f f i c u l t i e s experienced, and comments upon the method, may, however, be of i n t e r e s t . F i r s t l y , though some of the carbon from the arc may get into the glass when fusing i t i s s t i l l nearly always possible to use the glass since generally one small portion of the edge i s s u i t a b l e for a Becke l i n e t e s t . The glass was nearly always i s o t r o p i c , but occas-i o n a l l y i t was a n i s t r o p i c . The index, however, did not seem to vary an appreciable amount. Because of the conchoidal fracture of the glass, the grains were p e c u l i a r l y shaped and consequently the Becke l i n e was very d i f f i c u l t to see, and occasionally gave anomalous r e s u l t s , the Becke l i n e moving i n both d i r e c t i o n s . It was therefore necessary to use more than one grain and to observe the Becke l i n e i n a l l parts of the grain. It was also necessary to use the i n -clined l i g h t method, and thus obtain more than one check. In general i t was found that the lavas fused into a more or less colourless glass, whilst the t u f f s fused to brown glass. This difference may be due i n part to the more extensive a l t e r a t i o n experienced by the t u f f s . The rocks took d i f f e r e n t times to fuse, generally i n the order of 7 seconds. It was very important to watch the fusion through dark glasses so that i t could be stopped at the exact point of fusion. F a i l u r e to do this would result i n some of the v o l a t l l e s being driven o f f , on the other hand i f fusion was stopped too soon some of the more refractory elements would not be completely melted. This may sound easy i n theory but i t i s d i f f i c u l t i n p r a c t i s e . It i s thought that such discrepancies as occur i n the r e s u l t s most commonly are due to f a i l u r e to c o r r e c t l y i d e n t i f y the time at which to stop fusion. Some of the results noted are tabulated below: Lithology No. of Specimen Refractive Index of glass 6 1.54 Very eoarse t u f f 23 1.55 Keratophyre 8 1.49 Quartz Keratophyre 22 1.49 Porphyritic Keratophyre 11 1.48 Tuff 13 1.52 Keratophyre 15 1.50 Tuff fragments i n lava 18 1.52 Tuff 25 1.52 Tuff 20 1.50 Trachytic Keratophyre 21 1.49 As a general rule i t seems that the t u f f s have a higher Refractive Index than the flows. Specimens .collect-ed i n v i c i n i t y to one another are paired together, e.g. specimen 8 and 22. It i s seen that there i s a general s i m i l a r i t y between the r e f r a c t i v e indices of t h e i r glasses. It i s the writer's opinion that the indices are close enough to warrant the conclusion that the t u f f s were formed from flows. 34 THEORIES OF PETROGENESIS Turner and Verhoogen (page 201) state that i n f o l d mountain regions, such as the western mountain zones of America or the Alpine areas of Europe, there i s everywhere evidence of igneous a c t i v i t y broadly contemporaneous with orogeny. Once the tectonic and igneous h i s t o r i e s of such regions are worked out i n d e t a i l then i t becomes clear that during a single major orogenic cycle, the type of igneous a c t i v i t y and the nature of the corresponding rock assoc-iations vary considerably. Moreover, this v a r i a t i o n tends i n many instances to conform to a single broad pattern. (1) Eruption of dominantly basic (including s p i l i t i c suite) lavas, during the geosynclinal phase of the tectonic cycle. (2) Injection of ultrabasic and basic plutonic intrusions, during the early stages of f o l d i n g . In some cases t h i s overlaps stage 1 above. (3) Development of gran o d i o r i t i c and g r a n i t i c batholiths during and following the main period of f o l d i n g . (4) Surface eruption of basalts, andesites and r h y o l i t e s , during and following elevation of the folded mass. This phase i s t y p i c a l l y separated by a lengthy period of time from the main phase of fol d i n g and plutonic a c t i v i t y . Petrogenesis of the Spilite-Keratophyre Association: Opinion today i s s t i l l markedly divided upon the following points: (1) The possible existence of a d i s t i n c t s p i l i t i c parent magma. (2) Magmatic versus metasomatic o r i g i n of the a l b i t e . (3) The r e l a t i v e roles of r e s i d u a l igneous f l u i d s and of introduced f l u i d s i n metasomatism of s p i l i t i s rocks. The problem arises whether or not there exists a primary magma, of s p i l i t i c composition, which could d i f f e r -entiate to a keratophyre magma, i f i t be assumed that magma of s p i l i t i c components i s usually generated independently of b a s a l t i c magmas. It must be further assumed that i n orgenic b e l t s , and there alone, very sp e c i a l l o c a l conditions e x i s t which lead to this development. Turner and Verhoogen think this u n l i k e l y . It seems to them more l i k e l y that the s p i l i t e s are i n some way derived from magma of o l i v i n e basalt or t h o l e i i t i c composition, either by straight d i f f e r e n t i a t i o n or contamination. This is probable when the amount of normal basalts to be found i n geosynclinal areas i s considered. The question of the o r i g i n of the a l b i t e i n these rocks, i s therefore of considerable importance. There has been produced many times convincing petrographic proof of the replacement of pre-existing feldspar by a l b i t e , i n many keratophyres. Indications of soda metasomatism, are a l b i t e - f i l l e d v e s i c l e s , v e i n l e t s of a l b i t e and i n some cases widespread development of z e o l i t e s . It has been shown experimentally by Vuoristo, Rankama Eskdla, that at temperatures below 3 3 0°C and con-f i n i n g pressures i n the v i c i n i t y of 220 atmospheres anorthite r e a d i l y reacts with NaCO^ solut i o n and S I 0 2 i n a closed system to give the assemblage a l b i t e - c a l c i t e . I f labradorite i s replaced under l i k e conditions by a l b i t e , then since the reaction proceeds without appreciable volume change, i t involves the introduction of Na and S i , and complementary removal (presumably i n solution) of Ca and A l , according to some such equation as NaCaAl 3Si^O l 6 «• N a * r S i 4 «• 2 NaAlSi^Og r Ca*"* r Al 3 " " Labradorite A l b i t e It i s commonly found, possibly as a di r e c t r e s u l t of this equation, that Ca and A l , hydrous s i l i c a t e s (epidote prehnite etc) are often c l o s e l y associated with a l b i t l s e d rocks of the s p i l i t i c association. A l b i t i z a t i o n of the plagioclase i s commonly accompanied by development of c h l o r i t e , c a l c i t e , epidote and a c t i n o l i t i c amphibole at the expense of augite i n s p i l i t i c rocks. Although the secondary o r i g i n of a l b i t e i n these rocks cannot be doubted, there are also keratophyric rocks completely lacking petrographlc indications of secondary albltization. The origin of albite in these rocks is s t i l l un-certain. Petrological attention to the albite rich lavas was much increased by the appearance in 1911 of the classic paper by Dewey and Flett entitled "Some Bri t i s h pillow lavas and the rocks associated with them." It was suggested that a " s p i l i t i c suite" should be recognized, including p i c r i t e , albite diabase, keratophyre, quartz, keratophyre, soda f e l s i t e and albite granite. Dewey and Flett believed that the albite in these rocks was formed by albltization. This albltization was believed by them to be a juvenile alteration of rock masses caused by the same agents that produce the adinoles that are commonly associated with them. An excellent summary of the views of workers on the s p i l i t e problem, may be found in Gilluly's paper. It is argued by Eskola, Beskow, Daly and Sundius (1915) that they are derived from rocks of a normal alkali-calcic parentage. Dewey, Flett, Geijer, Wells, Backlund and Sundius (1930) argue that they are derived from independent magma suite. The soda rich character is thought to be determined in the magma by Dewey, Flett, Daly, Geijer, Wells, Backlund, Sundius, and Eskola, and as a product of post-magmatic influences by Termier, Sundius (1915) and Beskow. It was thought that their mineral composition (as distinct from chemical composition) was primary for the whole suite by Sundius ( 1 9 3 0 ) . Dewey, Flet t , Geijer, Wells, Beskow and Backlund thought the mineral composition was primary for the more siliceous members of the suite, viz. keratophyres. Termier, Daly, Sundius ( 1 9 1 5 ) , Eskola argued that the albite was metasomatic through a l l the suite. A l l the workers except Sundius ( 1930) believe the albite in the less siliceous members of the suite to be metasomatic. Gilluly states, furthermore, that the question of magmatic versus metasomatic origin of the albite of s p i l i t i c rocks, has two aspects. That referring to the saturated keratophyres and siliceous quartz keratophyres, and that referring to the sub-siliceous spilites and albite diabases. The production of siliceous rocks with notably high soda-potash ratio by crystallisation differentiation has been discussed by Goldschmidt in connection with the trondjhemitic Intrusions of West Norway. He postulated that a hydrous magma tended to the early formation of biotite with impoverishment of KgO. Any subsequent f i l t e r -ing off of the biotite tends to lower the potash content. It Is significant that many plutonic rocks found in the s p i l i t i c l o c a l i t i e s of the world have a high Na/KgO value. This would indicate a magmatic origin for the s p i l i t i c rocks. Keratophyres are generally found in geosynclinal areas. Localities from which these rocks have been re-ported are as follows: Australia (N.S.W.), East Indies, Germany, Czechoslovakia, Norway, Sweden, Wales, Cornwall, Devon, ( a l l of these are submarine occurrences). In America: the Triassic Keratophyres of Nevada are interbedded in marine sediments as are the Jurassic keratophyres of the Mother Lode country of California, and the Permian rocks of Eastern Oregon. On the other hand the Tertiary Keratophyres of Nevada are sub aereal, and so are the Ordovician Skomer Rocks of Wales, and at least some of the ultra-sodic halleflintas of Sweden. The following table compares some of the analyses from other parts of the world with the Br i t i s h Columbia samples: 40 1 2 1 4 5 6 sio2 75.04 81.33 66.05 56.95 75.10 74.63 A1 2G 3 13.39 9.21 13.29 17.87 12.84 12.47 F e 2 0 3 1.61 1.09 3.22 4.49 0.70 0.92 FeO .37 .74 5.07 6.00 1.36 1.28 MgO .18 .40 1.36 0.93 0.30 1.82 CaO .40 .25 .50 2.30 0.32 0.68 Na 20 6.36 3.25 6.67 8.80 5.12 4.59 K 20 .83 1.66 .87 0.38 2.39 1.47 H 0* 2 1.07 1.12 1.88 0.71 0.95 1.38 H 20- .24 .15 .96 0.38 0.27 0.07 T i 0 2 .10 .25 .49 0.89 0.22 0.25 P 0^ 2 5 .08 .04 .09 .04 0.10 MnO .05 .05 0.08 0.04 0.05 BaO 0.05 0.02 Fe'S2 0.04 FeJ) 2 3 co2 .10 .10 0.91 0.03 0.25 99.82 99.64 100.45 100.73 99.82 100.02 1. Quartz, Keratophyre S.W. 1/4, sec. 21, T. 7S, R41E, Oregon, J.G. F a l r c h i l d , analyst. 2. Quartz Keratophyre s i l i c i f i e d , S.W. 1/4 see. 23, T. 73, R41E, Oregon, J.G. F a l r c h i l d , analyst. 3. Keratophyre, Trevennen, Cornwall, No. I I , p. 209, (Dewey and F l e t t ) W. Po l l a r d analyst 4. Magnetite Keratophyre, Mundle, New South Wales, (Benson op. c i t . p. 139, 1915) 5* Quartz Keratophyre, Great King Is. New Zealand, Bartrum op. c i t . p. 417. 6. Quartz Keratophyre, Harrison M i l l s , B.C. (H.B. Wiik). The s a l i e n t features of these analyses are high Na 20/K 20 r a t i o , high alumina, low iron and calcium. The evidence from the studied area bearing on the o r i g i n of keratophyres i s as follows: (1) The rocks are altered by carbonate and c h l o r i t e with, however, only minor amounts of epidote. Furthermore there are no r e l i c t textures of more c a l c i c feldspar. This would indicate that the a l b i t e i s not the r e s u l t of saussur l t l z a t i o n . (2) The reaction rims around the quartz and feldspar can be interpreted as reactions i n an igneous melt. (3) The rocks were formed i n a geosynclinal environment. In the general area there are present a belt of ul t r a b a s i c i n t r u s i v e s , acid i n t r u s i v e s , and andesite-rhyolite lavas. It does not seem i l l o g i c a l to suppose that the keratophyres represent the f i r s t part of the above mentioned igneous a c t i v i t y . It may be stated, although the area has not been adequately studied that there has not been found any evid-ence of a l b l t i z a t i o n . In f a c t , the evidence appears to 42 indicate that the rocks may have formed by differentiation of an igneous magma in a geosynclinal environment. Turner and Verhoogen (p. 210) envisage the for-mation of a s p i l i t i c magma as follows: "Such a liquid could be formed by removal of early formed olivine, pyroxene and possibly basic plagioclase in appropriate quantities from an olivine basalt magma." This might be the f i r s t step. After this Turner (p. 210) argues that there may be: "Assimilative reaction with rocks situated in the basal levels of the geosyncline, concentration of magmatic water rich in soda, and chemical activity induced by entrapped sea water and rising connate waters squeezed up from deeply buried sediments, are a l l factors of possible significance in evolution of spilites and keratophyres." CONCLUSIONS A large body of volcanic rocks of quartz kera-tophyric composition are described. They are mineralog-i c a l l y and chemically s i m i l a r to quartz keratophyres from other parts of the world. Their mineralogy and textures suggest that they formed from keratophyric magmas and are not the re s u l t of a l b l t i z a t i o n . Such magmas are possibly the result of assimilation by andesite? of soda-rich sediments. 4 4 BIBLIOGRAPHY Bailey, E.B., and Graham, G.W. "Albltization of basic plagioclase feldspars". Geol. Mag. Dec. 5, V 5, 1909. Bauerman H., "Report on the Geology of the Country near the Forty-ninth Parallel of North Latitude West of the Rocky Mountains" Report of Progress 1882-4. Geological Survey of Canada 1885. Benson, W.N., "Spilite Lavas and Radiolarion Rocks in N.S.W." Geol. Mag. Dec 5 V. 10, 1913. Bowen, N.L. "A Geological Reconnaissance of the Fraser River Valley from Lytton to Vancouver, B r i t i s h Columbia", Summary Report for 1912, Geological Survey of Canada, 1914. Cox, A.H., "Geology of District between Abereiddy and Abercastle, Pembrokeshire", Quat. Jour. Geol. Soc. V 71, 1915. Crickmay, C.H., "The Structural Connection between the Coast Range of Br i t i s h Columbia and the Cascade Range of Washington", Geol. Mag. V 67, 1930. Cross, Whitman, "Constitution and origin of spherulites in acid emptive rocks". Bul l . P h i l . Soc. Washington, XI, 1891. Daly, R.A., "Geology of the North American Cordillera at the Forty-ninth Parallel", Memoir 38, Geological Survey of Canada, 1912. Dewey and Flett, "Some Bri t i s h pillow lavas and the rocks associated with them". Geol. Mag. Dec 5, v*8, 1911. Geijer, P., "Notes on albltization and the magnetite-syenite-porphyries", Geol. Foren. FSrhandl, 1916. Gilluly, J., "Keratophyres of E. Oregon and S p i l i t e Problem?1 Americ. Journ. S c i . March 1935 Gorai, M., "Petrological Studies on Plagioclase twins" Americ. Min. V3 6, 1931 James, A.V.G., "Factors producing columnar structure in lavas near Melbourne Australia". Journ. of Geol. V 28, 1920. 45 BIBLIOGRAPHY CONTINUED Mallet, R., "On the origin and Mechanism of production of the Prismatic (or columnar) Structure of Basalt, Phi l . Mag. Hand. 1875, p. 130. Mathews, W.H. "A useful method for determining approximate composition of fine grained igneous rocks" Am. Min. 1951, V 36. Pirsson^ "Microscopical characters of volcanic tuffs" Am. Journ. Sci. V40, 1915. Smith, G.O. and Calkins, F.C. "A Geological Reconnaissance across the Cascade Range near the Forty-ninth Parallel", B u l l . 235, U.S. Geol. Survey, 1904. Sundius, N., "Vetenskapliga och pratiska undersotningar Lappland" Uppsala 1915, Beitrage zur Geologie des siidlichen Tells des Kiruna-gebiets. Termier, P., "Sur d'elimination de l a chaux par metasomatose dans les rockes eruptives basique de l a region du Pelvoux". Bul l . Soc. Geol. France, T26, 1898. Temer and Verhoogen, "Igneous and Metamorphic Petrology" Turner, F.J., "Determination of Plagioclase with 4-axis stage". Am. Min. 1947, p. 389. T i l l e y , C.E., "Density, refractivity and composition relations of some natural glasses". Min. Mag. 19, 275-294. Vuoristo U., Rakoma K, and Eskola, P., "An experimental illust r a t i o n of the s p i l i t e reaction". Comm. Geol. Finlande Bul l . 119, pp. 61-68, 1937. Wells, A.K., "Nomenclature of the Keratophyres", Geol. Mag. V 59, p. 351, 1922. Wells, A.K., "Problem of the Sp i l l t e s " , Geol. Mg. V 60, 1923. APPENDIX The feldspars were determined by the use of a 4 - axis Universal stage. The method was that described by P . J . Turner No. (fwru). The nomenclature i n the paper was followed, and i s as follows: CPj_2 ~ Composition plane of subindividual (1-2) J_CP 1 - 2 ~ P ° 1 Q of C P 1 - 2 o r normal to CP^_ 2 CL^ - Cleavage i n sub in d i v i d u a l 1. 2QP^ - Second set of twin lamellae (transverse to CP"L..2) cutting across sub i n d i v i d u a l 1. ±(001) - Pole of (001) or normal to (001) [100] L010] - a b c c r y s t a l axis respectively. J.C0033 - Normal to the c axis within the plane (010) 010 •^ 2-3 " ^ ^ n n ^ f i axis f o r subindividual 2 and 3. A^ - Optic axis i n subindividual 1. The c r y s t a l was f i r s t roughly sketched and the various sub-individuals numbered. For each sub-individual, the three p r i n c i p a l directions X Y Z were then located, with reference to the plane of the micro section. The result i s recorded, e.g. Z, 79° 30 , Y i s also plotted, and X, found by l a t e r extrapolation. The completed projection shows X, Y, Z for each sub-individual, together with poles of a l l measured com-positions and cleavage planes, projected upon the plane of the th i n seetion. It was now possible to estimate the composition of the feldspars by measuring on the projection, the mean angles between X, Y and Z and the poles of observed composition and cleavage planes, using the curves given by Turner i n the above mentioned paper. planes may be attended by considerable error, i t i s pre-fer rable to use twinning axes as axes of reference with which to compare the r e l a t i v e positions of X, Y and Z i n each sub i n d i v i d u a l . To do this i t i s f i r s t necessary to i d e n t i f y the twinning law i n question before r e f e r r i n g to the curves used for determining composition. The readings obtained are as follows: S l i d e 8 F i r s t plagioclase Since direct measurement of the crystallographic X 1 247° 347*° 340i° 7l£° 4° 21° a. 32°| 315° a. 27°T o CPi_2 51° GP 2- 3 51° ll°i Results, (see figure 1 ) Feldspar i s a normal A l - 2 X - 65° twin A l - 2 y - 29° H-2~ z - 78° From Turners Curves this gives the feldspar as pure a l b i t e and a twin axis -L 001. The feldspar i s therefore probably Mannebach. 2V close to 78°t. This would indicate that the feldspar i s of the low temperature v a r i e t y . Since on Van der Kanden's curve, a l b i t e i s 2V = 45 G(-) a x 38°T 42°t See Figure .2. This makes i t a p a r a l l e l twin. z A l - 2 - 65° Y A A l - 2 - 21° Z A A - 2 - 84° This gives a composition of pure a l b i t e S l i d e 8. No. 3. Z, - 224° 10° Y x - 123° 14° Z 2 - 172° 4° Y 2 - 81° 3^° C P 1 - 2 i o i ° 47 49 Sl i d e No. 6. No. 1 "2 CP - 149° - 252° - 172° - 262° 1-2 69 C "4 28° —> 4° 2? G 12°l P a r a l l e l or complex twin. (See Figure 3) Z * Ax.2 " 76° Y A A 1 - 2 " 21° A 1-2 = 73c Close to sodic oligoclase AD^Q AJI^Q S l i d e No. 6 . No. 2 c p l - 2 " 3 1 1 ° c p2-3 " 3 H ° *1 - 54° 37° z l - 307° 22 *2 - 324° - 60° —) 4° 1° J3 - 36° 1° Z 3 - 121° 1° 16°1 27°i This i s a normal twin. (See Figure 4) X * Al-2 67° Y * Al-2 28° Z * A-2 76° 27°t <\ 5 l ° i «, This is pure albite, and is exactly the same as feldspar noted in slide 8, No. 1. These few descriptions w i l l serve to i l l u s t r a t e the method and the results achieved. It is estimated by Turner, and others, that these results are accurate to within An 2 or An,. 51 B V Geologica l C ross S e c t i on f rom A to B To H arrison 1 0 Mills r>jm or Olo • 1 Scale=r-4ooo OH To Agasslz t i W x A-B Volcanic Breccia Bedded Tuffs Poorly Bedded Tuffs Keratophyre s Porphyrit ic Keratophyres Talus Bedding Fault Positions of fused specimens L ine of section Pace and Compass Map showing the G e o l o g y of the A r e a 


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