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Acid rocks associated with an intrusive complex Coppermine River area, Northwest Territories 1960

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ACID ROCKS ASSOCIATED WITH AN INTRUSIVE COMPLEX COPPERMINE RIVER AREA, NORTHWEST TERRITORIES BY WILLIAM DONALD TEDLIE B.Sc. in Engineering (Geological) University of Manitoba 1958 THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE In the Department of Geology We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , I960 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f the requirements f o r an advanced degree a t the U n i v e r s i t y o.f B r i t i s h Columbia, I agree t h a t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and study. I f u r t h e r agree that p e r m i s s i o n f o r e x t e n s i v e copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department o r by h i s r e p r e s e n t a t i v e s . I t i s understood t h a t copying or p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be allowed without my w r i t t e n p e r m i s s i o n . Department o f Geology The U n i v e r s i t y of B r i t i s h Columbia, Vancouver 3, Canada. Date A p r i l 20, 196o'.'j ABSTRACT An intrusive complex in the Coppermine River Area, Northwest Territories appears to be a greatly elongated lopolith approximately 5 miles wide and 60 miles long. Multiple intrusion and magmatic differentiation have combined to produce layers of rocks which range in composition from dunite to granophyre within the lopolith. The acid rocks of the complex were emplaced as a number of separate injections of magma after the crystalliz- ation and cooling of the basic and ultrabasic rocks. The structural relations of the acid and basic rocks indicate that the acid intrusions were accompanied by faulting and subsidence of a part of the northern end of the lopolith. A prominent textural feature of the granophyre, an oscillatory mantling of nuclei of graphic quartz and potash feldspar by quartz-free potash feldspar and plagioclase, is believed to be the result of fluctuations' in water vapour pressure during crystallization of the magma. The fragments in a breccia cemented by granophyre were probably, in part, formed by fault movements which accompanied the intrusion of the acid magma. TABLE OF CONTENTS Page INTRODUCTION 2 Location of the Area 2 Previous Work 2 Methods of Investigation 3 General Character of the Area 5 GENERAL GEOLOGY 6 Principal Rock Types 6 Regional Structural Geology 7 The Differentiated Intrusion 8 BASIC AND ULTRABASIC ROCKS . 1 1 QUARTZ GABBRO AND ASSOCIATED ROCKS 14 ACID ROCKS 17 Granophyre 1.7 Intrusive Breccia 26 Petrogenesis of the Acid Rocks 29 Structural Relations of Acid and Basic Rocks . . 44 Origin of the Breccia Fragments 52 SUMMARY AND CONCLUSIONS 56 BIBLIOGRAPHY 58 LIST OF ILLUSTRATIONS Figure Page 1 Location map 1 2 Contact between c h i l l e d granophyre and gabbro. C h i l l e d granophyre shows mosaic texture of f i n e - grained quartz and feldspar, gabbro composed of p a r t i a l l y s e r l c i t i z e d plagioclase and u r a l l t i z e d pyroxene 13 3 General Textural Relations. P a r t i a l l y altered lath-shaped potash feldspar c r y s t a l s , a few subhedral quartz and potash feldspar crystals with quartz and feldspar i n graphic intergrowth . 18 4 General Textural Relations. Similar to Figure 3 but with lath-shaped cr y s t a l s of altered plagioclase surrounded by quartz and potash feldspar i n graphic intergrowth. . . . 19 5 Zoned Quartz-Feldspar Intergrowths. Inter- growth cores, feldspar s h e l l , i n t e r s t i t i a l quartz-feldspar intergrowth, as described above. Note o p t i c a l continuity of feldspar 20 6 Zoned Quartz-Feldspar Intergrowth. Core of the quartz and feldspar, and feldspar s h e l l surrounded by quartz and feldspar i n graphic intergrowth 21 7 S h e l l of S e r i t i c i z e d Plagioclase enclosing Quartz and Potash Feldspar 22 8 S e r i t i c i z e d plagioclase which appears to be p a r t i a l l y replaced by quartz, and surrounded by quartz and potash feldspar i n graphic intergrowth 23 9 Rim of c h l o r i t e and b i o t i t e around an i n - clusion of quartzite i n granophyre 24 10 Replacement of a quartzite i n c l u s i o n i n granophyre by quartz and potash feldspar i n graphic intergrowth 25 11 Texture of c h i l l e d granophyre. Fine-grained mosaic of quartz and potash feldspar 26 Figure 12 Breccia. Fine-grained mass of quartz and feldspar, quartzite inclusion, and a few sheaf-like aggregates of lath-shaped feldspar crystals 13 Aggregate of lath-shaped feldspar crystals in breccia matrix . 1 4 Normative Quartz, Orthoclase and Plagioclase in Acid Igneous Rocks 15 Alkali Feldspar Phase Relations and the Effects of Changing Water Vapour Pressure . . . . 16 Geologic Map of Area of Study . 17 Composite Sections Map of northern part of Intrusive Complex In pooliot Block Diagram of Intrusion in pooliot' ACKNOWLEDGMENTS The writer wishes to thank Dr. K.C.McTaggart and Dr. J .V. Ross for their direction of laboratory studies, their helpful suggestions, and discussions of the results obtained. The writer is also indebted to Dr. C H . Smith of the Geological Survey of Canada for permission to use this problem as the subject of a thesis. K F I G U R E I L O C A T I O N W A P O F A R E A 2 INTRODUCTION Location of the Area The rocks to be described are located approximately 45 miles south of the settlement of Coppermine in the District of Mackenzie, Northwest Territories. The names Desolation Lake, Desperation Lake, Isolation Lake, Valley Lake, and Canico Lake which wil l appear on the enclosed maps or in the text are not off ic ial ly recognized, but are used locally. Previous Work The first white man to visit the area was Samuel Hearne, who descended the Coppermine River in 1745 in search of the deposits of native copper which were reported to exist in this area. From that time until after World War II the only persons to visi t the area were a few trappers and hunters. After the discovery of nickel deposits in northern Manitoba, the International Nickel Company began a compre- hensive exploration program throughout the western part of the Canadian Shield. A differentiated intrusion was dis- covered as a result of this exploration program and investi- gation of sulphide deposits associated with it begun. The intrusion was mapped by geologists employed by the Inter- national Nickel Company, but none of the results of these investigations has been published. 3 Mapping of the intrusion on a scale of 1 inch to 1000 feet by C H . Smith of the Geological Survey of Canada was begun in 1959. Mapping during the field season of 1959 was completed on the southern portion of the intrusion as far north as Desolation Lake. The material for this thesis was collected while the writer was engaged in mapping the northern limit of the intrusion. At this time, the i n - trusion was mapped about 4 miles further north of the limit of the mapping done by the International Nickel Company geologists. The following problems arose as a result of the mapp- ing done by the author in the northern part of the intrusion. 1. The structure of the intrusion at its northern end, and in particular the structural relation of the granophyre to the other rocks in the intrusion. 2. The origin of the granophyre. 3. The origin of a breccia associated with the granophyre. These problems wil l be discussed in the forthcoming text along with the petrology of the granophyres. Methods of Investigation The majority of the specimens used in this study were collected during the field mapping of the intrusion. These were supplemented by specimens of the basic and ultrabasic rocks collected by Dr. J.V. Ross. Representative specimens of the acid rocks were collected at 50 foot intervals on two traverses across the intrusion. Specimens from other localities were collected primarily to study special features of the rocks at these points. The laboratory work consisted of examination of thin sections and the estimation of the proportion of constituent minerals. The compositions of feldspars were determined by means of the universal stage. For the alkali feldspars, the determination of 2V and the optic orientation were found to fix the compositions. In order to determine the compositions, the measurements were applied to curves published by Tuttle''" (1952). Errors in 2V due to differences in the indices of refraction of the feld- spar crystals and hemisphere did not amount to more than 2 degrees. Plagioclase composition determinations were made with 2 the universal stage using the method described by Turner (194-7). Checks of the composition by the Carlsbad-Albite twin method gave results which were reasonably consistent with those obtained by Turner's method. Curves published by Trogger^ (1952) gave inconsistent results. No detailed study was made of the basic and ultra- basic parts of the intrusion. 1 Tuttle, O.F. : Optical Studies on Alkali Feldspars. Am. Jour. Sc. , Bowen Volume, 1952, p. 553-567. 2 Turner, F . J . : Determination of Plagioclase with a 4-axis Universal Stage. Am. Min. Vol . 32, 194-7, p. 389-410. 3 Trogger, W.E.: Tabellen zur optischen Bestimmung der gesteinbildenden Minerale. E. Schweiaerbursche, Verlags- buchhandlung, Stuttgart, 1952. 5 General Character of the Area The area lies north of the limit of trees. The land surface is gently rolling with rock outcrops rising abruptly as much as 200 feet above grassy tundra. The only major topographic feature of the area is the valley of the Copper- mine River, the bottom of which lies 800 to 1000 feet below the level of the surrounding country. Most of the lakes and streams are small and shallow. 6 GENERAL GEOLOGY Principal Rock Types4" The oldest rocks in the area are Archean or early Proterozoic sedimentary rocks which have been folded and regionally metamorphosed. In the vicinity of Fontano Lake these rocks consist principally of quartz-muscovite schists with minor interbedded metaquartzites. The grade of meta- morphism increases northward. This progressive metamorphism is characterized by the successive appearance of biotite, garnet and plagioclase porphyroblasts, and pegmatitic veins. The progressive metamorphism reaches its maximum intensity in the vicinity of the Coppermine River where the schists have been converted into paragneisses. North of Valley Lake the intensity of metamorphism decreases until the quartzites and slates at the northern limit show only slight evidence of metamorphism. The oldest intrusive rocks in the area are grano- diorites and quartz diorites with minor granites. In zones of low-grade metamorphism the contacts of these intrusives with the country rocks are sharp and discordant with l i t t l e evidence of thermal metamorphism of the country rock. In zones of high-grade metamorphism the granitic rocks are more abundant and the contacts between the intrusives and the country rocks are gradational. These contacts are character- 4 Dr. J.V.Ross: Personal Communication. 7 ized by assimilation, l i t - p a r - l i t injection, and replacement of country rock by pegmatitic veins. This basement complex is overlain unconformably by the Epworth series which is believed to be late Proterozoic in age. The basal unit of the Epworth series is an ortho- quartzite with conglomeratic facies. The orthoquartzite is overlain by conglomerate which in turn is overlain by dol- omite. The total thickness of this sequence ranges from 8500 to 9000 feet. The Epworth series is overlain with possible dis- conformity by the Coppermine series which is composed of lavas and sedimentary rocks. The thickness of the Copper- mine series ranges between 1 8 , 0 0 0 and 2 8 , 0 0 0 feet. A swarm of basic dikes which are probably equivalent in age to the Coppermine lavas cuts the older rocks and some units of the Coppermine series. The dikes are for the most part gabbroic in composition. The general trend of the swarm is northerly. The differentiated intrusion is associated with the dike swarm and is of the same age to the dike swarm. Some dikes are cut by the intrusion but it is cut in turn by younger dikes. Regional Structural Geology The general structural trend in the basement complex is northward. The schistosity which is developed along the axial plane cleavage of the folds strikes consistently to- ward the north. The strike of bedding generally is parallel to that of the schistosity. Lineations also have a northerly trend. Dips of schistosity and bedding commonly exceed 60 degrees. The Epworth and Coppermine series have been folded into a broad regional anticline whose axis trends east-west. The limbs of this regional structure dip less than 10 degrees A few small cross-folds are present on the north limb of this fold. The Differentiated Intrusion The general shape of the differentiated intrusion resembles that of a much elongated lopolith, but is more like the hull of a ship, as illustrated in the accompanying block diagram. South of the Coppermine River, erosion has removed a l l of the intrusion except the dike-like feeder or "keel". The width of the feeder varies from 800 to 1,500 feet south of the Coppermine River. North of the river the width of the lopolith increases and reaches approximately 5 miles at Desolation Lake. North of Desperation Lake the width de- creases and it pinches out south of Melville Creek. Basic and ultrabasic rocks in the intrusion show well-developed layering. The chilled margin is composed of fine-grained norite. The central portion of the feeder and a small portion of the lower part of the lopolith itself are composed of picrite. This is overlain by layers of peridot- ite , dunite, pyroxenite, and gabbro in that order. The 9 overall aspect of the successive layers is that of a series of troughs stacked one inside another, the whole sequence plunging 2 to 3 degrees north. In plan, the layering results in a symmetry of distribution of rock types about the longitudinal axis of the intrusion, slightly distorted by the effects of relief of the land surface. Metamorphism of the rocks of the basement complex by the intrusion is limited to the conversion of schists to hornfelses for a few tens of feet out from the contacts of the intrusion. Pockets of sulphides, mainly pyrrhotite and chalcopyrite, occur in breeciated zones along the contacts of the.lopolith with the country rock. The only obvious structural control of the emplace- ment of the lopolith is the regional schistosity, which apparently acted as a surface of weakness along which the dike swarm and the lopolith Intruded. In the vicinity of Canico Lake there is a definite correlation between the de- velopment of schistosity and the feeder of the lopolith. Near the contacts of the feeder the axial plane cleavage on the country rock becomes so intensely developed that bedding in the country rock is obliterated. The origin of stresses which presumably opened the fractures along which the basic rocks were intruded is unknown. The wide part of the lopo- l i t h occupies the north limb of the anticline. In this portion of the intrusion successively higher layers of rock are exposed to the north, which strongly suggests that the lopolith has been folded along with the Epworth and Copper- mine series. 10 Structures associated with the acid rocks i n the northern part of the l o p o l i t h are more complex than those i n the southern part of the intrusion and w i l l be discussed i n d e t a i l below, but the d i s t r i b u t i o n of rock types may be summarized at th i s point. In the area to be described, the eastern margin of the l o p o l i t h i s formed by a band of p e r i - d otite up to 600 feet wide. This i s bounded on the west by a band of gabbroic rocks approximately 400 feet wide. At the southern edge of the area, a small band of p e r i d o t i t e about 1000 feet long and 20 to 50 feet wide occurs to the west of the gabbro band. This p e r i d o t i t e band w i l l be referred to subsequently as the small band of p e r i d o t i t e . A sequence of rocks which w i l l be referred to as the quartz gabbro group occurs i n the southern part of the map area to the west of the small band of p e r i d o t i t e . The quartz gabbro group i s ove r l a i n to the north by granophyre which i s c h i l l e d against the gabbro to the east. The granophyre i n turn i s overlain by a breccia composed of fragments of quartzite from the basement complex In a granophyric matrix. The r e l a t i o n of the intrusion to the Epworth series can be established. The walls of the l o p o l i t h are formed by rocks of the basement complex, but a large block or r a f t of Epworth quartzite occurs within the l o p o l i t h north of Desper- ation Lake. 11 BASIC AND ULTRABASIC ROCKS Basic and ultrabasic rocks in the lopolith are ex- posed south of Desolation Lake and along the eastern side of the intrusion over its f u l l length. Many rock types are present, but the information and specimens available are not sufficient to allow any detailed study to be made. At least three types of layering are present in these rocks, which along with the attitudes of the contacts of the lopolith define the shape of the intrusion. The largest scale layering is formed by the distribution of the different rock types. These layers l ie roughly parallel to the walls of the lopolith, forming canoe-shaped masses stacked one inside the other. Within each layer two small-scale types of layering occur. One type which may be termed igneous lamination, is formed by the parallel orientation of tabular crystals. This type of layering frequently occurs within the gabbroic rocks. The second type of small-scale layering is formed by variations in the mineralogical composition within any one rock type. This layering is best developed in a troctollte band at Desolation Lake where anorthositic and olivine-rich bands occur within the troctolite. The chilled margin of the lopolith is composed of norite. The layer above i t is composed of picrite and peri- dotite. The peridotite is overlain by dunite which is over- lain by pyroxenite which in turn is overlain by gabbroic rocks. Quartz gabbro and its associated rocks, and the 12 granophyre wi l l be discussed in detail at a later point. The occurrence of plagioclase and its changes in composition suggest that one, and possibly at least two separate injections of magma occurred in this part of the lopolith. Plagioclase of composition A n ^ is present in the chilled norite. It occurs in the picrite in the "keel" or feeder as a subordinate constituent, and as an accessory mineral in the peridotite. The plagioclase disappears in the dunite and pyroxenite, and reappears in the gabbro. The composition of the plagioclase ranges from A n ^ in the chilled norite to A n ^ in the gabbro, then the anorthite content i n - creases to Any^ in the uppermost layer of the gabbro. Extensive alteration has occurred throughout the basic and ultrabasic rocks. The alteration is characterized by the serpentinization of olivine and pyroxene, and the alter- ation of plagioclase to sericite and clay minerals. A red- brown biotite occurs throughout the basic and ultrabasic rocks associated with the pyroxene. The biotite has also been affected by the alteration of the basic and ultrabasic rocks, and is partially altered to chlorite. The alteration appears to be a secondary effect rather than the result of deuteric alteration. The red-brown biotite itself is probably the result of deurterlc alter- ation of pyroxene. The alteration of the gabbro adjacent to its contact with the chilled granophyre is distinct from the general alteration of the basic rocks. The plagioclase has been p a r t i a l l y s e r i c i t i z e d , and the mafic minerals completely a l t e r e d to u r a l i t e or c h l o r i t e . The i n t r o d u c t i o n of small q u a n t i t i e s of carbonate accompanies the a l t e r a t i o n at t h i s contact. The contact between the gabbro and c h i l l e d grano- phyre i s i l l u s t r a t e d i n Figure 2. U r a l i t i z e d Pyroxene A l t e r e d P l a g i o - c l a s e Curitaot - C o n t a c t Granophyre Figure 2. Contact between c h i l l e d granophyre and gabbro. C h i l l e d granophyre shows mosaic texture of f i n e - grained quartz and f e l d s p a r , gabbro composed of p a r t i a l l y s e r i c i t i z e d p l a g i o c l a s e and u r a l i t i z e d pyroxene. QUARTZ GABBRO AND ASSOCIATED ROCKS 14 This unit includes a variety of rock types, but for convenience in mapping i t has been subdivided into two main types, the quartz gabbros and the granogabbros. This group lies above a zone of layered gabbro, with the greater pro- portion lying west of the small band of perldotite. The lowest layer of the quartz gabbro group is a fine-grained black diabasic gabbro which does not extend across the f u l l east-west width of the group. The black fine-grained diabasic unit weathers brown. The texture ranges from hypautomorphic to sub-ophitic. The rock is composed of approximately 40 per cent plagioclase, An^i-», 35 per cent of a clinopyroxene, probably augite, 8 per cent biotite, 10 per cent sericite and clay minerals, and'4 per cent chlorite. The plagioclase occurs as euhedral lath- shaped crystals which show slight normal zoning at their rims. The pyroxene occurs chiefly as anhedral grains intersti t ial to the plagioclase, but a few large lath-shaped twinned crystals are also present. A red-brown and a greenish variety of biotite occur in the rock. The red-brown variety is found as shreds and flakes associated with the pyroxene. The green- ish variety is associated with the chlorite formed by the alteration of the pyroxene. The quartz gabbros show a wide variation in composition and texture. They weather brownish. The fresh surface is mottled. The grain size ranges from medium to fine. The texture is also variable. Granitic and sub-ophitic textures are the most common. Some of the rocks of this unit show a well-developed igneous lamination formed by the parallel orientation of tabular feldspar crystals. The principal minerals in the quartz gabbros are a greenish-black pyroxene, green plagioclase, and pink potash feldspar which is inter- grown with quartz. The general trend throughout the quartz gabbro group is toward an increase in both the grain size and the quartz- potash feldspar content from the base to the top of the unit. With the increase in the content of potash feldspar and quartz, the quartz gabbro grades into granogabbro. The granogabbros also exhibit a wide range of com- position. They a l l weather brown. The fresh surface is mottled. The grain size ranges from medium to coarse. The principal mafic mineral is greenish-black pyroxene which has been altered to uralite and chlorite. The plagioclase, of composition An^ 4 , which has been largely altered to sericite and clay minerals', occurs as greenish subhedral crystals. The third major constituent is pink potash feldspar which is intergrown with quartz. Some specimens contain plates of titaniferous hematite up to 3/4 of an inch in diameter. The variations in composition in the granogabbro appear to re- semble those described by Phemister^ (1937) in the transition zone of the Sudbury Nickel Irruptive. 5 Phemister, T.C.t A Review of the Problems of the Sudbury Irruptive. Jour. Geol. Vol. 54, No. 1, 1937» p. 1-47. 1 6 The quartz and potash feldspar in the upper part of the quartz gabbro group may have been introduced from the overlying granophyre, or they may have been concentrated in the last fraction of the differentiated gabbro magma. A metasomatic origin for the quartz and: potash feld- spar appears to be unlikely since the alteration of the gabbro adjacent to its contact with the granophyre is not accompanied by the introduction of quartz and potash feldspar. The upward gradation in composition and texture from a fine-grained diabasic gabbro to a coarse granogabbro sugg- ests that the granogabbro is probably the result of magmatic differentiation and crystal settling as discussed by Bowen ( 1 9 2 8 ) . The texture, which is characterized in part by intersti t ial micropegmatite, is typical of the texture Good- speed'7 ( 1 9 5 9 ) attributes to deuteric action rather than metasomatism. 6 Bowen, N . L . : Evolution of the Igneous Rocks. Princeton University Press, 1928. 7 Goodspeed, G.E. : Some Textural Features of Mag- matic and Metasomatic Rocks, Am. Min. Vol. 44, 1959, p.211- 2 5 0 . 17 ACID ROCKS The acid rocks associated with the intrusion include at least two types, a gray granophyre and an intrusive breccia. Since mapping of the lopolith was not completed north of Desolation Lake, the exact total extent of the acid rocks is unknown. Granophyre The granophyre weathers reddish or purplish-pink. The fresh surface is medium light gray with a slight pink or brownish tinge. The depth of weathering averages 3 to 4 inches, but may reach 6 inches. Grain size ranges from medium to fine, the latter being more common. The texture appears to be hypautomorphic. Irregular patches of quartz and potash feldspar in graphic intergrowth are scattered throughout the granophyre near its contacts with the intrusive breccia. Up to 35 per cent of the rock is composed of i n - clusions of gray fine-grained quartzite. The maximum size of these inclusions is about 1/2 inch in diameter. They are well-rounded, corroded, and embayed. Narrow black rims surround the inclusions. A typical inclusion with its dark rim is illustrated in Figure 19. Scattered inclusions of slaty material are also present in the granophyre. In thin section the gray granophyre was found to be composed of 20$ to 35$ quartz, 35% to 50% potash feldspar, and up to 7% red-brown biotite. Other primary minerals i n - clude plagioclase, sphene, ruti le , and pyrite. 18 The t e x t u r e i s t y p i c a l l y g r a n o p h y r i c , as d e s c r i b e d g and i l l u s t r a t e d by Johannsen ( 1 9 3 1 ) . I t i s c h a r a c t e r i z e d by subhedral or euhedral c r y s t a l s o f potash f e l d s p a r w i t h i n t e r s t i t i a l graphic intergrowths o f quartz and p o t a s h f e l d s p a r . The g e n e r a l t a x t u r a l r e l a t i o n s are i l l u s t r a t e d i n F i g u r e s 3 and 4. F i g u r e 3. General T e x t u r a l R e l a t i o n s . P a r t i a l l y a l t e r e d lath-shaped potash f e l d s p a r c r y s t a l s , a few subhedral quartz and potash f e l d s p a r c r y s t a l s w i t h quartz and f e l d s p a r i n graphic i n t e r g r o w t h . X16. 8 Johannsen, A.: A D e s c r i p t i v e P e t r o l o g y o f the Igneous Rocks, V o l . 1 , U n i v e r s i t y o f Chicago P r e s s , 1 9 3 1 . 19 F i g u r e 4. General T e x t u r a l R e l a t i o n s . S i m i l a r to F i g u r e 4, but w i t h lath-shaped c r y s t a l s o f a l t e r e d p l a g i o c l a s e surrounded by quartz and potash f e l d s p a r i n graphic i n t e r g r o w t h . X16. The f e l d s p a r and q u a r t z - f e l d s p a r intergrowths common- ly 1 show a r e l a t i o n s h i p that may be d e s c r i b e d as o s c i l l a t o r y m a n t l i n g . A t y p i c a l mantle r e l a t i o n s h i p c o n s i s t s o f a core o f quartz and potash f e l d s p a r surrounded by a s h e l l o f f e l d - spar c o n t a i n i n g no q u a r t z , which i s i n t u r n surrounded by quartz and f e l d s p a r i n graphic i n t e r g r o w t h . The f e l d s p a r s h e l l s show c r y s t a l o u t l i n e s , but may be penetrated by i n t e r - growth m a t e r i a l which i s continuous w i t h that i n the core o f the c r y s t a l . In some mantled c r y s t a l s , the i n t e r g r o w t h core i s not pres e n t . The r e l a t i o n i s then one o f a euhedral f e l d s p a r c r y s t a l surrounded by an i n t e r g r o w t h o f quartz and f e l d s p a r . In a l l types o f m a n t l i n g , the potash f e l d s p a r i s o p t i c a l l y continuous from the core to the outer i n t e r g r o w t h . 20 Some o f the f e l d s p a r c r y s t a l s show a s l i g h t p a t chiness under crossed n i c o l s , but p e r t h i t i c intergrowths and w e l l - d e f i n e d zoning as shown by o p t i c a l p r o p e r t i e s are not apparent un- l e s s more d e t a i l e d i n v e s t i g a t i o n s are made o f the f e l d s p a r s , such as det e r m i n a t i o n o f 2V i n the core and mantle. I n t e r - growth r e l a t i o n s are i l l u s t r a t e d i n F i g u r e s 5 and 6 . Quartz-Potash F e l d s p a r i n t e r g r o w t h core Potash F e l d - spar S h e l l Quartz-Potash F e l d s p a r i n t e r g r o w t h mantel F i g u r e 5. Zoned Quartz-Feldspar I n t e r - growths. Intergrowth c o r e s , f e l d s p a r s h e l l , i n t e r s t i t i a l q u a r t z - f e l d s p a r i n t e r g r o w t h , as d e s c r i b e d above. Note o p t i c a l c o n t i n u i t y o f f e l d - spar. X47. F i g u r e 6. Zoned Quartz-Feldspar I n t e r - growth, Core o f quartz and f e l d s p a r , and f e l d s p a r s h e l l surrounded by quartz and f e l d s p a r i n graphic i n t e r - growth. X150. The compositions o f the a l k a l i f e l d s p a r s were d e t e r - mined from the value o f 2V as measured wi t h the u n i v e r s a l stage. Determination o f i n d i c e s o f r e f r a c t i o n c ould not be made due to the s m a l l s i z e o f the c r y s t a l s . The use of 2V alone may y i e l d an ambiguous composition, but good r e s u l t s were obtained by s t a i n i n g the f e l d s p a r s with sodium c o b a l t - i n i t r i t e , hence they are probably p o t a s s i u m - r i c h v a r i e t i e s . The compositions are t a b u l a t e d i n F i g u r e 16 where the f e l d s p a r compositions are shown i n r e l a t i o n to the p o s i t i o n s of the specimens across the s e c t i o n of granophyre. The m a j o r i t y of the f e l d s p a r s are o r t h o c l a s e c r y p t o p e r t h i t e s w i t h a range i n composition from Or^ c^AI^to O r i o o A b Q . 22 P l a g i o c l a s e occurs i n the granophyre which l i e s be- tween the two c h i l l e d zones. The c r y s t a l s are e i t h e r l a t h - shaped or occur as s h e l l s around intergrown quartz and potash f e l d s p a r . Because o f the e x t e n s i v e a l t e r a t i o n , the composition of the p l a g i o c l a s e c o u l d not be determined w i t h any degree of accuracy, but the a n o r t h i t e content i s pro- bably not more than 15 per cent. P l a g i o c l a s e appears to precede potash f e l d s p a r and quartz i n the p a r a g e n e s i s , and appears to be p a r t i a l l y r e p l a c e d by q u a r t z . The t e x t u r a l r e l a t i o n s of p l a g i o c l a s e are i l l u s t r a t e d i n F i g u r e s 7 and 8. Quartz and Potash Feld- spar core S e r i c i t i z e d P l a g i o c l a s e F i g u r e 7. S h e l l o f S e r i t i c i z e d P l a g i o c l a s e e n c l o s i n g Quartz and Potash F e l d s p a r . X47 23 F i g u r e 8. S e r i t i c i z e d p l a g i o c l a s e which appears to be p a r t i a l l y r e - placed by q u a r t z , and surrounded by quartz and potash f e l d s p a r i n graphic i n t e r g r o w t h . X47 The b i o t i t e i s the common red-brown v a r i e t y , w i t h p l e o c h r o i c h a l o e s . These haloes p e r s i s t as gray patches i n c h l o r i t e where the b i o t i t e has been a l t e r e d to c h l o r i t e . A l t e r a t i o n of the primary m i n e r a l s has o c c u r r e d throughout the granophyre. F e l d s p a r s have been p a r t i a l l y a l t e r e d to s e r i c i t e and c l a y m i n e r a l s , b i o t i t e to a c h l o r i t e , p r obably p e n n i n i t e , and t i t a n i u m - b e a r i n g m i n e r a l s to leucox- ene. The a l t e r a t i o n o f the f e l d s p a r s occurs i n i r r e g u l a r patches whose boundaries show no r e l a t i o n to i n d i v i d u a l c r y s t a l o u t l i n e s . Instead one area o f a t h i n s e c t i o n may show more i n t e n s i v e a l t e r a t i o n than another. The s e r i c i t i z a - t i o n o f potash f e l d s p a r i s accompanied by the replacement o f the f e l d s p a r by tourmaline. The tourmaline i s v i s i b l e i n hand specimen as r o s e t t e s o f black a c i c u l a r c r y s t a l s on j o i n t p l a n e s . In t h i n s e c t i o n , the tourmaline occurs as r a d i a t i n g aggregates o f a c i c u l a r c r y s t a l s . The c r y s t a l s a re p l e o c h r o i c from very p a l e yellow to p a l e g r e e n i s h - b l u e . The pleochroism and low b i r e f r i n g e n c e of the tourmaline suggest that i t i s an a l k a l i v a r i e t y . The black rims around the q u a r t z i t e i n c l u s i o n s were found to be composed o f c h l o r i t e w i t h minor amounts o f b i o t i t e . The i n c l u s i o n s are penetrated and p a r t i a l l y r e - placed by q u a r t z - f e l d s p a r i n t e r g r o w t h s . However, t h i s r e - placement i s not p a r t i c u l a r l y common, nor i s the amount of replacement e x t e n s i v e . A c h l o r i t e - b i o t i t e r i m around an i n - c l u s i o n i s i l l u s t r a t e d i n F i g u r e 9. F i g u r e 10 shows r e - placement of an i n c l u s i o n by quartz and intergrown potash f e l d s p a r . F i g u r e 9. Rim of c h l o r i t e and b i o t i t e around an i n c l u s i o n o f q u a r t z - i t e i n granophyre. X47. 25 F i g u r e 1 0 . Replacement of a q u a r t z i t e i n - c l u s i o n i n granophyre by quartz and potash f e l d s p a r i n graphic i n t e r g r o w t h . X 4 7 . The c h i l l e d zone i n the granophyre resembles i n hand specimen, a f i n e - g r a i n e d a c i d l a v a or a h o r n f e l s . In o u t c r o p , i t weathers to a medium brown. The f r e s h s u r f a c e i s dark brownish-gray w i t h a subconchoidal f r a c t u r e . The q u a r t z - i t e i n c l u s i o n s present i n the c h i l l e d zone are i d e n t i c a l i n appearance to those i n the c o a r s e r granophyre. In t h i n s e c t i o n the c h i l l e d granophyre was found to be composed o f a f i n e - g r a i n e d mosaic o f anhedral quartz and potash f e l d s p a r w i t h t r a c e s o f b i o t i t e and c h l o r i t e . A few p a r t i a l l y a l t e r e d phenocrysts of potash f e l d s p a r are p r e s e n t . A l t e r a t i o n i n the c h i l l e d zone i s not as extensive as i n the c o a r s e r grano- phyre, but the i n t e n s i t y o f a l t e r a t i o n i n c r e a s e s near the contact o f the c h i l l e d zone wi t h the b r e c c i a . The t e x t u r e of the c h i l l e d zone i s i l l u s t r a t e d i n Figure 11. i 26 Figure 11. Texture of c h i l l e d granophyre. Fine-grained mosaic of quartz and potash feldspar. X47. The feldspar i n the c h i l l e d zone was found to be sanidine-anorthoclase, with a composition of O^oo Aby as determined from 2V=20°. Intrusive Breccia The intrusive breccia for the most part o v e r l i e s the gray granophyre. The breccia i s composed of fragments of quartzite and gneiss derived from the basement complex i n a quartzofeldspathic matrix. The weathered and fresh surfaces of the matrix are pink. The breccia fragments, which make up from 40 per cent to 60 per cent of the rock, are mainly angular to subangular 27 blocks of quartzite with minor amounts of gneiss, both de- rived from the basement complex. The maximum size of the fragments varies up to 2 feet in the greatest dimension. The size and shape of the fragments in the breccia contrast markedly with the small rounded fragments in the granophyre. Dark rims are present around the breccia fragments, as in the granophyre. The fragments in the breccia do not show the rounding, corrosion, and embayments which are characteristic of the fragments in the granophyre. Scattered vugs occur throughout the breccia and contain chlorite and specular hematite. One small quartz vein in the breccia was found to contain abundant graphite. In thin-section, the breccia matrix is composed of a fine-grained mass of quartz and potash feldspar in graphic intergrowth. Some crystals of potash feldspar occur as radiating aggregates of lath-shaped crystals. The feldspar is extensively sericitized and partially replaced by greenish- blue tourmaline. The feldspar is orthoclase cryptoperthite of composition 0 r 7 C ? . 2 8 The t e x t u r a l r e l a t i o n s of the b r e c c i a v m a t r i x are i l l u s t r a t e d i n Figures 1 2 and 1 3 . Figure 12. B r e c c i a . Fine-grained mass of quartz and f e l d s p a r , q u a r t z i t e i n c l u s i o n , and a few s h e a f - l i k e aggregates of lath-shaped f e l d - spar c r y s t a l s . X16. Figure 13. Aggregate of lath-shaped f e l d - spar c r y s t a l s i n b r e c c i a matrix. X47. 29 Petrogenesis of the Acid Rocks Intrusion of the granophyre magmas probably occurred at depths of from 6,000 feet to 20,000 feet, depending on the thickness of the Epworth rocks and the amount of Coppermine rocks which had been deposited up to that time. Since the granophyre is chilled against the gabbro, it appears that sufficient time passed after the intrusion of the gabbro for it to cool to a temperature no higher than that due to a normal geothermal gradient, hence the temperature at the depth at which the granophyre magma was emplaced was probably no higher than the temperature that would be due to a normal geothermal gradient. 9 Tuttle (1952) discusses the alkali feldspars in extrusive, hypabyssal, and plutonic rocks. The feldspars in the granophyres are of the type regarded as hypabyssal by Tuttle. The term hypabyssal appears to be used only in a relative sense in literature to denote an environment inter- mediate between the plutonic and extrusive environments. Laves1^ (1952) states that inversion of orthoclase to microcline occurs around 700°C. With microcline present, and assuming a geothermal gradient of 50°C per mile, em- placement of the acid magmas should then occur at a depth of 14 miles or less. The maximum possible depth of intrusion, assuming a maximum thickness of Epworth and Coppermine rocks, would be about 8 miles. 9 Tuttle, O.F. : Extrusive and Plutonic Sialic Rocks, Jour. Geol., Vol. 60, 1952, p. 107-124. 10 Laves, F . : Phase Relations of the Alkali Feldspars Jour. Geol., Vol. 60, 1952, p. 436-450, 549-574. 30 The composition o f the f e l d s p a r s i n the granophyre d i f f e r s from the composition o f the f e l d s p a r s i n the a c i d rocks d i s c u s s e d by T u t t l e 1 1 (1952) i n that the f e l d s p a r s i n the granophyre i n q u e s t i o n are more p o t a s s i c . However such d i f f e r e n c e s a r e to be expected as rocks i n d i f f e r e n t p e t r o - g r a p h i c p r o v i n c e s may show d i s t i n c t i v e d i f f e r e n c e s i n t h e i r 12 chemical and m i n e r a l o g i c a l c h a r a c t e r as d e s c r i b e d by Bowen (1926). A p l o t o f the average normative q u a r t z , o r t h o c l a s e , and p l a g i o c l a s e i n the granophyre i s shown i n F i g u r e 14, along w i t h the normative p r o p o r t i o n s o f the same m i n e r a l s i n o t h e r a c i d igneous ro c k s . The norm f o r the granophyre i s computed from estimated compositions, hence the e r r o r i s probably l a r g e , but the norm i s s t i l l i n d i c a t i v e o f an abundance o f potassium i n the granophyre. In view of the hypabyssal environment i n t o which the magma i n t r u d e d and the c h i l l i n g o f the magma a g a i n s t the w a l l s o f the chamber, h i g h temperature f e l d s p a r s might be and are present i n the c h i l l e d zone. However, i n t e r m e d i a t e temperature and low temperature forms o f the same f e l d s p a r are present i n the granophyre which i s enclosed by the c h i l l e d zone. In the l i g h t o f the i n f o r m a t i o n a v a i l a b l e , the occurrence of more than one polymorph o f the same f e l d s p a r i n the same rock cannot be s a t i s f a c t o r i l y e x p l a i n e d . 11 T u t t l e , O.F.- I b i d . 12 Bowen, N.L.: E v o l u t i o n o f the Igneous Rocks. P r i n c e t o n U n i v e r s i t y P r e s s , 1928, p. 3. F I G U R E 14 N O R M A T I V E Q U A R T Z , O R T H O C L A S E AND P L A G I O C L A S E IN ACID IGNEOUS R O C K S Q r — — - — — : — : — ~ ' — Figure 14 In this respect, Tuttle and Bowen1^ (1958) state that as many as four solid state phases may coexist in the same alkali feldspar crystal. These phases may include high, intermediate, and low temperature feldspars, but up to the 13 Tuttle, O.F. , Bowen, N.L . : Origin of Granite in the Light of Experimental Studies, Geol. Soc. Am. Mem. 74, 1958. present time, no details are known of the solid state phase relations of the alkali feldspars. The rate of inversion of a feldspar from the high to the low temperature form may be a factor responsible for the presence of two forms of the same feldspar in the same grano- 14 phyre. Laves (1952) states that the solid state feldspar phases are of an order-disorder type. The degree of disorder in the crystal lattice appears to be proportional to the temperature to which the feldspar has been heated. This is shown by the progressive variation in optical orientation from microcline through orthoclase to sanidine. It is possible that the feldspar in the chilled zone crystallized at a higher temperature than the feldspar in the main body of the granophyre and thus would require more adjustment in its crystal lattice than the feldspar in the main body of the granophyre. But since the feldspar in the chilled zone crystallized f i r s t , i t should have more time for the inver- sion from a high to a low temperature form to occur. Experimental results quoted by Laves 7 (1952) in - dicate that the rates of inversion of feldspars vary in specimens which apparently have the same composition, so additional research is s t i l l necessary on this subject. If information were available on the rates of inversion of feld- spars from high to low temperature forms, it might be possible to determine whether cooling could have taken place before 14 Laves, F , : Phase Relations of the Alkali Feld- spars. Jour. Geol. vol . 60, 1952, p. "436-450. 15 Laves, F , : op. c i t . 33 the inversion of the feldspar in the chilled zone. Data on the temperature of the magma within limits, thickness of the roof of the magma chamber, and thermal conductivity of the roof rocks are available or could be computed and the information used to determine the rate of cooling of the magma. The mantled intergrowth relations of quartz and feld- spar may have two possible modes of origin. One possibility is replacement of feldspar by quartz and/or quartz feldspar intergrowths. The optical continuity of the quartz and feldspar means l i t t l e as an argument either for or against replacement. However, the boundaries of the feldspar shells which enclose the intergrowths are rectilinear. It seems .improbable that replacement would be so selective unless the feldspar possessed well-developed zoning. Zoning is present, but it is so slight that i t can only be detected by means of the universal stage. Thus a replacement origin for the zoned quartz-feldspar intergrowths is unlikely. Some of the textural relations, and in particular the zoned intergrowths in which the cores connect with the inter- growths which fringe the feldspar shell , are suggestive of the corroded partially resorbed, and skeleton phenocrysts which occur in lavas. One mechanism by which resorption could occur is by a rise in temperature of the magma, but the source of heat necessary is not apparent. One possible source of heat may be from exothermic reactions which could result from material low in Bowen1s Reaction Series assimi- lating material high in the reaction series. Both these 34 materials are present in the form of basic and ultrabasic rocks and acid magma, but the solid chilled granophyre would prevent the acid magma from coming in contact with and assim- ilating gabbro and peridotite. If the compositional zoning of the feldspar in the intergrowths and the phase relations of the alkali feldspars are examined, resorption of feldspar by the magma appears 16 to be unlikely. Tuttle and Bowen (1958) found that an increase in the pressure of water vapour in the system a l - bite-orthoclase-quartz-water lowered the crystallization temperature of the other three components. If the pressure rose in the magma after crystallization of the feldspar began, i t is possible that the feldspar would be partially or wholly resorbed. However, the slight zoning present in the feldspars indicates that the feldspar in the intergrowths fringing the shells around the intergrowth cores is slightly enriched in plagioclase components, and that this zoning is not oscillatory. According to the phase relations of the alkali feldspars, the albite-rich portion should be the first to be resorbed. This may have occurred in the case of the outer portions of the crystals, but does not apply to the ortho- clase-rich cores, which also appear to have been partly re- sorbed. 16 Tuttle, O.F . , Bowen, N.L. : Origin of Granite in tie Light of Experimental Studies. Geol. Soc. Am. Mem. 74, 1958. 35 17 Spencer (194-5) summarizes the studies made of graphic granite and the conclusions drawn by a number of investi- gators. The early investigators, (Vogt, Makinen, Eskola, Holmes) concluded that graphic intergrowths were the result of the crystallization of quartz and feldspar in eutectic proportions. Later investigators, (Schaller, Andersen, Ailing Hess) attributed graphic intergrowths to replacement. More recent investigators (Switzer, Mitchell, Uspensky) favour the crystallization of a eutectic as the origin of graphic intergrowths. Spencer discusses the arguments pro and con replacement origin and concludes that there is no proof that graphic intergrowths are due to replacement. Investigations of the quartz content of graphic intergrowths show a range of 24 to 32 per cent quartz. The estimated quartz content of the granophyre under consideration ranges from 20 to 35 per cent. The oscillatory intergrowth-feldspar-intergrowth re- lations may be explained conveniently by the quartz-feldspar phase relations. Investigations of the system albite- 18 orthoclase-quartz-water by Tuttle and Bowen (1958) disclosed that an increase in water vapour pressure displaced the quartz- feldspar boundary toward the feldspar side of the ternary f i e l d . 17 Spencer, E . : Myrmekite in Graphic Granite and Vein Perthite. Min. Mag. Vol. XXVII, No. 189, 1945, p. 79-98. 18 Tuttle, O.F. , Bowen, N.L. ; Origin of Granite in the Light of Experimental Studies. Geol. Soc. Am. Mem. 74, 195S. 36 I f c r y s t a l l i z a t i o n of the granophyre began under r e l a t i v e l y high pressure with quartz and feldspar i n eutectic proportions, for that pressure, a graphic intergrowth might form. Release of pressure either by loss of v o l a t i l e s from the magma chamber or by movement of the magma, to a region of lower pressure would cause the boundary between the quartz and feldspar f i e l d s to move toward the quartz end of the f i e l d leaving the composition of the l i q u i d i n the f e l d - spar f i e l d . When thi s occurred, feldspar shells would begin to form around the c r y s t a l l i z e d quartz-feldspar intergrowths, as well as pure feldspar c r y s t a l s . As c r y s t a l l i z a t i o n con- tinued, the composition of the l i q u i d would move toward the quartz-feldspar boundary where quartz and feldspar in graphic intergrowth would again begin to c r y s t a l l i z e . The plagioclase she l l s which surround the quartz- potash feldspar intergrowths may have originated by direct c r y s t a l l i z a t i o n under r i s i n g pressure, or by resorption of the cores of the plagioclase c r y s t a l s . 1 9 Tuttle and Bowen (1958) proposed an o r i g i n for r a p i k i v i texture which involved the lowering of the liquidus surface of the system orthoclase-alblte-quartz-water by i n - creasing water vapour pressure. The shel l s of plagioclase which, surround cores of quartz-potash feldspar Intergrowths have some s i m i l a r i t y to r a p i k i v i texture. Their proposed mechanism may be applied to the mantled intergrowths i n the 19 T u t t l e , O.F., Bowen, N.L.: Origin of Granite i n the Light of Experimental Studies. Geol. Soc. Am. Mem. 74, 1958. 37 following way. At the star t of c r y s t a l l i z a t i o n , the l i q u i d probably had the composition of a quartz-feldspar eutectic, the c r y s t a l l i z a t i o n of which produced graphic intergrowths represented i n the granophyre by the cores of the mantled intergrowths. Conditions at this stage are represented by Figure 15A. With the concentration of v o l a t i l e s and the r i s e of pressure during c r y s t a l l i z a t i o n , the liquidus surface of the system was depressed u n t i l i t intersected the solvus, as i l l u s t r a t e d i n Figure 1 5 B . At t h i s stage, two feldspars began to c r y s t a l l i z e rather than one, r e s u l t i n g i n the man- t l i n g of the intergrowths, some by potash feldspar and others by plagioclase. S h i f t s i n the pos i t i o n of the quartz-feldspar boundary trough with consequent temporary c r y s t a l l i z a t i o n of feldspar unaccompanied by quartz may account for the quartz- free feldspar mantles around the intergrowth cores. With continuing c r y s t a l l i z a t i o n , the outer mantles of quartz and feldspar i n graphic intergrowth were then produced. An al t e r n a t i v e o r i g i n for the plagioclase mantles which surround the quartz-potash feldspar intergrowths i n - 2 0 volves resorption and replacement. Studies by Carr ( 1 9 5 4 ) showed that an increase i n pressure results i n a r a i s i n g of the temperature of c r y s t a l l i z a t i o n of plagioclase. The l a t h - shaped plagioclase c r y s t a l s appear to precede quartz and potash feldspar i n the paragenesis. If these crystals formed early i n the c r y s t a l l i z a t i o n of the magma under r e l a t i v e l y 2 0 Carr, J.M.: Zoned Plagioclase i n the Layered Gabbros of the Skaergaard Intrusion. East Greenland. Min.Mag. Vol. XXX, No. 2 2 5 , 1 9 5 4 , p. 367-37^. 38 FIGURE 15 A L K A L I F E L D S P A R PHASE RELATIONS AND THE EFFECTS OF CHANGING W A T E R ^ V A P O U R PRESSURE LOW P R E S S U R E B H I G H PRESSURE L 7 0 o ' C < 1 \ O r At E F F E C T S OF I N C R E A S I N G P R E S S U R E ON T H E S Y S T E M A L B I T E - O R T H O C L A S E - Q U A R T Z R E L E V A N T TO T H E C R Y S T A L L I Z A T I O N OF _ R A P I K I V I G R A N I T E A-B-C T R E N D O F / C R Y S T A L LIZATIO OF RAPIKIVI GRANITE G- COMPOSITION OF ^GRANOPHYRE t'v->- COMPOSITION OF l'->' RAPIKIVI GRANITE 5, 3 0 - Q U A R T Z - F E L D S P A R BOUNDARY AT 5 0 0 a 3 0 0 0 KG/CM 2 39 high pressure, a drop in pressure would develop reversed zoning. If the crystals were s t i l l near the melting point of plagioclase, the more sodic cores might be resorbed leaving hollow shells of more calcic plagioclase which could be f i l l e d later by quartz and potash feldspar in graphic intergrowth. In this respect, i t should be noted that the interior boundaries of the plagioclase shells are irregular and are penetrated by quartz along cleavage planes as con- trasted with the rectilinear boundaries of the potash feld- spar shells which also enclose graphic intergrowths. Two possible patterns of environmental changes during the crystallization of the granophyre magma emerge from the foregoing discussion. The first pattern involves rising pressure, probably due to expulsion of volatiles from the magma during crystalliz- ation. The rise in pressure might account for the origin of the shells of plagioclase which surround quartz-orthoclase intergrowths. This origin is essentially similar to that proposed by Tuttle and Bowen21 (1958) for rapikivi texture. Resorption of Potash feldspar crystals with rising pressure could also account for the oscillatory mantling of feldspar and graphic intergrowths, but this origin does not take into account the selectivity of the resorption that would be necessary to form the sharp boundaries of the quartz-free 21 Tuttle, O.F. , Bowen, N.L . : Origin of Granite in the Light of Experimental Studies. Geol.Soc. Am. Mem. 74,1958 40 potash feldspar, and conflicts with the zoning and phase relations of the alkali feldspars. The second pattern is one of falling pressure during crystallization of the granophyre magma. The origin of the plagioclase shells which surround the cores of quartz and orthoclase may be accounted for in this environment, but the explanation is cumbersome. The oscillatory zoning of potash feldspar and graphic intergrowths and the presence of high and low temperature feldspars in the chilled zone and the coarse granophyre may be accounted for by crystallization of the magma under conditions of falling pressure. The role of assimilation of country rock by the granophyre magma is difficult to evaluate. Assimilation of country rock has occurred, as is shown by the corroded and embayed inclusions of quartzite in the granophyre. Assimi- lation of phyllitic rock also appears to have occurred, as frayed inclusions of slatey material are also present in the granophyre. However, i t is not known how much country rock has been assimilated or how much the composition of the original magma has been altered by the assimilated material. Graphitic phyllite was noted in one.drill core, so assimila- tion of this phyllite may have supplied the graphite found in a quartz vein in the breccia. Alteration of the granophyre, which is characterized by the formation of sericite and tourmaline, may be deuteric or it may be a later effect, with the agents responsible coming from an outside source. If crystallization occurred under conditions of decreasing pressure, i t is reasonable to suspect that the volatiles in the granophyre magma may have been lost, which could account for the falling pressure, but would make deuteric alteration unlikely. The emanations responsible for the alteration are believed to have come from the breccia. A comparison of the chilled zone material shows that the alteration is more intense in the chilled zone near the contact of the chilled zone with the intrusive breccia.  43 FIGURE 17 COMPOSITE SECTIONS S C A L E 1 l " = I 0 OO' 44 Structural Relations of Acid and Basic Rocks Stream valleys in the northern part of the lopolith provide two well-exposed sections across the intrusion. The following observations are significant in the interpretation of the structure in this part of the intrusion. The contact between the layered gabbro and quartz gabbro is sharp but not chilled. If the gabbro and quartz gabbro were the products of the differentiation of a single magma, the transition from one to the other would probably be gradual. The quartz gabbro on the other hand grades up- ward into a coarse-grained granogabbro. The thickness of the quartz gabbro layer shows a marked variation from east to west. East of the narrow band of peridotite, the quartz gabbro layer is thin. West of the narrow band of peridotite, it is thick and the base of the layer is lower than It is to the east. Granophyre is chilled against, and intrudes gabbro. This relationship was observed at several locations. The intrusive breccia cuts the chilled zone in the granophyre along the granophyre-gabbro contact, but the breccia matrix is not chilled. The contact between the breccia and granophyre is characterized by irregular pene- tration of apophyses of breccia matrix into the chilled zone and the development of irregular patches of potash feldspar in the chilled granophyre adjacent to its contact with the breccia. 45 The contact between the c h i l l e d granophyre and gabbro di p s west at 80 degrees. At the same l o c a t i o n , l a y e r i n g i n the gabbro dips west at 35 to 40 degrees. The contact on the west between granophyre and country rock appears to d i p west at a shallow angle. T h i s dip i s i n f e r r e d from the trend o f the contact where i t c r o s s e s a stream v a l l e y . No b a s i c or u l t r a b a s i c rocks are exposed i n the western part o f t h i s v a l l e y although the e l e v a t i o n here i s lower than that to the east where gabbro and p e r i d o t i t e are exposed. A c h i l l e d zone occurs i n the middle o f the granophyre. (Lack o f outcrop has prevented mapping the f u l l extent of t h i s c h i l l e d zone). The c h i l l e d zone dips about 20 degrees west and grades i n t o granophyre to the east. The western boundary i s obscured by r u b b l e . Shearing occurs i n gabbro or p e r i d o t i t e at the western edge of the i n t r u s i o n . A l s o , s l i c k e n s i d e s f r e q u e n t l y occur i n blocks o f gneiss i n the b r e c c i a . Of these s t r u c t u r a l r e l a t i o n s , the asymmetric d i s t r i b u t i o n o f rock types across the i n t r u s i o n and i n par- t i c u l a r the l a c k of b a s i c and u l t r a b a s i c rocks along the western s i d e o f the body are b e l i e v e d to be most s i g n i f i c a n t . D r i l l h o l e i n f o r m a t i o n i n d i c a t e s that u l t r a b a s i c rocks extend west across part o f the width of the i n t r u s i o n below the granophyre. Two assumptions may be made as to the o r i g i n a l form of the i n t r u s i o n at i t s n o r t h e r n end. The elongated p a t t e r n 46 of the basic and ultrabasic rocks in plan suggests that they may have the form of a composite dike. The other possibility is that the intrusion may have been a canoe-shaped body as to the south. There are several objections to the dike hypothesis. The f irst concerns the attitude and origin of the layering in the gabbro, which might be flow layering in a dike. 22 Balk (1937) discusses flow layering in dikes, quotes numerous authors, and concludes that flow layering in un- deformed dikes commonly lies parallel to the walls of the dikes. The exception to this is layering which has been drag-folded by flow. No such parallelism occurs in the i n - trusion in question and there is no evidence of deformation or drag-folding during flow. The second possible origin of the layering is by crystal settling during crystallization. In this type, there is either a gravitative stratification of crystals according to density, or gravitative accumulation so that tabular crystals l ie flat on the surface of deposition. This type of layering could occur If the magma chamber had a floor. However, since gravity and the angle of repose are involved, such layering is commonly flat or dips at moderate angles. The layering in the gabbro dips at a fairly steep angle, so presumably it did not originate with- in a dike-like body. 22 Balk, R.: Structural Behavior of Igneous Rocks, Geol. Soc. Am. Mem. 5, 1937. 4-7 However, i f the intrusion originally had a canoe- shaped form at this point as it does to the south, then gabbro and peridotite should appear on the west, provided that the intrusion has not been deformed. This absence of gabbro and peridotite along the western side of the intrusion may be explained by subsidence of the western edge of the lopolith. Simple t i l t ing of the intrusion does not fully ex- plain a l l the attitudes of layering and contacts. In the canoe-shaped part of the intrusion, layering in the basic and ultrabasic rocks dips inward along the flanks. If the top of this part of the lopolith were essentially horizontal and i f the intrusion were tilted by subsidence along the western edge and granophyre magma f i l l e d the space between the top of the mass of basic and ultrabasic rocks and the roof of the lopolith, then the granophyre-gabbro contact would dip at a shallower angle than the layering in the gabbro in the eastern half of the intrusion, but in the same direction. The actual field relations indicate that the granophyre-gabbro* contact dips more steeply than the layer- ing in the gabbro adjacent to the contact. An alternative interpretation of the structure in this part of the intrusion explains a l l the observed struct- ural relations. The absence of gabbro and peridotite could be explained by subsidence of the western part of the i n - trusion, but i f subsidence,of the western side occurred by downfaulting of a block bounded on both sides by faults, then no anomalous structural relations remain to be ex- plained. 48 The strongest evidence of subsidence of a fault- block is the discordant relations of the quartz gabbro. In the eastern portion of the intrusion the quartz gabbro layer is thin and the base of the layer is at a relatively high elevation. West of the narrow band of peridotite, the quartz gabbro layer is thicker and the base of the layer lies at a lower elevation than to the east. The structural relations of the granophyre are similar to those of the quartz gabbro, and the same arguments as to the interpretation of the structure apply. The presence of a fault along the western boundary of the lopolith is shown by the sheared gabbro or peri- dotite. The other fault is presumably located along the granophyre-gabbro contact, near the axis of the lopolith. Subsidence and intrusion of the quartz gabbro and acid rocks appear to have occurred in the following sequence. First the two faults formed and the block along the western edge of the lopolith sank. This subsidence was accompanied by the intrusion of the quartz gabbro magma. With further subsidence, the first injection of granophyre magma occurred. This magma was chilled against the walls and roof of the chamber. With more subsidence, this chilled zone was torn loose from the roof of the magma chamber and a second in- jection of granophyre magma occurred into the space west of the chilled zone. Emplacement of the breccia was the last event, since the breccia intrudes the chilled granophyre. The feeder along which the quartz gabbro magma i n - truded was not found, but the granophyre appears to have intruded along the fault near the center of the lopolith. The presence of a feeder at this location is suggested by the occurrence of chilled granophyre below the level of the base of the granophyre sheet and along the strike of the fault. The same fault presumably served as a feeder for the breccia, since chilled granophyre is cut by breccia along this l ine. One objection to the subsidence of the mass of basic and ultrabasic rocks is the lack of evidence of the collapse of the roof of the intrusion. Presumably the in- trusion of the basic magma stopped at the unconformity be- tween the basement complex and the Epworth series, but there is no conclusive evidence that the overlying Epworth series subsided with the block below. It is possible that the faults terminated at the unconformity between the Epworth series and the older rocks. While fragments of the Epworth rocks are conspicuous by their absence from the breccia, there is evidence that some parts of the roof did collapse or that blocks were stoped from i t , as is shown by the quartzite raft near Desperation Lake. However, i f the entire roof did subside, it would involve a block with a maximum width of about 3000 feet. If a section is considered across the intrusion, with a min- imum thickness of 6,300 feet of Epworth rocks, i t could be regarded as a beam with both ends fixed and a depth/span ratio of at least 2/1, assuming that none of the Coppermine rocks were present. If any of the Coppermine rocks were present, their effect would be to Increase the depth/span ratio of the roof. It is possible that the eruption of the Coppermine lavas would be accompanied by the injection of dikes and s i l l s into the Epworth series. Such a system of dikes and s i l l s might serve to bind the roof together in a manner similar to rock bolts or stitching. No intrusions with similar structural relations have been described in any of the literature examined. The only ones which have any resemblance, to the one at Desolation Lake are the Tertiary ring-dikes of the northern British Isles. In the ring-dikes, subsidence of a more or less cylindrical block bounded by a fault was accompanied by the intrusion of magma along the fault. Observed field relations indicate that intrusion of magma along faults has occurred. Some of the circular faults reached the surface, in which case the fault enclosed an area of cauldron subsidence. In other instances, the faults did not reach the surface. Structures of this type are referred to as "piston-faults" or subsurface cauldron subsidences. In these structures, the fault terminated at some level below the surface, probably along bedding plane. The block enclosed by the fault sub- sided then magma intruded along the fault and f i l l e d the space between the sunken block and the roof. 5 1 2 3 24 Papers by Bailey and Maufe ( 1 9 1 6 ) , and Anderson ( 1 9 3 7 ) describe ring-dikes and piston-faulting, but few structural data are provided as to how these investigators arrived at their structural interpretations. The Ben Buie intrusion on the Island of Mull has also been interpreted as 25 the result of subsurface cauldron subsidence. Lobjoit ( 1 9 5 9 ) arrives at a different interpretation of the structure of the intrusion, but this interpretation also involves sub- surface subsidence of a block of country rock accompanied by the intrusion of magma into the space left by the sub- sidence. If these structural interpretations are correct, the intrusion at Desolation Lake is analogous to the subsurface cauldron subsidences of the British Isles in that the faults, terminated at some level below the ground surface while the roofs over the intrusions remained undisturbed. In the case of the intrusion at Desolation Lake, the surface of the un- conformity between the basement complex and the Epworth series appears to be a more favourable level for the termination of faults than bedding planes in the case of the subsurface cauldron subsidences. 2 3 Bailey, E.B. , Maufe, H.B.: The Geology of Ben Nevis and Glen Coe. Mem. Geol. Surv. of the United Kingdom, 19167 24 Anderson, J .G.C. : The Etive Granite Complex. Q.J .G.S. , Vol. XCIII 1937, p. 487-533. 2 5 Lobjoit, W.M.: On the Form and Mode of Emplace- ment of the Ben Bule Intrusion. Island of Mull, Argyllshire, Geol. Mag. Vol. XLVI, No. 5, 1 9 5 9 7 p. 393-402. 5 2 The absence of fragments of Epworth rocks from the breccia is one of the principal arguments against subsidence of the roof of the intrusion. However, when the amount of subsidence and the amount of material removed by erosion are considered, this argument loses some of its weight. The minimum amount of material removed from the top of the intrusion by erosion is estimated at from 200 to 500 feet. This estimate is arrived at by projecting the dip of the basal member of the Epworth series back to the top of the intrusion and estimating elevations. The total amount of subsidence, as shown by the section of acid rocks, is estimated at 300 to 500 feet. Thus, i t is possible that the roof over the intrusion did collapse, but any fragments of the roof were removed by erosion. Origin of the Breccia Fragments The breccia fragments are composed mainly of ortho- quartzite with a small amount of gneiss derived from the basement complex. No fragments of Epworth rocks, gabbro, or peridotite were found. The fragments may have several possible modes of origin. They may be the result of explos- ive activity, rockbursting, magmatic stoping, or of tectonic activity. The extensive deuteric alteration of the breccia matrix, the presence of vugs, and the manner in which apophyses of breccia matrix penetrate the country rocks suggest that the matrix was a highly mobile liquid rich in volatiles. Such conditions might be expected to favour ex- plosive activity. In this respect, Richey2^ (1940) states that subsurface explosive activity may occur at depths down to 6000 feet. This is the minimum estimated depth at which this intrusion was emplaced. However, i f explosive activity did occur and the country rock were shattered, fragments of the basal member of the Epworth series should be present in the breccia as well as quartzite from the basal complex. For this reason, the breccia is not believed to be due to an explosive origin. The depth of the intrusion (over 6000 feet) is in the zone at which rockbursting begins in mines, though this varies according to the rock types present. In general, the brittle siliceous rocks such as form the country rock around the intrusion are more susceptible to rockbursting than basic and ultrabasic igneous rocks. Presumably, i f rockbursts did occur, open spaces would be necessary, and there is no evidence that open spaces existed below the ground surface. However, such spaces could be formed by the subsidence of the fault block. If rockbursting did occur, i t is difficult to explain why fragments of the basal member of the Epworth series are not present in the breccia. The unit in question is also a siliceous rock like the quartzose rocks of the basement complex, and would presumably be as susceptible to rockbursting as the metaquartzite. 26 Richey, J . E . : Association of E&plosive Breccia- tion and Plutonic Intrusion in the British Tertiary Igneous Province. Bul l . Volcan. Ser.2, vol . 6,1940, p. 157-176. 54 A third possibility is that the breccia fragments originated during movements along the faults which bound the sunken block. Presumably some shattering of the country rock occurred along the faults during their formation. When movement occurred on the faults, grinding of irregularities on the fault surface occurred which tore loose fragments of the country rock which were incorporated into the breccia. Since the faults are believed to terminate at the surface of the unconformity between the Epworth series and the basal complex, there is no reason to expect that fragments of Epworth rocks should be present, so the absence of Epworth fragments presents no objection to this origin for the frag- ments. A fourth possible origin of the breccia fragments is magmatic stoping of country rock from the sides of the magma chamber. In the case of this intrusion, the exact role and 27 mechanism of stoping are difficult to evaluate. Gates (1959) attributes stoping in breccia pipes to a pumping action of the magma accompanied by pressure changes, rock- bursting, and collapse of the top of the magma conduit. In this intrusion, where the breccia is in contact with slaty country rock, the country rock has been spalled off , but anastomosing veins in the country rock which are characteristic of brecciated granite contacts are not present. In addition 27 Gates, 0,: Breccia Pipes in the Shoshone Range. Nevada. Econ. Geol. Vol. 54, No. 5, 1959, p. 790-815. the feldspars in the breccia matrix show no zoning. This may not be indicative of the absence of pressure changes, but reduces the probability of such pressure changes. Gneissic fragments are also present in the breccia although no gneiss was found near the contacts between this part of the intrusion and the country rock. The fragments of gneiss may be derived from a gneiss which occurs at depth, and were carried up by the rising magma. The absenc of gabbro fragments may be explained by the difference in specific gravity between the gabbro fragments and acid magma. The denser gabbro fragments would presumably sink in the magma. However, some gabbro fragments might be held in the chilled zone of the granophyre, but they are not present. To sum up, the majority of the breccia fragments appear to have originated by the brecciation of the country rock during fault movements. Explosive activity and rock- bursting probably did not occur. Magmatic stoping may have contributed to the brecciation of the country rock, but was probably a subordinate agent of brecciation. The quartzite fragments in the granophyre probably originated in the same fashion as the breccia fragments. 56 SUMMARY AND CONCLUSIONS Apparent reversals of differentiation trends and the structural relations of basic and ultrabasic rocks in the lopolith suggest that multiple intrusion as well as differ- entiation contributed to the development of layering in the intrusion. The chilling of the granophyre against gabbro and the structural relations of the granophyre and the basic and ultrabasic rocks indicate that the acid rocks represent separate intrusions of magma rather than products of the differentiation of a basic magma in situ. The intrusion of the acid magmas was accompanied by the downfaulting of an elongated block in the western part of the lopolith. The faults which bound the block probably terminated at the surface of the unconformity between the basement complex and the Epworth series. The depth at which the acid magmas intruded (below 6000 feet) is within the hypabyssal environment. The presence of high and intermediate temperature forms of alkali feld- spar in the acid rocks is consistent with this environment. The oscillatory mantling of the alkali feldspars and quartz-feldspar intergrowths is believed to be the result of pressure changes during the crystallization of the acid magmas. Two trends of pressure change are possible. Increasing pressure during crystallization is one possibility but the textural relations of some of the feldspar crystals are not typical of resorption which would occur i f the pressure rose. Falling pressure during crystallization is a more satisfactory explanation for the origin of the oscillatory mantling of the feldspar and quartz-feldspar intergrowths as i t does not 'conflict with theories for the origin of other features in the granophyre. The breccia fragments probably originated as a fault breccia during subsidence of the downfaulted block. However other possible origins should not be discounted completely. No chemical analyses were available, so variation diagrams could not be constructed and the genetic relation i f any of the acid rocks to the main body of the lopolith established. However, the mineralogical composition of the various rock types strongly suggests that the entire sequence is deficient in sodium, which may be indicative of a possible genetic relationship. BIBLIOGRAPHY Anderson, J . G . C . , : The Etive Granite Complex. Q.J.G.S. Vol. XCIII, 19377 PP. 457^533. Balk, R. , : Structural Behavior of Igneous Rocks. G.S.A. Mem. 5, 1937. Bailey, E .B. , Maufe, H.B . , : The Geology of Ben Nevis and Glen Coe. Mem. Geol. Surv. of U.K., 1916. Bowen, N . L . , : Evolution of the Igneous Rocks. Princeton University Press, 1928~I Carr, J . M . , : Zoned Plagioclases in the Layered Gabbros of the Skaergaard Intrusion, East Greenland. Min. . Mag. Vol. XXX, No. 225, 1954, pp. 3^7-375. Gates, 0.,: Breccia Pipes in the Shoshone Range, Nevada, Econ. Geol. Vol. 54, No. 5, 1959, pp. 790-815. Goodspeed, G . E . , : Some Textural Features of Magmatic and Metasomatic Rocks. Am. Min. Vol. 44, No. 3, 1959 pp. 211-250. Johannsen, A . , : A Descriptive Petrology of the Igneous Rocks. Univ. of Chicago Press, 1931, Vol. 1-4. Laves, F . , : Phase Relations of the Alkali Feldspars. Jour. Geol. Vol. 60, 1952, pp. 436-450. Lobjoit, W.M.,: On the Form and Mode of Intrusion of the Ben Buie Intrusion, Island of Mull r Argyllshire, Min. Mag. Vol. XLVI, No. 5, 1959, pp. 393-402. Phemister, T . C . , : A Review of the Problems of the Sudbury Irruptive. Jour. Geol. Vol. 54, No. 1, 1937, pp. 1-47. Reynolds, D . L . , : Fluidization as a Geological Process and its Bearing on the Problem of Intrusive Granites. A.J .S. Vol. 252, 1954, pp. 577-614. Richey, J . E . , : Association of Explosive Brecciation and Plutonic Intrusion in the British Tertiary Igneous Province. Bul l . Volcan. Ser. 2, Vol7~6"7 1940, pp. 157-176. 59 Rust, G.W.,: Preliminary Notes on Explosive Volcanism in Southeastern Missouri. Jour. Geol. Vol. 437 No. 1, 1937, PP. 48-75. Spencer, E . , : Myrmekite in Graphic Granite and in Vein Perthites. Min. Mag. Vol. XXVII, No. 189, 1945, PP. 79-98. Trogger,: W.E., : Tabellen zur optlschen Bestimmung der gesteinbildenden Minerale. E. Schweizerbarsche, Verlagsbuchhandlung, Stuttgart 1952. Turner, F . J . , : Determination of Plagioclase with a 4-Axis Universal Stage. Am. Min. Vol. 32, 1947, pp.389- 410. Tuttle, O . F . , : Extrusive and Plutonic Salic Rocks. Jour. Geol. Vol. 60, No. 2~1952, pp. 107-124. •- Optical Studies on Alkali Feldspars. Am. Jour. Sc. , Bowen Volume, 1952, pp. 553-567. Bowen, N . L . , : The Origin of Granite in the Light of Experimental Studies. G.S.A. Mem. 74, 1958.

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