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Geology and metamorphism of the Mount Breakenridge area, Harrison Lake, British Columbia Reamsbottom, Stanley Baily 1974

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GEOLOGY AND METAMORPHISM OF THE MOUNT BREAKENRIDGE AREA, HARRISON LAKE, BRITISH COLUMBIA. by STANLEY BAILY REAMSBOTTOM B.Sc., Aberdeen University, Scotland, 1968. M.Sc, University of British Columbia, 1971. A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in the Department of Geological Sciences We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA April 1974 I n 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 r e q u i r e m e n t s 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 C olumbia, 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 s t u d y . I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by the Head o f 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 u n d e r s t o o d t h a t c o p y i n g o r 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 n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Department The U n i v e r s i t y o f B r i t i s h Columbia Vancouver 8, Canada i i ABSTRACT. The Mount Breakenridge area i s underlain by metamorphosed strata of the Upper Paleozoic (?) or older Breakenridge Formation, the Upper Paleozoic (?) or Mesozoic Cairn Needle Formation and the Lower Cretaceous Peninsula Formation. These rocks and enclosed ultramafic pods were subjected to Barrovian-type metamorphism prior to the mid-Cretaceous. Locally, p e l i t i c schists developed andalusite and sillimanite i n contact aureoles around Late Cretaceous quartz diorites. Ultramafic and related rocks i n the map-area and i n the Southern Coast Crystalline Complex and the Northern Cascade Mountains may have formed i n a marginal basin which was i n existence from Mississippian to Late Triassic. Rock and mineral chemistry of pelites indicates that the composition of white mica i s sensitive to metamorphic grade, and composition of plagioclase i s controlled by rock bulk chemistry. No obvious relationship exists between bi o t i t e and staurolite compositions and rock chemistry or metamorphic grade. Distribution of chemical species between coexisting chlorite, b i o t i t e , garnet and staurolite implies a close approach to chemical equilibrium. Linear regression analyses of minerals indicate that assemblages i n the sillimanite and kyanite zones are dependent on rock bulk chemistry and that apparent univariant reaction assemblages act as local f H Q buffers. The minerals talc (T), forsterite (F), ehstatite (E), anthophyllite (A) i n ultramafites may have formed by the following reaction sequence. The reaction T + F = E produced E at 7kb. U p l i f t and subsequent reduction i n pressure rendered enstatite unstable and anthophyllite formed by the reaction T + F = A. Thermodynamic calculations confirm that the equilibrium temperature of the vapour-absent reaction E + T = A i s extremely sensitive to iron s o l i d solution so that the topologies of reactions i n the system MgO-SiO2-H20 may be a function of the Mg- content of the system. i i i TABLE OF 'CONTENTS. Page INTRODUCTION 1 I. THE GEOLOGY' OF THE MOUNT BREAKENRIDGE AREA, HARRISON LAKE, BRITISH COLUMBIA . . . 2 ABSTRACT 3 REGIONAL GEOLOGY BETWEEN HARRISON LAKE AND THE FRASER RIVER ........ 9 GEOLOGY OF THE MOUNT BREAKENRIDGE AREA 14 DESCRIPTION OF LITHOLOGIC UNITS 15 Breakenridge Formation 15 Cairn Needle Formation.. 16 Peninsula Formation 18 Mount Breakenridge Plutonic Complex 19 Scuzzy Pluton 19 Dacite Porphyry 19 Ultramafic Rocks 20 STRUCTURE 24 Relationship between metamorphic recrystallization and deformation.. 30 METAMORPHISM 32 SEQUENCE OF EVENTS 36 STRUCTURAL SYNTHESIS AND RELATION TO PLATE TECTONIC THEORY 38 ACKNOWLEDGEMENTS 43 REFERENCES 44 II. CHEMICAL PETROLOGY OF PELITIC ROCKS FROM THE MOUNT BREAKENRIDGE AREA, HARRISON LAKE, BRITISH COLUMBIA 48 iv Page ABSTRACT............... ^  v 49 INTRODUCTION.............. 51 BULK ROCK. CHEMISTRY......................... 51 MINERAL CHEMISTRY..................... 5 7 Chlorite............................... 57 Muscovite ..............^..............• 61 Biotite 6 6 Garnet .^ 73 Staurolite 76 Plagioclase 8 2 Ilmenite ......... 8 2 DISTRIBUTION STUDY • ' 8 6 LINEAR REGRESSION ANALYSES.................. S9 Staurolite-kyanite zone 91 Sillimanite zone. 93 GRAPHICAL REPRESENTATION OF MINERAL ASSEMBLAGES...... 96 DISCUSSION 100 CONCLUSIONS 103 ACmKLFJDGEMENTS 104 REFERENCES 105 APPENDIX 1.... 109 III. METAMORPHISM OF ULTRAMAFITES FROM THE MOUNT BREAKENRIDGE AREA, HARRISON LAKE, BRITISH (XfLUMBIA... 113 ABSTRACT.... 114 INTRODUCTION 116 Description and Petrography 120 Mineral Chemistry 123 Metamorphi sm 126 'Cp.tM3QtJ3&^3^IOI^. ' ••• 137 ACKNOWLEDGEMENTS... 138 REFERENCES.. ^  • 139 APPENDIX 1 141 APPENDIX II... 150 SUMMARY AND CONCLUSIONS 153 V LIST OF TABLES. Table Page. I . 1. Iitportant units i n the Coast-Cascade and Insular Belts , and their relationship to rocks of the Mount Breakenridge Area. 21 2. Style elements of small-scale folds i n the Mount Breakenridge Area. 25 3. Relationship between metamorphic recrystal l izat ion and deformation i n the Mount Breakenridge Area. 29 I I . 1. Relationship between specimens and metamorphic grade. 52 2. Modal analyses of schists. 53 3. Chemical analyses of schists. 54 4. Microprobe analyses of chlor i te . 58 5. Microprobe analyses of muscovite. 60 6. Microprobe analyses of b io t i t e . 65 7. Microprobe analyses of garnet. 69-72 8. Microprobe analyses of staurolite. 77 9. Relationship between zinc, iron and magnesium i n stauroli te. 78 10. Microprobe analyses of plagioclase. 80 11 .Microprobe analyses of ilmenite. 83 12. Possible reactions i n p e l i t i c rocks as defined by the l inear regression technique.. 90 13. Regression models between bulk rock compositions and mineral assemblages i n Specimens 6 and 9. 95 Table. v i Page. I I I . l . Mineral assemblages i n ultramafites of t±ie Mount 119 Breakenridge Area. 2. Microprobe analyses of o l iv ine . 144 3. Microprobe analyses of t a l c . 145 4. Microprobe analyses of enstatite. 146 5. Microprobe analyses of tremolite. 147 6. Microprobe analyses of anthophyllite. 148 7. Microprobe analyses of chlorite and magnesite. 149 8. Relationship between assemblages i n metamorphosed 125 ultramafites and surrounding p e l i t i c schists. VIX LIST OF FIGURES. Figure. page I. 1. Geological Map of the Coast Momtains, British Columbia. 5 2. Geological sketch map of the Southern Coast Mountains between Harrison Lake and Fraser River, B.C. 8 3. Metamorphic Map of Mount Breakenridge Area. 31 4. Petrogenetic grid showing P.T. conditions of regional (R) and contact (C) metamorphism in Mount Breakenridge area. 34 5. Space and time distribution of lithological assemblages interpreted in idealistic terms within the various physiographic and geological belts of the Canadian Cordillera37 6. Hypothetical synthesis which relates the development of the Southern Coast Crystalline Complex and the Northern Cascade Mountains to Plate Tectonic Theory. 40 v i i i LIST OF FIGURES. Figure. Page I I . 1. Metamorphic map of Mount Breakenridge Area, B.C. 50 2. Ray diagram of Fe^^/FeO ratios of p e l i t i c rocks. 55 3. Plot of bulk FeO, MgO, MnO, MgO/MgO + FeO i n p e l i t i c rocks versus rock oxidation ra t io . 56 4. A l k a l i content of white mica. 59 5a) Plot of Na20 content of rock versus Na20 content of white mica. 62 b) Plot of Na20 content of white mica versus Ab- content of coexisting plagioclase. 62 6. White micas plotted i n A l 2 0 3 - (K20 + Na20) : FeO + MgO + T i 0 2 : Na20 + K 2 0 triangle. ' 63 7. Ternary plot of muscovite, paragonite and phengite content of white mica. 64 8. Octahedral s i te occupancy of coexisting biot i te and white mica. 67 9. Relationship between MgO/MgO + FeO ratios of b io t i te and rock. 68 10. Zonation profiles of garnets from p e l i t i c rocks. 74-75 11. Plot of CaO content of schists versus An- content of plagioclase. 81 12. CaO/Na20 content of rock versus An- content of plagioclase. 81 13. Plot of chemical composition of ilmenite versus rock oxidation ra t io . (O.R.). 84 14. Distribution diagrams of to ta l Fe, Mg, Mn and Ca among coexisting chlori te , b io t i t e , garnet (edge composition) and stauroli te. 87-88 15. Relationship between bulk composition and composition of minerals i n hypothetical specimens 6 and 9. Minerals B and C form so l id solution series. 94 ix Figure. Page II. 16a) Stereographic plot of garnet (edge composition), biotite, muscovite and plagioclase in the tetrahedron MnO-Na20-K20-CaO. Rock bulk compositions are also plotted within the tetrahedron. 97 b) Simplified stereographic plot of garnet, biotite, muscovite, plagioclase and bulk compositions of specimens 2,6,9 in the tetrahedron MnO-Na20-K20-CaO. 97 17. Stereographic plot of garnet, muscovite, plagioclase in the tetrahedron MnO-Na20-K20-CaO. 99 X LIST OF FIGURES. Figure. Page IIL'. 1. Distribution of ultramafites i n Br i t i s h Columbia (after McTaggart, 1971). 115 2. Metamorphic map of the Mount Breakenridge Area, B.C. 117 3. Plot of Talc, Anthophyllite, Enstatite, Olivine and Magnesite i n the system MgO-Si02-H20. 122 4. Distribution diagram of Xmg i n coexisting olivine, t a l c , chlorite, tremolite, enstatite, anthophyllite magnesite. 124 5. Metamorphic reactions i n the system CaO-44gO-Si02-K20-C0 2 (after Evans and Trommsdorff, 1970). 127 6. Progressive change i n topology of reactions i n the system MgO+si~o2+H20 as the position of the equilibrium talc + enstatite = anthophyllite moves to successively higher T (configuration B) relative to other equilibria involving anthophyllite. 129 Schematic phase relationships among the phases forsterite (F); enstatite (E); talc (T); anthophyllite (A); magnesite (M) i n the system MgOfSi02+H20+C02. p^ and P 2 correspond to pressures shown i n configuration A, Figure 6. (P^™ = P u (-.+PrY~1 ) 130 ±\j± 2 ^^2 Plot of Function t(X}) (See Text) versus Xmg, olivine for n6r9:0;j.or7 0$P, +7110 ° G. 5A +W (1) - 134 Univariant reactions which emanate from the isobaric invariant points EFTA i n the system MgO-Si02-H20-CO2 with calculated Ternary reaction F + T = E + A i n the system Fe0^yigO-Si02-H20-C02. 136. xi LIST OF PLATES. Plate. Page I. IA. Rootless F isocline in Caim Needle Fo:rmation schists. 26 A IB. F A isocline refolded by F c small-scale fold in Breakenridge Formation striped amphibolite. IC. Subisocline F_, folds in Breakenridge Formation amphibolite. ID. Small-scale similar to concentric F^ folds in Breakenridge Formation striped amphibolites. IE. Small-scale similar to concentric F^ folds in Breakenridge Formation amphibolites. II. IIA. Garnet porphyroblast which has crystallized before F_ 2 8 schistosity. X 25 IIB. Garnet porphyroblast which overprints small-scale F^ folds. X 25 IIC. Kyanite and plagioclase which overprints small-scale F c folds. X 25 IID. Staurolite porphyroblast which overprints F c strain-slip cleavage. X 25 IIE. Polygonal arc of sillimanite mimetic on F c small-scale fold. X 25 IIF. Aggregates of sillimanite with good(010) cleavage pseudemorphous after andalusite. Opaque inclusions define the chiastolite cross of the andalusite. X 25 x i i LIST OF PLATES. Plate. P a g e IIIvlA. Enstatite set in.platy talc matrix, h 25 118 IB. Irregular shaped, enstatite. set i n talc.matrix.. Seme of the talc may be retrogressive, after .enstatite. X 25 IC. Sprays of tremolite and associated olivine set i n talc matrix. X 25 ID. Anthophyllite and associated magnesite set. i n talc matrix. IE. Sprays of anthophyllite set i n talc matrix. X 25 IF. Anthophyllite piercing olivine and enstatite i n talc matrix. x i i i MAPS IN POCKET MAP 1 G e o l o g i c a l map of Mount Breakenr idge Area 1A G e o l o g i c a l map of Mount Breakenr idge Area (De t a i l ) 2 S t r u c t u r a l map of Mount Breakenr idge Area xiv AQQSDWLE3IX5EMENTS. I thank Dr. H.J. Greenwood under whose supervision this work was done. His helpful advice and constructive criticism have contributed a great deal to this study. Helicopter support during field studies, rock thin sections and rock chemical analyses were provided by Drs. W.W. Hutchison and J.A. Roddick of the Geological Survey of Canada, Vancouver, B.C. I greatly appreciate the encouragement given to me by the above gentlemen. Microprobe analyses were obtained at the University of Washington, Seattle, U.S.A., under the supervision of Dr. B.W. Evans, Miss L.Leitz and E. Mathez. Drs. H.J. Greenwood, K.C. McTaggart and P.B. Read read and greatly improved the manuscript during preparation of the thesis. The research was supported by a National Research Council of Canada post-graduate fellowship from 1969 to 1971 and by a Killam pre-doctoral fellowship from the University of British Columbia in 1973. Research funds were provided through National Research Council grant (A67-4222) to Dr. H.J. Greenwood. Finally, I thank my wife, Gillian, not only for typing the manuscript but for the encouragement and inspiration she gave me throughout the duration of this study. 1 INTRODUCTION. The Mount Breakenridge Area was f i r s t mapped on a reconnaissance basis by Roddick and Hutchison (1969). They outlined a roof pendant i n quartz dior i te plutons made up of highly metamorphosed gneisses and schists. Reconnaissance mapping however, proved inadequate to completely relate the geological history of the area to that of the Southern Coast Crystalline Complex and Northern Cascade Mountains i n Hope map-area. The distr ibution of metamorphic isograds, estimation of physical conditions of metamorphism and temporal relationships between deformation, metamorphism and plutonism required detailed f i e l d and laboratory studies. The present dissertation considers the above problems with emphasis on the metamorphism of p e l i t i c schists and ultramafic rocks i n the region. I t i s presented i n three manuscripts. The f i r s t describes the geology of the Mount Breakenridge area and relates i t to the geologicalhhistory of the Southern Coast Crystalline Complex and Northern Cascade Mountains. Metamorphic petrology of p e l i t i c rocks, with emphasis on chemical equilibrium and possible reactions at the s i l l imanite isograd i s discussed i n the second manuscript, while the th i rd defines possible reactions and conditions of metamDrphism i n ultramafites of the region. The complete study describes the geological and metamorphic history of this part of the Coast Mountains. THE GEOLOGY OF THE MOUNT BREAKENRIDGE AREA, HARRISON LAKE, BRITISH COLUMBIA . by Stanley B. Reamsbottom, Department of Geological Sciences, The University of Br i t i sh Columbia, Vancouver 8, B.C. 3 ABSTRACT. The Mount Breakenridge Area i s underlain by metamorphosed strata of the Upper Paleozoic (?) or older Breakenridge Formation, the Upper Paleozoic (?) or Mesozoic(?) Cairn Needle Formation and the Lower Cretaceous Peninsula Formation. Granodiorites and quartz diorites of the synkinematic Mount Breakenridge Plutonic Complex and the Late Cretaceous Seuzzy Pluton intrude the metarrDrphic rocks. Ultramafic pods outcrop sporadically mainly within the Cairn Needle Formation. Four phases of folding haye been recognised. Phases A and B, the earl iest recognized, are known only as rootless refolded isocl ines. Map-scale folds of these phases cannot be traced. F^ folds are the most important major folds i n the map-area and have northwest trending subvertical axia l surfaces. Qilminations of F^ northwest trends caused by northeast trending F^ folds or upward emplacement of the Mount Breakenridge Plutonic Complex have produced broad domes cored by gneisses of the Breakenridge Formation. A northwest-trending fault zone near Harrison Lake brings Breakenridge Gneiss structurally over Cairn Needle Schist and Cairn Needle Schist structurally over Peninsula Formation gri ts and conglomerates. The fault zone i s correlated with the mid-Cretaceous Church Mountain Thrust. A deep-seated c lass ica l Barrovian metamorphic sequence developed i n p e l i t i c rocks before the mid-Cretaceous. The main period of metamorphic recrystal l izat ion post-dated F folding, Although plutonic rocks were intruded before and after the climax of deep-seated metamorphism they do not appear to have been the direct cause of the regional metamorphism. The Late Cretaceous Scuzzy Pluton only loca l ly produced andalusite and s i l l imanite hornfelses i n narrow contact zones. Oceanic-type assemblages of the Southern Coast-Cascade system are considered to have developed i n a marginal basin 4 which was i n existence from the Mississippian to Late Triassic . Lawsonite and crossite schists developed i n a west-dipping subduction zone which developed at the junction between volcanic arc and volcaniclastic rocks of Chilliwack Group and Cultus Formation from oceanic rocks of Hozameen and Fergusson Groups. Ultramafic and related p e l i t i c rocks i n the Mount.Breakenridge Area wre metamorphosed at depths of 23 km. between the Late Triassic and mid-Cretaceous. These rocks were subsequently uplifted and contact metamorphosed by Late Cretaceous Plutons. G e o l o g i c a l Map o f the Coast Mounta ins, B . C . V v v v v V LEGEND Sediments Miocene I n t r u s i o n s . T e r t i a r y - R e c e n t V o l c a n i c s a E < * * • * • • t • . . • 111 a Lower T e r t i a r y In t rus ions , ENLY MESOZOIC Quartz Monzonite G r a n o d i o r i t e Quartz D i o r i t e D iora te • a • a O o O O Gabbro ^ U l t r a m a f i c Rock MAINLY LOWER MESOZOIC I I Sedimentary and V o l c a n i c Rocks, MAINLY MES. & UPPER PAL.? K l; ;''":jSchist & Gneiss SdATSLY UPPER PALEOZOIC jlj^jUnmeta. o r weakly metamorphosed rocks | | MAINLY PALEOZOIC Gne iss & Migmatite 10 O 20 40 Miles. Unmapped A f t e r J . A . Roddick and W.W. H u t c h i s o n , 1 9 7 2 . »* * i % Author's Thesis Area. / — I u 6 INTRODUCTION. The Mount Breakenridge area (N. Lat. 49°35' - 49Q50'; W.Long. 121°35' - 122°): i s situated within the Southern Coast Crystalline Complex close to the boundary between, the Coast and Cascade Mountains. The Coast Plutonic Complex of .British Columbia, formerly known as the Coast Range Batholith, covers an area of 43,000 square miles and is composed of a long narrow belt of plutonic and metamorphic rocks which extends 1,100 miles through British Columbia, Southeast Alaska and Yukon. (Figure 1). The belt varies in width from 50 to a maximum of 120 miles opposite the north coast of Vancouver Island. Plutonic rocks of the belt are flanked mainly by Mesozoic volcanic and sedimentary rocks, though locally, rocks of Permian, Pennsylvanian and Mississippian ages are encountered. On Prince of Wales Island, in the Alaskan Panhandle, Ordovician and Silurian rocks crop out, while in the San Juan Islands of Northern Washington State, pre- Middle Devonian rocks are found .Mosttcl-astic rocks flanking the complex contain 'granitic' debris although this is mainly concentrated in post- Upper Jurassic, especially Upper Cretaceous, rocks. Emplacement of plutonic rocks in the Coast Mountains continued intermittently from the early Paleozoic to mid-Tertiary. Many large plutons in the south were emplaced in mid or late Cretaceous time. Metamorphism within the belt ranged in age from Paleozoic to late Cretaceous and was broadly contemporaneous with Plutonism. Major events took place in pre- Middle Devonian, Triassic, Jurassic, mid-Cretaceous and possibly: early Tertiary time. The Mount Breakenridge area offered a unique opportunity to map and study the metamorphism of migmatitic gneisses which may have been formed by one of the earlier metamorphic events in the Coast Mountains. Field mapping. ((1::2500:O) was carried out in the summers of 1969,1970, 1971 and 1972, for a total of approximately six months. A preliminary report of structural and .petrographic studies to 1970 was presented.by the author for an M.Sc Thesis at the University, of British Columbia in 1971. Figure 2. Geological sketch map of the Southern Coast Mountains between Harrison Lake and Eraser River, B,C LEGEND. E Eocene Conglomerate. P Quartz diorite and granodiorite Plutons. U Ultramafite. M Mesozoic clastic and volcaniclastic rocks. H Upper Paleozoic (?) or Mid-Triassic Hozameen Group. C Chilliwack Group. S Schist of uncertain age. G Gneiss of uncertain age. B Pre-Devonian(?) Basement. (See text for description). Fault. 9 REGIONAL GEOLOGY BETWEEN HARRISON LAKE AND THE FRASER RIVER. Geology of the Southern Coast Crystalline Belt between Harrison Lake and Fraser River (Figure 2) represents the more deeply exposed level of a tectonic belt which, south of Fraser River, continues into the Northern Cascades of British Columbia and Washington State. The evolution of this orogenic belt which reached a tectonic climax in mid-Cretaceous has been described by McTaggart (1970). The Northern Cascade Region is composed mainly of folded and faulted sedimentary, volcanic and metamorphic rocks. There easterly and westerly spreading mid-Cretaceous thrusts are rooted within a central crystalline and metamorphic axial zone. Traced northward into the Coast Mountains, only the steep root zones of these thrusts are preserved in deeply eroded roof pendants within massive quartz diorite plutons. ROCK UNITS. (See Figure 2). Basement (B). Tectonic slices of metamorphosed igneous rocks, which are similar to the pre-Devonian crystalline basement of northern Washington State (Misch, 1966), occur within the root zone of the mid-Cretaceous Shuksan Thrust, east of Harrison Lake. (Lowes,1972). Gneiss (G). Migmatitic gneiss with small areas of schist outcrops in the Mount Breakenridge area and within the Fraser River fault system. The gneiss along the Fraser River is an extension of the Custer Gneiss (McTaggart and Thompson, 1967), which is equivalent to the Skagit Gneiss in Washington State (Misch, 1966). These latter rocks constitute the Cascade Metamorphic Suite. The Skagit Gneiss is considered by Misch to be the highly metamorphosed equivalent of the upper Paleozoic or Triassic Hozameen Group (McTaggart and Thompson, 1967); (Cameron and Monger, 1971). 10 The age of metamorphism of this suite is problematic. Misch thought that i t was pre-Jurassic, probably pre-Mesozoic, while McTaggart (1970) did not dismiss the possibility that the Custer Gneiss was not much older than the mid-Cretaceous Spuzzum Pluton. Zircons within the Skagit Gneiss have pre-Cambrian (1200 m.y.) U/Pb ages (Mattinson, 1972). These are possibly detrital minerals derived from the pre-Cambrian Swakane Gneiss, as the original parent rocks of the Skagit Gneiss are considered to be immature greywakes (Misch, 1966). In addition, zircon in pegmatites of the Skagit migmatites yielded mid- to Late Cretaceous ages (90-60 m.y.), so that rocks of the Cascade Metamorphic Suite have had a long and complex metamorphic history. Schist (S). Dominantly pelitic schists form roof pendants and screens in the plutonic rocks northwest of Boston Bar, west of Hope Fault and in the Mount Breakenridge area. Metamorphism of these rocks has been studied by Duffell and McTaggart (1952); Read (1960); Hollister (1969 a,b); Roddick and Hutchison (1969); Reamsbottom (1971); Pigage (1972) and Lowes (1972). The Mesozoic metamorphism which affected these rocks was Barrovian in type. Local low or high pressure (Hollister, 1969 a) contact metamorphism accompanied mid-and Late Cretaceous plutonism. The age of these rocks is uncertain. Granitoid clasts in conglomeratic horizons within the schsts may indicate that they are Mesozoic as there are no pre-Jurassic plutons in the region (Duffell and McTaggart, 1952; Reamsbottom, 1971). Lowes (1972) correlated the Settler Schist east of the root zone of the Shuksan Thrust with the Chiwaukum Schist of the Upper Paleozoic Skagit Metamorphic Suite. Hozameen Group (H). The Hozameen Group consists of more than 20,000 feet of ribbon chert, volcanic greenstone, limestone and argillite. Its age is uncertain but i t was generally considered to be Upper Paleozoic. 11 Recently i t has been correlated with the mid-Triassic Fergusson Group (Cameron and Monger, 1971). Chilliwack Group (C). Rocks of the Chilliwack Group east of Harrison Lake have been metamorphosed locally to upper epidote amphibolite facies. They consist of two main units; a lower metasedimentary series of phyllite and greywake overlain by schistose metavolcanic rocks (Lowes, 1972). Unlike the Pennsylvanian - Permian type section of the Chilliwack River Valley (Monger, 1966) these rocks are unfossiliferous and contain l i t t l e or no limestone. Mesozoic Rocks (M) . Paleozoic rocks within the high grade, axial zone area are in fault contact with Mesozoic clastic rocks. On the eastern margin \' of the axial zone the mid-Cretaceous Hozameen Fault juxtaposes rocks of the Hozameen Group against the Lower and Middle Jurassic Ladner Group, the Upper Jurassic Dewdney Creek Group and the Lower Cretaceous Jackass Mountain Group. These rocks consist of sequences of sandstones, pelites and conglomerates containing granitic debris. To the west, on Harrison Lake, the northern extension of the root zone of the mid-Cretaceous Church Mountain Thrust (Lowes, 1972)brings the Chilliwack Group against the Middle Jurassic Mysterious Creek Formation and Lower Cretaceous Brokenback H i l l and Peninsula Formations. Eocene Conglomerates (E). Granitic bearing Eocene conglomerate outcrops between Hope and Yale Faults. Plutonic Rocks (P). Potassium - argon radiometric ages of plutonic rocks within the region range from earliest Jurassic to Tertiary.(Baadsgaard, Folinsbee and Lipson, 1961); (White, et al., 1967); (Richards and White, 1970); (Richards, 1971) and (Hutchison, 1971, p. comm.). The Scuzzy and Spuzzum Plutons (72±4 m.y.) underlie an area 12 of approximately 1370 sq.km. and form one of the largest plutons within the Coast Mountains. Ultramafic Rocks (U). An eastern belt of ultramafic and associated basic igneous rocks follows the probable trace of the Hozameen Fault. This Coquihalla Serpentine Belt (Cairnes, 1930.) and its extension northwest of Boston Bar (Duffell and McTaggart, 1952) consists of serpentine, serpentinized peridotite and gabbroic and dioritic rocks. A western, less clearly defined belt of ultramafic rocks occurs along the root zone of the Shuksan Thrust (Lowes, 1972). The belt consists of small bodies of pyroxenite, peridotite, hornblendite, dunite and gabbroic rocks. Peridotites are partially serpentinized or altered to talc schists with associated tremolite and carbonate. Contacts of these ultramafites are reaction zones which contain -tremolite, actinolite and chlorite. A zoned ultramafic body 12 kilometers northwest of Hope is of considerable economic importance as i t contains pyrrhotite with subordinate pentlandite and chalcopyrite (Aho, 1956). This body consists largely of pyroxenite with cores of peridotite and patches of hornblendic pyroxenite with a hornblendite margin. The absolute age of the ultramafites is unknown but many were emplaced along mid-Cretaceous faults. SUMMARY OF TECTONIC HISTORY. The tectonic history of the Coast Cascade Region includes formation of a pre-Devonian Basement Complex; (Yellow Aster and Turtleback Complexes, Swakane Gneiss), (Mattinson, 1972), development of a eugeosyncline which lasted from Devonian to late Mesozoic times; (Chilliwack, Hozameen, Cultus, Ladner, Dewdney Creek and Jackass Mountain Groups), (Monger, 1970), generation of an axial zone of gneiss probably pre-Mesozoic, a tectonic climax in mid-Cretaceous with accompanying metamorphism and thrusting, emplacement of granitic plutons; (Scuzzy and Spuzzum Plutons), and finally, uplift of the Coast Crystalline Belt in Early Tertiary. Emplacement of granitic plutons continued from earliest Jurassic to Miocene. The Fraser River fault system was intermittently active frcm the late Mesozoic through early Tertiary Time. 14 GEOLOGY OF THE MOUNT BREAKENRIDGE AREA. The Mount Breakenridge area is underlain by a triangular shaped septum of metamorphosed schists and gneisses bounded to the east and west by quartz diorite plutons of the Southern Coast Crystalline Complex. Geology. Two major formations of metamorphosed rocks form two elongate domes, produced by the culminations of two northwesterly trends. (Maps 1 and 1A). The eastern dome and the more complex western one are both cored by the grey gneiss, migmatite, amphibolite and pelite of the Brekenridge Formation. Enveloping the domes, and probably stratigraphically above the Breakenridge Formation, are rusty weathering pelitic schists, calc silicates, minor limestones and conglomerates of the Cairn Needle Formation. Northwest striking faults of mid-Cretaceous age brought Breakenridge gneisses over Cairn Needle Formation and, in turn. Cairn Needle Formation over low grade grits, chlorite schists and conglomerates which are lithologically similar to the Lower Cretaceous Peninsula Formation. The deep seated regional metamorphism which affected these rocks reached its climax after the main folding event and prior to mid-Cretaceous faulting. To east and west granitoid plutons of the Coast Crystalline Complex intrude the metamorphic rocks. The Upper Cretaceous Scuzzy Spuzzum Pluton, which is composed of quartz diorite and granodiorite, and the accompanying metamorphic rocks form the axial zone of the Hope map-area. (Monger, 1970). On Cairn Needle this quartz diorite pluton has produced chiastolite and sillimanite in the surrounding schists by contact metamDrphism. A syntectonic quartz diorite to granodiorite cores the western dome which underlies Mount Breakenridge. Dark, biotite-rich, pencil-shaped inclusions in the marginal zone of the pluton are oriented subparallel to lineations within the surrounding schists and gneisses. The boundary between foliate pluton and gneiss is gradational. Younger, 15 epizonal dacite porphyries cut the foliated|piutoh.F^t-teending andesite dykes, probably related to Quaternary vulcanism, intrude a l l other rocks. Ultramafic rocks outcrop as a series of small pod-like bodies mainly within the Cairn Needle Formation on the western flanks of Mount Urquhart, on the ridge north of Clear Creek, and at the head of Hunger Creek. A brief description of the units encountered follows. A more complete petrography of these is given by Reamsbottom (1971). DESCRIPTION OF LITHOLOGIC UNITS. (See Map 1). Breakenridge Formation. The Breakenridge Formation is composed mainly of homogeneous grey gneiss and amphibolite. Migmatitic, banded to irregularly banded gneiss, pelitic schist and skarn form heterogeneous zones between the grey gneisses and amphibolites. The formation may have originally been a eugeosynclinal sequence of volcanic and sedimentary strata. Amphibolites are presumably metavolcanic and the intimate association and stratigraphic persistence of horizons of grey gneiss and pelitic schist strongly suggest a sedimentary origin for the gneiss. Metamorphism of mixtures of greywake and basic volcanic rocks have produced a similar migmatitic terrain in Southwest Greenland (Kalsbeek, 1970). Grey gneiss, migmatite and skarn (1). The grey gneisses are medium-grained with an allotricmorphic granular texture. They consist of biotite, quartz and plagioclase (An1Q_24) locally with muscovite, garnet or microcline. Myrmekite commonly develops between grains of microcline and plagioclase. Several types of migmatite exist in the complex zones between the homogeneous grey gneisses and amphibolites. These include grey gneiss with abundant mafic inclusions; amphibolite with leucogneiss screens and inclusions; grey gneiss with ptygmatic veins and worm-like quartzofeldspathic stringers and, most abundant, banded to irregularly banded gneiss composed of leucocratic biotite gneiss, amphibolite and pegmatites. Within the rnigmatitic zones brightly coloured red-green skarn outcrops as a series of discontinuous boudins. These are composed of orange andradite, bright green hedenbergitic pyroxene, quartz and epidote. Amphibolite (2 and 3). Amphibolites in the Breakenridge Formation are of two varieties. One is homogeneous with a well-defined foliation, whereas the other is more schistose and striped in appearance. They are hornblende j-.-plagioclase-^. quartz amphibolites and may b'\ have abundant biotite or minor cgametcand-£drops£de\dc^ as a primary mineral or after plagioclase and also in cross-cutting veins. Plagioclase for the most part is unzoned and ranges in composition from An-LQ^ -An.^ . Pelitic schist and gneiss (4). Rusty weathering, medium- to coarse-grained, pelitic schists and gneisses are distinguished by bladed blue-grey kyanite, coarse garnet and twinned brown staurolite. These . -continuous mappable units proved invaluable in defining the structure of the northern part of the region. A complete description of the schists is presented in a companion paper on the chemical petrology of these rocks. Cairn Needle Formation. The Cairn Needle Formation is composed mainly of rusty weathering pelitic and garnet hornblende schists and minor but diagnostic banded calc silicates, crystalline limestone and oc~-':o: conglomerates with granitoid clasts. A dyke-like layer of hornblende, garnet and cummingtonite granulite extends 20 kilometers from Hunger Creek to Fir Creek. The presence of limestones and conglomerates indicates this formation developed in a continental to shallow water marine environment. 17 Conglomerates ( 5 ) . The boundary between Breakenridge and Cairn Needle Formations, south of Cairn Needle, is marked by six thin conglomeratic horizons which range from 1 to 3 meters in thickness. This zone of conglomerate and associated calc silicate and kyanite gneiss can be traced around the nose of an antiform for a distance of three miles. Other, less continuous horizons of conglomerate are found in the schists west of Mount Urquhart. The clasts are granitic rocks, quartz diorite, amphibolite and quartzite and in the Cairn Needle a area many have been strained into prolate ellipsoids. Calc silicate (6). T T Thin hcbedsins ( 1 0 - 3 0 cm.) of banded calc silicate, <z possibly formed by reactions between carbonate-rich rocks and pelites, occur within the formation. Bands are crudely symmetrical about a calc silicate-rich central zone which is composed of garnet, plagioclase, sphene, hornblende, epidote, diopside and clinozoisite. Outwards from this zone are thin bands ( 2 - 1 0 m.m. thick) alternately rich in plagioclase, hornblende, epidote or diopside. Two thick hcuriitsis ( 1 5 0 meters) near Harrison Lake are not so obviously banded and contain garnet, sheaves of hornblende, plagioclase (An2Q_4Q)» epidote, biotite and muscovite. Crystalline limestone (7). A 9-meter-wide hcuriit>n of metamorphosed dolomitic limestone occurs immediately west of Hunger Creek. Associated with this white limestone are red-green calc silicates. The limestone contains forsterite and tremolite in addition to calcite, whereas the associated calc silicate contains grossular, diopside, clinozoisite, calcite and quartz. 18 Pelitic and garnet hornblende schists (8). Garnet hornblende schists are a major constituent of the Cairn Needle Formation. These rusty weathering schists are of wide areal extent and in Map 1 have not been differentiated from pelitic schists. Characteristic mineral assemblages are biotite-hornblende-garnet-plagioclase. The plagioclase is homogeneous and varies from specimen to specimen within the range An 3 2-An 4 4. Pelitic schists studied are described more fully in a companion paperr.on chemical petrology. These rusty weathering rocks contain index minerals characterisitc of Barrovian metanorphism. Metamorphic zones within which the schists contain chlorite, biotite, garnet, staurolite, kyanite and sillimanite are shown in Figure 3. In the Cairn Needle Aureole the schists contain chiastolite, some of which is pseudonorphed by sillimanite. Schists on the ridge north of Clear Creek contain biotite, garnet and radiating sheaves of gedrite. Granulite (9). The 1 by 12 kilometer layer of basic granulite within the Cairn Needle Formation is presumably a metamorphosed basic igneous dyke or s i l l . It is a biotite, hornblende or cummingtonite granulite with minor hornblendite. This unit, like most of the metamorphic rocks, is extensively veined by white gneissic pegmatite. Peninsula Formation (10). A sequence of low grade, metamorphosed conglomerate, grit and chlorite schist occurs on Harrison Lake Shore northwest of Big Silver Creek. This formation is similar to clastic rocks within the Lower Cretaceous Peninsula Formation. It is in fault contact with the Cairn Needle Formation. Conglomerates have rounded to discoid pebbles of granitic rock, black or white chert and quartzite, set in a fine micaceous matrix. The grits contain angular clasts of feldspar and opalescent blue quartz set in a matrix of pale green chlorite and quartz. 19 Plutonic Rocks. Mount Breakenridge Complex (11). The plutonic complex underlying Mount Breakenridge has a central core of medium-grained biotite hornblende granodiorite with 10 per cent rounded or elongate biotite-rich inclusions. To the east, on Big Silver Creek, the granodiorite grades quickly into a garnet-bearing, hornblende biotite quartz diorite. A well-defined foliation pervades the rock but is more pronounced towards the margins where i t is subooncordant to the surrounding gneisses. Contacts between foliate plutonic rock and gneiss are gradational. Cigar-shaped mafic inclusions in the plutonic rock have long axes sub-parallel to lineations within the surrounding schists. In plutonic style this complex is intermediate between the autochthonous and para-autochthonous plutons described by Hutchison (1970) in the northern Coast Mountains. Apparently emplacement was synchronous with F c folding and may have been partly responsible for the drsharmonic synformal structure which wraps around the northern margin of the complex. Scuzzy Pluton (12). • In the map-area the marginal zone of the Scuzzy Pluton is mainly a medium-grained hornblende biotite quartz diorite with a foliation. Around Cairn Needle the contact dips steeply to the northwest and transects structures within the schists. Locally pronounced marginal foliation has been drag folded during emplacement. The core of the pluton is mainly massive granodiorite (Roddick and Hutchison, 1969). Dacite porphyry (13). An elongate, pink, fine- to medium-grained dacite porphyry intrudes the core of the Mount Breakenridge Complex. Marginally i t grades into a mesocratic quartz diorite porphyry. Miarolitic cavities f i l l e d with crystalline quartz, calcite and black biotite are common within this epizonal intrusion. 20 ultramafic Rocks (U). Small elongate or pod-like bodies of ochre-weathering ultramafite occur within a north-north-westerly trending zone which extends from the western slopes of Mount Urquhart to the head of Hunger Creek. They are similar to ultramafites mapped in the Old Settler Region (Lowes, 1972). Most contain forsterite, tremolite and talc, but those within the sillimanite zone contain anthophyllite and enstatite. A detailed description of their mineralogy, textures and metamorphism is given in a companion paper on the chemical petrology and metamorphism of the ultramafic rocks. Table 1. Plate Tectonic Evolution Insular Belt JR TR P P M Westward jump of subduction zone Blueschists in W. dipping sub. zone Development of Marginal basin Bonanza Karmutsen Sicker Gp. S 0 C P.C. Turtleback Complex W. Paleozoic and Mesozoic Belt Peninsula Form. Coast-Cascade Axial Core E. Paleozoic + Mesozoic Belt Tertiary Plutons ex Chilliwack Bath. Jackass Mtn. Group. Scuzzy-Spuzzum Plutons Ladner Group Plutonism High-level plutonism Meta-thrusting emplacement of ultramafites Cultus Em. Chilliwack Gp. (Shuksan Suite) Cairn Needle Form? Cairn Needle Fm.? Nicola Group Fergusson Grp. = ? Hozameen Gp. Breakenridge Fm.? Hozameen Gp.? = Custer Gneiss = Skagit Gneiss C r e e k <*• Blueschists 218-259 m.y. Meta. 415 m.y. Swakane Gneiss; Yellow Aster Comp.? = B.F.? 22 CORRELATION AND AGE OF ROCK UNITS. Breakenridge Formation. (Table 1). The Breakenridge Formation is similar to the Custer and Skagit Gneisses. As previously discussed, these gneisses possibly represent Upper Paleozoic strata which were metamorphosed and migmatised in pre- Jurassic (?) and mid- to Late Cretaceous time. (McTaggart and Thompson, 1967; Misch.,1966; McTaggart,1970; Mattinson, 1972). Cairn Needle Formation. The Cairn Needle Formation passes southward into rocks of the Settler Schist and Chilliwack Group (Lowes, 1972). The root zone of the Shuksan Thrust which separates the Settler Schists from the Chilliwack Group south of the map-area has not been recognised in the map-area. If the thrust zone were projected northwards into the area, the only difference in schists of the Cairn Needle Formation across this proposed extension would be one of metamorphic grade, not lithology. Consequently, the Cairn Needle Formation may be upper Paleozoic because the Settler Schist is equivalent to schists within the upper Paleozoic Skagit Metamorphic Suite (Lowes, 1972), while the Chilliwack Group is Pennsylvanian-Permian in age (Monger, 1966). The abundance of granitoid clasts in conglomerates of the formation may indicate that the formation is in part of Mesozoic age. Peninsula Formation. Rocks on Harrison Lake Shore northwest of Big Silver Creek are similar to clastic rocks in the lower Cretaceous Peninsula Formation (Monger, 1970). Plutonic Rocks. Scuzzy - Spuzzum Complex. Potassium-argon ages of the Scuzzy and Spuzzum plutons range from 70-74 ± 4 m.yrs. (Hutchison, 1971, p.comm.). In the south, near Hope, the Spuzzum Pluton yielded K-Ar ages of between 80-102 m.yrs. (Richards, 1971). Provided these ages represent the time of emplacement, this plutonic complex is of mid- to Late Cretaceous age and younger than the Barrovian metamorphism of the Cairn Needle and 23 Breakenridge formations. Breakenridge Plutonic Complex. The gneissic granodiorite on Mount Breakenridge is syntectonic with F c folding. This is presumably pre-mid-Cretaceous as F trends are truncated by faults on Harrison Lake Shore and c J the Scuzzy - Spuzzum pluton. Thus, depending on the age of the Caii Needle Formation, this intrusion is either post-Jurassic to Early Cretaceous or post-Late Paleozoic to pre-mid-Cretaceous. The epizonal porphyry on Mount Breakenridge is probably of similar age to Tertiary intrusions such as the Hells Gate Pluton. (Baadsgaard, Folinsbee, Lipson, 1961; Hutchison, 1971, p.a Ultramafic Rocks. The ultramafic rocks have a complex metamorphic history which suggests their emplacement within the Cairn Needle Formation before the climax of regional metamorphism in pre-mid-Cretaceous time. 24 STRUCTURE. Deformation within the map-area is polyphase and took place between the deposition of Cairn Needle Formation and Late Cretaceous. Four phases of folding have been recognised (F^, F B, F c and F D in order of decreasing age). Large-scale folds of the f i r s t two phases have not been recognised but the mapped distribution of lithologies delineates F c and latest FQ folds (Maps 1 and 2). The style and elements of small-scale F A, F B and FQZ folds are given in Table 2. Rootless F A isoclines (Plate IA) have been refolded by younger folds (Plate IB). These F A folds occur in both Breakenridge and Cairn Needle Formations (Map 2) so that both formations have had the same deformational history and a l l recognised folding events occurred after the deposition of Cairn Needle Formation, The angular variation between F B subisoclinal folds (Plate 1C) and the local foliation or compositional layering varies between 2 and 19 degrees. Small-scale F g folds have been refolded by large-scale F c folds in the area southeast of Cairn Needle and around the dome structure which straddles Clear Creek. Although marker horizons in the Breakenridge Formation cannot be traced to large-scale F B fold closures, the shear senses of observed F g minor-folds southeast of Cairn Needle indicate a large-scale F B axial surface located approximately in the position shown in Map 2. The most important folds in the map-area {FQ) have northwest-trending subvertical axial surfaces (Map 2). Culrninations of F c folds result in dome structures cored by gneisses of the Breakenridge Formation. The dome which straddles Clear Creek results from the intersection of F c and latest F D antiforms. The dome-like nature of Breakenridge Formation gneisses west of Big Silver Creek may in part be due to continued upward emplacement of the synkinematic Mount Breakenridge Plutonic Complex. FQ small-scale fold axes north of this pluton plunge to the northwest and grey gneisses 25 Table 2. Style and elements of minor folds. l a Rootless Similar Sub-isoclinal, limbs loc a l l y sheared o f f . Similar Open to chevron. Similar, but where layer anisotropic more concentric. Tight curved hinge. Straight to broadly curved limb. Folded compositional layering with axial schistosity. Broadly curved hinge. Broadly curved limb. Moderately harmonic. Folded compositional layering with axial schistosity. Sharp hinge. Straight limb. Moderately harmonic. Hinge line may be short and straight or long and curved. Mainly folded compositional layering. Some stra i n - s l i p axial schistosity. PLATE 1. Rootless F A isocline in Cairn Needle Formation. F A isocline refolded by F c small-scale fold in Breakenridge Formation striped amphibolite. Subisocline Fg folds in Breakenridge Formation amphibolite. Small-scale similar to concentric F c folds in Breakenridge Formation striped amphibolites. Small-scale similar to concentric F c folds in Breakenridge Formation amphibolites. 26 Plate I 27 to the south, immediately west of Big Silver Creek, have been folded into a southeast plunging antiform (Map 2). In style minor folds are open to chevron, similar to concentric (Plates 1D,E). Their axial planes have a rorthwest subvertical trend which compares favourably with major F c folds. The curvilinear nature of F c fold hinges is manifested by the attitudes of small-scale F^ , fold axes. Fold axes southeast of Cairn Needle have a northwest trend and plunge (317/37 NW.), but between Mount Breakenridge and the dome on Clear Creek they are subhorizontal or plunge shallowly to the southeast and west of Mount Urquhart they again trend and plunge to the northwest (Map 2). Thus domal culminations have apparently been produced by interference between northwest-trending FQ folds and later northeast-trending folds and upward emplacement of plutonic rocks. The northwest-trending fault zone close to Harrison Lake is the extension of the fault zone on Cascade Peninsula which is the continuation of the root zone of the mid-Cretaceous Church Mountain Thrust (Lowes, 1972). In this fault zone rocks of the Cairn Needle Formation are fault-bounded by gneisses of the Breakenridge Formation and clastic rocks of the Peninsula Formation. Previously the fault zone was considered to be the extension of the root zone of the Shuksan Thrust (Lowes, 1972). However, east of Big Silver Creek, rocks of the Cairn Needle Formation continue southwards out of the map-area without obvious fault ;0ff-set, so that there can be no direct link between this fault and the Shuksan Thrust. The eastern fault in the Shuksan Thrust root zone separates Settler Schist from Chilliwack Group and is marked by a belt of ultramafites (Lowes, 1972). This fault apparently does not extend into the map-area. The only difference in Cairn Needle Formation across the northern extension of this fault is one of metamorphic grade not lithology. Ultramafites in the region are not a l l bounded^byimagor faults as small pods outcrop within the Caim Needle Formation 7 kilometers northeast of the Shuksan Thrust as mapped by Lowes (1972). 28f PLATE II. A . Garnet porphyroblast which has crystallized before F D hi schistosity. B. . Garnet porphyroblast which overprints small-scale F^ folds. C. Kyanite and plagioclase which overprints small-scale F c folds. D. Staurolite porphyroblast which overprints F^ strain-slip cleavage. E. Polygonal arc of sillimanite mimetic on F c small-scale fold. F. Aggregates of sillimanite with, good (010) cleavage pseudomorphous after andalusite. Cpaque inclusions define the chiastolite cross of the andalusite. Plate II 29 Table ,3. Folding episode. F A F B Fc Index Mineral. BIOTITE GARNET STAUROLITE KYANITE SILLIMANITE Relationship between crystallization of metamorphic index minerals and deformation. 30 However, a prominent physiographic lineament along Hunger Creek may .'mark" a fault along which an ultramafic pod has been emplaced. Relationship between metamorphism and deformation. Relationships among minor folds, schistosities, cleavages and porphyroblasts of index minerals in metamorphic rocks can be used to relate metamorphic recrystallization to deformation (Zwart, 1962; Spry, 1969). In the map-area maximum metamorphic recrystallization occurred after F c folding (Table 3). Garnet crystallization spans F B and F c folding. Porphyroblasts have crystallized before Fg ss schistosityes (Plate 11 A) and after small-scale F c folds (Plate 11 B). Staurolite crystallization is mainly post-Fc as porphyroblasts have overprinted strain-slip cleavage associated with F^ minor folds (Plate 11 D). However, some staurolite crystals are included in pre-F B garnets so that i t too had a complex crystallization history. Crystallization of the aluminosilicates is post-F^, as kyanite and sillimanite invariably overprint (Plate 11 C) or form polygonal arcs around FQ minor folds (Plate 11 E). 31 Figure 3. 32 METAMORPHISM. Metanorphism in the map-area is of the classical Barrovian type with development of zones of garnet, staurolite-kyanite and sillimanite (Figure 3). Fault-bounded rocks on Harrison Lake shore are of chlorite or garnet grade. The development of the Barrovian metamorphism was broadly contemporaneous with folding. Although locally, rocks had reached garnet grade prior to F B-folding, the maximum grade of metamorphism was reached after F c- folding and before mid-Cretaceous faulting. Local contact metamorphism, which accompanied the emplacement of the Late-Cretaceous Scuzzy Pluton produced chiastolite and sillimanite hornfels in a 300 meter wide aureole around Cairn Needle. Prismatic crystals of andalusite with well-defined chiastolite crosses have been pseudomorphed by aggregates of sillimanite (Plate 11 F). In the map-area emplacement of plutonic rocks took place both during and after the climax of metamorphism. Intrusion of the Mount Breakenridge Plutonic Complex was synkinematic with FQ- folding which pre-dated the metamorphic climax. The Scuzzy Pluton post-dates the deep-seated regional metamorphism. The heat source for the deep-seated metamorphism lay in the area now underlain by the Scuzzy Pluton and was undoubtedly a zone within which plutonic rock was being generated. Whether this plutonic rock was the direct cause of the Barrovian metamorphism or an end product of the metamorphic process is undetermined, but certainly by Late Cretaceous the effect of the Scuzzy Pluton on its surrounding schists was minimal as • ;'.. metamorphism was confined to thin localized zones around its contacts. Physical conditions of metamorphism can be estimated by comparing natural mineral assemblages with experimental data. The physical conditions of the regional and contact metamorphic events in the map-area as defined by experimental data for the a aluminosilicates (Richardson, e t a l . , 1969), staurolite (Hoschek, 1969) and muscovite (Evans, 1965) are given in Figure 4. Barrovian metamorphism took place at pressures above 33 Figure 4. Petrogenetic grid showing P.T. conditions of regional (R) and contact (C) metamorphism as defined by a) Muminosilicate curves and t r i p l e point with uncertainty area. (Richardson, Gilbert and Bell.1969) b) Ch. + muse. = St + bio. + qtz. + H^ O . (Hoschek, 1969). c) St. + muse. + qtz. = A l - S i l . + bio. + fc^O (Hoschek, 1969). d) Muse. + qtz. = K-feldspar + A l 2 S i 0 5 + H20. (Evans, 1965). 34 the aluminosilicate invariant point and reached temperatures of approximately 700°C. Contact metamorphism occurred at pressures of between 4 and 5.5 kb and possibly reached temperatures similar to the Barrovian event. 36 SEQUENCE OF EVENTS. Uncertainty in the ages of the main formations precludes assigning absolute times to events in the region, but a well-defined relative timing is apparent. i) The presence of conglomerates between Breakenridge and Cairn Needle Formations may mark a minor hiatus between their deposition. Structural evidence which would indicate that the Breakenridge. Formation was subjected to deformation and metamorphism before the Cairn Needle Formation is lacking as both contain the earliest recognised folds. ii ) Determination of the age of deep-seated metamorphism and deformation is dependent on determination of the age of the Cairn Needle Formation. The climax took place either between the Jurassic and mid-Cretaceous or upper Paleozoic and mid-Cretaceous. Emplacement of the Breakenridge Plutonic complex was synkinematie with FQ folding, which predates the metamorphic climax. Ultramafic rocks were emplaced in the Cairn Needle Formation prior to this deep-seated metamorphic event. i i i ) High-angle faults between Breakenridge, Cairn Needle and Peninsula Formations probably developed in the mid-Cretaceous, as these faults are the northern extension of a fault zone around Cascade Peninsula on Harrison Lake which has been correlated with the mid-Cretaceous Church Mountain Thrust (Lowes, 1972). Low grade metamorphism which represents the waning stages of the main metamprphic event probably accompanied this phase of faulting. iv)) Local contact metamorphism resulted from the emplacement of the Scuzzy - Spuzzum Pluton between the mid- and Late Cretaceous. v) Emplacement of porphyries on Mount Breakenridge possibly occurred in the Tertiary, but andesite dykes probably accompanied Quaternary vulcanism. 37f Figure 5. Space and time distribution of lithological assemblages interpreted in idealistic terms within the various physiographic and geological belts of the Canadian Cordillera (after Monger et a l . 1972). 1. = Rocky Mountain Belt. 2. = Qnineca Crystalline Belt. 3. = Intermontane Belt. 4. = Coast Plutonic Complex. 5. = Insular Belt. O.C. = Oceanic Crust. C.C. = Continental Crust. P-M = Plutonic Metamorphic Complex. V.A. = Volcanic Arc. B. = Basalt. P.B. = Plateau Basalt. S.B. = Successor Basin. F.D. = Fore Deep. S-S = Shelf and Slope deposits. 38 STRUCTURAL SYNTHESIS AND RELATION TO PIATE TECTONIC THEORY. The geological evolution of the Mount Breakenridge area i s intimately related to the evolution of the Southern Coast Crystalline Complex and the Northern Cascades. Structural syntheses of the Coast-Cascade Complex i n Br i t i sh Columbia and the Noerthern Cascades of Washington State have been presented by McTaggart (1970) and Misch (1966). Rocks i n the Coast - Cascade system have been metamorphosed under varying P - T regimes which range from high P - low T ('Blueschist') through intermediate P - T (Barrovian) to low P - high T (Buchan) (Monger, 1966; Monger and Hutchison, 1971; Misch, 1971; Read, 1960). These metamorphic facies types have been related to plate tectonic theory (Dewey and Bi rd , 1971) so that interpretation of the metamorphic and structural history of the Coast -Cascade system i n terms of a unifying plate tectonic model i s most desirable. Monger et a l . (1972) presented a plate tectonic model of the Canadian Cordil lera based on interpretation of the l i tho logica l assemblages i n terms such as oceanic crust; island arc and continental r i se ; slope; shelf deposits (Figure 5). The occurrence of oceanic crust i n the Intermontane and Omenica Crystalline Belts i n Mississippian to Middle Triassic time was explained by a model which involved oceanward stepping of east-dipping subduction zones from Permian to mid-Triassic. A westward stepping of subduction zones occurred between the deposition of the Permian to Upper Triassic Karmutsen volcanics (oceanic pi l low basalts and breccia) (Sutherland-Brown, 1966) and the Upper Triassic to Early Jurassic Bonanza volcanics (island arc andesites, dacites and pyroclastics). The Cordil lera evolved from a system of island arcs i n the Late Triassic and Early Jurassic through an intermediate stage with deposition i n increasingly restricted basins to a continental Cordil lera i n Late Cretaceous and Early Tertiary. Within the Insular Bel t , island arc assemblages seem to have developed on pre-existing continental basement, a feature which one would not have expected i f , as postulated by Monger, the overlying 39 pre-Jurassic arc or oceanic rocks had developed on or near oceanic crust and were rafted continentwards. The following speculative synthesis i s presented i n an attempt to explain this anomaly and set the evolution of the Coast -Cascade system within a plate tectonic framework. 1. The pre-Devonian basement (Yellow Aster complex, Swakane Gneiss, Turtleback complex) was metamorphosed i n Upper Si lur ian (415 m.y.) (Mattinson, 1973). Though no structural or metamorphic events older than Mesozoic have been recognized within the Breakenridge Formation, the poss ib i l i ty that i t also i s pre-Devonian i n age cannot be precluded. 2. The metamorphism preceded and possibly heralded the development of a marginal basin (Karig, 1971) i n Mississippian times i n which upper Paleozoic chert, basalt oceanic assemblages (Cache Creek Group, Hozameen Group?) were deposited. Island arc and volcaniclast ic flysh deposits (Sicker Group, Chilliwack Group) developed oceanward from this basin and on top of the pre-Devonian basement rocks (Turtleback complex). Rocks from which various metamorphic rocks were later made (Custer Gneiss; Skagit Gneiss; Cascade River Schist; Breakenridge Formation and possibly Cairn Needle Formation) (Figure 6A) were also deposited at this time. 3. The slow diffuse spreading process which involved separation of the island arc from the continent margin continued into the Lower Triass ic . At this time spreading may have terminated and a west-dipping subduction zone may have developed at the marginal basin - island arc transi t ion. Within this subduction zone Chilliwack Group and equivalent rocks (Shuksan Metamorphic Suite i n Washington State) developed crossite and lawsonite or 'blueschist' mineralogy. The Middle Triassic oceanic assemblages within the Fergusson Group (Cameron and Monger, 1971) indicate that the marginal basin persisted to this time. Blueschist metamorphism certainly continued to the Triassic and possibly the Jurassic as Crossite schists i n the Shuksan Suite are dated at 259-218 m.yrs. and Monger reports ' incipient ' and fibrous lawsonite i n the Triassic to Jurassic Cultus Formation. 40f Figure 6. Hypothetical synthesis which relates the development of the Southern Coast Crystalline Complex and Northern Cascade Mountains to plate tectonic theory. (See text for discussion). 40 MISS. SICKER GP CHILLIWACK GP CACHE CREEK. , . i i i i i i i i r r CONT. REMNANT * W *~\ " t >^ MARGINAL-BASIN. For i l l u s t r a t ive " ^ i i - ^ v purposes, the Arc-Trencl*^ Gap i s reduced i n length. CONT S. L. BONANZA L.TRIASSIC. CULTUS FM. B M CRETACEOUS SHUKSAN I i HOZAMEEN F. Ultramafite \J fP-M COMPLEX. ./ AXIS OF HIGH [ HEAT FLOW 50 ml C 41 The Upper Triassic Karmutsen volcanics were generated close to the main east-dipping subduction zone and the calc-alkaline Nicola volcanics were extruded i n island arcs west of the marginal basin which by now was almost fu l ly contracted. 4. Between Upper Triassic and Early Jurassic the main east-dipping subduction zone jumped westwards (Monger et a l . 1972) and the Bonanza Group was formed i n island arcs associated with this new subduction zone (Figure 6b). 5. From the Upper Jurassic to the mid-Cretaceous the Coast -Cascade system developed from a system of island arcs and restricted basins i n which volcaniclastic and c las t ic rocks were deposited (Ladner Group, Jackass Mountain Group). 6. The climax of metamorphism and deformation occurred i n mid-Cretaceous. Migmatization and metamorphism of the Skagit gneiss i n Washington State has been dated at 90 - 60 m.yrs.(Mattinson, 1972). Metamorphism of the Custer Gneiss may have accompanied intrusion of the Late Cretaceous Spuzzum Pluton (McTaggart, 1970). The climax of deep-seated Barrovian metamorphism i n the Mount Breakenridge area occurred before mid-Cretaceous faulting and later low pressure contact metamorphism accompanied emplacement of the Late Cretaceous Scuzzy and Spuzzum Plutons. At this time ultramafic rocks were emplaced upwards along major faults or thrusts such as the Hozameen or Shuksan (Figure 6c). Discrete pods and sl ivers of ultramafites i n the Mount Breakenridge Area, which along with their enclosing schists, had been metamorphosed at depths of 23 km. were also uplifted during this major orogeny. 7. Emplacement of high level intrusions continued through the Tertiary (Richards, 1971; Richards and Whire, 1971) as the Coast - Cascade system developed into a t ruly continental Cordi l lera . The development of a marginal basin and associated island arc i n the Upper Paleozoic i n the Coast - Cascade and Insular Belts may thus explain the anomaly i n the scheme presented by Monger et a l . (1972) and account for the development of 'Blueschists 1 i n Permian to Jurassic time within the Chilliwack Group, the Cultus Formation and Shuksan Metamorphic 42 Suite. As stated by Dewey and Bird (1971); "Island arcs developed in this way, with the opening and closing of marginal basins, are likely to have a continental foundation since they originate near or on continental margins; therefore, the occurrence of old continental basement within an orogenic belt near, or at the continental margin does not constitute an objection to continental accretion and to the previous existence of marginal basins". 43 ACKN0WLEDGEMFJS1TS. The author would like to thank Dr.'sW-W. Hutchison and J.A. Roddick of the Geological Survey of Canada who provided helicopter support,, rock thin-sections;and invaluable guidance throughout this study. Dr. R.L. Wheeler and C.J. Duffy provided able assistance and pleasant companionship in the field. The work was carried out under the supervision of Dr. H.J. Greenwood at the University of British Columbia.Research funds were provided by National Research Council grant (A67-4222) to Dr. H.J. Greenwood. The author gratefully acknowledges financial support provided by a National Research Council Postgraduate Fellowship and a Killam Pre-Doctoral Fellowship from the University of British Columbia. 44 REFERENCES. Aho. A.E. 1956. Geology and, genesis of ultrabasic rdckel-copper pyrrhotite deposits at.the Pacific Nickel Property, Southwestern British Columbia. Econ. Geol., vol. 51, pp 444-481. Baadsgaard, H., Folinsbee, R.E. and Lipson, J. 1961. Potassium-argon dates of biotites from Cordilleran granites. Bull. Geol. Soc. Amer., vol. 72, pp 689-701. Cairnes, CE. 1930. The serpentine belt of the Coquihalla Region, Yale district, British Columbia. Geol. Surv. Can., Sum. Dept. for 1929, pt. A., pp 144-197. Cameron and Monger, 1971. Middle Triassic Conodonts from the Fergusson Group, northwestern Pemberton map-area. Geol. Surv. Can. Rept. of Act. paper 71-1, pt. B, p 94. Danner, W.R. 1966. Limestone resources of Western Washington. Washington Div. of Mines and Geol. Bull. 52. Dewey and Bird, 1971. Origin and emplacement of Ophiolite Suite: Appalachian Ophiolites in Newfoundland. J. Geophysical Research, vol. 76(14). pp 3180-4000. Duffell, S. and McTaggart, K9G1 1952. Ashcroft map-area, British Columbia. Geol. Surv. Can., Mem. 262. Evans, B.W. 1965. Application of a reaction rate method to the breakdown of muscovite and musoovite plus quartz. AM. J. Sci., vol. 263, pp 647-667. Hollister, L.S. 1969 a. Metastable paragenetic sequence of andalusite, kyanite and sillimanite, Kwoiek Area, British Columbia. Am. J. Sci., vol. 267, pp 352-370. Hollister, L.S. 1969 b. Contact metamorphism in the Kwoiek Area of British Columbia: an end member of the metamorphic process. Geol. Soc. Am. Bull. 80, pp 2465-2494. 45 Hoschek, G. 1969. The stability of staurolite and chloritoid and their significance in metamprphism of pelitic rocks. Contr. Mineral and Petrol., yol.22rpp 208-232. Hutchison, W.W. 1970. Metamorphic framework and plutpnic styles in the Prince Rupert region of the Central Coast Mountains British Columbia. Can. Jour. Earth Sci., vol. 7, no. 2, pp 376-405. Kalsbeek, F. 1970. Petrography and origin of gneisses, amphibolites and migmatites in the Quasgialic area, S.W. Greenland. Med. cm Gron. B.D. 189, N.R.I. Karig, D.E. 1971. Origin and development of marginal basins in the Western Pacific. J. Geophys. Research, vol.76(11) 1971. Lowes, B.E. 1972. Geology between Harrison Lake and the Fraser River. Unpubl. Ph.D. thesis. Univ. of Washington. Mattinson, J.M. 1972. Age of zircons from the Northern Cascades, B.G.S.A. vol. 83, pp 3769-3784. McTaggart, K.C. and Thompson, R.M. 1967. Geology of part of the Northern Cascades in Southern British Columbia. Can. Jour, of Earth Sci., vol. 4, pp 1199-1228. McTaggart, K.C. 1970. Tectonic history of the Northern Cascade Mountains. Geol. Assoc. Can., Spec. Paper 6, pp 137-48. Misch, P. 1966. Tectonic evolution of the Northern Cascades of Washington State. Can. Inst. Mining Met., Sp. vol. 8, pp 101-148. Misch, P. 1971. Metamorphic facies types in North Cascades. . in "Metamorphism in the Canadian Cordillera", Cord. Sect., G.A.C., pp 22-23. Miyashiro, A. 1961. Evolution of metamorphic belts. J. Petrology, vol. 2, pp 277-311. Monger, J,W.H. 1966. The stratigraphy and structure of the type area of the Chilliwack Group, southwestern British Columbia, unpubl. Ph.D. thesis, Univ. of Br.'Columbia. Monger, J.W.H. 1970. Hope Map area. West Half. British Columbia. G.S.C. paper 69-47. Monger, J.W.H. and Hutchison, W.W. 1971. Metamorphic Map of the Canadian Cordillera. G.S.C. paper 70-33 pp 43-61. 46 Monger, J.W.H.; Souther,J.G,; and GabrielsefH. 1972. Evolution of the Canadian Cordillera: A plate.tectonic model. A.J.Sci., vol. 272, pp 577-602. Pigage,L.C. 1973. Metamorphism southwest of Yale, British Columbia. unpubl. M.Sc. thesis, Univ. of British Columbia. Read, P.B. 1960. Geology of the Fraser Valley between Hope and Emory Creek, British Columbia, unpubl. M.A.Sc. thesis, University of British Columbia. Reamsbottom, S.B. 1971. The Geology of the Mount Breakenridge Area, Harrison Lake, British Columbia. unpubl. M.Sc. thesis, University of British Columbia. Richards, T.A. 1971. The Chilliwack Batholithic Complex of south-western British Columbia, unpubl. Ph.D. thesis, Univ. of British Columbia. Richards, T. and White, W.H. 1970. K-Ar ages of plutonic rocks between Hope, British Columbia and the 49th. parallel. Can.Jour, of Earth Sci., vol. 7, pp 1203-1207. Richardson, S.W., Gilbert, M.C. and Bell, P.M. 1969. Experimental determination of kyanite-andalusite and andalusite-sillimanite equilibria; the aluminum silicate triple point. Am.J.Sci., vol.267, pp 259-272. Roddick, J.A. and Hutchison, W.W. 1969. Northwestern part of Hope Map area, B.C. (92H/W%). G.S.C. paper 69-1, part A., pp 29-38. Roddick, J.A. and Hutchison, W.W. 1972. Plutonic and associated rocks of the Coast Mountains of British Columbia. 24th. Int. Geol. Congress. Field Exc. A04-C04. Sutherland-Brown, A. 1966. A tectonic history of the Insular Belt of British Columbia: In Gunning and White: ed. 'A Tectonic History of the Western Cordillera 1. Can. Inst. Min. Met. sp. vol. 8, pp 83-100. 47 White, W.H., Erickson, G.P., Northcote, K.E., Dirom, G.E. and Harakel, J.E. 1967. Isotope dating of the Guichon batholith, B.C. Can. J. Earth Sci., vol. 4, pp 677-690. Zwart, H.J. 1962. ®n the deteimnation of polymetamorphic mineral associations and its application to.the Bosost area (Central Pyrenees). Geol. Rdsch., vol. 52, pp 38-65. II. CHEMICAL PETROLOGY OF PELITIC ROCKS FROM THE MOUNT BREAKENRIDGE AREA, HARRISON LAKE, BRITISH COLUMBIA. by Stanley B. Reamsbottcm, Department of Geological Sciences, The University of Bri t i s h Columbia , Vancouver 8, B.C. 49 ABSTRACT. Relationships between bulk rock compositions, mineral chemistry and metamorphic grade i n p e l i t i c rocks from Mount Breakenridge area, B r i t i sh Columbia, are examined. Rock oxidation rat io does not control bulk rock chemistry but does affect the chemical composition of ilmenite. Plagioclase composition i s dependent on host rock composition not metamorphic grade and the Pa/Pa + Ms + Ph content of white mica decreases with increase i n metamorphic grade. Contrary to ear l ier findings, staurolite i s not confined to rocks with F e 2 0 3 / A l 2 0 3 ratios of about 0.4. Compositional zonation profiles i n garnet complicate the study of variation i n mineral composition with grade of metamorphism. The distr ibution of chemical species between coexisting chlori te , b io t i t e , garnet and staurolite implies a close approach to chemical equilibrium between these phases. Use of a linear regression technique to search for reactions i n the p e l i t i c rocks indicates that single assemblages i n the kyanite-staurolite zone are possibly univariant reaction assemblages whose wide regional distr ibution i s the result of buffering and internal control of water fugacity by the mineral assemblage. Search for reactions between kyanite-staurolite and si l l imanite zone rocks indicates that assemblages i n these zones are not only the result of different physical conditions but also of rock bulk chemistry. Representation of mineral assemblages i n the tetrahedron - CaO-Na20-MnO-K20 i l lus t ra tes how the compositions of minerals i n mineral assemblages such as garnet-muscovite-plagioclase may be controlled by variation i n f . 50 Figure 1. 51 INTRODUCTION. Pelitic rocks in the Mount Breakenridge Area, Harrison Lake, British Columbia have mineral assemblages characteristic of the chlorite, garnet, staurolite-kyanite and sillimanite zones of regional metamorphism (Figure 1; Table 1).. Because the mineralogy of ipelitic rocks is very sensitive to changes in metamorphic grade these rocks are ideal for studying the effects of variable pressure (pTOTAL' PH20 ?02^ temperature and bulk composition. Chemical analyses of twenty-one specimens were determined by the Geological Survey of Canada, Ottawa. The compositions of constituent minerals from thirteen of these specimens were determined by the electron microprobe. These data allow a study of: o a) control of rock composition on itiineral compositions; b) effect of metamorphic grade on mineral chemistry; c) approach to chemical equilibrium; d) metamorphic reactions at the kyarute-sillimanite isograd as defined by linear regression analysis. Bulk Rock Chemistry. Modal and chemical analyses of twenty-one schists are shown in Tables 2 and 3. In comparison to the average pelite (Shaw, 1956), the suite has greater weight per cent T1O2, AI2O3, FeO, MgO and CaO and less Si02, F e2°3' J^O, 1^0, ^0 and CO2. This particular suite has a high alumina to alkali ratio and a low content of MgO and CaO which are typical of pelites. Unlike typical pelites, a majority of the suite has more Na20 than K^ O and more CaO than MgO. The wide range of Fe20^ / FeO ratios are represented by the change in slope of the lines from 0.075 to 1.0 in Figure 2. The oxidation state of pelitic rocks has been expressed as the oxidation ratio (Molar 2Fe203 x 100/2Fe2O3+FeO) by Chinner (1960). Oxidation ratios are listed in Table 2 and range from about 17 to 68. Oxidation ratios may be a function of the oxidation state of the original sediment or, i f i t contained carbonaceous material the oxidation state could have been modified by reactions of the type: Table 1.  Specimen No. 1 2,3,6,5,7,8,15,16,17,18, 20,21 4,9,10,13,14 11,12,19 Metamorphic Zone. Garnet Staurolite-kyanite zone. Sillimanite zone. Cairn Needle Contact Aureole. T a j i ! ? . Mo-ia.1 a ^ a l y s ^ s of o c h l G t o :;o. OO 35 25 71 244 235 453 1*68 1+65 1*27 503 459 460 1000 159 H-7 264 384 462 431 447 1 2 3 1* 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 23.2 36.7 65.0 18.4 27.2 15.1 2l*.6 28.1* 30.6 7.5 33.2 48.8 51.0 25.0 20.0 20.0 30.0 20.0 20.0 35.0 30.0 0.5 33.8 5.6 22.9 26.0 15.6 23.0 0.8 10.7 8.0 11.0 10.1 11.7 10.0 15.0 20.0 15.0 10.0 15.0 20.0 10.0 Chlorite 6.6* 1.6* 1.3* 0.6 0.2 1.1 1.3 7.3 0.3 7.0 5.0 1.0 1.0 ' 5.0 1.0 1.0 Muscovite 60.4 1.4** 2.8 0.1 8.7 7.2 27.7 10.7 0.2 * 5.7 24.0** 4.2** 20.0 2.0 10.0 5.0 30.0 20.0 10.0 Biotite 0.0 l4.8 13.1 0.8 1.2 6.9 4.2 13.2 15.8 16.7 19.3 0.3 23.7 10.0 1.0 15.0 5.0 5.0 25.0 4.0 20.0 Garret 7.8 4.0 1.2 31.9 13.3 7.5 30.7 5.6 31.2 8.1 3.1 1.0 5.0 15.0 30.0 20.0 30.0 8.0 10.0 5~s.u-rolite 1.4 3.2 23.4 0.2 34.1* 6.1 4.7 0.5 0.3 5.0 3.0 5.0 10.0 1.0 Kyar.ite 0.2 0.3 8.3 7.8 13.7 10.4 10.0 10.0 10.0 10.0 15.0 1 ir.ar.ite 1.0 3^.7 34.0 21.4 4.4 Ar.-iai~i-te 3.4 Ecrr.tl^r.de 25.0 Cfcirite 4o.o 1.7 5.0 7.5 0.9 4.4 0.1* 0.3 0.8 1.0 0.3 4.0 3.8 5.0 5.0 3.0 1.0 II=er.ite X X X X X X X X X X X X 1.0 5.0 1.0 X X X H ^ t i t e X X Graphite X X Pyrrriotite X ft-rite X Chalcopyrite Spawns X 0.3 0.3 1.0 1.0 1.0 F.-tilo 0.6 X X 0.1* X ' X X 1.1 X X Tourzalir.e 0.5 0.6 Apatite 0.1 1.0 1.0 1.0 Chlorite cainly prograde. Muscovite i n part pseudomorphoua after staurolite and alumino-silicates. 1'a'ols . Chemical analyses of schists and gneisses. F i e l d No. 80 35 25 71 21*1* 235 i*53 ^53 1*68 1+65 1*27 1*59 1*60 503 1000 159 H-7 261* 381+ 1*62 ^31 1*1+7 Sceeinen No. 1 2 3 1* 5 6 •.7 .7 8 9 10 11 12 13 l l * 15 16 17 18 19 20 a SiO. 57.6 7k.l 56.it 1*3-0 58.3 52.3 63.O 62.1 60.9 66.2 5k.k 63.5 59-5 61.2 51.9 55.9 6O.9 61*. 1. 62.1 59.0 70.1* 61.0 1.13 0.1(0 0.76 1.37 1.62 1.17 0.55 0.62 1.07 o.79 O.87 • 0.91 O.89 O.89 0.92 0.95 1.61 I . 0 5 1.19 0.75 0.36 0 . 9 : Ai.Cu 19.6 10.2 20.lt 27.3 20.0 21*.1* 23.6 21*.1 21.7 17.6 19.2 16.9 19.1 22.1* 21.7 19.8 19.8 20.1 28.1 28.8 13.0 19.1 F e 2 0 3 1-9 0 . 8 . 0.9 6.1* 1.0 5.h 1.2 1.2 3.9 ••0.5 12.1 1.1* 1.6 1.7 1.6 k.O 0.7 1.6 "1.5 1.7 0 .8 1.5 ?e0 6.0 2.7 5-0 8.8 7.6 5-2 1.8 1.7 it.l* 5.0 0.0 1*.6 k.9 k.3 8.3 5.1* l * . l 3-1+ 6.1* k.7 3-9 6.1* MnO 0.10 0.11 O.llt 0.27 O.07 O.09 o.oi* 0.05 0.01* 0.06 0.23 0.10 0.06 0.11 0.17 0.08 0.07 O.03 0.16 0.07 0.17 0.11 M~0 1.80 1.5 1.1* 2.0 0.9 1.6 0.7 0.7 1.7 1.6 3.6 2.1 2.5 0.9 5.0 2.1 1.1+ 0.8 . 1.2 2.2 2.3 2.6 CaO 1.20 1.7 5.0 2.7 2.8 3.2 1.1 1.2 0.3 1.8 2.8 • 0.60 2.8 1.2 1*.6 3.5 2.9 1.3 3-k 1 .9 1.7 1.2 X-0 1.7 0.7 1.2 0.2 0.9 2.0 1.9 2.1 2.8 1.5 0.9 2.9 2.6 1.0 0.2 2.8 0.6 3.1 0 .8 3.9 1.0 3-6 Na.,0 1.6 2.3 3-h 1.6 1.7 2.1 2.1 2.2 0.1* 2.1 2.2 1.5 2,6 2,1 1 .3 1.6 1.7 1.5 2.1* 1.9 3-3 0 .6 ?-0_ 0.08 0.07 0.15 0.03 0.07 0.09 0.08 0.08 0.02 0.07 0.13 0.15 0.2 0.03 0.03 0.13 O.00 0.02 0.33 0.35 0.08 o.ie co2 0.1+ 0.00 0.1 0.00 0.3 0.1 0.2 0.00 0.2 0.00 0.00 0.00 0.00 0.1 0.1+ 0.00 0.00 0.00 0.00 0.00 0.1* 0 . 6 H 20 4.0 0.8 1.1 1.1* 0.9 1.5 1.6 1.7 2.0 , 1.8 1.3 3.»* 2.2 2,1* 2.1 2.1 0.7 1.7 0.6 2.6 1.2 1.7 Total 97.1 . 95.'t 96.0 95-1 96.2 99-2 97.9 97.8 99->* 99.0 96.1* 98.1 99-0 98.3 98.2 98.1* 97.6 98.7 98.2 97.9 98.6 99.5 Analyses by Geological Survey of Canada, Ottawa. FeO Figure 2. Ray-diagram of Fe203/FeO ratios of pelitic rocks. Slope varies from 0.075 to 1.0 . 56f Figure 3. Plot of bulk FeO, MgO, MnO, MgO/MgO + FeO in pelitic rocks versus rock oxidation ratio.(M olar 2Fe 20 3 x 100/2Fe2O3+FeO). 56" OS 6 5 6 s *8 6 Q 8 J ooi * o u W 57 C + 4Fe + 3 + 202~ = C02 + 4Fe+2 (Hounslow and Moore,1967) The Fe and 0 are derived from coupled reactions involving silicates and oxides. If a l l the graphite was oxidized,the.oxidation state would be dependent on the original oxidation state and the amount of carbon in the,rock, provided hydrogen and oxygen did not migrate. In the rocks studied the variation in oxygen state over relatively short distances indicates that i f hydrogen or oxygen migration did take place i t was insufficient to homogenize the oxidation state. Previous studies (Chinner, 1960; Hounslow and Moore, 1967) have observed relationships between bulk rock chemistry and oxidation ratio. In pelites from Glen Cova, Scotland, Chinner observed that bulk MnO increased and FeO and MgO decreased with increase in oxidation ratio. In Grenville schists studied by Hounslow and Moore an increase in A^ O-j/MgO ratio accompanied an increase in the rock oxidation ratio. The spread of bulk MgO, FeO and MnO data in Figure 3 is too great to demonstrate the type of relationships observed by Chinner. In this study bulk MnO tends to decrease rather than increase with respect to rock oxidation ratio. No relationship between Al202/MgO ratio and oxidation ratio exists in these rocks. Mineral Chemistry. Microprobe analyses of coexisting chlorite, biotite, garnet,staurolite, plagioclase, muscovite and ilmenite from thirteen specimens were carried put on the Applied Research Laboratory microprobe at the University of Washington, Seattle under the supervision of Professor B.W. Evans and Miss L. Leitz. Details of analytical technique, operating conditions and standards used are given in Appendix 1. Chlorite. Microprobe analyses of chlorites from four specimens are listed in Table 4. Chlorite of specimens 1,2 and 3 appears texturally stable, but that of 4 is retrogressive after other Table 4. Microprobe analyses of C h l o r i t e . 4 Spec. No . 1 2 3 S i 0 2 24 .83 25.93 25 .77 25.23 A1 20 3 23.25 23.02 23.40 23.04 T i 0 2 0.08 0.10 0.10 0.08 FeO * 28 .84 22.57 23.09 26 . 18 MnO 0.11 0 .08 0.12 0.07 MgO 10.59 15 .74 15.12 13 .00 K 20 0.20 0.05 0 .06 0.07 Total ** 99 .2 98.99 99 . 16 99.17 No . of ions 28 (0) . Si 5 . 30 5 . 37 5 . 34 5 . 32 Al 2 .70 2.63 2.66 2.68 T i 0.0 0.0 0.0 0.0 Total 8.00 8 .00 8 .00 8 .00 Al 3.16 2.99 3 .06 3.06 T i 0.01 0.02 0.02 0.01 F e + 2 5.16 3.91 4.0 4.63 Mn 0.02 0.01 0.02 0.01 Mg 3.37 4.86 4.67 4.08 Total 11.72 11.8 11.77 11.8 K 0.05 0.01 0.02 0.02 * Total iron expressed as FeO. ** 11.5 wt.% water added. Analyst:- S.B. Reamsbottom. 59 Figure 4. Alkali content of white mica. Specimen 1 = Garnet zone; Specimens 2,3,5,6,7,8 = Staurolite-kyanite zone; Specimens 9,13 = Sillimanite zone; Specimens 11 and 12 = Contact aureole. 6 0 Table 5 . Microprobe analyses of muscovite.  field Ho. 80 25 35A 244 J3J_ 1*68 * 71 JEL. 459M 459P 460M 4ft)P Bxclmen So. - i l . s i o 2 A12°3 T10„ FeO » MnO KgO CaO Ha^ O V HgO *» Total St Al Ti Total Z Al T i ' f . 4 2 No KB Total T Ca l a K Total x *5.92 35.61 0.26 1.20 0.00 0.81 0.00 1.84 8.14 4.50 l»5.45 35.99 O.36 1.12 0.00 0.78 0.00 1.13 9.28 4.50 "•3.79 35.65 0.88 1.22 0.00 0.73 0.00 1.37 8.33 4.50 47.77 35.33 0.54 1.5* 0.00 0.73 0.00 1.04 9.43 4.50 46.68 3>*.97 0.69 1.00 0.00 0.89 0.00 1.06 9.25 4.50 47.0 32.55 0.55 2.95 0.00 O.89 0.00 1.00 9.06 4.50 46.S 33-82 0.80 1.49 0.00 0.82 0.00 0.86 9.39 4.50 45.69 34.61 0.78 1.25 0.00 0.73 0.00 1.25 8.88 4.50 46.89 35.58 0.80 1.05 0.00 0.77 0.00 1.13 9.37 4.50 •*5.95 34.81 0.35 2.43 0.00 0.59 0.00 0.84 9.70 4.50 44.9 35.2. 0.87 1.08 0.00 0.67 0.00 0.98 8.92 4.50 U5.85 35.09 0.80 1.40 0.00 0.64 0.00 1.02 9.61 4.50 <*5.85 35.28 O.83 I.03 0.00 0.88 0.00 1.05 9.70 4.50 44.76 35.04 0.97 1.10 0.00 0.85 0.00 0.96 9.76 4.50 6.14 1.86 0.00 8.00-3.76 0.03 0.13 0.00 0.16 4.08 0.00 0.48 1.39 1.87 6.09 1.91 0.00 8.00 3.77 0.04 0.13 0.00 0.16 4.10 0.00 0.29 1.59 1.88 6.00 2.00 0.00 8.00 3.75 0.09 0.14 0.00 0.15 0.00 0. 36 1. U5 1.81 6.25 1.75 0.00 8.00 3.69 0.05 0.17 0.00 0.14 4.05 0.00 0.26 1.57 1.33 6.22 1.78 0.00 8.00 3.70 0.07 0.11 0.00 0.18 4.06 0.00 0.27 1.57 1.84 6.24 1.76 0.00 8.00 3.51* 0.06 0.34 0.00 0.18 4.12 0.00 0.27 1.60 1.87 6.18 1.82 0.00 8.00 3.65 0.08 0.17 0.00 0.17 4.07 0.00 0.23 1.64 1.87 6.18 1.82 0.00 8.00 3.69 0.08 0.14 0.00 0.15 4.06 0.00 0.33 1.53 1.86 6.18 1.82 0.00 8.00 3.70 0.08 0.12 0.00 0.15 4.05 0.00 0.29 1.57 1.86 6.17 1.83 0.00 8.00 3.68 0.04 0.27 0.00 0.12 4.11 0.00 0.22 1.66 1.88 6.12 1.88 0.00 8.00 3.73 0.09 0.12 0.00 0.14 4.08 0.00 0.26 1.56 1.82 6.13 1.87 0.00 8.00 3.66 0.08 0.16 0.00 0.17 4.07 0.00 0.26 1.64 1.90 6.12 1.88 0.00 8.00 3.68 0.08 0.12 0.00 0.18 4.06 0.00 0.27 1.65 1.92 6.07 1.93 0.00 8.00 3.67 0.10 0.12 0.00 0.17 4.06 0.00 0.25 1.69 1.94 46.1? 35.15 0.88 1.10 0.00 0.84 0.00 O.96 9.85 4.50 98.30 98.61 96.47 100.88 99-24 ^ 98.7 98.6 97.69 100.09 99.17 96.3* 97.1 99.12 97.94 99.47 Bo. of loas on basis of 22(0). 6.15 1.85 0.00 8.00 3.67 0.09 0.12 0.00 0.17 4.05 0.00 0.2s 1.67 1.92 * Total iron expressed as FeO. Muscovite 59.3 72.3 69.7 64.3 65.0 58.4 68.5 64.1 67.1 70.2 70.1 69.9 71.0 74.2 Faragoalte 30.1 19.2 24.2 19.9 20.2 20.5 16.8 22.6 20.1 15.8 19."> 17.6 17.3 15.6 Jbengita 10.7 8.5 6.2 15.8 14.8 21.2 10.5 13.3 12-.8 14.0 10.5 12.5 11.7 10.2 * Total iron expressed aa F'.-O. U.5 vt.f H20 ADDED. Analyst). 3 . B . Hesnshottom. 61 ferramagnesian minerals. On addition of 11.5 weight per cent 1^0, analysis totals f a l l within 1.5 per cent of 100 per cent. The calculated chemical formulae indicate that the number of silicon and total iron atoms are characteristic of the ripidolites (Hey, 1954). The total octahedral site occupancy is within 0.3 of the ideal 12, so that these chlorites are members of the ideal serpentinepamesite series. Muscovite. Variation in composition of metamorphic white micas can be expressed in terms of two series; muscovite - paragonite muscovite - phengite The f i r s t series involves substitution of Na for K in interlayer sites in the mica structure and the second involves a coupled substitution of (Mg, Fe + 2) Si for Al^Al" 1^ such that the Si/Al ratio is greater than 3 to 1 for phengite. The white micas have no margarite component as no lime was detected. Many workers have found that as the metamorphic grade increases the composition of white mica commonly approaches muscovite 'sensu stricto 1 (Butler, 1967; Lambert, 1959; Evans and Guidotti, 1970). The alkali content of analysed muscovites (Table 5) is plotted in Figure 4. This figure shows X group totals less than the ideal of 2 and may indicate the presence of the hydronium ion (H30+) in this structural site (Evans and Guidotti, 1966). It is also clear from this figure that with increase in grade of metamorphism the paragonite content of the mica decreases (See Table 1). Although the amount of data from the garnet zone is inadequate for proof, i t appears that this variation is most pronounced between garnet and kyanite zones and less so between kyanite and sillimanite zones of metamorphism. The sodium content of muscovite could be a function of the sodium content of the rock or of plagioclase and not only related to metamorphic grade. The bulk rock Na20 content ranges from 62 .3 0 2 z o 5.1 12 13 2» «2 •6 *7 5» 11* 1 • Wt0/oNa2O Ms. too, 80 60 o J3 40 20 / / 9» 12«J  4 * / / Wt°/o Na20 Ms. Figure 5. a) plot of Na20 content of rock versus Na20 content of white mica. b) plot of Na20 content of white mica versus alhite content of coexisting plagioclase. 63 Figure 6. 64 Figure 7. Ternary plot of muscovite, paragonite, and phengite content of white micas. 65 Table 6 . Microprobe analyses cf biotite. Fie ld Mo. 35 25 235 468 453 427 465 71 503 460 Specimen Ho. 2 3_ 6 8 7 10 9 4 13 12 S102 38.22 37.53 37.66 37.58 38.71 38.80 37.93 38.67 38.11 38.12 Al20 3 18.1*0 19.19 19.25 19. 14 19.26 19.76 19.28 18.32 19.06 19.87 T102 1.87 2.05 1.30 2.02 2.18 1.34 2.14 1.72 2.50 2.39 FeO * 15.99 17.26 17.37 18.83 18.92 11.80 19.21 16.31 19.51 17.84 MnO 0.06 0.08 0.04 0.03 O.Olf 0.01 0.04 o.o4 0.04 0.06 MgO 11.1* 10.53 10.27 9.56 9.17 14.15 9.27 11.80 7.14 9.52 CaO 0.03 0.05 0.00 0.00 0.11 0.00 0.11 0.10 0.11 0.08 Na20 0.13 0.17 0.20 0.30 0.25 0.23 0.24 0.15 0.31 0.17 K2O 8.62 9-59 8.1*7 8.55 7.60 8.14 8.72 8.62 8.12 8.89 HgO * • 4.00 4.00 If.00 If. 00 lf.00 4.00 4.00 4.00 4.00 4.00 Total 98.76 100.1(5 98.56 ioo.01 100.2lf 98.31 100.94 99.73 98.90 100.94 No. of ions on basis of 22(0). Si 5.68 5.56 5.67 5-59 5.70 5.66 5.60 5.70 5-73 5.59 A l 2.32 2.44 . 2.33 2.4l 2.30 2.34 2.40 2.30 2.27 2.41 Ti 0.00 0.00 0.00 0.00. 0.00 0.00 0.00 0.00 0.00 0.00 Total Z 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 A l 0.91 0.90 0.97 0.95 i.o4 1.05 O.96 0.88 1.11 1.02 T i 0.21 0.23 0.15 0.23 0.24 0.15 0.24 0.19 0.28 0.26 Fe +2 1-99 2.14 2.22 2.34 2.33 1.44 2.37 2.01 2.45 2.19 Mn 0.01 0.01 0.01 0.00 0.01 0.00 0.01 0.01 0.01 0.01 Mg 2.54 2.32 2.33 2.12 2.01 3.07 2.04 2.59 1.60 2.08 Total Y 5.66 5.6O 5.68 5.64 5.63 5.71 5.62 5.68 5A5 5.56 Ca 0.00 0.01 0.00 0.00 0.02 0.00 0.02 0.02 0.02 0.01 Na 0.04 0.05 0.06 0.09 0.07 0.06 0.07 0.04 0.09 0.05 K 1.64 1.81 1.65 1.62 1.43 1.51 1.64 1.62 1.56 1.66 Total X 1.68 1.87 1.71 1.71 1.52 1-57 1.73 1.68 1.67 1.72 XMg 0.56 0.519 0.511 0.475 0.462 0.681 0.462 0.562 0.394 0 .486 X F e 0.438 0.479 0.489 0.525 0.535 0.319 0.536 0.436 0.603 0.512 XMn 0.002 0.002 0 .002 0.0 0.002 0.0 0.002 0.002 0 .003 0.002 * , Total iron expressed as FeO X ^ - Mg atoms /Mg+Fe+Mnf' ** 4 wt.$ 1^ 0 added. X K e =Fe atoms /Mg+Fe+Mn XMn" M n o t o m s /Mg+Fe+Mn Analyst:- S.B. Beamsbottom.' 0.4 t o 3.4 weight per- cent but the Na 2 0 content o f muscovite i s more o r l e s s constant . (Figure 5 ) . The sodium content o f muscovite i s independent o f the sodium, content o f the rock (Figure 5) but s e n s i t i v e to the A b - content o f the c o e x i s t i n g p l a g i o c l a s e (Figure 5b) . The p l a g i o c l a s e i s i n t u r n c o n t r o l l e d by the CaO content o f the r o c k . The white micas form an a l k a l i d e f i c i e n t s e r i e s which p a r a l l e l s the t h e o r e t i c a l muscovi te-phengite j o i n on a modi f ied A . K . F . diagram (Figure 6) . They do not tend to the pure muscovite composit ion wi th increase i n grade. In f a c t , mmscovites from the s i l l i m a n i t e zone p l o t i n the middle o f the s e r i e s , those from kyani te zone span the composi t ional range and muscovite from the garnet zone has a smal le r phengite content than many h igher grade white micas . However, a ternary p l o t o f muscovi te , paragonite and phengite content o f the white micas (Figure 7) i n d i c a t e s t h a t , i n s p i t e o f the s t rong c o n t r o l exerted by the An - content o f c o e x i s t i n g p l a g i o c l a s e , there seems t o be a good c o r r e l a t i o n o f Par / Ms + Par + ph wi th increase i n metamorphic grade i f the l a t e r contact metamorphic specimens are i g n o r e d . The phengite content o f the white mica precludes use o f the muscovite - paragonite so lvus to est imate metamorphic temperatures. B i o t i t e . Microprobe analyses o f b i o t i t e s (Table 6) show tha t the major v a r i a t i o n i n b i o t i t e composit ion i s i n the p r o p o r t i o n o f i r o n t o magnesium (Figure 8 ) . In other a r e a s , workers (Naggar and A ther ton , 1970; B u t l e r , 1967) have demonstrated tha t host rock composit ion and iron-magnesium r a t i o s o f c o e x i s t i n g minera ls con t ro l the i r o n to magnesium r a t i o i n b i o t i t e . The r a t i o may not be a f u n c t i o n o f metamorphic grade. There i s no c l e a r dependence between Mg/Fe r a t i o s o f b i o t i t e s from the study area and t h e i r host r o c k s . (Figure 9). 67 Al • Ti • Mn Figure 8. Octahedral site occupancy of coexisting biotite and +2 white mica. Total iron expressed as Fe . Garnet zone white mica plots in the centre of the cluster formed by white micas from kyanite and sillimanite zones. 68 0.6r CO 10 8* O • 1 3 7 9 13 M / M F Rk. 0.5 Figure 9. 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Microprobe analyses of garnet. Field No. 1*59 459 459 >*59 459 460 46o 460. Specimen No. 11 e 11 f 11 « 11 h 11 1 12 a .12 b 12 c S10 2 37.45 37.70 37.88 37.51 37.90 37.96 36.92 37.28 A1203 21.09 20.33 . 20.47 20.38 21.01 21.34 20.43 20.47 T102 0.08 0.05 0.04 0.06 0.07 0.05 0.08 0.05--FeO * 32.42 32.25 32.35 33.01 33.34 32.72 32.40 32.82 MnO 2.87 2.17 2.20 2.65 2.30 I.89 2.72 2.07 MgO 4.28 3.84 3.88 4.36 3.53 4.25 •^39 3.46 CaO 2.10 3.26 3-35 1.95 2.64 2.01 2.10 3.23 Total 100.29 99.60 100.17 99.92 100.79 100.22 99.14 99.38 No. of ions on basis of 24(0). Si 5-97 6.06 6.05 6.02 6.03 6.03 5.97 6.02 A l 0.03 0.00 0.00 0.00 0.00 0.00 0.03 0.00 A l 3.85 3.85 3.85 3.93 3.98 3.87 3.89 T i 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 +3 Fe 0.0= 0. 14 0.14 c.i4 0 .Oo 0.01 0.12 0.10 Total Y 4.oo 4.00 4.00 4 ioo 4.00 4.00 4.00 4.00 Fe +2 4.27 4.18 4.18 4.29 4.37 .^33 4.26 .^33 Mn 0.39 0.30 0.30 0.36. 0.31 0.25 0.37 0.28 Mg 1.02 0.92 0.92 1.04 0.84 1.01 1.06 0.83 Ca 0.36 0.56 0.57 O.34 0.45 0.34 0.38 0.56 Total X 6.04 5.96 5.97 6.03 5.97 5.93 6.07 6.00 Moi j. Almandine 71.03 70.93 70.66 71.82 73.53 73.06 70.75 72.58 Andradite 1.55 3.74 3.63 3.56 1.58 0.18 3.24 2.58 Grossular 4.34 5.^ 5 5.75 1.88 5.87 5.57 2.91 6.58 Pyrope 16.71 15.05 15.10 16.91 13.88 16.91 17.08 13.64 Spessartine 6.37 4.83 4.87 5.84 5.14 4.27 6.02 4.64 XMg 0.17 XFe 0.731 XMn 0.04 XCa 0.057 * Total iron expressed as FeO. Analyst:- S.B. Roomsbottom. Xj^ = Mg atcms / Mg + Fe + Mn + Ca = Fe atoms / Fe + Mg + Mn + Ca = Mn atcms / Mn + Fe + Mg + Ca = Ca atoms / Ca + Mg + Fe+Mn 73 Wenk (1970) demonstrated that in metamprphic micas of the Central Alps, octhahedral aluminum was an important index of VI VI metamorphic grade. The value of Al ms / Al ^ in coexisting muscovite and biotite in the study area is between 3 and 4 and is similar to upper amphibolite facies micas studied by Wenk. Garnet. The composition of garnet from pelitic rocks commonly shows a marked variation with increase in metamorphic grade (Miyashiro, 1953; Engel and Engel, 1960; Sturt, 1962; Nandi, 1967). The ratio (FeO + MgO) / (CaO + MnO) in garnets rises steadily from chlorite through sillimanite grades. Microprobe studies of garnets (Hollister, 1966; Harte and Henley, 1966; Atherton, 1968; Brown, 1969; Okrush, 1971) have shown that many garnets from pelitic schists are campositionally zoned with respect to Mn, Mg, Fe and Ca due to the mineral's failure to internally re-equilibrate with its surroundings during growth. This phenomenon complicates the study of compositional variation with increase in grade as well as attempts to demonstrate chemical equilibrium between garnet and coexisting rrdnerals. The garnets studied here were traversed by electron microprobe to check for possible zoning and the results are given in Table 7 and Figure 10. Because Si, Al and Ti are uniformly distributed throughout the garnets they have not been plotted. Characteristics of garnet zoning. i) Many but not a l l profiles are symmetric. Complex irregular or asymmetric zonations exist in garnets in staurolite-kyanite and sillimanite zone rocks. ii ) Garnets from garnet and staurolite-kyanite zones may stav? the well-documented bell-shaped distribution of Mn which has been explained by Rayleigh fractionation (Hollister,1966) or or Pfann equilibrium segregation (Atherton,1968). In addition (Mn + Ca) decreases from core to rim and is antipathetic to Fijure 10. Zonation profiles of garnets from pelitic rocks. Chemical• compositions at selected points in traverses are given in Table 7":, e.g. analysis 6g of Table 7O equals 6c.g. of Figure 10. 1 0 a . 10b. 10c. 11a Tib 11c MnO ' i oL FeO "V"\-.. MgO CaO / V / A \ \ v -/ V / - - ^ . . . . A a b c 6 a / \ ,~-'\--. A A / -\ \ / y \ J V Al VA MnO 3 IL MnO 2 " 33, FeO I 32L 33 FeO 4' '4 MgO H3 3 CaO J 2 2 d e f . 6b 6c 8 Hi j 12 1.3X1 13 b MnO A FeO \ a b C d MgO I'CQO MnO f, v . . / -FeO .r . . 30 / V MgO 4 Y V As CaO 100^ b c d 7 ^ A .A r - \ / ~ f g MnO 9a ./V V FeO FeO h i a . 4 0 0 ^ | 4 MflO Y /• x. CaO \ |00>, 9b \ / i Figure 10. (contd.). in 76 (Fe + Mg). Increase in Mn at garnet rims may be due to retrogressive metamorphism, or continued garnet growth during cooling. i i i ) Garnets in the same rock need not display similar zonation profiles (E.G. specimen 6) . iv) Lack of zoning in some sillimanite zone garnets is presumably due to higher temperatures promoting internal equilibration during growth of the garnet; Staurolite. Because staurolite is a mineral of restricted chemical composition with a limited stability field (Ganguly,1973; Hoschek, 1969; Richardson, 1968), i t is a good index of metamorphic grade. Workers such as Williamson (1953) and Ellitsgaard-Rasmussen (1954) believe host rock composition may control the occurrence of staurolite and Atherton (1965) stressed the importance of low MgO values and F e2 03^M2 03 r a t i ° °f around 0.40. Juurinen (1956) demonstrated that in staurolite-bearing rocks silica and alumina may vary from 28 to 90 per cent and 4 to 42 per cent respectively. The microprobe analyses (Table 8) and calculated formulae of eleven staurolites from the study area compare favourably with other staurolite analyses (Deer et.al., 1966; Hollister and Bence, 1967; Hounslow and Moore, 1967; Guidotti, 1970). Much of the F e + 3 reported in staurolite analyses may be the result of oxidation of Fe* during analysis. (Shreyer and Chinner, 1966). Data from Mossbauer spectroscopy (Bancroft et al., 1967; Smith, 1968) indicate no +3 Fe in the staurolite structure. Consequently iron in the analysis has been treated as FeO. Examination of bulk rock analyses of both staurolite-and non-staurolite-bearing rocks (Table 3) indicates that MgO values are less than 2.6 weight per cent and Fe^^/Al,^ ratios are between 0.03 and 0.22. Specimens 4 and 6, with abundant staurolite have the highest Fe 20 3/Al 20 3 values but other staurolite-bearing rocks with lesser values are not systematically different from 77 table 8 . Microprobe analyses of staurolite. (Fe, Mg, Mn, Zn) 4 (Al, T i ) l f l Oj^ (Si04)g(0, OH)^ Pleia Ho. 3? 25 24UA 244B 235 453 468A 468B 71 4?7 465 Specimen Ho. 2 3. 5. 5. 6 J 8 8 4 10 sio2 27.79 27.70 27.36 26116 27.62 26.86 26.87 27.12 27.66 27.17 26.79 T102 0.66 0.80 0.80 0.79 0.55 0.74 0.74 0.81 0.80 O.SO 0.77 Al20 3 53.72 54.38 53.78 55.64 52.42 53.24 52.60 53.49 53.96 54.01 54.29 TeO • 11.91 11.54 12.33 12.65 13.78 13.31 13.15 13.11 13.22 10.31 11.66 MgO 1.46 1.34 1.40 O.63 2.15 1.84 1.74 2.05 2.56 2.37 1-39 KoO 0.15 0.22 0.03 0.03 0.11 O.09 0.03 0.20 0.07 0.05 O.05 ZnO 0.75 0.80 0.80 O.92 0.02 0.02 0.07 0.00 0.15 0.07- 1.4o HjO ** 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 Sotal 98.44 99.27 98.62 98.83 98.66 98.27 97.20 98.78 100.42 97.91 98.35 no. of ions on basis of 48(0.OH). SI 7.74 7.70 7.69 7.59 7.83 7.55 7.65 7.90 7.66 7.90 7.82 A l 17.56 17.78 17.77 I8.89 17.30 17.57 17.60 18.21 17.48 18.52 18.56 XI 0.02 0.17 0.17 0.17 0.09 0.17 0.17 0.18 0.17 0.18 0.18 Total A l + T i 17.58 17.95 17.94 19.06 17.39 17.74 17.77 18.39 17.65 18.70 18.74 Fe 2.86 2.69 2.87 3.14 3.23 3.22 3.10 3.16 2.30 2.47 2.81 Kg 0.67 0.51 0.51 . 0.35 0.85 O.85 0.69 0.88 1.00 1.06 0.53 Mn O.03 0.05 0.01 0.01 0.03 0.02 0.01 0.05 0.02 0.02 0.02 Zo. 1 O.17 0.17 0.17 0.17 0.00 0.03 0.02 0.00 0.03 0.02 0.35 total 3.73 3.42 3.56 3.67 4.11 4.12 3.82 4.04 3.35 3-57 3.71 OB 3-70 3-70 3.72 3.84 3.74 3.73 . 3.78 3.87 3.66 3.88 3.87 **« 0.188 0.157 0.150 0.207 0.208 0.182 0.301 0.299 0.158 Xte 0.80 0.828 0.847 0.786 0.787 0.816 0.693 0.696 0.836 XMn 0.008 0.01S 0.003 0.007 0.005 0.003 0.006 0.006 0.006 * Total iron expressed as FeO *» 2 wt.jt 1^ 0 added. Analyst:- S.B. Reamsbottom. » Mg atoms / Mg + Fe + Mn Jfj^ - Fe atoms / Fe + Mg + Mn. Xj^ • Mn a tans / Mn + Fe + Mg 78 Table 9. Relationship between zinc, iron and magnesium i n staurolite. Spec. No. ZnQ FeO MgO FeO+MgO wt.% wt.% wt.% wt.% 8B 0.00 13.11 2.05 15.15 6 0.00 13.78 2,15 15.93 8A 0.07 13.15 1.74 14.89 10 0.07 10.31 2.37 12.68 4 0.15 13.22 2.56 15.78 7 0.20 13.31 1.84 14.15 2 0.75 11.91 1.46 13.37 3 0.8 11.54 1,34 12.88 5A 0.8 12.33 1.40 13.73 5B 0.92 12.65 0.36 13.28 9 1.40 11.66 1.39 13.05 Specimens Specimens 8,6,7,2,3,5 equal staurolite-kyanite zone, and 4,9 and 10 equals s i l l imanite zone. 79 non-staurolite-bearing schists. Thus no rocks have Fe2Q2/&3-2P3 r a t i ° s similar to those indicated by Atherton. Naggar and Atherton (.1970) showed that non-kyanite-bear ing, staurolite rocks of the Donegal granite aureoles are iron^-rich. Kyanite-staurolite schists on the other hand were shown to be restricted to Mg)-rich rocks with MgO/MgO-FeO ratios greater than 0.50. The staurolite-kyanite schists of this study have MgO/MgO-FeO ratios which range from 0.96 to 0.306 and show no such host rock control. A recent compositional study of staurolite (Guidotti, 1970) has demonstrated an almost tenfold increase in the amount of zinc in staurolite as the boundary between lower and upper sillimanite zones of metamorphism is approached. In this area six specimens from the kyanite zone and three from the lower sillimanite zone have been analysed. Table 9 shows the staurolite analyses arranged in order of increasing zinc content. The zinc content does not systemafeieally^rise towards the sillimanite isograd although the staurolite with highest zinc content is within the lower sillimanite zone. Examinations of the areal distribution of specimens with respect to the isograd indicates instead that as the isograd is approached and crossed the zinc content decreases. An inverse relationship exists between ZnO and (FeO + MgO) in the staurolite and indicates as Guidotti suggested that Zn is concentrated with Fe and Mg in the fourfold cation site of staurolite. As discussed by Guidotti, a systematic though non-linear relationship should be expected between octahedrally co-ordinated +3 Ti in staurolite and coexisting micas. Comparison between rocks studied by Guidotti and those of the map-area indicates similar Ti02 of biotites and almost double the amount of Ti02 in staurolite and muscovite from the Breakenridge areaarBecause a l l the staurolite schists considered in this study contain rutile and ilmenite the Ti0 2 content of coexisting micasaand staurolite can be compared. Ti02 is most concentrated in biotite and least in muscovite. Also, the .'' ^ y con 80 Table • Microprobe analyses of plagioclase. F i e l d No. " 35 25 244 235 453 465 427 71 503 459 460 Specimen No. 2 3 5 6 7 9 10 4 13 11 12 S i 0 2 62. .11 57.^5 57.36 57.25 62.08 60. 23 57.98 57.65 58.18 65. 83 58.41 AI2O3 23. • 55 26.74 25.63 26.91 23.22 24. 90 25.32 26.70 23.94 19. 60 26.30 T i 0 2 0, .01 0.01 0.01 0.00 0.01 0 . 00 0.00 0.01 0.00 0 . 00 0 .01 FeO * 0. .12 0.05 0.01 0.00 0 .01 0 . 04 0.01 0.03 0 .01 0 . 46 o.o4 CaO 4. .96 8.66 7.27 8.73 3.92 6. 53 7.25 8.69 5.78 0 . 45 7.82 Ka 2 0 8. .87 6.92 7.72 6.78 9.37 8. 10 7.65 6.89 8.23 11. 21 7.28 K 20 0. .06 0.03 0.05 o.o4 0.09 0 . 05 0.02 o.o4 0 . 0 6 0 . 03 0.11 Total' 99. .68 99-86 98.05 99.75 98.72 99- 85 98.23 100.01 96.20 .97. 58 •99-97 No. of ions on basis of 32(0) S i 11.05 10.31 10.47 10.29 11.12 10.74 10.55 10.33 10.77 11.83 10.45 A l 4.94 5.66 5.51 5,70 4.90 5.23 5.43 5.64 5.22 4.15 5-55 Fe 2 0.02 0.01 0.00 0.01 0.00 0.01 0.00 0.00 . 0 . 0 0 9.07 0.01 Ca 0.95 1.67 1.42 1.68 0.75 I . 2 5 1 . 4 l 1.67 1.15 0.09 1.50 Na 3.06 2 . 4 l 2.73 2.36 3.26 2.80 2.70 2.39 2.95 3.91 2.53 K 0.01 0.01 0.01 0.01 0.02 0 .01 0.00 0.01 0 .01 0 .01 0.03 Z 15.98 15.97 15.98 15.99 16.03 15.98 15.97 15.97 15.99 15.98 15.99 X 4.02 4.08 4.16 4.05 4.03 - 4.06 4.12 4.07 4 . i i 4.00 4.05 Moi % Ab 76.13 59.02 65.59 58.29 80.81 68.99 65.56 58.80 71.79 97.66 62.36 An 23.53 4 o . 8 i 34.13 41.48 18.68 30.73 34.33 40.98 27.86 2.17 37.0.2 Or 0.34 0.17 0.28 0.23 0.51 0.28 0.11 0.22 0.34 0.17 0.62 Total i r o n expressed as FeO. Analyst:- S.B. Reamsbottom. 81f Figure 11. Plot of CaO content of schists versus An - content of plagioclase. Specimens 14 - 20 An- content determined optically. Figure 12. Cao-Na20 content of rock versus An- content of plagioclase. Specimens 14-20 An- content of plagioclase determined optically. o CJ 3 ^ 2 11 14 15,. 6 '18 J 2 » 1 6 10 4 ™ 1 9 2 7 13 8 20 40 60 An % 80 100 o z N 4 0 3 u u 15 19 '• • 2 20 40 60 An % 80 100 82 TiC^ content of the rock i s not systematically related to the TiC^ content of staurolite, muscovite, b io t i te or garnet. Plagioclase. Microprobe traverses show individual plagioclase grains i n schists to be unzoned. Anorthite contents of eleven analysed plagioclases range from 2.17 to 41.48 per cent (Table 10). A l l specimens have a minor amount of orthoclase component. The anorthite content of plagioclase does not increase with grade. Rocks of the staurolite-kyanite zone have plagioclase which range from An, „ , n to An, , .„ and those of the s i l l imanite 3 18.68 41.48 zone have anorthite content of 27.86 to 40.98 per cent. The andalusite hornfels i n Cairn Needie contact aureole has An 2 but the sillimanite-bearing hornfels has 02 Examination of bulk rock analyses indicates that host rock chemistry controls the composition of plagioclase. Because plagioclase i s the dominant calcium-bearing phase i n the schists, a roughly linear relationship exists between the CaO content of the schists and the An content of the plagioclases (Figure 11). In the section on muscovite i t was noted that a linear relationship exists between the Na20 i n the plagioclase and mica but none exists among the Na2© of the rock, mica or plagioclase. However the CaO/TS^O rat io of the rock aO-so controls the amount of An i n the plagioclase (Figure 12). The CaO content of the rock controls the An content of the plagioclase and therefore i t s Ab component. The Ab i n plagioclase affects i n turn the paragonite i n white mica so that indirect ly the CaO content of the rock controls the amount of paragonite i n muscovite. Ilmenite. The dominant opaque phase i n the p e l i t i c schists i s ilmenite. Microprobe analyses of ilmenites are l i s t ed i n Table 11. The chemical compositions of coexisting ilmenite and magnetite can i n some favourable cases give unique values of oxygen fugacity (f ) and temperature (T) of formation. (Lindsley, °2 Table . Micr o p r o b e a n a l y s e s o f l l m e n i t e . F i e l d No. 35 2j> 71 2kk 235 ^53 ^68 465 427 460 $03 Specimen No. 2 3 h 5 6 7 8 9 10 12 13 T i 0 2 53-21 51.22 50.58 52.31 44.21 50.77 48.96 51.69 52.38 51.82 49.58 A1 20 3 0.55 0.59 0.45 0.43 0.49 O.49 0.49 0.47 0.65 0.53 0.57 FeO * 44.89 he.Ik 48.23. U5.48 49.26 46.12 ^5.52 45.49 46.45 44.63 MgO 0.35 0.10 0.43 O.05 0.03 0.12 0.08 0.13 0.73 0.10 0.17 * T o t a l i r o n e x pressed as FeO. A n a l y s t : - S.B. Reamsbottom. co 84f Figure 13. Plot of chemical composition of ilmenite versus rock oxidation ratio, (O.R.) O.R. = Molar 2Fe0 ., x 100/2Fe~O-+FeO °1. in m o LL O LO O tO ^ ^ ' 301XO % \N\ o O % I M if) CD 00 \ CO 00© ID , g O) 84 o o \r Q : O O 85 1963; Budding-ton and Lindsley, 1964). Consequently i f the composition of ilmenite is dependent on the temperature and, oxygen fugacity prevailing at the time of crystallization a correlation should, and does, exist between oxidation ratio of host rock and the ccmposition of ilmenite. As the ferric iron content could not be determined by microprobe analysis i t was calculated stoichiometrically. The molecular proportion of Ti0 2 necessary for the pyrophanite (MnTiO^) and geikielite (MgTiO^) components of ilmenite was subtracted from the analysed TiO~. The remaining Ti0 0 would combine with iron (Fe ) to form ilmenite and excess iron, i f any, is then treated as Fe + 3. For host rock oxidation ratios greater than 60 the T?e.£)^ increases markedly. (Figure 13). Thus the Xincrease in hematite content of ilmenite within rocks of similar metamorphic grade as rock oxidation ratio increases is in good agreement with the experimental results of Lindsley (1963). 86 Distribution of Elements between Minerals. When considering the atomic distribution of an element between two mineral phases which probably formed under essentially similar physical conditions, distribution diagrams of the type f i r s t used by Roozeboom (1891) and later by Kretz (1959) prove invaluable. Smooth distribution curves for coexisting mineral pairs which crystallized at the same pressure and temperature imply a close approach to chemical equilibrium (Phinney, 1963; Hounslow and Moore, 1968). Under equilibrium conditions for coexisting mineral phases oC - (A,B)M and £ = (A,B)N, where A,B,M and N are chemical species, the distribution coefficient with respect to species A i s : K, = X. v (1-XA a ) / X. . (1-X, ) where X A = mole fraction of A, equal to A/A+B. The distribution relations of total Fe, Mg, Mn and Ca, octahedrally co-ordinated elements among coexisting biotite, garnet and staurolite are shown in Figure 14. The straight line whose slope is equal to for the mineral pair is a- least squares f i t to the plotted points (X / 1-X ) versus (X / 1-X ) in the diagram. Conclusions. i) The good straight line relationships displayed in the plot of X/l-X, iron and magnesium in coexisting biotite garnet and staurolite indicate a close approach to chemical equilibrium between these mineral species. The compositional range of manganese in biotite is very limited and may be too small to demonstrate equilibrium distribution of manganese between coexisting garnet, staurolite and biotite. i i ) The distribution coefficients are independent of temperature differences between the stauroiite-kyanite and sillimanite zones. 87f Figure 14. Distribution diagrams of total Fe, Mg, Mn and Ca among coexisting chlorite, biotite, garnet (edge composition) and staurolite. Kj^ = (X/l-X^/ (X/1-X)^ Xmg Ga = + Fe + Mn + Ca Xng,st,bio,ch = Mg/Mg + F e + M n A= X/1.-X Closed dots= sillimanite zone Open dots= staurolite-kyanite zone A x 100 G A R N E T mn fD m o d 3. 8 CD D O D CO 8 D O T o • o <° .0) K5 A m r«100 G A R N E T O o > 3 :> O O § - i m o A m nt100 S T A U R O L I T E O O 1 r 0) 88 89 Linear Regression Analyses. Linear regression analysis of metamorphic rocks can be of assistance in deciding whether or not a particular mineral assemblage is a multivariant, divariant or univariant reaction assemblage (Fletcher, 1971; Pigage, 1972). The technique assesses the number of independent components necessary to adequately describe a mineral assemblage. By accounting for a l l components in a l l phases at the same time, the technique elegantly enables a consideration of mineral systems in N- dimensions rather than in the standard 2 dimensions of graphical analysis. The technique uses a least squares approach to test for significant linear dependencies between minerals in one or more equilibrium assemblages. Consider the equilibrium assemblages A and B composed of minerals A^, A 3 a n c^ B^, B 2 and B^ respectively. If one of the minerals can be modelled in terms of the others such that; A l = c2*2 + C 3 A 3 + C4 B1 + C5 B2 + C 6 B 3 where the c coefficients may be positive or negative, a relationship exists between assemblages A and B. If the equation, on rewriting to give positive coefficients, is of the form A 1 + C 2 A 2 + C 3 A 3 = C4 B1 + C5 B2 + C 6 B 3 the two assemblages have a possible reaction relationship and must have formed under different physical conditions. If the equation can not be rewritten with only positive coefficients for assemblage:, A and only positive coefficients for assemblages B then assemblages A and B result from differences in bulk composition. The reactions defined by this method are only 'possible1 ones between the assemblages; they may not actually have occurred within the rocks in question. Textural and modal relationships between the minerals involved reduce the possible reactions to the most probable reactions. Computer programmes PROTEUS and REACT (Greenwood, pers. coitm.) were used for the linear regression analyses of the pelitic rocks. Mineral compositions were entered as weight per cent oxides. 90 Table 12. Assemblage React. Basis . Regression Equation S i A l . Fe Mn M2 .03 .53 Ca Na K H Zn .0 .0 Ti .34 2.9 Rock 2 1A 8 lMs2+.24Rut2+.21Qu2+.44Ch2 .09 =2.43Ky2+.06Ilm2+.89Bi2+.10 1.83 Pc2+3.83H20 .05 1.54 .05 6.7 -.004 -2.2 -.66 -.69 .13 .58 .01 .91 .004 .42 Residual 2 Error Rock 2 IB 8 lSt2+2.33Rut2+1.39Qu2 = 8.5Ky2 -.02 +1.22Ilm2+.002Bi2+.0007Pc2+.12ch2 -1.05 -.01 -1.0 -.01 -4.0 .14 1.29 -.004 -.3 -.002 -.4 -.003 -.33 -.005 -.52 .0004 .21 .75 1.19 -.06 -1.8 Residual Error Rock 3 2 9 lMs3+.66Rut3+.37ch3+.19Ga3 = 2.5Ky3 -.04 +.79Qu+.26Ilm3+.88Bi3+.09Pc3+3.2H2O -1.9 -.02 -1.6 -.02 -7.1 .59 2.3 -.01 -.55 -.30 -.72 + .10 .6 -.01 -.9 .0 .43 .0 .0 -.15 -3.1 Residual Error R2 v R3 3 lGa3+.0008Bi3+1.37Rut+.78H2O = .4Ky .005 +1.27QU+. 64Ga2+. 69Ilm2+. 0 9ch3+. 007p3.. 97 .003 .77 .002 3.69 ^.1 -1.2 -.003 -0.3 -.044 .36 r-.006 -0.3 .002 .46 .008 .2 .(! • .0 .02 1.6 Residual Error R5 v R6 4 lSt5+.25Ms(5)+1.27Qu+.04Sp = 9.2Ky6 .39 +.26Bi6+.007Pc5+.79Mag6+.08Rat+1.94 1.11 "2° .27 1.08 .31 4.2 .02: 1.4 .10 .31 .10 .43 .08 .35 .06 .54 .01 .22 .8 1.24 1.5 1.80 Residual Error R6 v R9 5 8 lpc6+.59Rut+.32H20 = .63Sill(9) -.24 +.003Mag6+.63Sp+.01Bi6+.85Pc9 -1.1 -.15 -.84 -.19 -3.7 .0 -1.2 -.05 -.27 -.05 -.4 -.04 -.34 -.05 • -.48 -.01 -.19 .0 .0 -.94' -1.6 Residual Error R6 v R9 6 9 lMs6+Sill9+.04Rut+.01Pc9 = 1.01MS9 -.07 + .01Bi6+.01Sp+.14Qu+.07H2C> 1.1 +.06 .9 .05 3.9 .0 -1.3 .02 .29 .01 .4 .01 .34 .01 .54 .0 .23 .0 .0 .25 1.7 Residual Error R6 v R9 7 9 lGa6+.89Rut+.37H20+.47Ms9 = 2. 4 S i l l .45 +1.07Mag6+2. 7QU+. 81Sp+. 44Bi6+. 03Pc9 1.4 .30 1.1 .36 5.0 1.1 1.6 .11 .37 .10 .51 .08 .41. .08 .63 .02 .25 .0 .0 1.76 2.14 .Residual Error R6 v R9 8 9 lSt6+1.23Qu+.05Sp+.43Ms9 = 9.1Sill9 .39 +. 03Rut+. 78Mag6+l. 9H20+ .39Bi6+.02Pc9 1.2 .26 1.1 .3 4.5 .1 1.4 .09 .35 .08 .45 .07 .37 .06 .58 .01 .24 .02 1.33 1.5 1.93 Residual Error R6 v R9 9 9 lBi9+.llsill9+.07H 2O+.0lPc9 = .05Rut .12 + .14Mag6+.04Qu+.02Sp+.85Bi6+.13Ms9 .97 .09 .76 .09 .36 .0 1.1 .03 .26 .03 .36 .02 .30 .02 .49 .01 .20 .0 .0 .46 1.5 Residual Error R6 v R9 10 40 lGa9+2.8Rut+.llH20+.41Ms9 = 2.4SU19 .01 +-26Mag+3.2Qu+.03Pc9+.4Bi6+.26Sp 1 4 +1.41Ita .01 1.1 .03 -5.2 .95 1.6 .0 -.37 .0 .52 .0 .43 .0 -.65 .0 .26 .0 .0 .31 2.3 Residual Error Reactions are as numbered i i i text. 1. Basis of system i s the mirmnum number of components required to adequately describe the system. 2. Residual i s the difference, expressed as weight per cent oxide, between oxides of the mineral and oxides of i t s regression model. Caption. Linear regression equations obtained by 'reacting' mineral assemblages i n individual rocks, or pairs of rocks from the same (staurolite-kyanite) or different (staurolite-kyanite - sillimanite) zones of metamorphism. Mumino-silicates, rutile, magnetite and quartz were considered to be stoichiometric and water was included in the reactions. A regression equation is considered to be significant only i f residuals are less than calculated error limits for the modelled, mineral. Residuals are differences (expressed in weight per cent) between oxides of the mineral and oxides of its regression model. In the computer analyses the error limits applied to each component in each mineral were calculated using a linear scale which was directly ~ ' proportional to the amount of the component present. The maximum permissable error allowance for each component in the equation was the sum, over a l l minerals, of error limits for each mineral times the amount of mineral present in the equation. Maximum error limits for each component were scaled to correspond to ± 1 weight per cent in 40 per cent SiGv, and the minimum limits were defined as one third of the maximum. The best regression equation was the one which satisfied the constraints imposed by the field evidence. In this study linear regression equations obtained by reacting assemblages within the staurolite=kyanite zone and between staurolite-kyanite and sillimanite zones are given in Table 12. Staurolite-kyanite zone. The mineral assemblage quartz-muscovite-biotite-garnet-staurolite-kyanite plotted in an AFM projection represents a possible reaction assemblage as tie-lines between the minerals cross. The above assemblage with or without chlorite is widespread in the kyanite-staurolite zone of the Mount Breakenridge area. (Table 2). The minerals chlorite, muscovite, biotite, garnet, staurolite and kyanite are coirmonly in physical contact. Kyanite commonly overprints muscovite and chlorite or is intimately associated with early-formed garnet, biotite and staurolite. Rutile and ilmenite are ubiquitous and ilmenite commonly mantles rutile. The following are possible reactions within individual specimens 2 and 3. 92 ms2 + rut2 + qu2 + ch2 « ky-2 + ilm2 + bio2 + pc2 + H20 *' (IA) ms3 + rut3 + ch3 + ga3 = ky3 + qu3 + ilm3 + bio3 + pc3 + H20 (2A) These reactions are essentially of the form ms + ch = ky + bio ± qu + H20 In the Fe-free system this reaction is equivalent to ms + ch = ky + phlogopite + qu + E^ O (Shreyer and Seifert, 1970) Garnet in these rocks is commonly idioblastic and shows no textural evidence such as corroded rims which would suggest that i t was involved in the formation of kyanite so that reaction 2A may not have taken place in these rocks. The reaction st2 + rut2 + qu2 = ky2 + ilm2 + bio2 + pc2 + H20 (IB) takes place within the staurolite + quartz + kyanite stability field (Richardson, 1968) at temperatures between 530°C and 600°C. In specimen 3 ilmenite mantles rutile and kyanite occurs in close association with staurolite in quartz-rich lenses. Reacting minerals of assemblage 2 with those of assemblage 3, both within the staurolite-kyanite zone, indicated a possible additional reaction: ga3 + bio3 + rut + H20 = ky + qu + ga2 + ilm2 + ch3 + pc3 (3) As phases of assemblage 3 occur on both,sides of the equation this could be interpreted as being due to bulk compositional differences. However, as the quantities of ch3 and pc3 involved in the equation are very small (Table 12),the equation more likely represents a reaction relationship , and hence impliespphysical differences between the two assemblages. There thus appears to be a dehydration reaction with formation of garnet and biotite as the sillimanite isograd is approached. If reaction 3 is valid, physical conditions varied between individual assemblages in the staurolite-kyanite zone. The univariant reactions written for assemblages 2 and 3 indicate that these are either disequilibrium assemblages or that the reactions have been buffered on the reaction curves. As the distribution of 93 of chemical species between coexisting minerals implied equilibrium these may be buffered assemblages. Buffering i s possible i f the reacting system was closed to water. Alternatively in a system s t i l l open to water but with P PTOT' a t ^ given -Pr^p/ the equilibrium temperature of the reaction would be lowered by an amount proportional to the difference between P-™ and P (Thompson, 1951, Greenwood, 1961). Therefore at>a fixed P.pQrp, f h e observed assemblages could have crystallized at different temperatures under the influence of a range of water fugacities. Sillimanite zone. Kyanite and sillimanite overprint minor folds associated with the third phase of folding in the map-area so that relative to deformation they crystallized over the same period of time. The simultaneous crystallization of these minerals implies a temperature gradient between staurolite-kyanite and sillimanite zones of metamorphism. Sillimanite is not seen to form directly from kyanite but rather is intimately associated with biotite as fibrolitic mats or as needles in biotite-rich areas around staurolite and garnet. A search for possible reactions between rocks of the .- - >. staurolite-kyanite and sillimanite zones (Specimens 6 and 9) produced the following: pc6 + rut + H20 = sil l 9 + sp + bio6 + pc9 + mag6 (5) ms6 + si l l 9 + rut + pc9 = ms9 + bio6 + sp + mag6 + qu + H20 (6) ga6 + rut + ms9 + H20 =• sil l 9 + bio6 + pc9 + mag6 + qu + sp (7) st6 + qu + sp + ms9 = si l l 9 + bio6 + pc9 + rut + mag6 + H>0 (8) bio6 + ms9 + mag6 + sp + qu + rut = sil l 9 + bio9 + pc9 + H20 (9) bio6 + pc9 + ilm + mag6 + sp + qu + si l l 9 = ga9 + rut + ms9+ H20 (10) (See Table 12). As phases of the same assemblage occur on the opposite side of these reactions a significant difference in bulk chemistry exists between rocks of the staurolite-kyanite and sillimanite zones in the map-area, so that the difference in mineralogy across the sillimanite isograd is not merely a function of temperature but 94 Figure 15. Relationship between bulk composition and composition of minerals in hypothetical specimens 6 and 9.. Minerals B and C form solid solution series. (See text for discussion). / Table 13. Bulk Min. Rock Ass. +2 Regression Equation Si Al Fe Mn Mg Ca Na K H Zn T i C P 6 v 6 R6 = .001Bi6+.17Me6+.04Ga6+.23Stfr +1.3Ky6+.17PC6+2.33Qu+.06Sp + .25H20 6 v 9 9 v 9 9 v 6 -.02 -.03 -.004-.004-.007-.01 +.001-.002-.009-.001-.005.02 .009 Residual .04 .04 .03 .03 .03 .03 .03 .03 .03 .02 .03 .03 .03 Permit Error R6 = .15Pc9+.32Ms9-.13Bi9+.6Ga9 .02 +.2Qu-.3Sill9-.03St9-.7911m .05 +1.7Rot+.25H20 R9 = .15Pc+3.8Qu+.09Ms9+.03Bi9+.01 .02 Ga9+.23Sill9+.01St9+.01Sp .03 +. 006I1IIH-. 35Rut+. 43H20 .03 -.01 -.02 -.002 .03 -.01 .001 .019 .01 -.02 .02 .01 .04 .04 .04 .04 .04 .04 .04 .04 .04 .04 .04 .04 .01 .01 .003 .016 .017 .02 .02 .02 .02 .02 -.004.01 .015 .003 -.3 .02 .02 .02 .02 .02 R9 = .17Pc6+4.8Qu+.06Ms6+.06Bi6-.08Ga6-.01 -.01 -.004.01 -1.3Ky6+.21St6 .04 .04 .03 .03 -.01 -.01 T004-.001 -.01,.-. 001.03 .03 .03 .03 .03 .03 .03 .03 .006 .02 .01 .03 Residual Permit Error Residual Permit Error Residual Permit Error Caption: Least squares regression models of bulk rock compositions of specimens 6 and 9 and their respective mineral assemblages. (See Text for discussion), vO also of rock bulk chemistry. Because the bulk compositions of rocks 6 and. 9 are known, a simple and elegant test can be applied to check for compositional dependence. If the bulk composition of rock 6 i s modelled with a) minerals of rock 6 and b) minerals of rock 9 and i t is found that the bulk composition of 6 lies inside the phase volume from rock 6 (all positive coefficients in the regression equation) but outside the phase volume of minerals from rock 9 (seme negative coefficients in the regression equation) bulk composition controls the mineral assemblage. A two dimensional analog of this situation is given in Figure 15 and the models of bulk compositions 6 and 9 would be as follows: (a) (b) (c) (d) Such models would also prove that the bulk composition of specimen 9 is incapable of crystallizing to the phases of specimen 6. Regression models between bulk rock compositions and mineral assemblages of 6 and 9 (Table 13) prove that bulk composition controls mineral assemblages across the sillimanite isograd in the Mount Breakenridge area. Graphical representation of mineral assemblages: In discussing chemical equilibria in metamorphic rocks, petrologists have relied heavily on ACFK and AFMK diagrams. Commonly these projections do not consider a l l the components present in the phases under discussion. The linear regression technique simultaneously considers a l l the components in a l l phases, defines the nummum number of components required to adequately describe the system in question, and enables one to devise a projection which more truly represents the phases. B C 9 = -A6 + B 6 + C 6 Bc9 = D9 + B 9 + C g Bc6 = A g + B 6 + C g Bc6 = + B g + Cg - Dg Figure 16a. Stereographic plot of garnet (edge composition), biotite, muscovite, plagioclase in the tetrahedron MnO-Na20-K20-CaO. Rock bulk; compositions also plotted within the tetrahedron. Figure 16b. Simplified stereographic plot of garnet, biotite, muscovite, plagioclase and bulk compositions of specimens 2, 6 and 9 in the tetrahedron Mn0-Na2O-J^O-CaO. -f-QU. 98 The projection used in this study was based on the following assumptions: I) Regression analyses reguired from eight to ten components to adequately describe mineral assemblages. In most cases at least as many phases as analytical constituents were needed. Thus a ten component system is required to describe adequately a l l phases. II) By projecting from alumino-silicate, quartz, ilmenite, rutile, water and an analysed staurolite of average composition without manganese, rocks in the staurolite-kyanite and sillimanite zones could be reduced to a system of four components - CaO, MnO, K,,0 and Na20. Use of the plot option in the program PROTEUS results in a stereographic plot ofethe remaining minerals - garnet (edge composition), biotite, muscovite and plagioclase (Figure 16). Features to note in the figure include: a) The compositional variation is greatest in garnet and plagioclase and least in the micas. b) The plotted four-phase assemblages in the four-component system are divariant; therefore at a fixed pressure and temperature garnet-plagioclase-muscovite-biotite should be cartpositionally invariant. That the four-phase assemblages have a range in composition implies, i f the projection is valid, the existence of physical gradients not only between the staurolite-kyanite and-sillimanite zones but within the staurolite-kyanite zone. The validity of the projection can be confirmed by testing for least squares reactions between specimens which contain a l l the mineral phases and outcrop close to one another (5 and 6; Figure 1 and Table 2) and hence were at the same pressure and temperature. If a small amount of plagioclase on the wrong side of the regression equation (Reaction 4, Table 12) can be ignored the following reaction relationship exists between the specimens: st5 + ms5 + qu + sp = ky6 + bio6 + (pc5) + mag6 + rut + H20 Hence the fugacity of water must have been different between the specimens so that each acted as a local buffer. Figure 17. Stereographic plot of garnet, muscovite, plagioclase in the tetrahedron MnO-Na20-K20-CaO. Explanation. Ms-Pl-G forms a volume with 1 degree of freedom, f i l l e d with subparallel coexistence triangles. The only triangles that can be at equilibrium with another mineral too are those of either the top or bottom of the stack. Only 1 triangle should have joins leading to biotite. The joins for rock 2 to biotite contradict triangles 9 and 6. The projection thus proves either disequilibrium, buffering or inadequacy of projection. 100 c) The remarkable parallelism of the three-phase triangle garnet-musoovite-plagioclase implies a close approach to chemical equilibrium between these phases. (Figure 17). d) The compositions of minerals show no systematic variations with grade but seem to be a function of host rock composition. For example, as the host rock composition tends to the CaO apex of the tetrahedron, the composition of plagioclase becomes more CaO-rich and muscovite more potassic. e) Addition of phases which do not require an additional component such as sphene or chlorite render the assemblages invariant. Some of these are similar to assemblages involved in the reactions discussed above. f) Some crossed tie-lines may also indicate disequilibrium or reaction assemblages. Discussion. Consider a three-phase equilibrium assemblage of garnet-muscovite-plagioclase in the system CaO-MnO-K20-Na20-^yigO-FeO-Si02-H20 which is at fixed P and T (st, qu, ky, ilm, rut, H20 a l l present). The minerals of this assemblage would have compositional freedom and plotted on the diagram would form a set of three-phase triangles which are parallel to the ones shown in Figure 17 but closer to the MnO apex of the tetrahedron due to the absence of biotite. The location in the projection of three-phase triangles such as these could be controlled either by variation in P, T or a H20 at constant bulk composition or by different bulk compositions at any fixed P and T. Such a system may be viewed in the following manner: Increase of T promotes reaction (1). Paragonite + qu = Ab + Ky + H20 (1) K l = fab * %0 a 2 par 101 This reaction is coupled with reaction (2). Gross + 2ky + qu <= 3An (2) K2 = a_L aGross Looking at the effect of variation of on other phases, through coupling leads to; (1) d I n a = d I n a„ Ab Pa (2) d I n a^„_ = -3 d I n G r o s s - « a A b T h e r e f o r e d I n a G r o s s -3 d I n a„ Pa so that as reaction (1) goes i t produces a) more K-rich muscovite b) more Ca-rich plagioclase c) more Ca-rich garnet Thus in spite of producing Ab by reaction (1) the net effect is to make the plagioclase more calcic ,in agreement with observations. At lower temperature we then should have Bi + Na - Ms + Na - Plag + Mn - Gar and at higher T (or lower f H Q) we should have Bi + K - Ms + Ca - Plag + Ca 2- Gar For any fixed P, T, P R Q we should have unique compositions in the phases Bi - Ms - Gar - Plag. As nearby specimens have different compositions apparently f^ ^  i s not the same from place to place at the same T and P. This cou?d be due to local variations in gas phase composition (say CO,,) or to buffering of f R Q by the mineral assemblage.- The reaction relation which exists between specimens 5 and 6 indicates that these assemblages are local buffers. One interesting consequence of the shift of the three-phase triangles in response to variation in f ^ is that variation in garnet H2° l 102 composition could be a function of variable f R Q f so that complex garnet zonation profiles of the type presented2in this study could be solely due to differences in f . 103 CONCLUSIONS. The following conclusions can be drawn from a chemical study of pelitic rocks from Mount Breakenridge area, British Columbia. a) The bulk rock <±iemistry is independent of rock oxidation ratio. b>)) Oxidation ratios greater than 60 increase hematite solid solution in ilmenite. c) The composition of plagioclase is independent of metamorphic grade but dependent on host rock CaO and CaO/Na20 ratios. d) The Pa/Pa + Ms + Ph content of white micas decreases with increase in metamorphic grade. e) Staurolite is not confined to rocks with Fe20.j/Al203 ratios of 0.4. f) Garnets have complex zonation profiles many of which show typical bell curve distribution of manganese. Garnets in the s same rock need not have similar zonation profiles. g) A distribution study of Fe, Mg, Mn and Ca between garnet, staurolite, biotite and chlorite implies a close approach to chemical equilibrium between these phases. The Kp's are independent of temperature differences between the kyanite and sillimanite zones. h) Linear regression analysis defines reaction relations within individual specimens which contain the minerals biotite, muscovite, garnet, staurolite, kyanite or sillimanite, ± chlorite, plagioclase, ilmenite, sphene, rutile or magnetite. These reaction relations therefore represent disequilibrium, or more likely, they are the result of buffering of f„ by the mineral assemblage. Regression analyses also indicates significant bulk chemical differences between rocks of the kyanite and sillimanite zones. i) Representation of mineral assemblages in the tetrahedron CaO - Na20 - MnO - K20 emphasises how mineral assemblages such as garnet-muscovite-plagioclase may be controlled by variation in fH20' 104 ACKEPWLIilDGEMENTS. The author greatly appreciates helicopter support, rock, thin sections and bulk rock analyses provided through the generosity of Drs. W.W. Hutchison and J.A. Roddick of the Geological Survey of Canada. Microprobe analyses were carried out at the University of Washington, Seattle, U.S.A. under the supervision of Dr. B.W. Evans, Miss L. Leitz and Mt. E Matthez. Dr. R.L. Wheelers arid C.J. Duffy provided able assistance in the field. Research funds were provided by National Research Council grant (A 67-4222) to Dr. H.J. Greenwood and the author was financially supported by a National Research Council post-graduate fellowship from 1969-1971 and by a Killam pre-doctoral fellowship from the University of British Columbia in 1973-1974. I would like to thank Dr. H.J. Greenwood whose constructive criticism greatly improved the content of this paper. 105 REFERENCES,. Atherton, M.P. 1965 Reck composition and the isograd in W.S. Pitcher and G.W. Flinn Ed. Controls of Metamorphism/ John Wiley & Sons, New York. Atherton, M.P. 1968. The variation in garnet, biotite and chlorite composition in medium grade pelitic rocks from the Dalradian, Scotland, with particular reference to the zonation in garnet. Contr. to Mineralogy and Petrology, vol. 18, pp 347-371. Bancroft, G.M., Maddock, A.G. and Burns. R.G. 1967. Application of the Mossbauer effect to silicate mineralogy. I. Iron-silicates of known crystal structure. Geochem. et. Cos. Acta, v.31 p 2219. Brown, E.H. 1969. Some zoned garnets from the greenschist facies. Am. Min. vol. 54, pp 16662-1677. Buddington, A.E. and Lindsley, D.H. 1964. Iron-titanium oxide minerals and synthetic equivalents. J.Pet. vol. 5, pp 310-357. Butler, B.C.M. 1967. Chemical study of minerals from the Moine Schists of Ardnamurchan area, Argyllshire, Scotland. Journal of Petrology, vol. 8, pp 233-368. Chinner, G.A. 1960 Pelitic gneisses with varying ferric-ferrous ratios from Glen Cova, Angus, Scotland. Journal of Petrology, vol. 1, pp 178-217. Deer, W.A., Howie, R.A. and Zussman, J. 1966. An introduction to the rock forming minerals. Longmans. Ellitsgaard-Rasmussen, K. 1954. On the geology of a metamorphic complex in West Greenland. Meddr. Gronland. 136, No. 6. 106 Engel, A.E.J, and Engel, C.G. 1960 Progressive metamorphism and granitization of; the major paragneiss, northwest Adirondack Mountains, New "Jtork, Part 11.. Mineralogy Bull. Geol. Soc.-Am., vol. 71/ pp 1-58. Evans, B.W. and Guidotti, CV. 1966. The sillimanite-potash feldspar isograd in Western Maine, U.S.A. Contr. to Mineralogy and Petrography, vol. 12, pp 25-64. Fletcher, C.J.N. 1972. Geology of the Penfold Creek area, Quesnel Lake, British Columbia. Unpubl. Ph.D. thesis, Univ. of British Columbia. Fletcher, C.J.N. 1971. Local Equilibrium in a two-pyroxene amphibolite. Can. J. Earth Sci., vol. 8, pp 1065-1080. Ganguly, J. 1973. Staurolite stability and related parageneses. Journal of Petrology, vol. 11, pp 338-363. Greenwood, H.J. 1961. The system Nepheline - ^ 0 - Argon in total pressure and water pressure in metamorphism. J. Geoph. Res. vol. 66, pp 3923-3946. Guidotti, CV. 1970. The petrology of the transition from lower to upper sillimanite zone, Oquossoc area, Maine. J. Petrology, vol. 11, pp 287-335. Harte, B. and Henley, K.J. 1966. Occurrence of compositionally zoned almanditic garnets in regionally metamorphosed rocks. Nature, vol. 210, pp 689-692. Hey, M.H. 1954. A new review of the chlorites. Min. Mag., vol. 30, p 277. Hollister, L.S. 1966. Garnet zoning: an interpretation based on the Rayleigh fractionation model. Science, vol. 154, pp 1647-1651. Hollister, L.S. and Bence, A.E. 1967. Staurolite: sectoral compositional variations. Science, vol. 158, No. 3804, pp 1053-1056 Hollister, L.S. 1969 b. Contact mel^mDrphism in the Kwoiek Area of British Columbia: an end member of the metamorphic process. Geol. Soc. Am. Bull. 80, pp 2465-2494. 107 Hoschek, G. 1969. The stability of staurolite and chloritoid and their significance in metamorphism of pelitic rocks. Contr. Mineral, and Petrol. 7 vol.22, pp 208-232. Hounslow,A.W. and Moore, J.M. Jr., 1967. Chemical petrology of Grenville schists near Fernleigh, Ontario. J. of Petrology, vol. 8, pp 1-28. Juurinen, A. 1956. Composition and properties of staurolite. Ann. Acad. Sci. Fenn., Ser.A. HI Geol., vol.47, pp 1-53. Korzinskii, D.S. 1959. Physiochemical basis of the analysis of the paragenesis of minerals. New York Consultants Bur. Kretz, R. 1959. Chemical study of garnet, biotite and hornblende from gneisses of southwestern Quebec, with emphasis on distribution of elements in coexisting minerals. Jour. Geology, vol. 67, pp 371-402. Lambert, R. St. J. 1959. The mineralogy and metamorphism of the Moine Schists of the Morar and Knoydart districts of Inverness-shire. Trans. Roy. Soc. Edinburgh, vol. 63, pp 553-588. Lindsley, D.H. 1963. Equilibrium relations of coexisting pairs of Fe-Ti oxides. C.I.W. Year Book 62 for 1962-1963. Miyashiro, A. 1961. Evolution of metamorphic belts. J. Petrology., vol. 2, pp 277-311. Naggar, M.H. and Atherton, M.P. 1970. The composition and metamorphic history of seme aluminum silicate bearing rocks from the aureoles of the Donegal granites. J. Petrology, vol. 11, p 549. Nandi, K. 1967. Garnets as indices of progressive regional metamorphism. Min. Mag. vol. 36, pp 89-93. 108 Okrusch, M. 1971. Garnet - cordierite - biotite equilibria in the Steinach. Aureole, Bavaria. Contr. Mineral. and Petrol., vol, 32, pp 1~23, Phinney, W.G. 1963. Phase equilibria in the metamorphic rocks of St. Paul Island and Cape North, Nova Scotia. Journal of Petrology, vol. 4, pp 90-130. Pigage, L.C. 1973. Metamorphism southwest of Yale, British Columbia, unpubl. M.Sc. thesis, Univ. of Br. Col. Richardson, S.W. 1968. Staurolite stability in part of the system Fe-Al-Si-O-H. J. of Petrol., vol. 9, pp 467-488. Roozebocm, H.W.B. 1891. Uber die Loslichkeit von Mischkrystallen seziell zweier isomorpher kroper. Zeit. Phys. Chemie., vol. 8, pp 504-530. Shaw, D.M. 1958. Geochemistry of Pelitic rocks. Part III. Major elements and general chemistry. Bull. Geol. Soc. Am., vol. 67, pp 919-934. Shreyer, W. and Chinner, G.A. 1966. Staurolite-quartzite bands in kyanite quartzite at Big Rock, Rio Arriba County, New Mexico. Contr. Min. Pet. vol. 12, pp 223-244. Shreyer, W. and Seifert, E. 1970 Compatability relations of the aluminum silicates. Am. J. Sci. vol. 267, pp 371-388. Smith, J.V. 1968. The crystal structure of staurolite. Am. Min. vol. 53, pp 1139-1155. Sturt, B.A. 1962. The compositions of garnets from pelitic schists in relation to the grade of regional metamorphism. J. of Petrology, vol. 3, pp 181-191. Thompson, J.B. 1955. The thermodynamic basis for the mineral facies concept. A. J. Sci. vol.-253, pp 65-103. Wehk, 1970. The distribution of Muminium between coexisting micas in metamorphic rocks from the Central Alps. Contr. to Min. and Petr. vol. 26, pp 50-61. Williamson, D.H. 1953. Petrology of chloritoid and staurolite rocks north of Stonehaven, Scotland. Geol. Mag. v.90, pp 353-361. 109 'APPENDIX I I . Microprobe standards used, in mineral analyses. Up to five elements were measured simultaneously using an accelerating potential of 15 k.v. on an Applied Research Laboratories EMX-SM unit. Specimen current varied from 0.06 to 0.09 a for major elements and 0.2 a for trace elements. The beam diameter was 2 or greater (15) when trace or volatile elements were being analysed. Between 10 and 20 spots per mineral in each thin section were measured. In seme garnet traverses, more than 20 spots were measured. Biotite, staurolite and plagioclase were checked for chemical homogeneity. Corrections for deadtime, background and drift were carried out on UWPROBE, while additional corrections for X-ray absorption, characteristic fluorescence and atomic number effect were accomplished by means of the EMX2A or the RUCKLIDGE (Electron Microprobe Data Reduction EMPADR VII), computer programs. UWPROBE and EMX2A are computer programs in the Geology Department Library at the University of Washington, Seattle. The following standards are from the microprobe collection at the University of Washington. 110 PELITIC ROCKS. Chlorite. Standard Elements LA 10 Q S i Fe Mg A l MG 7 Si Fe Mg A l LA 10 P Si Fe Mg A l LA 10 K S i Fe Mg A l LA 25 C S i Fe Mg A l Nuevo Garnet Mn Biotite #1 Ti K Bioti te #3 Ti K Technique:- Working curve f i t by UWFR0BE. Muscovite. Standard Elements MascoMte #1 K Fe A l S i Feldspar #7092 Wa Biotite #3 Mg Ti Technique:- Straight l ine f i t by UWPR0BE then f u l l correction by EMX2A. I l l Biotite. Standard Elements Biotite # 3 Si A l Fe Mg K Mn Biotite # 1 Ti Feldspar # 7 0 9 2 Wa Ca Technique:- Straight line f i t UWPROBE f u l l correction on EMX2A. Garnet. Standard Elements S 1 7 Si Ti A l Fe Mg Mn Ca Sturbridge Si AA1 Fe Mg Ca 22kk2 Si A l Fe Mg Ca Nuevo garnet Si A l Fe Mn Diopside 2% T i Ti Technique:- Working curve f i t with UWPROBE. SiJauEeM'fS. ( Standard Elements Kyanite from sp. 5 {zkk) Si A l Hess # 3 0 Mg Hess # 1 8 Fe Franklinite Zn E 22 Ti Wuevo garnet Mn Technique:- Straight line f i t on UWPROBE, f u l l corrections by Rucklidge. 112 Feldspars. Standard Elements An # 7 0 9 2 K Ca Na S i Orthoclase # 1 K Wa Si Albite Tiburon Wa Si Crystal Bay Ca Na S i Anorthite Glass Ca Si Technique:- Working curve f i t by UWPROBE. Ilmetiites. Standard Elements Synthetic Spinel A l K 1 3 Fe Ti Mg Sawyer Fe Ti Technique:- Working curve f i t by UWPROBE. / 113 III. METAMORPHISM OF ULTRAMAFITES FROM THE MOUNT BREAKENRIDGE AREA, HARRISON LAKE, BRITISH COLUMBIA . by Stanley B. Reamsbottom, Department of Geological Sciences, The University of Br i t i s h Columbia , Vancouver 8, B.C. 114 ABSTRACT. Metamorphosed ultramafites in the Mount Breakenridge area contain the minerals olivine, talc, tremolite, anthophyllite, chlorite and magnesite. Because microprobe analyses of these minerals indicate significant amounts of iron in the minerals, metamorphism of these rocks is discussed in terms of the system MgO^FeO-Si02-H20-C02. Thermodynamic calculations indicate that the equilibrium temperature of the vapour-absent reaction talc + enstatite = anthophyllite i s extremely sensitive to magnesium - iron solid solution and that the dehydration equilibria forsterite + talc = anthophyllite + water and anthophyllite + forsterite = enstatite + water are insensitive to magnesium - iron solid solution so that the location i n PT space of the isobaric invariant point formed by the inter section of these reactions- (enstatite, forsterite, talc, anthophyllite) depends / on the Mg- content of the system. The assemblage talc + forsterite + anthophyllite + enstatite noted in the ultramafites may have formed by the following reaction sequence discussed in terms of the system MgO-SiC^-^O-CG^ 1) The isobaric univariant reaction talc + forsterite = enstatite + vapour took place at 7kb. Uplift and subsequent reduction in pressure rendered this reaction metastable and anthophyllite formed by the reaction talc + forsterite = anthophyllite + vapour. Textural relationships in the ultramafites and the metamorphic history of the Mount Breakenridge Area substantiates this reaction sequence. 115 YUKON TERRfFORY pinchi Lake fault BRITISH COLUMBIA ^4ff, Polaris Complex Will « Coquihalla Deix. * Tulameen complex WM'*, — -U.S.A. | j Zoned Ultramifie Complex j V > } Alpine Ultramafic Bodies I V v v | Upper Triassic (and Lower Jurassic? ' V v ' basalt and andesite. Late Paleozoic Sedimentary and Volcanic rocks. Figure 1. Distribution of ultramafites in British Columbia, (after McTaggart, 1971). 116 INTRODUCTION. Alpine ultramafites in British. Columbia (Figure 1) are mainly peridotites and dunites,of lenticular r elongate, equidimensional or even batholithic form which are commonly spatially related to major faults (McTaggart, 19711. Serpentinization is common and pervasive in the smaller bodies. The ultramafites l i e within rocks which range in age from Upper Paleozoic to Upper Triassic or Jurassic. Many l i e along faults which separate rocks of the Cache Creek Group from younger strata. The ultramafites in the study area (Figure 2) l i e in rocks of uncertain age. The Cairn Needle Formation, a unit of pelitic and calc silicate schists, amphibolites, minor limestone and conglomerate with granitoid clasts may be Mesozoic, probably Jurassic (Reamsbottcm, 1971). However, this unit can be continuously traced to the south into both the Upper Paleozoic (?) Settler Schist and Upper Paleozoic Chilliwack Group, so that i t may be composed of both Upper Paleozoic and Mesozoic strata. Ultramafic rocks due west of the Old Settler Mountain l i e on the sole of the Shuksan Thrust which separates the Settler Schist from the Chilliwack Group (Lowes, 1972). In the map-area, the ultramafic pods straddle the northern extension of this thrust although the fault as such has not been recognised. As most ultramafic rocks in southern British Columbia are confined to Triassic or older strata (McTaggart, 1971) the Cairn Needle Formation may indeed be Paleozoic and hence McTaggart's hypothesis that the ultramafites are complementary to volcanics of the Triassic Nicola Group could well hold in this region. Lowes (1972) considered ultramafites immediately to the south to be mantle derivatives tectonically emplaced along the Shuksan Thrust. The ultimate origin of these rocks is unresolved. In the map-area the ultramafites have been thoroughly recrystallized under the physical conditions of the staurolite-kyanite and sillimanite zones. This metamorphism which is the subject of this paper, pireventsiy direct deductions from their mineral chemistry Figure 2. 118f PLATE I! . A . Enstatite set in platy talc matrix. B. Irregular shaped enstatite set in talc matrix. Some of the talc may be retrogressive after enstatite. C. Sprays of tremolite and associated olivine set in talc matrix. D. Anthophyllite and associated magnesite set in talc matrix-. . r E. Sprays of anthophyllite set in talc matrix. F. . Anthophyllite piercing olivine and enstatite in talc matrix. I 1 / mm. Plate I Table 1. Mineralogy of Ultraroafic Rocks. Spec. No. CH. OL. TA. TR. ANTH. ENST. MAG. CPY. 01 5A X X X X X 5D X X X X X X 5E X X X X X X 18A X X X X X 18B X X X X X X X X X 18C X X X X X X X X 18D X X X X 18E X X X X 18F X X 18G Hornblende - spinel - garnet 181 Act inol i te - b io t i te - chlorite 18J X X X X X X 19 matrix X X X 19 vein X X X D X X X X X X 497 X X x • X CH - Chlorite, OL - Olivine, TA - Talc, TR - Tremolite, ANTH - Anthophyllite, ENST - enstatite, MAG - Magnesite, CPY - Chalcopyrite, OP - Magnetite. i—1 120 on possible mantle origin. Description and. Petrography. An ochre, brown weathering surface characterizes the j. ultramafic lenses .They, ^occur tas, a series of lenses up to approximately 200 by 600 meters within a north-northwest trending zone from west of Mount Urquhart to Hunger Creek. The mineral assemblages in each of the bodies are listed in Table 1 and their field locations are indicated in. Figure 2. Most have books of silvery talc, green chlorite, radiating sprays of tremolite and anthophyllite, honey-brown stumpy enstatite and brown-weathering olivine. Some.are extensively net-veined by talc, chlorite and anthophyllite. Irregular lenses and layers from 25 cm. to 2 meters wide of fine-grained green chlorite - talc - anthophyllite or dark green amphibole-rich rock are distributed sporadically throughout body 18. Within this body is an irregularly shaped inclusion of garnet - hornblende -spinel granulite. In olivine - tremolite - talc rocks radiating sprays of tremolite (1 cm. to 3 cm.) commonly pseudomorphed by talc are set in a mosaic of granoblastic to tabular olivine (1 mm. to 3 mm.). The fabric is characterized by equilibrium triple junctions or peculiar comb in comb texture in which areas of the rocks are composed of interlocking tabular olivine crystals more or less in optical continuity. Small amounts of talc, chlorite and opaque minerals make up,the rest of the rock. , Enstatite-antliophyllite-olivine-tremolite-talc-magnesite assemblages have textural features which suggest that enstatite crystallized in olivine-and talc rich-areas of the rock before amphiboles and magnesite. Irregularly-shaped enstatite crystals (0.5 cm.) form as optically continuous1 islands' in a platy talc matrix (Plate 1 A,B) or as intimate inter/growths with olivine. Apparently nucleating and growing out.of the talc rich areas are needles and sprays of anthophyllite and tremolite (Plate 1 C,E) which pierce or overprint the olivine and enstatite (Plate 1 F). Magnesite in close intergrowth with anthophyllite overprints both olivine-and talc-rich areas which surround enstatite fragments (Plate 1 D). The sequence of development of these minerals suggests that a reaction to form enstatite from olivine and talc took place in the ultramafites before a reaction which formed anthophyllite and magnesite. Figure 3. Plot of Talc, Anthophyllite, Enstatite, Olivine and Magnesite i n the system MgO-FeO-Si02. 123 Mineral Cheirdstry. Microprobe analyses of ol i v i n e , enstatite, anthophyllite, tremolite. t a l c , chlorite and magnesite are given i n Tables 2 to 7.^  Five elements or less were measured simultaneously on the Applied Research Laboratories EMX-SM microprobe at the University of Washington, Seattle, under the supervision of Professor B.W. Evans and Miss L. Leitz. A l l analyses were corrected for deadtime, d r i f t and background. Pyroxenes and amphiboles were corrected for x-ray absorption, characteristic fluorescence and atonic number effect. Operating conditions and standards used are given i n Appendix 1. The compositions of coexisting t a l c , anthophyllite, enstatite, olivine and magnesite are plotted i n the ternary MgO-FeO-SiC^ diagram Figure 3. Tables 2 to 7 also l i s t atomic proportions of Fe, Mg, Mn, Ni and Cr noted i n the minerals with Xmg = Mg atoms / Mg + Fe+Mn + Ni + Cr. The distribution coefficient, K^, between olivine and coexisting minerals i s given by / K,. = (X _ , o l / 1-X .ol) / (X ,min / 1-X ,min) = A/B D mg ' mg' ' mg' ' mg' ' The was calculated by f i t t i n g a least squares li n e to the plot A vs. B with U.B.C. computing centre routine SIMREG. the curves of Figure 4 were drawn to f i t the calculated Kp's. The preference for Mg relative to Fe i n the minerals i s as follows: t a l c > chlorite > tremolite > olivine > magnesite = enstatite> anthophyllite and agrees with the data from the Malenco ultramafite (Trommsdorff and Evans, 1972). . 1..... See Appendix 1. 124f Figure 4. Distribution diagram of Xmg i n coexisting olivine, talc,chlorite, tremolite, enstatite, anthophyllite, magnesite. Xmg = Mg atoms / Mg + Fe + Mn + Ni + Cr 124 Specimen Number h 5 D r 'D 19 M 8 C / A / 0 O / A ©/ / / © / I / Q / A Q © M 8 B A OB / / / -d. L 0.8 X mg. © Talc n Chlor i te A Tremol i te 0.9 o Ens ta t i te • Magnes i t e A Anthophyl l i te. 1.0 > 0.9 O CD E x 0 8 Table 8. Relationship between assemblages i n metamorphosed and surrounding p e l i t i c schists. P e l i t i c metamorphic zone. staurolite-kyanite s i l l imani te Ultramafic assemblage. olivine-tremolite=talc enstatite-olivine-anthophyllite-chlorite Equivalent ultramafic zones. zone C of Bergell tonalite (1) zone D of Bergell tonalite (1) enstatite-forsterite-chlorite-schist. Val Cama, Eastern Lepontine Alps. (2) (1) Trommsdorff, V. and Evans, B.W. 1972. (2) Troitmsdorff, V. and Evans, B.W. 1972. 126 Metamprphism. In metamorphosed ultramafic rocks of the map-area an olivine-talc-tremplite assemblage develops within the staurolite-kyanite zone of the surrounding pelitic schists. Enstatite-olivine-anthophyllite .is the ultramafic assemblage surrounded by pelitic rocks of the sillimanite zone (Table 8 and Figure 2). The sillimanite isograd marks the incoming of metamorphic enstatite in metamorphosed ultramafic assemblages. In the sillimanite zone the presence of magnesite and veins rich in hydrous phases such as talc, chlorite and anthophyllite indicate metamorphism under variable compositions of the fluid phase. A high pressure kyanite-sillimanite facies series was developed in pelitic rocks of the map-area before emplacement of the Late Cretaceous Scuzzy Pluton produced andalusite and sillimanite hornfels in surrounding contact rocks (Reamsbottom, 1971). The metamorphic history of the region thus indicates a pressure reduction between the regional and contact metamoiphic events. Textural criteria of the ultramafites indicates that enstatite eery stalliz^^^ anthophyllite and magnesite. Any model which accounts for the metamorphism of these ultramafites must incorporate this reaction sequence. The following discussion explores the conditions under which such a reaction sequence could take place, a) Metamorphic reactions in the system CaO-MgO-SiO^^O-CG^ (Figure 5) which define the stability of pure magnesian anthophyllite (Greenwood, 1963) would with increase in temperature at constant pressure involve the formation of anthophyllite from talc and forsterite before the formation of enstatite. Uncertainty in the tJiermodynamic properties of anthophyllite (Zen, 1971; Greenwood, 1971) and hence in the calculated position of the solid-solid reaction 5 8 4 5 Temperature, °C x 100 Figure- 5. Metamorphic reactions i n the system CaO-MgO-Si02-H2Q-CC>2. S = Serpentine; Ta= Talc; B = Brucite; F = Forsterite; D = Diopside; Tr = Tremolite; A = Anthophyllite; E = Enstatite; Q =. Quartz; W = Water, (after Evans and Trommsdorff, 1970). to 128 t a l c + enststite = anthophyllite could, as discussed by Zen (1971) and Greenwood (1971) render the reactions talc + forsterite = anthophyllite + E^O anthophyllite + forsterite = enstatite + 1^ 0 me tastable and the reaction talc + forsterite = enstatite + 1^ 0 stable (Figure 6). In this way enstatite could form stably before anthophyllite. Thermodynamic data are not sufficiently accurate to define the PT conditions of the vapour - absent reaction enstatite + talc = anthophyllite (1) in the system MgO- Si02~H20 (Zen, 1971; Greenwood, 1971). An estimate of maximum PT conditions of metamorphism in the Mount Breakenridge area, based on mineral assemblages in surrounding pelitic schists, indicates pressures of 7kb and temperatures of 600°-700°C. The expression T = RT 2.303 log K A S R where S_ and K are the entropy chancre and equilibrium constant of reaction (1), allows an estimate of the temperature of equilibrium of the reaction i n the pure Mg-system. Compositions of talc, enstatite and anthophyllite i n specimen 18B are X^ = 0.971; 0.847; 0.809 respectively. The entropy change of reaction (1) at 7kb and 900°K is 3.73 cal deg - 1 (Zen, 1971). Therefore the equilibrium temperature of reaction (1) in the pure Mg-system will l i e at a temperature greater than 900 K by an amount given by • U 1 taMg,enstatite; ' ^ ^ a l c T = RT A S R a'Mg,anthophyllite = RT In (XM . ) 4 . (X. . , J 3 r-^ - Mg,enst. Mg,talc ^Xi,^ -= RT In (0.847)4 . (0.971)3  A S R (0.809)7 = 350°C. 129 Anthophyllite T Q = Quartz W = Water Figure 6. Progressive change i n topology of reactions i n the system MgO + SiO^ + H 20 as the position of the equilibrium ta lc + enstatite = anthophyllite moves to successively higher temperatures (configuration B) relative to other equi l ibr ia involving anthophyllite. 130 H 2 0 X C O CO, Figure 7. Schematic phase relationships among the phases forsterite (F); enstatite (E); talc (T); anthophyllite (A); magnesite (M) i n the system MgO + SiO» + H„0 + C0 o. P1 and P„ correspond to pressures shown i n configuration A Figure 6. P, - P + P TOT rH 20 C0 2 131 Thus the composition effect on reaction (1) i s so great that a more reasonable topology for reactions in the pure Mg-system i s that of Figure 6 (B). This would imply that the assemblage pure Mg - anthophyllite plus pure Mg - forsterite does not have a stability f i e l d . This result agrees with the conclusions of Zen (1971) and Evans and Troimisdorff (1974). The effect of variable substitution of iron for magnesium on the equilibrium temperatures of the dehydration reactions talc + forsterite = anthophyllite + H2<D (2) and anthophyllite + forsterite = enstatite + H20 (3) has been demonstrated to be minimal. (Troirmsdorff and Evans, 1972). Therefore the topology of reactions shown in Figure 6A will only be valid i f the phases participating in the reactions are Fe/Mg solid solutions. In particular the location of invariant point EFTA wil l be particularly sensitive to the compositions of coexisting enstatite. talc and anthophyllite. (b) Metamorphism of these ultramafites can be discussed in terms of the system MgOSi02-H20-C02 (Figure 7). Combinations of sillimanite zone minerals, forsterite (F)-enstatite (E) - talc (T) -tremolite - anthophyllite (A) - magnesite (M) are representative of isobaric invariant points MAFT, MAFE and MEET. The invariant point EFTA can be treated as an isobaric invariant point provided the iron content of the phases T + E + A is known and fixed. The assemblage EFTA is isobarically and compositionally invariant. The author considers that the observed assemblages are unlikely to represent unique invariant points but are more reasonably the end product of a complex sequence of self - buffering, univariant reactions which took place as the physical conditions changed in P-T-X space. v Consider the metamorphic history of specimen 183 which contains enstatite (E) - forsterite (F) - talc (T) - anthophyllite (A) -magnesite (M) - tremolite. At pressures at which EFTA and MEET are stable perhaps above the aluminosilicate invariant point (P^ Figure 6A), a talc - forsterite schist (a) (Figure 7A) reacts to form enstatite and vapour. If the mineral assemblage buffers the composition of the fluid 132 phase, continued heating with the reaction proceeding at equilibrium could drive the reaction along the univariant curve to EFTA and hence anthophyllite could form after enstatite. However i f PpL U I D i - s f a l l i n g as the reaction t a l c + forsterite = enstatite + vapour proceeds, the system could change to pressures at which MAFT and MAFE are stable. The tal c - forsterite - enstatite ultramafite would then be within the s t a b i l i t y f i e l d of anthophyllite at (b) (Figure 7B) so that enstatite would be metastable and talc and forsterite would react to form anthophyllite. I f the f l u i d phase composition within the resultant talc - forsterite - anthophyllite - r e l i c t enstatite ultramafite (b) was externally controlled and varied to the 00^ - ri c h side of the diagram at (c), forsterite could react to form additional anthophyllite and magnesite. Although complex, the above sequence of reactions agrees with textural evidence and i s i n accordance with the metamorphic history of surrounding p e l i t i c schists. These were regionally metamorphosed at pressures above the aluminosilicate invariant point and then uplifted and contact metamorphosed at lower pressures. (c) The experimental data of Greenwood (1963) define the st a b i l i t y of pure Mg - anthophyllite i n the system MgO-SiG^-^O. The equilibrium P-T data for the reactions 4F + 9T = 5A +4W (2) and A + F = 9E + W (3) can be used to study the effect of iron solid solution on these reactions and define physical conditions of reactions i n the system Fe0-Mg0-Si02-H20. However, Trommsdorff and Evans (1972) have demonstrated that the equilibrium temperatures of these dehydration equil i b r i a are v i r t u a l l y independent of iron s o l i d solution. 133 The following theoretical discussion i s therefore presented as an alternative method of studying the effect of s o l i d solution on the equilibrium temperatures of pure end-member reactions whose physical conditions have been experimentally defined. The relationship A = -&H ( 1 - 1 ) = £ n. log a 2.303R ( T T Q) 1 for reaction (2) at 7kb equals A = -AH2 ( 1 - 1 ) = (8.75 log X^'- 2 log Xp 2 .303R ( T TQ ) - 6.75 log ( see Appendix 2) where T Q = equilibrium T at 7kb = 690° ± 5°C AH 2 = AH of reaction 2 at 7kb R = gas constant X^ = X ^ , anthophyllite 18B *F = *Mg ' f o r s t e r i t e 1 8 B *T = *Mg ' telc 1 8 B This relationship coupled with partitioning data from specimen 18B VT/F = % / *F = 6 ' 1 5 ^D,F,A, = * F / *A = i " 2 8 leads to the expression F ( X ) 2 = Xj, - ( Xp ) 4 * 3 8 . ( 6.15 Xp T 3 * 8 8 . 1 0 - A / 2 1.28-0.28Xp 1+5.15 Xp where Xp = ol i v i n e for any given A. 135 Thus the composition of coexisting olivine, talc and anthophyllite can be related to temperature. Similarly for reaction (3) the compositions of coexisting olivine, anthophyllite and enstatite can be related to temperature. (See FJX)^ in Appendix 2). F(X) 2 and F(X)^ for reactions (2) and (3) were plotted for values of X between 0 and 1 over the temperature range 690°C - 710°C (Figure 8). The results obtained clearly indicate that the reactions are insensitive to iron solid solution as zero values of F(X) were obtained only at = 1.0 at the equilibrium temperatures of the respective pure Mg- reactions. This approach therefore substantiates the conclusions of Trommsdorff and Evans (1972). If the equilibrium temperature of the reactions had proved to be sensitive to iron solid solution the sweep of the three -phase triangles TFA and AEF across the triangle FeO-MgO-SiC^ with varying temperature could have been defined. This hypothetical situation is shown in Figure 9 which depicts the univariant reactions which emanate from the isobaric, isocompositional invariant point EFTA in the system MgO-FeO-Si02-H20-CC>2. Where Xp of the three - phase triangle TFA is equal to Xp in the three -phase triangle AEF the theoretically possible but improbable reaction F + T = E + A + vapour would take place. (The slope of this reaction in T-X^ space has been calculated on. the basis of an assumed equilibrium temperature of 698°C at 7kb in the MgO-Si02-H20 system). Thus i f the dehydration equilibria had been sensitive to iron solid solution the following sequence of reactions could have taken place with increase in temperature in the ultramafites at P = 7kb and 1) T + F = E + vapour in magnesia rich rocks (1-2 Fig 9) 2) Shift in the three-phase triangles TEF, TFA to the iron-rich side of the MgO-FeO-Si02 triangle (2-3 Fig 9) 3) Univariant reaction T + F = E + A + vapour ( 3 Fig 9) 136 t ' : I— : . . "1 : < i . 01 0-2. 0 3 0-4 X c o 2 • Figure 9. Univariant reactions which emanate from the isobaric invariant point EFTA i n the system MgO-Si0 2-H 20-G0 2 with calculated-ternary reaction F + T = E + A i n Lie .%;,-<•!tan Fthejy!§y^tem.,FeX3-MgXD-Si02-H20-C02 (See text for discussion) . P = 7kb. 137 This final reaction assemblage looks invariant when considered in terms of the system MgO-Si02-H20 but is not because of the presence of iron. Thus although the reactions discussed in Figure 9 are purely hypothetical the presentation is valid because a) It substantiates the conclusion, that equilibrium temperatures of dehydration equilibria (2) and (3) are insensitive to iron solid solution and b) presents an alternative method for studying the influence of solid solution on equilibrium temperatures of pure end-member reactions. Conclusions. Thermodynamic calculations confirmed the conclusions of Trommsdorff and Evans (1972) and Evans and Trommsdorff (1974) that the equilibrium temperatures of dehydration equilibria in the system MgO-Si02-H20 are insensitive to iron solid solution and that because the vapour absent reaction T + E = A is extremely sensitive to magnesium-iron solid solution, the topology of the equilibria which define anthophyllite stability is a function of the Mg - content of the system. A possible sequence of reactions (discussed in terms of configuration Figure 6A and Figure 7) results in the assemblage talc (T), forsterite (F), enstatite (E), anthophyllite (A) and magnesite (M) as follows: a) Isobaric univariant reaction T + F = E + vapour takes place at 7kb. This mineral assemblage buffers the composition of the fluid phase on the reaction curve, b) With uplift and reduction in Ppyjjjy the E is metastable within the A stability field so that T + F reacts to form A. c) Additional late A and M may have formed from F i f the fluid phase was externally controlled and varied to 00 2 - rich values. 138 ACKNOWIEDGEMENTS. The author greatly appreciates helicopter support provided by Drs. W.W. Hutchison and J.A. Roddick of the Geological Survey of Canada. Thin sections were prepared by the technical staff of the University of British Columbia, under the supervision of Mr. E. Montgomery. Microprobe analyses were carried out at the University of Washington, Seattle, U.S.A. under the supervision of Dr. B.W. Evans, Miss L Leitz and Mr. E. Matthez. The author was supported by a National Research Council post-graduate fellowship from 19.69-1971 and by a Killam pre-doctoral fellowship from the University of British Columbia in 1972-1974. Research funds were provided by a National Research Council grant (A 67-4222) to Dr. H.J. Greenwood. The constructive criticism and advice of Dr. H.J. Greenwood greatly improved the content of this paper. 139 REFERENCES,. Evans, B.W. and TrcmmsdQrff, V. 197Q. Regional netamorphism of ultramafic rocks in the central, Alps: Parageneses in the system CaO-MgO-SiO^ H-jO. Schweizer Mineralog. Petrog. Mitt. vol. 50, pp 481-492. Gr^nwccd, H--I* 19f4. Stability of Enstatite and Tale, and C02 metasomatism of Metaperidotite, Val d'Efra, Lepontine Alps. 5*5.. SUftWt$@&.^OvoI^ Greenwood, H.J. 1963. The synthesis and stability of anthophyllite. Jour. Petrology, vol. 4, pp 317-351. Greenwood, H.J. 1971. Anthophyllite. Corrections and comments on its stability. Am. Jour. Sci., vol. 270, pp 151-154. Greenwood, H.J. 1967. Mineral equilibria in the system MgO-SiO,,-H20-C02. Researches in Geodiemistry, Abelson, P.H. ed: J.Wiley and Sons, pp 542-567. Johannes, W. 1969. An experimental investigation of the.system MgO-Si02-H20-C02. Am. Jour. Sci. vol.267,pp 1083-1104. Lowes, B.E. 1972. Geology between Harrison Lake and the Fraser River. unpubl. Ph.D. thesis, Univ. of Washington. McTaggart, K.C. 1971. On the origin of ultramafic rocks. Bull. Geol. Soc. Am. vol. 83, 1972. p.30. Reamsbottom, S.B. 1971. The Geology of the Mount Breakenridge Area, Harrison Lake, British Columbia, unpubl. M.Sc. thesis, Univ. of British Columbia. Robie, R.A. 1966. Thermodynamic properties of minerals. In. Geol. Soc. Am. Memoir 97, Handbook of Physical Constants, pp 437-458. Trommsdorff, V. and Evans, B.W. 1969. The stable association enstatite-forsterite-chlorite in amphibolite facies ultramafics of the Lepontine Alps. Schweizer Mineralog. Petrog. Mitt. vol. 49, pp 325-332. 140 Tranmsdorff, V. and Evans, B.W. 1972. Progressive metamorphism of antigorite schist in the Bergell Tonalite Aureole (Italy). Am. J. of Sci., vol. 272, pp 423-437. Zen, E-AN, 1971. Comments on the tiierjtKxJynamic constants and hydrothermal stability relations of anthophyllite. Am. Jour. Sci., vol. 270, pp 136-150. 141 APPENDIX I . Microprobe standards used i n mineral analyses. Up to five elements were measured simultaneously using an accelerating potential of 15 k .v . on an Applied Research Laboratories EMX-SM unit . Specimen current varied from 0.06 to 0.09 a for major elements and 0.2 a for trace elements. The beam diameter was 2 or greater (15) when trace or vo la t i l e elements were being analysed. Between 10 and 20 spots per mineral i n each thin section were measured. In some garnet traverses, more than 20 spots were measured. B io t i t e , staurolite and plagioclase were checked for chemical homogeneity. Corrections for deadtime, background and d r i f t were carried out on UWPROBE, while additional corrections for X~ray absorption, characteristic fluorescence and atomic number effect were accomplished by means of the EMX2A or the RUCKLIDGE (Electron Microprobe Data Reduction EMPADR VTI), computer programs. UWPROBE and EMX2A are computer programs i n the Geology Department Library at the University of Washington, Seattle. The following standards are from the microprobe collection at the University of Washington. ULTRAMAFIC ROCKS.  Talc. Standard Elements E n a l 10 A l R 62 Fe E n s t a t i t e g l a s s S i Mg Technique:- L i n e f i t "by UWPROBE. E n s t a t i t e and Amphiboles.. Campolungo Tremolite. S i Ca A c t i n o l i t e 11 B Fe Enal 20 Mg A l Nuevo garnet Mn 52 - ML - 11 Cr N i 0 N i A l b i t e Tiburon Na Technique:- L i n e f i t UWPROBE, f u l l c o r r e c t i o n EMX2A. O l i v i n e . Standard Elements MG 7 S i Fe Mg Quebec S i Fe Mg O l i v i n e T S i Fe Mg N i 17^.1 S i Fe Mg Y.S. 2k S i Fe Mg Mh Fro n d e l S i -Fe Mg 19.7 S i Fe Mg Technique:- Working curve f i t on UWPROBE. C h l o r i t e . Standard Elements LA 10 Q S i Fe Mg A l MG 7 S i Fe Mg A l LA 10 P S i Fe Mg A l LA 25 C Fe Mg Technique:- Working curve f i t on UWPROBE. Magnesite. Standard Elements Dolomite Ca Mg Magnesite Fe Mg Technique:- L i n e f i t on UWPROBE, C0 2 by d i f f e r e n c e . 144 Table 2. Microprobe analyses of olivine. Specimen No. 18C 19 D 18B 5D S i 0 2 1+0.06 4o .31 4o .59 39.58 39.98 FeO * 11.95 10 .16 9 .29 1^.36 7-95 MgO U7.81 48 .95 49 .94 45.09 50.55 MnO 0.17 0 .13 0 .14 o . i 4 0.11 NiO 0.26 0 .42 0 .15 0.22 0.28 Total 100.25 99 .97 100 .11 99.49 98.87 no. of Ions on bas: is of 4(0) . S i 0.991 0 .992 0 • 992 0.998 0.986 Fe 0.247 0 .209 0 .190 0.303 0.164 Mg 1.763 1 .796 1 .820 1.694 1.857 Mn o.oo4 0 .003 0 .003 0.003 0.002 Ni 0.005 0 .008 0 .003 o.oo4 0.006 X 2.019 2 .016 2 .016 2.004 2.029 XFe 0.122 0 .104 0 .094 0.151 0.081 XMg 0.873 0 .891 0 .902 0 . 845 0.915 XMn 0.002 0 . 002' 0 . 002 0 . 002 0 . 001 XNi 0.003 0 . 004 0 .002 0 . 002 0 . 003 * Total iron expressed as FeO. Analyst:- S.B. Reamsbottom. 145 Table 3. Microprobe analyses of talc. Specimen Wo. 18C 19 , D 18B 5D SiOp 62.28 62.21 61.87 63.15 60.52 AlgO 0.15 0.00 0.07 0.07 0.01 FeO 0 * 1.37 1.37 1.37 1.5^ 1.17 MgO 29.76 29.5^ 30.02 29.37 30.16 HgO ** k.l h.l h.l h.i Total 98.26 97.82 98.03 98.83 96.56 Wo. of ions on basis of 24(0,OH). Si 8.O38 8.059 8.008 8.098 7.952 Al 0.023 0.00 0.011 0.011 0.002 Fe 0.142 0.148 0 .148 0.165 0.129 Mg 5.725 5.704 5.791 5.613 5.906 OH 4 .646 4.061 4 .058 4.02 . 4.119 XFe 0 . 024' 0.025 0.025 0.029 0.021 XMg 0.9 76 0.9 75 0.9 75 0 . 9 7'1 0.979 * Total iron expressed as FeO. •^Stoichiometric 4 .7wt.$ water added to each analyses. Analyst:- S.B. Reamsbottom. 146 Microprobe analyses of enstatite. Specimen No. 18C D 18B 56.18 57.02 56.26 A I 2 6 3 0.16 0 .16 0.l4 FeO * 7 .89 7.19 10.99 MgO 37-95 3 6 . 8 6 34.00 MnO 0.12 0 .71 o.o4 CaO o.ok 0.06 0 .06 Na 20 0.00 0.00 0.12 Cr 2 0 0.00 0 .06 0.73 NiO ° 0.12 0 .71 o.o4 Total 102.46 102.77 102.48 No. of ions on basis of 6(0) Si 1.916 1.938 1.934 A l 0.006 0.006 0.006 A l 1.922 1.944 1.940 0.00 0.00 0.00 Fe 0 . 2 2 5 0.204 0.316 Mg 1.929 I . 8 8 5 1.771 Ca 0.001 0.002 0.002 Mn 0.003 0.020 0.001 Cr 0.00 0.002 0.002 Ni 0.00 0 .00 0.00 Na 0.00 0 .00 0.00 2.158 2.113 2.110 XFe 0 . 1 0 4 0 . 0 9 8 0 . 1 5 1 XMg 0 . 8 9 3 0 . 8 9 1 0 . 8 4 7 XMn 0 . 0 0 1 0 . 0 0 9 0.001 XCr 0.00 0 . 0 0 1 0 . 0 0 1 Total iron expressed as FeO, Analyst:- S.B. Reamsbottom. Table 5. Microprobe analyses of tremQlite. Specimen No. l8c D 18B 5D Si0 2 57.02 57.68 53.65 57.10 A l o O o 1.05 0.77 5.09 0.55 FeO J * 3.10 3.05 3.36 2.17 MgO 24.68 24.17 21.74 25.01 MnO 0.08 0.92 0 . 0 0 0 . 0 0 CaO. 12.05 12.06 11.79 10.99 Na 20 0 . 0 0 0 . 0 0 0 . 0 0 0 . 2 7 Cr 9 0 o 0 . 2 1 0.1k 0 . 0 0 0.03 Ni6 0 . 0 0 0 . 0 0 0 . 0 0 0 . 0 0 H^ O ** 2 . 2 2 . 2 2 . 2 2 . 2 Total 100.39 100.99 97.83 98.32 No. of ions on basis of 24(0,0H). Si 7 . 7 8 3 7.843 7 . 5 0 9 7.891 A l O .OI69 0 . 1 2 3 0 . 4 9 1 0.090 7 .952 7.966 8.00 7.981 A l 0 . 0 0 " 0 . 0 0 0.349 0 . 0 0 Fe ' 0 . 3 5 ^ 0 . 3 4 7 0.393 0 . 2 5 1 Mg 5.021 4.900 ^•535 5 .151 Ca 1 .762 1.757 1 .768 1 .627 Na 0 . 0 0 0 . 0 0 0 . 0 0 0 . 0 7 2 Cr 0 . 0 2 3 0 . 0 1 5 0 . 0 0 0 . 0 0 3 Ni 0 . 0 0 0 . 0 0 0 . 0 0 0 . 0 7 2 Mn 0 . 0 0 9 0 . 0 1 6 0 . 0 0 0 . 0 0 7 . 1 6 9 7.035 7.045 7.176 OH 2 . 0 0 3 1 .995 2 . 0 5 4 2 .028 XFe 0.065 0.066 0.08 0 .046 XMg 0.928 0.928 0.921 0.94 XMn 0. 002 0.003 0 . 0 0 0 . 0 0 Mi- 0 . 0 0 0 . 0 0 0 . 0 0 0.013 XCr 0.004 0.00 3" 0 . 0 0 0.001 * Total iron expressed as Feo. ** Stoichiometric 2.2wt.$ water added. Analyst:- S.B. Reamsbottom. 148 Table 6. Microprobe analyses of anthophyllite. Specimen No. 18c 19 D 18B SiO 57.68 58 .1 56.29 58 .03 Al 20o 0.00 0 .06 0.00 0 .14 FeO * 11.48 11 • 35 11.98 11 • 55 MgO 29.32 29 .49 29.79 29 .12 MnO 0.22 0 • 59 0.35 0 .15 CaO 0.53 0 • 32 0.45 0 • 33 Na20 0.00 0 .00 0.00 0 • 23 C r 2 ° 3 0.00 0 .00 0.00 0 .27 NiO 0.00 0 .00 0.00 0 .01 H20 ** 2.2 2 . 2 2.2 2 .2 •Total 101.43 102 , .11 100.06 102 .03 No. of ions on basis of 24(0,0H). Si 7.788 7.806 7.526 7.795 A l 0.00 0.009 0.00 0.022 7.788 7.815 7.526 7.817 A l 0.00 0.00 0.00 0.00 Fe 1.296 1.254 1.340 1.280 Mg 5-901 5.905 5.937 5.856 Ca 0.077 0.045 0.064 0.048 Na 0.00 0.00 0.00 0.059 Cr 0.00 0.00 0.00 0.028 Ni 0.00 0.00 0.00 0.059 Mn 0.025 0.066 0.033 0.017 7.299 7.270 7.374 7.289 OH 2.252 2.204 . 2.230 2.210 XFe 0.18 . XMg 0-817 XMn 0.004 XCr 0.00 XNi 0.00 0.174 p.18 0.177. 0.817 "0.812 0.809. 0.009 0.005 0.002-0.00 0.00 0.008: 0.00 0.00 0.0004 * Total iron expressed as FeO. ** Stoichiometric 2 . 2- wt$ H20 added. Analyst:- S.B. Reamsbottom. Table 7. Microprobe analyses of c h l o r i t e and magnesite 149 Specimen No, C h l o r i t e l8C Magnesite l8B SiO A l 6 FeO MgO H2O T o t a l 33-73 15.10 3.03 34.05 13.0 99-91 CaO FeO MgO COo 0.45 13.85 42.74 42.96 1 0 0 . 0 0 No. of ions 36 (0 ,OH), No. of ions 6 ( 0 ) , S i 6 . 3 6 4 Mg 1 . 9 8 0 A l 1 . 6 3 6 Fe O . 3 6 O 8 . 0 0 Ca 0 . 0 1 5 A l 1 . 7 2 2 2 . 3 5 5 Fe 0 . 4 7 8 C 1 . 8 2 3 Mg 9 . 5 7 6 1 1 . 6 7 6 OH 1 6 . 3 6 1 XFe 0.048 0. 154 XMg ° - 9 A 3 0.846 150 APPENDIX 2. a) 4F + 9T « 5A + 4W (2) A HR(298 1) = 2 6 , 2 X 1 q 3 x 4 1 , 8 3 b a r c 0 ' ^ 1 niole"1 ^^(Tkb) = 9 9 7 - 5 2 x 1 q 3 b a r c c mole"1 From A Hp = A.^ +A.V(P-1) A VSOLIDS = ~ 1 9 - 7 8 c c ^o 1" 1 H20 Volume data from Robie (1966). Reaction ( 2 ) = F + 2.25 T = 1.25 A + W K. * a 1-25 ^ „ , _ _ 2.25 A fH20 / ap . e^' and = (X 7 ) 1 - 2 5 f / (x 2} fx V - 2 5 <Amg,A ' ' fH 20 ' % , F } (Xmg,T ) From -AH (1 - 1) =< n log a 2.303R (T To) -AH (1 - 1) = A = 8.75 log X. - 2 log X^-6.75 log 5 27303R (T To) A * D^,F,A = *F / \ = 1.28 we get log Xp = 4.38 log X A - 3.88 log - A F 4.38 , ,v x -3.88 (X) 2 = Xp - ( Xp T * J O ( 6.15 :Xp) ( 1.28z0.28Xp) (1+5.15 Xp} This function is solved for Xp for the following values of A. T°C. T°K 1/T°K (1/T-l/To) A 690 963.13 1.04 x 10~3 0 x 10~3 0 700 973.13 1.03 -0.01 0.051 710 973.13 1.02 -0.02 0.102 To for reaction 1 at 7kb = 690° ± 5°C. b) A + F =9 E + W (3) A HR(298 1) = 2 , 7 9 x 1 0 x 4 1 - 8 3 8 3 c c b a r deg^mole" ^.Hj^kb) = '-ll.S * x 10^ cc bar deg \nole 1 from A Hp = A E± +AV(P-1) /^ Vg = - 25.23 cc. Reaction (3) = A + F = E + W 9 K'3 ~ *E ' fH 20 / aA ' *F 9 7 2 = *E • fH 20 / XA * *F from A^H (1 - 1 ) = £ n log 2.303R (T. To) -4H a - 1 ) - A =9 log Xp - 7 leg XA - 2 leg X and 2.303R (T To) w - ^ / % = 0 , 9 8 4 i-Xp i - x A we get F(X) 3 = X, - X, 4' 5 - X ^ " • 10-^ 2 = Q F ( 0.984+0.016^) (1.28-0.28^) This function is solved for for the following values of A. T T°K 1/T 1/T-l/To A 690 963.13 1.04 x 10~3 0.02 x IO - 3 -1.203 700 973.13 1.03. 0.01 - 0.602 710 983.13 1.02 0 0 The functions were solved using subroutine RZFUN stored in U.B.C. Computing Centre Library. 153 SUMMARY AND CONCLUSIONS. The Mount Breakenridge Area, Harrison Lake, B r i t i sh Columbia, i s underlain by metamorphosed strata of the Upper Paleozoic (?) or older Breakenridge Formation, the Upper Paleozoic (?) or Mesozoic(?) Cairn Needle Formation and the Lower Cretaceous Peninsula Formation. These gneisses and schists have been intruded by quartz-diorite and granodiorites of the synkinematic Mount Breakenridge Plutonic Complex and the Late Cretaceous Scuzzy Pluton. Small pods of ultramafite outcrop mainly within schists of the Cairn Needle Formation. Prior to mid-Cretaceous the rocks were complexly, folded and metamorphosed. The p e l i t i c schists developed index minerals characteristic of Barrovian Metamorphism (P 5.5kb; T=600-700°C) and associated ultramafites recrystal l ized to ol ivine + talc + tremolite + chlorite ± anthophyllite ± enstatite ± magnesite schists. Locally, p e l i t i c schists of the Cairn Needle Formation were recrystal l ized to andalusite and si l l imanite hornfels i n a contact aureole around the Late Cretaceous Scuzzy Pluton. The Breakenridge Formation i s correlated with the Upper Paleozoic Custer and Skagit Gneisses but i t may also be equivalent to the Pre-Cambrian Yellow-Aster Complex i n Washington State. Cairn Needle Formation i s correlated with the Upper Paleozoic Chilliwack Group but granitoid clasts i n dis t inct ive horizons of meta-conglomerate close to i t s boundary with the Breakenridge Formation may indicate that i t i s i n part of Mesozoic age. Ultramafic and related rocks i n the Southern Coast Crystalline Complex and Northern Cascade Mountains formed i n a marginal basin which was i n existence from the Mississippian to the Triass ic . Permo-Triassic "Blueschists' i n the Chilliwack Group and the Shuksan Metamorphic Suite formed i n a west-dipping subduction zone which separated the marginal basin from western island arc rocks. Ultramafic rocks of the marginal basin, such as those i n the Mount Breakenridge Area, were metamorphosed at depths of 23'km. and now ; outcrop as pods and s l ivers within their Upper Paleozoic (?) host rocks. 154 Other less matamorphosed Alpine peridotites were enplaced along mid-Cretaceous faults and thrusts. Mineralogical study of pelitic schists in the map-area indicates that the Pa/Ms + Pa + Ph content of white mica is sensitive to metamorphic grade; plagioclase composition is controlled by host rock composition, not metamorphic grade, and that biotite and staurolite compositions are independent of host rock composition and metamorphic grade. Distribution of total Fe, Mg, Mn and Ca between coexisting chlorite, biotite, garnet and staurolite imply a close approach to chemical equilibrium. Linear regression analyses of rock and mineral chemistry indicates that rock bulk chemistry significantly controls mineral assemblages across the sillimanite isograd. The mineral assemblage biotite-garnet-staurolite-kyanite or sillimanite-muscovite-quartz-plagioclase-rutile-ilitieniteimagnetiteisphene which, in the AFM triangle represents a univariant reaction, may be the result of local buffering of fjj Q by the mineral assemblage. A plot of minerals in the tetrahedron K20-Na20-MnO-CaO reinforces this concept. In the map-area metamorphosed ultramafites are composed of talc + forsterite + tremolite ± enstatite ± anthophyllite ± magnesite. Thermodynamic calculations indicate that the equlibrium temperature of the vapour - absent reaction talc + enstatite = anthophyllite is extremely sensitive to magnesium - iron solid solution and that the dehydration equilibria forsterite + talc = anthophyllite + water and anthophyllite + forsterite = enstatite + water are insensitive to magnesium - iron solid solution so that the location in PT space of the isobaric invariant point formed by the intersection of these reactions (enstatite, forsterite, talc, anthophyllite) depends on the Mg- content of the system. The assemblage talc + forsterite + anthophyllite + enstatite noted in the ultramafites may have formed by the following reaction sequence discussed in terms of the system MgO-Si02-H20-C02 1) The isobaric univariant reaction talc + forsterite = enstatite + vapour 155 took place at 7kb. Uplift and subsequent reduction in pressure rendered this reaction metastable and anthophyllite formed by the reaction talc + forsterite = anthophyllite + vapour. Textural relationships in the ultramafites and the metamorphic history of the Mount Breakenridge Area substantiates this reaction sequence. 

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