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A study of contact metamorphism at Harrison Ridge, Harrison Hot Springs, B.C. Grove, Edward Willis 1955

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A awm of CONTACT MBTmmmim AT HABHISOH REXHB, MHBI30H HOT SpRSf&S, B.G. by EDWARD WILLIS GROVE A THESIS SUBMITTED IK PAH2II&,- FBLFmBBUT Of THS HffiiUIR»WITS FOR THE DBGKEB OF MASTER Of 3CIMCS in th© Department of Geologjr and Geography tfe accept this thesis as conforming to the standard re quire d from ©aii&i&utes for the degree of MASTISR OF SCH2IGE ..•ill « H . . I , „ , ; , ! > • „ „ . • , . . „.,..•. • I i, \ .-. -M@mh@r3 of tlM Department of GEOLOGY AND GEOGRAPHY mi mvrmaiTT OF BRITISH COLUMBIA A p r i l , 1985 mtmm A petrographic study Is mad© of th© granitic and related metamorphic rocks at Harrison Hot Springs, B.C. this thesis contains a general treatment of th® regional and structural geology of th® general area. The petrography ©f th® various rock types is discussed under two main headings? the granitic rocks, and th® metamorphic rocks. Heat given of? by the intrusive magma converted the adjacent unmetamorphosed sediments to hornfelses of th® cordlerite-anthophyllit® subfacies and pyroxene-hornfels facies. These rocks are arbitrarily subdivided Into rock types on the basis of their mineraloglcal compositions, rather than on the field occurrence. These assemblages are discussed with reference to the facies classification and the mineraloglcal phase rule. It is concluded that the hornfelses were derived by thermal stetamorphisra of aluminous-chloritic argillaceous sediments; that the cordierite-anthophyllite hornfelses were originally rich in Kg© and FeQj metasomatism was not an important process In the formation ©f th® aetaBtorphie assemblages. OHitPTSf! I 2M£ Introduction . . . . . General Statement Ideation and: Fature of Area . . . . . . . .... .... 1 1 2 Field Work and Laboratory Investigation© . . . . . 3 Acknowledgments . » . » » . * * • . • » • » • • • 3 Previous Work . . . . . . . . . . . . . . 4 CHAPTER II General Geology of the Area . . . » $ Regional Geology . . . . . . . . . 5 Regional Structural Geology . ... CBAPTEB H I Geology of Harrison Ridge Sedimentary Boeks . . . » * • • • * Age and Correlation . . • . • . » Internal Structure • » » • < » • Granitic locks , * * * * « . + * * * Occurrence . . . . . . . . . . . Petrography . # * . * . » * . » * Detailed Contact Relationships . Xeaoliths in the Granitic locks . Mode of Emplacement of the Magna • « Metamorphic locks . . . . . . . . . General Statement . . The Phase Rule and Facies Classification Conditions of Contact MetamorphlsBi . . Terminology of Contact Metamorphic Rocks Petrography . . . . . . . . Cordierite-Anthophyllite Hornfels . 10 10 10 11 12 12 22 23 24 24 30 32 33 37 Cordl*rlttt^JLnthoph^lltt«^AIaandln« Hornfsls, Cord ler it «*M t hophy 11 it «<*Ctuniiag t OR It* H o r n f t l s B o m f a l s • Cor<U«rit«*Orthi»elAS«*Blotii* 8oraf®ls • * . Biops ido-^la giocl&sA Hor:u ois QtIfiy'tlJ}l1»® **,*-* * . • « » » • * » « • * • '>natHMtolyti«' Book® • • • « . « • « • . . Haseovlta Bocks , . . « . . . , * . . * . Slssra Boates » * • . . . » < . , * , « * # CHAPTER If Discussion of Cortt&et Mdta&orphlsja at I s prison Ridge , "hyslcsl Conditions . » . Influence of S t r t u s . . . . Source of Hsat « * • « . » • « • » . « » . . • Zmm of Contact M«taft&?phiiB « » » « * « • • • • Zo&lng ia Flftgloelftse * • * « • * • « * * • « • • OvlginftX C©a|>©sitioa m& Blstrlbutlon of Mttaetorphlc ¥yp«* • • « • » « • » » » * • • • « Ketasscrphlc History? Summary . . . 0$xi«?&l Gofielasioa® . . . * . » » » * * * » # . * * # Bibliography 1. Pyrcxene-hornfels facies : AC? diagram 27a 2 , ft«l*tlontfalp of tsetaworphic fuelM to temper-ature and pressure » « • « » * * » • • • • • 27a a 4. Asphlbollt* f a c i e s t AC? diagram, rocks d e f i c i e n t lb XgO . . . . . . 35a 5. J a p h i b e l i t ® f a c i e s 1 i f ? diagr&a * . , . . . « . . 39a 6. V a r i a t i o n of optic' angle o f o o r d l e r i t e w i t h a l k a l i content » « . « • » , * . • « , » • * 39a 7. f t t r e h e d r o n i l l u s t r a t i n g wat«i>deflei®iit and #xt#ss»vat#r regions * « • • * . . • » • » * 40a 8» AsssaWagei In the excess-water and water-deficient regions i n the aystea MgO*AlgDj » Sldg - HgO • « « • • * » • * • • 40a 9. Biairaa® I l l u s t r a t i n g zoning i n plagiool*®® » . • g2a XOm Ohestioal c h a r a c t e r i s t i c s of 4o<Urlt«s and greenstones . . . . . . . . . . . . . . . . . 87a Map 1* Geological Map of Harrison Ridge (In Pocket) m s m t m Plat® . fh® south slope of Harrison lidg® (Fyontlaelae*) I * 95 H* 96 i n * • 97 If, 96 A STUDY OF COSTAC? KETAII0RPHI3I! AT HARRISOH alBQll, HAI181SOH HOT SJPK2H0S, B.C. CHAPTER 1 INTRODUCTION General Statement The present investigation was undertaken primarily to study the nature of contact metaaorphissi and related contact effect® at Harrison Ridge. Th® metamorphosed sedimentary and volcanic rocks on Harrison Ridge ar® cut by tm granitic plutons. The larger of thee®, and also th® cost Important in this study 1® exposed on th® east end of th® ridg®, ii»ediately west of Harrison Hot Springs. Although this paper Is essentially concerned %dth th© description and explanation of the features relating to the a®t®eorphi® rocks, th® local general geology i s con-sidered. The principal chapters deal with th® nature and petrography ©f th® granitic rocks, the aetaaorphie rocks, and the interpretation of th® contact zone. Briefly, the evidence points to the passive intrusion of a hot dioriti® 2 magma into a terrain of slightly metamorphosed sediments and volcanics. Diffusion of heat from th© magma caused the formation of a moderate-grade hornfels contact aureole. Location and Mature of Area fhe area studied lies on Harrison Ridge immediately south-west of Harrison Hot Springs. Harrison Ridge is isolated from the mountainous surroundings by the Harrison River on the north side, and on the south side by the alluvial plain of the Fraser River, The ridge is separated from Bear Mountain to th® east by a f l a t , half mile wide valley extend-ing from the southern end of Harrison Lake to the valley of the Fraser* The ridge trends south-westerly and is approx-imately eight miles long and from two to three miles wide. It rises abruptly from the flat valley floor to a height of over 3,000 feet. Locally, the southern end of the ridge is known as Woodside Mountain. The slopes are generally steep and rough, A cover of soil and brush is present almost everywhere in spite of the steepness of the slopes, and a l l but parts of the south-ern slopes are heavily forested by mixed conifers. Many of the slopes are covered with talus overgrown by maples and alder, In general the exposures are unsatisfactory for detailed mapping. The heavy brush and lack of continuous outcrops have handicapped th® present study, making it almost impossible to obtain complete sequences of rock specimens along the strike of the sediments as they approach the granitic contact. In addition th® study has been further complicated by the fine-grained nature of the rocks in which sedimentary or metamorphic structures are only difficultly recognizable. Field Work and Laboratory Investigations This study of contact metamorphism is largely based on the petrographic investigation of rock specimens collected by Mr. B.A* Kretz during May, 1954. The rock-forming minerals were identified by microscopic study of thin-sections and also by X-ray powder photograph methods, Acknowled gmsnts The writer wishes to express his thanks for helpful discussions and suggestions to Professor K.C. McTaggart of the Department of Geology of the University of British Columbia under whose guidance this study has been made. The writer is indebted to Mr. l.A. Kretz who kindly turned over his rock collection and field notes for study. Thanks are also due Mr. J,A. Donnan who prepared the necessary thin* sections. 4 Previous Work Much of the general area has been g e o l o g i c a l l y mapped and described i n reports by H. Bauerman (1885)> G.O. Smith and P.C, Calkins (1904), P.A. Daly (1912), N.L. Bowen (1914), C.H. Crlckmay (1929-30), and r e c e n t l y the Geological Survey of Canada has published the Hope Sheet (737k) compiled by C.E. Cairnes (1942). B.J. Burley (1954) examined in d e t a i l the Middle J u r a s s i c s p i l l i t e s and agglomerates along No. 1 Highway at the base of WoodsIda Mountain, Apart from this report no other d e t a i l e d g e o l o g i c a l work in the immediate area has come to th© a t t e n t i o n of the w r i t e r * CBAPTEH II GKNERAL GEOLOGY OF THE AREA Regional Geology The H a r r i s o n Lake area o f southern B r i t i s h Columbia l i e s wholly w i t h i n th© physiographic province of the Canadian C o r d i l l e r a , The F r a s e r R i v e r In t h i s area geograph-i c a l l y marks the boundary of th© Coast Mountains of B r i t i s h Columbia and the northern extension o f the Cascade Range of Washington S t a t e , G e o l o g i c a l l y , however, there appears t o be no s t r i c t b a s i s f o r t h i s d i v i s i o n , Crickraay (1930), concluded that the formations and s t r u c t u r e s of the Cascade Mountains and the Coast Mountains east of H a r r i s o n Lake are continuous and s t r u c t u r a l l y one. He a l s o s t a t e d that they were formed as a continuous chain e a r l y In th® Cenozoic. The Fraser V a l l e y which cuts through the Cascade Range and across i t s s t r u c t u r e s Is considered merely an e r o s i o n a l gap, not a t e c t o n i c l i n e , which had I t s o r i g i n much l a t e r than the mountains i t t r a v e r s e s * The rocks exposed In the general area, apart from the -'-recent sediments, i n c l u d e rocks ranging i n age from Late P a l e o z o i c t o T e r t i a r y . The Paleozoic rocks c o n s i s t l a r g e l y o f Carboniferous (and p o s s i b l y younger), s t r a t a assigned to 6 the Chilliwack Group* This group Is predominantly sedi-mentary and consists mainly of crystalline limestones, sand-stones, argillites and agglomerates with their oetamorphlc derivatives, and Intercalated volcanic rocks. Other Carbon-iferous rocks include acid lavas and basic intrusions, and the Hozameen Group, This last mentioned group has been dated as "probably Carboniferous or Permian," and consists largely of cherts, argillites, phyllites, and limestones and intercalated volcanics, fhe fries ozoic rocks range in age froa Upper Triassic to Lower Cretaceous and are regionally the most abundant. The notes on the Hope Sheet (737A) describe these rocks as follows. The Upper Triassic includes the Cultus Formation which is essentially sedimentary, and the Tulameen group of schists and argillite. Middle Jurassic strata which con* sist largely of tuffs and agglomerates are conformable on the Upper Triassic sediments. On Harrison Hidge th® Middle Jurassic lavas are overlain conformably by Middle and Uppar Jurassic tuffs and tuffaceous argillite. These are succeeded, unconforaably, by the Upper Jurassic or Lower Cretaceous Agassiz Croup. The contact met amorphism of the Agassiz Group sediments will be considered in som© detail in this paper. A stratlgraphie table adapted froa C.H. Crlckmay (1930) showing his subdivisions of the rocks around Harrison Lake will be found on the following page. tmm m FOBK&TXOKS mmmm mMMi Lowe? Gmtmmm® } Brafcaatock 8111 ) Penningala . Spper Jlmursle M i d i © J u r a s s i c ) Triasste Agassis? Prairie Billhook Hystericus Cr»ek ) Eobo Island } Harrison Lake !toi*am*d rocks ) at Cent Cova aggloawjratas sandst©s# a r g i l l l t e eongloaarats toff a r g i l l i t ® t u f f s , ss. etc. agglomerates and lavas various 1,200 5*000 3*000 1,000 2t5©0 2,700 9,200 3*000 Aucalla crasslcollls Au©«lla ©aoadtaaa taMMtrdloes-ras perriai "Cadoeeras" sp. *Ca4oe«ra»* seJbttldtl L i l l o e t t i a l i l l o e t e n s i s Cylindr© tent his tbamls T o t a l Tnicfctt«ss 32,000 feet, >3 8 Igneous rocks found In th© area range in age from Jurassic to Cenozoic. The Jurassic Custer granite-gneiss and the Slesse horablende-diorite form a minor portion of th® granitic rock exposed in the area. The greater part of the granitic rocks are Post-Lower Cretaceous granites, grano-diorites, quartz-diorites, and diorites. Small bodies of quartz-diorite or diorite, of Cenozoic age are found in the area. On® of these, lies across the Harrison River at the south end of Harrison Lake, and extends south-easterly to the north of Agassiz, and according to Crickaay (1930), crosses the Fraser River Valley at Cheaa View and continues Into the mountains beyond Wahleach Lake. The portion of this stock exposed on Harrison Rldg© will be considered in some detail in the following portions of the paper. Regional Structural Geology: The following discussion of the structure of the region has been taken largely from C.H. Crickaay (1930)* The area under discussion lies approximately seven miles from the eastern contaet of th© Coast Range Batholithie complex, Th© structures of the Coast Mountains and the Cascade Range east of Harrison Lake are continuous. Th© folds, faults, and foliation structures trend generally north-westerly through the area, but swing to th® south In th© 9 southerly sections, Crickmay considered the folding of the region to be associated with the Laramide orogeny, and to p a r a l l e l approximately the eastern edge of the Coast Range intrusIves west o f Harrison Lake* Crickmay expressed the b e l i e f that Harrison Lake l i e s mainly along the trace of a large thrust f a u l t , called the Harrison Lake Overthrust. He postulated thrusting of the formations against the Coast Range Bathollth. This over-thrust apparently dips steeply to the east. Crickmay be-lieves the maximum displacement o n this overthrust was about 10 miles. This overthrusting is described i n part as a simple break as at the south end o f Harrison Lake, and partly a series of breaks separated by shuppen. These structures were suggested by f o s s i l evidence on Long Island i n Harrison Lake. Lower Cretaceous and e a r l i e r bads appear to have been deformed to an almost equal extent, although the pre-Jurassie formations exhibit a greater degree of alteration. The Tertiary formations, however, are almost undeformed. Thus the p r i n c i p a l periods of deformation appear to have occurred in Upper Cretaceous or Early Tertiary times, CHAPTER III GEOLOGY OF HARRISOK RIDGE Sedimentary Rocks The sedimentary series exposed on Harrison Ridge Is composed of a great variety of sediments* This succession i s known l o c a l l y as the Agassiz Group. The Agassiz Group consists of a basal conglomerate about 3*000 feet thick -Crlekmay's lent formation, which Is overlain by 5,000 feet of sandstones and a r g i l l i t e s * Crickaay's Agassiz P r a i r i e . This group of sediments succeed unconformably Upper Jurassic volcanics and sediments - Crickmay's Billhook formation. Age and Correlation of the Sediments The age of the Upper Jurassic Billhook formation was determined by i t s unconformable relationship to underlying Middle Jurassic volcanics exposed on Harrison Ridge and on the wast side of Harrison Lake, and by f o s s i l evidence. The f o s s i l s , genus Cadocsras, were found by Crick»ay and others mainly on the north slope of Harrison Hid.;;a above the- Harrison River, and -also on the western shore of Harrison Lake immed-i a t e l y north of the outlet of the lake. On Harrison Ridge these early Upper Jurassic tuff s and a r g i l l i t e s are succeeded unconf©ratably by the Upper 11 Jurassic or Lower Cretaceous Agasslz Group. I d e n t i f i a b l e f o s s i l s have not yet been found i n these rocks i n t h i s l o c a l -i t y , but they have been c o r r e l a t e d with a group of s i m i l a r a r g i l l a c e o u s beds found on the western edge of Cascade Pennin-sula In H a r r i s o n Lake. Thasu a r g i l l i t e s c o n t a i n an Upper Jurassic ammonite Anacpr d ioe eras perrlnl (Crickmay 1930). fhe basal conglomerate member of the Agassis: Group i s con* sidered by C a i r ties (1942), to represent an e r o s i o n a l i n t e r v a l probably talcing place during part o f Upper Jurassic time. I n t e r n a l S t r u c t u r e o f the Sediments The determination of the structure i n these rocks i s d i f f i c u l t . F i r s t , the sediments of the Agasslz Group are e i t h e r massive, with l i t t l e evidence of bedding as i n the thick basal conglomerate, or they are t h i n l y bedded with no recognizable key ho r l s o n s . Secondly, tho sediments of this group have high d i p s , and although c l e a r evidence of overturning and complex folding was not observed, other com-plications such as l o c a l i s o c l i n a l f o l d i n g have been noted. The t h i r d d i f f i c u l t y a r i s e s from the fact that the rocks are nowhere s u f f i c i e n t l y exposed. Fourth, the sediments have been metamorphosed with the r e s u l t that the rocks are out-wardly vary s i m i l a r ©specially i n the zone of intense meta-morphism. F i f t h l y , the sediments now appear as an isolated unit with i t s r e l a t i o n s p a r t i a l l y o b l i t e r a t e d by g r a n i t i c Intrusions and by erosive processes, 12 For these reasons, the detailed structural picture of th© sediments and volcanics i s as yet:Incomplete. fhe structure of the rocks as interpreted by the author Is as follows, fhe members of the Agasslz Group strike north 20 west and dip steeply east. The st r i k e varies from about I 40° ¥ to almost due north at the main contact, fhe rocks exposed on the southern slopes of Harrison "Ridge are an apparently conformable sequence grading froa west to east through a thick massive conglomerate, through arkoses, Into a thick series of thinly bedded a r g i l l i t e s , qu&rtzites, tuffs and scattered calcareous sediments, fhe Agasslz Group succeeds unoonformably a group of massive to thin bedded volcanic rocks called the Billhook by Crickmay. fhe structure of this formation car. be observed best on th© point northwest of Harrison Hot Springs, fhe volcanics here have a general low 20° - 30° southwesterly dip and i n general they s t r i k e i n a northwesterly di r e c t i o n , fhe portion of this formation vest of the main d i o r l t e con-tact Is only s l i g h t l y known but the formations appear to str i k e northwesterly and clip gently eastwards into the contact. Granitic Bocks Occurrence Two outcrop areas of granitic rocks have been ex~ amined on the east end of Harrison Ridge (see Map 1), fhe largest of th© two appears on the Geological Survey of Canada's Hope Sheet (727A). The smaller was found s l i g h t l y west of the main ssdirr-snt-rranitic contact on the southern slope of the ridge. If the portions of the stock on north side of Harrison Biver and southeast of Harrison Hot Springs are alt© considered the body i s e l l i p t i c a l in shape. The major axis of the stock trends In a northwesterly direction, and i s roughly p a r a l l e l to the s t r i k e of the sediments heading into the contact. Crickmay (1930) considers this body to be part of the northern extension of a large Cenozcic stock p a r t i a l l y exposed In th© mountains around Wahleaeh Lake, On the Hope Sheet (737A) the two have been differentiated by intrusive relationships which suggest th® Harrison body is younger than the mass around Vahleach take. In plan the smaller of the two granitic bodies i s e l l i p t i c a l . The axis of this stock also l i e s p a r a l l e l to the trend of the sediments. Petrography The main mass Is composed of a medium-grained blotite-hornblende d i o r i t e , consisting of b i o t l t e , hornblende, and andesine-labradcrite plagioclase, with subordinate quartz, apatite, r u t i l e (?), and apatite. In some of the specimens examined minor amounts of pyroxene were found assoc-iated with the amphlbole. 14 ?ery l i t t l e variation In grain size or mineralogy was observed throughout the part of the stock examined on Harrison Ridge, In general the diorlte has widely spaced horizontal jointing and dykes or quartz veins are rare. Surface outcrops of the diorlte are usually weathered to a depth of one-half Inch or less, but some specimens of the rock have been deeply weathered to a ehalky-whlte friable rock. This process has apparently caused the partial break-down of the feldspar to clay minerals. The mafics do not appear to have suffered as extensively as th® feldspar. The fresh diorlte varies in color from a creamy-gray to a greyish-white, the latter predominating. The mafics together for® about 20 to 30 per cent of the rock, and the remainder Is largely feldspar. In the specimens of the diorlte examined, the feldspar is almost Invariably andesine-labradorlte. The feldspar is generally clear and colorless a l -though in some sections masses of hair-like inclusions of unknown composition lying in definite directions were found, Zoning is not exceptionally abundant in th© plagio-clase, but both the continuous and oscillatory types have been observed. Less than five per cent of the feldspars are zoned and of these the greater proportion are of the oscilla-tory normal type that is the zones become progressively more sodic outwards froa the cores. Th© range In composition was found to be fro© Ab5o M 5 0 in the core to Ab&5 An^tj at the rims In the crystals studied. So®® of these zoned grains 15 contain narrow bands of ser i c l t i e material marking uncon-formities within the grains, Alblte or multiple twinning is invariably developed in a l l but the well gonad plagio-clases, and Pericline and Carlsbad twinning are in general rare. The Plagioclase crystals commonly show a poor to well developed protoelastic texture. This is readily observable where th® grains are surrounded and veined along irregular fractures by plagioclase of a more sodic composition than the main grains or by quartz and potash feldspar. This feature of rimming by material of a different composition is very common in these rocks. Commonly the contacts on the borders of the plagioclase grains have extremely irregular myrmekitic zones. The fracturing of the plagioclase feld-spar does not extend into the i n t e r s t i t i a l minerals. The feldspar content of the diorite varies from 60 to 70 per cent of the rock. Only small amounts of potash feldspar have been found. Quartz is present in a l l specimens and may locally amount to more than 10 per cent of the diorite, although i t Is usually less. It is i n t e r s t i t i a l and xenomorphic with respect to tho other minerals. The individual grains are clear and colorless, lack strain shadows, and are unfraetured. Th® ferromagnesian minerals found in th© rooks i n -clude hornblende, b i o t i t e , c h l o r i t e , and pyroxene. The hornblende Is of the dark brown variety. The pleochroic scheme i s entirel y in shades of brown, the absorp-tion greatest p a r a l l e l to Z and least p a r a l l e l to X, The amount of hornblende appears to range from 10 to 20 per cent of the d i o r i t e . In some specimens simple twinning was found to be common i n the hornblende. The habit of the hornblende i s i n general prismatic but cr y s t a l faces are not commonly well developed. It forms crystals up to 3 ma i n length p o i k i l i t i c a l l y enclosing smaller anhedral crystals of plagio-clase, cllno-pyroxene, and accessory minerals. A few grains of a colorless hornblende were found in one Specimen of the d i o r i t e taken from the contact. In part this colorless variety was found intimately associated with brown hornblende crystals, and i n part as separate masses. It forms the bulk of the amphibole found i n this contact specimen. This mineral has inclined extinction, ZA.C * 1 8 ° , moderate birefringence, moderately high (*) ve r e l i e f , length slow, 2? a 80° o p t i c a l l y positive, and i s commonly coarsely twinned. The habit i s prismatic and more fibrous than the brown hornblende. This mineral was tentatively Identified as cuamingtonlte rather than treisolitej because of i t s large optic angle and positive sign. Th© clino-pyroxene i s a pale green to colorless variety and is found In some specimens as cores or inclusions in brown hornblende. These inclusions are usually surrounded by colorless reaction rims in the hornblende. Many of the pyroxene grains exhibit a linear structure which appears to be due to Inclusions of thin plates of an acicular colorless anistroplc mineral, and grains of a black isotropic mineral, possibly magnetite, arranged parallel to the cleavage dir-ection. This pyroxene constitutes only a minor portion of the rock, and was not found In a l l sections* The biotlte which constitutes from 10 to 20 per cent of the diorlte, is a dark brown variety. It is generally scattered at random through the rock. The pleochrolsm in the biotlte Is as followsJ X * greenish-yellow, Y « Z = dark brown. In some specimens the biotlte is partially altered to chlorite, probably the variety penninite. The accessory minerals in the diorlte ar© apatite, magnetite, pyrite, epicote, sphene, and rutlie (?). Apatite Is common In a l l thin-sections. It exhibits its hexagonal form in basal sections, in some thin-seetions the apatite grains are up to 5 mm long, and are pleochroic. The pleochrolsm is confined to the central portions of the grains. The pleochroic formula for these areas appears to be 0<E, and the intensity ranges from a bluish-black to a brown-ish-black. This pleochrolsm may be due to numbers of minute Inclusions confined to the central portions of the grains. 18 Sphene has been found as scattered irregular grains associated with the mafic minerals i n only a few specimens. Magnetite Is present i n most of the rocks primar-i l y associated with b i o t l t e and hornblende as Inclusions. The grains are small and irregu l a r . In some thin-sections the magnetite made up approximately one per cent of the d i o r l t e . Pyrite Is rare i n these rocks but some small grains were found. The pyrite occurs as the cores of larger magne-t i t e grains and also as separate small anhedral grains. Epidote i s sporadic in Its occurrence. It was found associated with b i o t l t e In specimens of the d i o r l t e from the contact zone. Long, acieular c r y s t a l s , possibly r u t i l e , were found scattered throughout the mafles and feldspars of the rocks i n minor quantities. The texture and granularity of the d i o r l t e does not vary appreciably throughout the main body. In general the grain size i s medium, but coarse and fine variations are present. Small rounded Inclusions of coarse-grained aiaphi-b o l i t e are present but only i n small amounts. The average grain Size of the d l o r i t e appears to be about 4 to 6 mm. The texture is generally hypidiomorphic granular and somewhat p o i k i l i t i c . P o i k i l i t l e feldspar Is present but not common. Hornblende and b i o t l t e enclose p o i k i l i t i c a l l y 19 small grains of feldspar, quartz and the accessory minerals, Both the mafics and the feldspar are randomly oriented and lineatlon or f o l i a t i o n are both missing i n the d i o r l t e . There i s a suggestion however, of f o l i a t i o n in some of the dark fine-grained inclusions in the d i o r l t e . Hare the mafics are aligned parallel to the xenolith borders. This structure i s only weakly developed and not present i n the majority of the inclusions examined. The basic nature of the feldspar, the moderate pro-portion of dark minerals, the small amount of quartz, and the general preponderance of hornblende and b i o t l t e j u s t i f i e s the c l a s s i f i c a t i o n of this granitic rock as a b l o t i t e -hornblend e-diorite. The small granitic body previously mentioned, Is ©f e s s e n t i a l l y the same nature as the above d i o r l t e . However, several minor variations have been observed. The border zone Is composed of a darker rock of finer grain than the central portion. This zone does not appear to be continuous along the periphery, and does not see®, to extend more than 20 to 30 yards Inwards from the contacts. In this dark border phase the b i o t l t e constitutes from 10 to 20 per cent of the rock. It Is a dark brown variety with pleochrolsm as follows! 1 • yellowish, Y • % * dark brown. Apatite inclusions are common i n the b i o t l t e , A small proportion of the b i o t l t e i s altered to chlorite (pennlnite). 20 The hornblende Is a dark green variety with a pris-matic habit moderately developed. The hornblende constitutes from 20 to 30 per cent of this rock. Together the maflcs comprise on th© average from 40 to 50 per cent of the diorite. The hornblende commonly has a mottled appearance due to a variation in th® color of the grains. This Is most noticeable where the hornblende is intimately associated with chlorite and biotite. Many inclusions of needle-like rutile (?) grains are present in the hornblende, as well as scattered magnetite grains, The feldspar of this border phase is a Labradorite -Ab^e; A*1^) which is somewhat more calcic than that of the main body. It is clouded with numerous small inclusions. Zoning in the feldspar is common and very well developed, T&e observed variation from core to rim i n some of the grains was found to range from Ab^ Ang^ to Abgsj Narrow zones of sericlte-like Inclusions are commonly present in these zoned plagioclases marking compositional unconformities. Many of the plagioclase grains are surrounded by narrow rims of untwinned feldspar noticeably more sodlc than the enclosed grains. This material often veins th© plagio-clase grains along fractures. The protoclastle texture of the feldspar observed In the main bo«y of diorite is also commonly present In this rock, Quartz forms up to 10 per cent of the rock. The 21 individual grains ar© usually free of s t r a i n shadows and show l i t t l e or no fracturing, fhe accessory minerals i n this d i o r l t e ar© apatite, magnetite, and r u t l i e (?), fhe small body of d i o r l t e Is generally a medium-grained hypldiomorphic granular assemblage of mafics and plagloclase. fhe mafics appear to be p o i k i l i t i c , i n general, and are randomly oriented. Bo d i r e c t i o n a l textures such as f o l i a t i o n were encountered throughout the mass, fhe s i m i l a r i t y of the two bodies both texturally and miner&logically and the close s p a t i a l relationship indicates a similar derivation. It i s quite possible that they are i n fact part of a single body which extends to the west under the sediments, Detailed Contact Relationships fhe mstazaorphic-gr&nitie contact, not very well ex-posed, seems to be. sharp (Plate I, a), Hornfelses distant froa the contact are commonly greenish-black, or bluish-black but at a distance o f about 150 yards on the average the metamorphies take on a d i s t i n c t reddish color, f h l s color persists, with l o c a l variations, up to the granitic contact, fhe d i o r l t e , however, remains more or less unchanged. A variation of this color effect was found near the small d i o r l t e stock. Here the hornfelses are a buff-whit© 22 at the margin of th© d i o r i t e and gratia outwards to the normal dark color over a distance of about 300 feet. These 'bleached' hornfelses are also noticeably vuggy and appear to contain a large proportion of white mica, suggesting intense pneumatolytic alt e r a t i o n by th© granitic intrusive. Xenoliths i n ths Oranltlc Rocks In general, d e f i n i t e l y recognizable xenoliths are not abundant, an Intensive search of the greater part of the area disclosing only a scant half dozen. These were commonly a foot or less i n length, although one body was seen which was of much larger dimensions. Bone of these hornfelsic xenoliths was available for thin-saction study. They appeared to consist of a reddish, raed11 jm-r r a in ed aggregate of b i o t i t e , quartz, and feldspar, l o c a l l y f o l i a t e d . These In-clusions e s s e n t i a l l y resemble the inetamorphic rocks found at or near the main contact. Much more abundant than the above are ghost-like inclusions. These are rounded, dark, fine-grained granitic masses which might be interpreted either as inclusions or as fine-grained phases of th© d i o r i t e . In places, they appear to comprise up to 15 per cant of the d i o r i t e . These xenoliths, i f they can be called such, range i n size from an inch to several feet i n diameter. Their outline against the leucocratle d i o r i t e i s apparently commonly rounded and smooth, They are composed of a granoblastic aggregate of 22 fIne-to-medius^gr&ined b i o t l t e , hornblende, and feldspar. In thin-section this material is an ldiomorphie mosaic of the above minerals plus a minor quantity of xenoblastic quartz. This rock Is ess e n t i a l l y equivalent to the d i o r l t e host except for a larger proportion of mafics, The feldspar has an average composition of Ab^ 2 An4 3 , and i t i s commonly zoned. It may be that these rounded bodies ar© only a d i s -persed, more c a l c i c phase of the d i o r l t e , but the p o s s i b i l i t y exists that they are Inclusions of almost entir e l y recon-stituted sediments. Mode of Emplacement of th© Magma F i e l d and laboratory investigations Indicate that the d i o r l t e was "disharmonious" with the surrounding country rocks. The term, "disharmonious", as defined by Walton (1955) refers to the discontinuity in the temperature of a magma and i t s surrounding as compared to "discordant", which refers to a structural discontinuity. The evidence for the disharmony of a pluton i s represented i n low temperature country rocks by a v e i l defined, high temperature, contact metamorphic aureole. The development of this aureole i n -dicates that the thermal energy of the magma was greater than that of the country rock* Walton suggests that strong disharmony with low temperature country rock i s proof of the magmatic emplace-ment of a granite. 24 The disharmony o f the d i o r i t e on Har r i s o n Kiclge w i t h till® r e l a t i v e l y umaatamorphoaed Agassiz Group i n d i c a t e s a magmatie o r i g i n f o r the p l u t o n . The method of emplacement o f the d i o r i t e must how-ever, b® l e f t to s p e c u l a t i o n * I t i s evident though, t h a t the magma exerted no l a r g e deforming s t r e s s e s on the w a l l rocks. The absence of p e r i p h e r a l s c h i s t o s i t y or g n e i s s i c s t r u c t u r e , and the discordance of the contacts are evidence f o r the absence of f o r c e f u l i n j e c t i o n . The p o s s i b i l i t i e s f o r emplacement seem t h e r e f o r e , to be reduced to those of a passive or quiet r i s e of magma by a process o f st o p i n g or by a s s i m i l a t i o n . The presence of small angular i n c l u s i o n s i n the d i o r i t e i n d i c a t e s that p l a c e -meal s t o p i n i was e f f e c t e d t o some extent. However, these statements are based upon the assumption that the w a l l rocks were n e i t h e r e n t i r e l y d i g e s t e d , nor were the o r i g i n a l e f f e c t s impressed upon the w a l l rocks destroyed. Metamerpbie Rocks General Statement The-term Bmetamorphlc M, was f i r s t introduced In 1883 by L y e l l to whom i t meant "transformed 1', and to whom ^ i n d i c a t e d a c l o s e a s s o c i a t i o n with igneous a c t i v i t y . In the Principles of Geology i t i s s t a t e d t h a t , i n jsetamorphism, 2J The transmutation has been affected by th© Influence of subterranean heat acting under great pressure, and aided by thermal water, or steam, and other gases permeating the porous rocks, and giving r i s e to various chemical decomposition and new com-binations. In 188?, Rosenbusch published his c l a s s i c account of the thermal metamorphic aureole around the Barr-Andlau gran-i t e in the fasges, His findings Indicated that there had been l i t t l e or no transfer of material into the surrounding country rocks and he then applied this conclusion to a l l granite contacts. In modern usage metamorphism applies to trans-formations i n rocks without melting and exclusive of th© processes of weathering and sedimentation (Barth, 1952). Thermal or contact metamorphism with which the remainder of this paper i s l a r g e l y devoted, involves a chemical recon*» stitution of rocks primarily i n response to a change i n temperature, other conditions such as confining pressures, and metasomatism exert only a minor Influence. The Phase Rule and Pacies C l a s s i f i c a t i o n Goldscbmidt was f i r s t to rscoaiize that i f the miner-a l assemblages of meta&orphic rocks were to approach e q u i l i -brium, they must confora to the acquirements of the phase rule . For a system In equilibrium the following reaction holds between the number of coexisting phases, p, components c, and degrees of freedom, fit 26 p • C 2 - f (1) This is th® phase rule of M i l l a r d GIbbs (1874). This phase rule places a l i m i t on the number of minerals which can occur i n equilibrium In a given rock. In such studies the maximum number of phases can be obtained only i n an invariant systems one in which both P and T are fixed invariently, corresponding to a single point on a P-T diagram. However, during the processes of metamorphlsm P and T are believed to be variable, f i s equal to 2 and equation 1 reduces to p » c ( 2 ) , This equation has been called Goldschmidt's "mineralogical phase rule". According to this r u l e , the number of different minerals i n a rock should not exceed the number of components. Actually the number of minerals may be less because of s o l i d solution. Ooldsehmidt (in Turner and Verhoogen, 195D applied this p r i n c i p l e of the phase rule to th© high temperature hornfelses of the Oslo region where he observed certain minerals occurring i n various combinations. The composition of these assemblages was stated In terms of the ten main constituent oxides: S10 2, T10 2, AlgO^, Fe 20^, FeO, Mg©, Cao, Ka 20, K20 and H20. In these rocks, Ra 20 always entered into plagioclase, Fe 20^ i s always capable of being accomodated in place of A1 20^, as i n co r d l e r i t e , FeO i s capable of sub-s t i t u t i n g for Kgo, and assuming ( S i , T i ) 0 2 Is one component, six Independent components can be selected. Thus by the mineraloglcal phase rule th© number of any associated mineral phases should not exceed s i x . This requirement was met i n the Oslo hornfelses, where assemblages of two to s i x minerals were found. This suggests a general attainment of equilibrium. Ooldschmldt ( i n T and V», 1951) c l a s s i f i e d the horn-felses In terms of three components, CaO, (Mg, Pe)0 and AlgO^, fhe possible assemblages are shown In the following ACF diagram, Figure l j quartz may be an additional phase i n each ease, orthoclase Is a possible f i f t h , and b i o t l t e may appear i a a l l but the most c a l c i c rocks. T i l l y has added two noncalcareous assemblages (IA and IB) In his account of the Comrie area. Ooldschmidt succeeded in show-ing that seven minerals could form that would be stable under the conditions resulting from eontact metamorphismj andalusite, AlgSiO^i c o r d l e r l t e , Mg2Al4 S i ^ O ^ j dlopside, Ca (Mg,Fe) S l 2 0 o ? grossularite, Ca^ A l 2 S i ^ 0 1 2; wollastonite, CaSiO^? hypersthene, (Mg, Fe) SiO^; anorthite, Ca A l 2 S l 2 Og. However, of these seven, only three or less can occur together i n addition to quartz b i o t l t e and an a l k a l i -bearing mineral* fhe combinations of these minerals, corresponding to hornfelses, are shown i n the preceding ACF diagram. Muscovite and amphibole are notably absent from these rocks, but the former may occur as a product of retrogressive metamorphlsm., Sskola (In T. and V.) o r i g i n a l l y defined five metamorphic facies* the sanldinite (low pressure), hornfels I 27a W o l l a s t o n i t e D i o p s i d e H y p e r s t h e n e F i g . 1. P y r o x e n e - h o r n f e l s f a c i e s : A C F d i a g r a m f o r r o c k s w i t h e x c e s s S i 0 2 . ( a f t e r V . M. G o l d s c h m i d t . ) F i g . 2. R e l a t i o n o f M e t a m o r p h i c Facies and I g n e o u s F a c i e s ( g i v e n in i t a l i c s ) t o T e m p e r a t u r e and P r e s s u r e A c c o r d i n g t o P . E s k o l a , 1939 T e m p e r a t u r e i n c r e a s i n g D e v e l o p e m e n t o f z e o l i t e s i n i g n e o u s r o c k s S a n i d i n i t e f a c i e s ( d i a b a s e f a c i e s ) G r e e n s c h i s t f a c i e s K p i d o t e - a m -p h i b o l i t e f a c i e s A m p h i b o l i t e f a c i e s ( h o r n b l e n d e -g a b b r o f a c i e s ) P y r o x e n e - h o r n f e l s F a c i e s ( g a b b r o f a c i e s ) G r a n u l i t e f a c i e s G l a u c o p h a n e - s c h i s t f a c i e s E c l o g i t e f a c i e s ( e c l o g i t e f a c i e s ) 28 (moderate pressure), and ecloglte f a d e s (high pressure), a l l representing high-temperature metamorphlsffij the amphi-b o l i t e (moderate temperature), and greenschist facies (low temperature), corresponding to moderate pressure conditions* The relationship of these facies with reference to temper-ature and pressure can be seen in Figure 2, Bskola's scheme has been expanded and modified by Turner to give the follow-ing c l a s s i f i c a t i o n , adopted In this papers 1. Sanidinite facies s maximum temperatures and minimum pressures (pyrometasoma11sm). 2. Pyroxene-hornfels facies s high-temperature contact metamorphism. 3. Amphlbollt© facies i moderate temperatures and high pressures. (a) Cordiorito - anthophyllite subfacies % moderate-temperature contact metamorphlsm. 4 . Albite-epidote-amphibolite facies : moderate to low temperature, high pressure, 5. Greenschist facies : low temperature, moderate pressure, 6. Oranulite facies J deep-seated regional metaraorphism at very high temperature and pressure. 7. Ecloglte facies : deep-seated metamorphism at very high temperature and extreme pressure, A metamorphic facies has been defined by Turner and Verhoogen (1951) to include a l l rocks, of any chemical com-position, and hence of widely varying mlneralogical compos-i t i o n , which have reached chemical equilibrium during metamorphlsin under a particular set of physical conditions. In addition, for a given metamorphic facies the mineral assemblage that develops In a rock of given chemical compos-i t i o n Is constant. Assemblages of minerals that are stable over a wide rang© of temperature and pressure may belong to several aetamorphlc facias. For © precise d e f i n i t i o n of a facies a c r i t i c a l sensitive mineral assemblage, stable only within r e l a t i v e l y narrow H a l t s of temperature and pressure, i s selected, fhe facies so defined Is named after the c r i t i c a l rock type or aft e r a c r i t i c a l mineral assemblage. Bocks belonging to the same metamorphic facies are termed i s o f a c i a l or Isograde (isogradic)• fhe l a t t e r term was suggested by f i l l a y to indicate that the rocks i n question had attained the same metamorphic grade. fhe facies c l a s s i f i c a t i o n i s founded upon the petro-graphic experience that many mineral assemblages in metamor-phic rocks, though by no means a l l , obey th® laws of chemical equilibrium^ and the recognition of ideal equilibrium assemblages i s essential In applying and extending th© c l a s s i -f i c a t i o n * In this paper the study of the hornfelses w i l l be limited to the Inner contact rocks. It Is i n these horn-felses that true equilibrium conditions w i l l have been most closely approached. In subjecting the hornfelses of the Inner contact zone to the facies c l a s s i f i c a t i o n a l l secondary minerals must fee excluded, and only those minerals belonging 30 to a common sphere of di f f u s i o n can be considered as part of a single reacting system, fhe sphere of diff u s i o n i s of course d i f f i c u l t to acess. The assumption made her© is that th© range of d i f f u s i o n between essentially s o l i d phases under th® influence of thermal activation i s very limited, Harker ( 1 9 3 2 ), has pointed out that in metamorphic rocks which have retained their o r i g i n a l sedimentary structures the l i m i t of dif f u s i o n must have been extremely narrow, Bowen (in T, and ¥„) has found that the diffusion coefficients for s i l i c a t e s are only i n the order of 10 or 10 ' em, /see, at 1500°CU Diffusion in s i l i c a t e minerals seems to be excessively slew even at jaetamorphic temperatures. Apart from these considerations, the fact that many metamorphic assemblages can be f i t t e d Into the facies scheme indicates that, i n general, equilibrium i s frequently approached i n th© inner zone of hornfelses. Conditions of Contact Metamorphlsm The fundamental factors controlling contact or thermal metamorphiam are pressure, temperature, and t o t a l composition at the time of metamorphism. The Influence and a c t i v i t y of water and other volatile© must also b© considered, Two kinds of pressure most be considered? confining pressure, or equal pressure, and directed pressure or ©tress. In thermal mataaorphlsm th© absence of directed pressure is usually considered to favor the growth of th© so-called 31 antlstress minerals eordierite and andaluslte whose fields of s t a b i l i t y are apparently restricted by nonhydrostat ic stress* The influence of temperature is recognized to be the most Important factor i n thermal or contact metamorphism. Many of the transformations Involved in thermal metamorphiSHj are s p e c i f i c a l l y i n f l u e n c e d and controlled by temperature, In most casus the solubility of s o l i d s i s i n -creased with rising temperature (T. and 7. 195D* In addition the rates of reaction or diffusion varies directly with increasing temperature. The importance of bulk composition Is evident from the d e s c r i p t i o n s of the hornfe lses of the contact a u r e o l e . In Eskola's original explanation of the facias principle i t was assumed that the reacting system was i^ocheialeal, dis-regarding the water content, loder ( 1 9 5 2 ) has taken a definite stand against this concept and pointedly emphasised that It is not valid. He found that changes of only a few per cent in bulk composition, Including water, may produce great changes In mineralogy, He has emphasised that what had been interpreted by many authors as pressure and temper-ature changes may be the result of a change in bulk compos-i t i o n . With this emphasis on composition he does not f i n d It d i f f i c u l t to accept the occurrence of eclogite with amphlbolite or lower grade rocks. He pointed out by way of 35 i l l u s t r a t i o n that i n material of th© same composition, as the water content Is increased the sequence of changes oclogite to amphibolite to green schists takes place without pressure or temperature changes, Terminology of Contact Metamorphie Rocks In the older l i t e r a t u r e hornfelses are usually referred to as chorty-lookin.? rocks* A hornf els as defined by Holmes (1920) Is as follows: A contact metamorphosed rock, usually of speckled granular appearance, but not t y p i c a l l y schistose, nor s t r i c t l y granulosa, consisting essentially of quartz, micas, and feldspars, with or without garnet, andalusite, or cord i e r i t e , and more rarely pyroxene or araphlbole. Any cleavage or Incipient sehistosity possessed by the parent rock i s o b l i t -erated by a naw structure which may bo described as maculosa, In l a t e r descriptions the term, hornfsis (or hornstone), has been extended to include not only cherty-looking rocks with no s c h i s t o s i t y , but also those with a granoblastlc texture, Shand (1951) has described the hornfels as being a fine-grained or dens© rock requiring the microscope for i t s study. It generally lacks schistosity, f i s s i l i t y , or lamination, breaking with an irregular splintery fracture, The average size of the particles i s 0.1 to 0,2 mm. Under the micro-scope the rock i s made up of Interlocking grains of a uniform size which are commonly p o i k i l i t i c (or have sieve texture). These characters are according to Shand typical of "horn-stone structur®tt. 33 Grout (1933) suggested that the term* hornf e l s , covers a wide range of contact r o c k s , some coarse, some f i n e , soma massive, others s c h i s t o s e . However, he r e s t r i c t e d the term to exclude "schistose hornfelses," i n d i c a t i n g that I f so f a r extended, i t would lose I t s value as a term f o r the massive, coarse-grained contact rocks. This preceding d e f i n i t i o n i s e s s e n t i a l l y that i n which the term h o r n f e l s , w i l l be used In this paper. Petrography The rocks of the inner zone of the Harrison contact are with a few notable exceptions, essentially f r e e - s i l i c a , potash-poor hornfelses showing v a r i a b l e development of g r a i n - s i z e and o b l i t e r a t i o n of primary sedimentary f e a t u r e s . The hornfels specimens c o l l e c t e d from this zone have been c l a s s i f i e d on the basis of mine r a l o g i c a l cc ^ positions Into the f o l l o w i n g assemblages* 1, C o r d l e r l t e - a n t h o p h y l l i t e - b l o t i t e - p l a g i o e l a s e - q u a r t z . 2, C o r d i e r i t e - a n t h o p h y l l i t e - b l o t i t e - g a r n e t - p l a g i o c l a s e - q u a r t z , 3, Cordierite-anthophylllte-cujflMlngtonlte-plagloclas©-quartz, 4, Cordierite-:>:>iscovit e - b i o t i t -?-pls gioclase-quartz. 5, Cordierlte-muscovite-blotite-gamet-plagioclase-quartz. 6* Cor&lerite-orthoelasa-biotite-plagloclase-quartz. 7. Diopslde-plagioclase. 0. Quartzite (quartz-amphibcle)* These mineral assemblages w i t h some exceptions, are 34 t y p i c a l hornfelses of the amphibollte f a c i a s . Further, they can be generally grouped i n the cordierlta-anthophy-l l i t e sub*facies. This facies corresponds to medium grade contact matamorphlsm (outline on p. 28 ) . The maximum, temperatures attained i n this facies appears to be around 500°C (Barth, 1952), The relations between the chemical and mlneraloglcal compositions i n th© amphibolite facies were f i r s t described i n Eskola's c l a s s i c a l Investigations of the O r i j a r v i region of southwest Finland (Eskola, 1914), In this facies amphi-bole always appears I f the i n i t i a l rock composition permits. The formation of the minerals of the amphibolite facies depends largely upon the I n i t i a l bulk composition of the rock. The metamorphic temperatures indicated at O r i j a r v i appear to be lower than i n the pyroxene-hornfels facias attained at both Comrie and Oslo, Many of the O r i j a r v i rooks are composed of what are usually regarded as contact assemblages, including commonly such antlstress minerals as andaluslte and co r d i e r i t e . Turner and Terhoogen (1951) have adopted the O r i j a r v i region as the type l o c a l i t y for the cordierite-anthophyllite sub-f a c i e s , Th© development of this assemblage appears to be conditioned by stress conditions of a low magnitude. The characteristic members of this subfacies are hornfelses, skarns, and other contact rocks containing either hornblende or muscovlte as Important constituents. 35 The p o s s i b l e mineral assemblages of the amphibolit® f a c i e s i n rocks s u f f i c i e n t In s i l i c a and potash are shown i n Figure 3» an A C P d i a g r a m . These assemblages are of course subject to m o d i f i c a t i o n r e s u l t i n g from d e f i c i e n c i e s either i n s i l i c a or potash, or both, For exaiapla, the format i o n of o i o t i t e depends on the proportions KgQAMgjFeWAlgO^ (Barth 1952}• Thus the amount of mica formed i s determined by the proportion of these oxidas. I f there i s excess KgO, potash f e l d s p a r i s formed* In rocks having I n s u f f i c i e n t amounts of K^O r e l a t i v e to AlgO^ £ o T *fcila f o r B a * i ° n o f Mica, then one or two of the f o l l o w i n g minerals can forms anda-l u s i t a , c o r d i e r i t e , a n t h o p h y l l i t e (or ctimminetonite). With the exception of the high-temperature sili3.man.ite - alraandine subfacies , potash feldspar i s unstable i n th© presence of non-potassic, litae-fr®@ minerals such as a n d a l u s i t e , c o r d i e r -i t e , almandine, and anthophyllite, In place of which auseovite and b i o t i t e are a stable combination. Two groups of f r e e - s i l i c a hornfelses can be d i s t -inguished on the basis of potash content, Th® paragenesis of th® rocks w i t h s u f f i c i e n t KgO to prevent the formation of andalasite, c o r d i e r i t e , almandine, or a n t h o p h y l l i t e (or cui a s i n g t o n l t s ) , i s shewn In Figure 3« Mineral assemblages of t h i s group can a l l include a l c r o c l i n e and quartz i n a d d i t i o n to the ptoses i n d i c a t e d by the diagram. In the second group potash i s d e f i c i e n t and as a r e s u l t i t i s not a possible member of any of the e q u i l i b r i u m assemblages. 35a f o l l a s t o n i t e Diopside Actinolite F i g . 3. Amphibolite facies, cordierite-anthophyllite subfacies: ACF diagram for rocks with excess Si02 and K20. (after P. Eskola) Actinolite itathophyllite (Curamingtonite) F i g . 4. Amphibolite facies, cordierite-anthophyllite subf acies: ACF diagram for rocks with excess SiC>2 and deficient in K20. (after P. Eskola) 36 The p&ragenesis of these five-phase assemblages are shown in the ACF diagram Figure 4. Here, any of the phases, andalusite, c o r d i e r i t e , and anthophyllite can coexist with a mica. In addition almandlne garnet can coexist with co r d i e r i t e i f the FeO/MgO r a t i o in the bulk composition of the rock exceeds a certain value, and MgO and PeO are then considered as separate components (T. and V.). Spessartite can also be present i f MnO i s present* In the following descriptions, the hornfelses of the inner zone are divided into compositional groups ir r e s p -ective of their stratigraphic position. This method of c l a s s i f i c a t i o n i s complicated, as i t does not necessarily indicate the spa t i a l relationships with the intruslves. But since i t i s based on both o r i g i n and bulk composition i t appears to be the best method for describing the motamorphic rocks. The distri b u t i o n between certain groups may seem arbitrary, but thi s i s unavoidable in the s p l i t t i n g of a complex group of rocks for purposes of description. Bo real attempt has been made to draw lines on the map to Indicate mineral zones other than the two basic divisions made previously i n th© paper. The d i f f i c u l t y of correlating changes exactly i n rocks of varied bulk compositions has been well appreciated by workers in other metamorphic terralnes. The general occurrences of each type and the evid-ences and theories for their origins w i l l be dealt with In a separate section. 37 Cordierite-Anthophyllite Hornfelses The cordierite-anthophyllite hornfels considered here belong to assemblage 1, outlined i n Figure 4. These hornfelses consist of fine-grained grano-b l a s t i c aggregates of plagioclase feldspar, quartz, b i o t l t e , c o r d i e r i t e , anthophylllte and minor accessory minerals. In general this rock type appears to be confined to a narrow zone along the main contact on the south slope of Harrison Ridge. The rock i s usually stained pink to red with hema-t i t e and i n general primary sedimentary structures are obliterated or poorly preserved. The rock i s characterised by scattered radiating groups of fibrous anthophylllte, (Plate II,a), some aggre-gates measuring up to 20 mm i n diameter. This amphlbole usually has a fine columnar habit, but short stubby anhedral grains are common. The interstices of these r a d i a l aggreg-ates are usually f i l l e d by large quartz or plagioclase grains measuring 5 a® i n diameter. In some of the plagio-clase grains the anthophylllte needles have apparently grown along cleavage traces, Cordierite Is also present i n these coarse aggregates, but It Is perfectly devoid of any crystallographle development, and is poeciloblastlc. Small amounts of poeciloblastlc brown b i o t l t e flakes are present throughout the rock but they are commonly absent from the 38 groups of coarse anthophyllite, quartz and feldspar. Small amounts of ch l o r i t e are associated with the b i o t i t e and poeciloblastic anthophyllite, and It i s probably a secondary alte r a t i o n product of the b i o t i t e . The accessory minerals present i n these rocks are apatite, magnetite, and pyrlte. Hematite (?) stains many of the minerals i n the rock impart-ing a yellow cast i n thin-section in plan© l i g h t . The cordlerite i s commonly unaltered and contains inclusions. The pleochroic halos, so characteristic of this mineral, are generally absent. Most of th© cordlerite grains are i n complex groups showing irregular multiple twinning, somewhat resembling that of plagioclase. In cordlerite a pseudohexagonal twinning i s commonly developed on 110 or 130 (Winehell, 1951). In basal sections the masses of cordlerite are c l e a r l y observable as i n t r i c a t e intsrgrowths of many Individuals showing the c y c l i c twinning. The cordlerite was not Identified megascopically but i n thin-section i t was observed as colorless non-pleo-chroic grains. The indices of refraction were not determined exactly because of the fine-grained nature of the rocks, but in general the Indices were found to be greater than those of quartz but lower than those of andeslne plagioclase. According to Winehell (195D» the values of the refractive indices range widely. Optic angle of the cordlerite was determined using the universal stage microscope. The average 39 of the values obtained was calculated to be 2T_ » 73°t(**)ve>, Rankin and Merwin (1918, in Yoder, 1952) f i r s t suggested that two variations of cordierite may exist. Recent investigations by Yoder (1952) have disclosed the existence of the a and ja forms, fhe high temperature a for® was considered the stable phase. Variations in the s i l i c a content were once thought to influence the optical properties of cordierite, but experiments performed by Yoder have dis-proved this hypothesis. Pollinsbee (1941), attributed the variation In optic angle and sign of cordierite to the percentage of alkalies. These variations are shown in Figure 6 , Follinsbee (1941), found that the effect of alkalies on the optical properties was to 1, Increase the indices 2, change the birefringence, and 3, change the optic sign. He also found that the presence of CaO tends to increase the optic angle. These effects on the optical properties are graphically illustrated in Figure 6. Follinsbee concluded that optically positive cordlerltes are low in alkalies and relatively high in calcium, Anthophylllte, (Mg,Fe)y (0H)2 ^igQgg » is found i n two habits In these rocks, fhe previously mentioned radial Microcline Fig. 5. Amphibolite facies, cordierite-anthophyllite subfacies: AKF diagram for rocks with excess SiC>2 and A I 2 O 5 . 40 aggregates consist of long, blade-like crystals associated with the coarse grains of quartz and plagioclase feldspar. In the fine-grained quartz-plagioclase groundmass of the rocks, coarse l d i o b l a s t l c anthophylllte p o i k l l i t i c a l l y en-closes quartz and feldspar grains, fhese porphyroblastic grains are up to 10 mm i n length and 4 mm wide. In thin-section the anthophylllte i s colorless, non-pleochroic and unaltered. The optic sign of the antho-p h y l l l t e was determined using the universal stage, and was found to be 2Y • 78° (*-)v®. It has positive elongation, and Z C * 0®. Although the anthophylllte i t s e l f i s usually unaltered i t i s commonly intimately associated with c h l o r i t e l a t h s , and often has a yellowish coloration under the micro-scope due to the hematite (?) stain. From his studies (1952), in the MgQ-AlgO^ * SiOg -HgO system, Yoder concluded that experimentally anthophylllte cannot be obtained In the presence of excess water. But, because It Is found so frequently i n nature associated with cordierite and chlorite Yoder thought It should occur some-where i n this system as a stable phase. The assemblages stable in the presence of water vapor are shown In Figures 7 and 8. Yoder has also isolated i n this tetrahedron a region deficient In water, which he c a l l s the water-deficient region, within which he believes the phases anthophylllte and also pyrope are stable. The /+0a V c Fig. 7. Tetrehedron illustrating the phases stable in both the water-deficient and excess-water regions at ca. 600°C and 15,000 psi for the system MgO - A120^ -Si02 - H2O . The planes which are lined divide the excess-water regions and the water-deficient regions. The points An and T l i e on the back surface; Pr and Co on the base, and CI lies within the tetrehedron. (after H. S. Yoder, 1952.) Fig. 8. Assemblages in both the excess-water (solid and dashed lines) and water-deficient region |dotted lines) believed to stable at ca. 600°and 15,000 psi in the system MgO - AI2O3 - Si02 - H2O. occurrence of enstatite i n this water deficient region, i s explained by Yoder as du© to an i n s u f f i c i e n t quantity of water needed to convert i t to hydrous compounds. Above 660° C however, the estatite Is stable i n the presence of excess water, whereas the anthophyllite decomposes at some temper-ature below 660° C. At this unknown temperature the antho-p h y l l i t e probably decomposes according to the equation Anthophyllite * Enstatite * t a l c , i n the water deficient region, and i n the excess water region the equation would be Anthophyllite * vapor • t a l c r f o r s t e r i t e the excess-water assemblages were determined experimentally by Yoder and these water-deficient phasas are based on observed f i e l d associations. Thus i t appears that anthophyllite bearing hornfelses may have formed under r e l a t i v e l y 'dry' conditions. As has been b r i e f l y mentioned, the c o r d i e r i t e -anthophyllite hornfels contains a substantial proportion of plagioclase. It i s intimately associated i n the groundmass with quartz, anthophyllite, and cordlerite, The grains of plagioclase are anhedral and xenoblastic towards the other minerals In the assemblage. It is generally clear and color-l e s s , and unaltered. Twinning i s very common, but the P y r i -d i n e and Carlsbad types are minor-in most specimens. The plagioclase i s not abundantly zoned but i t is conspicuous where present, The average composition of the plagioclase 42 feldspar In this hornfels was determined from a number of observations generally making use of the grains showing sections 1 a, fhe values obtained correspond to a compo-s i t i o n Ab^g A a42* f h l s composition is somewhat more sodlc than that of the plagioclase In the d i o r l t e which was found to average about Ab^tj An^, Two d i s t i n c t variations in the grain size of the plagioclase were observed* In the matrix or groundmass of the rock the feldspar maintained an average diameter of about 0,2 mm. In. the coarse-grained areas of quartz, and antho-p h y l l l t e however, the plagioclase averages from 0,5 Rim to 1,0 mm In diameter. This variation i n grain s i z e , noticeable megascopically, may be related to the o r i g i n a l grain size of th© sediments when vague bedding features are preserved. The zoning i n the plagioclase of these metamorphic rocks i s one of the most Interesting phenoiaenons observed. The geologic l i t e r a t u r e Is almost devoid of detailed accounts of zoning of metamorphic plagioclase. Brief references to the composition of the zones have been found i n the older descriptive l i t e r a t u r e but l i t t l e has been found on the process by which the soned plagioclase i s formed. This subject w i l l be treated i n some d e t a i l , In a later section. Quartz, an Important constituent of the c o r d i e r i t e -aathophylllte hornfelses, occurs i n much the same associations 43 and textwral relationships as the plagioclase feldspar just described, the quartz forms clear, colorless, unfractured grains of random orientation. The average grain size of the quartz i n the granoblastic matrix d i s t i n c t l y p arallels that of the plagioclase which i s about 0,2 mm on the average. In the coarse-grained aggregates of this rock the diameters of the quartz grains reach maximum diameters of from 3,0 to 5.0 mm,, which i s i n excess of the coarse feldspar grains. These large quartz grains contain small r a d i a l aggregates of the fibrous anthophyllite either within a single grain or scattered around i t s peripheral portions. Wavy extinction, suggesting s t r a i n , i s commonly observed i n some of the large grains but Is not recognizable i n the smaller grains. The elongation of the wavy extinct-Ion zones are p a r a l l e l within each grain and extend from grain to grain in a p a r a l l e l orientation. This is probably the result of a low stress environment during or after meta-morphism. Thin plates of b i o t i t e ar© scattered randomly throughout this rock type. It i s invariably a l i g h t brown with pleochroism X * yellowish, T « Z • brown, and 2? i s small, B i o t i t e i s idiomorphic in most orientations. Most grains contain small Inclusions of accessory magnetite and apatite, The b i o t i t e i s commonly closely associated with the anthophyllite but this r e l a t i o n does not appear to be of any special significance. The flakes of b i o t i t e attain dimensions of 20 mm i n diameter, which is similar to the size of the b i o t i t e flakes i n the granitic intrusive. These large b i o t i t e grains give the hornfels a somewhat "g r a n i t i c " appearance. The proportion of the b i o t i t e In the hornfels varies from specimen to specimen but on the whole i t appears to form about 10 per cent of the rock. The accessory minerals In the cordierite-antho-p h y l l i t e hornfelses are magnetite, pyrite, and apatite. Magnetite i s a ragularly occurring accessory commonly found as minute rounded to anhedral grains scattered Irregularly through a l l phases of the rock except the coarse-grained aggregates of quartz and plagioclase feldspars. In the fine-grained matrix of the hornfels swarms of the mag-netite grains cloud irregular area's. Some of the magnetite grains form large "porphyroblasts" attaining sizes of up to 2 mm. The average size of the magnetite i s about 0.01 mm. In some of the speclaens studied the magnetite comprises at least 1 to 2 per cent of the rock. Small subhedral crystals of apatite are found evenly distributed through the rock. The averag© sis® of the grains is about 0,1 mm, while the largest-Individuals may approach diameters of 0,2 mm. On the whole the amount of apatite in the rock Is less than, one per cent, 4f Pyrite is a rare constituent of the rock and is generally absent. As previously suggested this cordierite-antho-p h y l l i t e hornfels can be f i t t e d into the amphibolite f a d e s , cordierite-anthophyllite subfaeies representing medium temperature contact metamorphism possibly conditioned by stress of low magnitude, fhe mineral assemblage found in the above hornfels includes quartz, plagioclase feldspar, b i o t i t e , cordlerite, anthophyllite plus minor accessory and secondary minerals. The deficiency of potash in this assem-blage Is indicator! by the absence of potash feldspars. Quartz and plagioclase are in excess and can be considered as possible members in every assemblage of the cordierite-anthophyllite subfaeies. The minerals c r i t i c a l to this assemblage then, are cordlerite, anthophyllite, and biotite. The mineral paragenesis of this five-phase assemblage i s i l l u s t r a t e d in Figure 4, an ACF diagram, representing a@ta-moyphie rocks with excess SiOg and deficient In KgO, Cordierite-Antfaophylllte-Alaaadine Hornfels The cordierite-anthophyllito-almaudine hornfels is basically similar mineralogleally and taxturally to the previously described cordierite-anthophyllite hornfels. The essential difference Is that in this rock type an additional phase, almandlns garnet, is present along with the cordlerite, anthophyllite, biotite, plagioclase feldspar, and quartz. 46 The s p a t i a l relationships are a l s o somewhat c l i f f -©rent as t h i s m i neral assemblage was found f u r t h e r west of th© main d i o r l t e contact than the c o r d l e r i t e - a n t h o p h y l l l t e rock. Macroscopically t h i s h o r n f e l s Is a tough, compact, f i n e - g r a i n e d or "horny" textured rock commonly st a i n e d a l i g h t r e d d i s h color by hematite ( ? ) . The coarse-grained phases described i n the cordlerite-anthophyllite rock are a l s o present i n t h i s rock type but thay are n e i t h e r so con-spicuous i n the f i e l d nor 30 abundant in the rock. These coarse-grained zones are not e a s i l y seen except in t h i n -s e c t i o n s . Under the microscope t h i s rock c o n s i s t s of a f i n e -grained quartzofeldsphathic mosaic with i r r e g u l a r l y s c a t t e r e d ' i s l a n d s * of the coarse-grained phases. The matrix of the rock c o n s i s t s of an intimately i n t e r l o c k i n g a s s o c i a t i o n of p l a g i o c l a s e f e l d s p a r , quartz, b i o t l t e , c o r d i e r i t e and minor accessory minerals. The coarse phases are e s s e n t i a l l y quartz w i t h p l a g i o c l a s e and r a d i a l groups of a n t h o p h y l l l t e . Cor-d i e r i t e i s present i n small amounts. The mineralogies! composition of the rock as determined f r o a t h i n s e c t i o n s t u d i e s i s as follows* 47 Quartz Plagioclase Anthophylllte Cordierite Almandlne Biotlte The quarts is generally clear, colorless and un-fracvured. In the coarse-grained phases th© quartz makes up more than 75 - 80 per cent of the aggregate. The average diameter of these quartz grains is about 1.0 mm. The quartz grains in the groundmass has an average diameter of about 0,2 mm. Generally there Is no transition froa the fine-grained groundmass to the coarse material and the transition from on© phase to another is abrupt. towards a l l other mineral components of the hornfels. It consists of coarse to fine irregular grains up to 2 mm in diameter in the coarse-grained quartz clusters* It has an irregular twinning resembling that of plagioclase, and appears to consist of a series of twinned individuals giving rise to a complex sectional twinning. Basal sections of the cordierite masses are conspicuous by the complexity of pseudohexagonal twins. The mineral is colorless and non-pleochroie and contains few inclusions other than quartz or feldspar grains. The characteristic pleochroic halos ar® generally lacking, bat the mineral is usually Identifiable by its birefringence which is slightly higher than either The cordierite is poeciloblastlc, and xenoblastic 46 quartz or feldspar, Its twinning, and i t s characteristic wavy fracture. The cordierite i s somewhat altered along grain boundaries and fractures to small zones of white mica or a yellowish isotropic mineral, probably chlorite (?). The anthophylllte Is also commonly poeciloblastlc in this rock type. Its habit i s coarse columnar and i d i o -b i a s t i c towards the other rock components. The o p t i c a l properties are e s s e n t i a l l y similar to those described for the cordlerlte-anthophylllte hornfels. The b i o t l t e i s of the common dark-brown type, Th® majority of the grains are i d i o b l a s t i c and they ar© commonly poeciloblastlc. The average grain size of the b i o t l t e is about 0.1 mm but some of the crystalloblasts attain diameters of 5 HUB. In general the b i o t l t e i n t h i s rock i s not as con-spicuous a component as i n the hornfelses nearer the contact. Plagioclase feldspar is not as abundant i n t h i s rock as i n the cordlerite-anthophyllite assemblage. The grains are xenoblastle, clear and colorless, and completely unaltered. The plagioclase forming a large part of the granoblastle groundmass usually shows multiple twinning in thin-sections between crossed n i c o l s , ^ s r i c l i n a and Carlsbad type twinning Is generally rare. The maximum extinction angles i n sections r showing albi-te twinning and basal cleavage Is about 21°, which corresponds to the composition A b ^ A n 4 Q or andesine. Zoned plagioclase is common in the rock but appears to make up less than 10 per cent of the total feldspar* The zoning Is oscillatory-normal, with calcic cores and sodic rims. Th© range in composition of these grains is often d i f f i c u l t to calculate because of a lack of twinning and cleavage. Although most of the plagioclase grains are completely un-altered a few show extensive replacement by poeciloblastlc cordierite. These feldspar grains are clouded by numerous minute Inclusions of iron ores and other unidentifiable materials. These grains may represent the last remains of the original clastic plagioclase now largely r©crystallized, Garnet is a conspicuous component of this hornfels in thin-section but not readily observable macroscopically. The grains are xenomerphle and usually have a well developed poeciloblastlc texture (Plate III,a). The garnet is actually present only as a minor component in this hornfels, but its presence as a sixth phase must be explained and the assem-blage subjected to the facies classification and phase rule. It was not found possible to Identify the garnet because of its very small dimensions and sporadic nature, The average size of the garnet porphyroblasts is about 0,2 am. It is colorless In thin section, unaltered, aad contains up to 30 per cent of inclusions, Th© chief occurrence of the garnet in the rock is in the fine-grained jaatrix and is associated with a l l the other mineral phases. 50 The occurrence and mineral components of t h i s rock type s t r o n g l y suggests that i t belongs to the c o r d l e r i t e -a n t h o p h y l l i t e subfaeies of the amphibolite f a d e s . Almandine garnet i s often found i n t h i s a s s o c i a t i o n , although I t i s not a t y p i c a l member of t h i s s u b f a e i e s , and i t w i l l be assumed that t h i s unknown garnet i s almandine. Although not common In the c o r d i e r i t e - a n t h o p h y l l i t e h o r n f e l s e s , i f the FeO/MgO r a t i o i n th© bulk composition of the rock exceeds a c e r t a i n v a l u e , the MgO and FeO must be considered as separate com-ponents. Thus almandine may c r y s t a l l i z e s i d e by side w i t h c o r d l e r i t e . This has been explained by Eskola (1914), as du© to th® high r a t i © of FeO/MgO In almandine while In c o r d l e r i t e the same r a t i o i s u s u a l l y low. The i r o n i n excess of that taken up by the c o r d l e r i t e can then appear as the a d d i t i o n a l phase, almandine, or i f there i s s u f f i c i e n t K 20, b i o t i t e . Th© lack of orthoclas© i n the rocks studied however, s t r o n g l y suggests that a d e f i c i e n c y of SgO e x i s t s . The f i r s t premise Is therefore the most probable. The presence of another oxide may also induct the formation of garnet. Spessortite-almandine may appear i n the rock i f s u f f i c i e n t KnO i s present, Xoder (1952), has suggested that pyrope garnet, l i k e a n t h o p h y l l i t e , Is formed i n a region of water d e f i c i e n c y (Figure 1 0 ). The arguments previously outlined f o r th® formation of a n t h o p h y l l i t e are a l s o applied to garnet. His i n v e s t i g a t i o n s and arguments are open to question however, 51 His use of pyrope or the a l l inclusive term "garnet" may be an oversimplification and misleading. The point i s that pyrope i s not a common mineral constituent i n thermally metamorphosed rocks. It i s , instead, usually characteristic of eclogites. The accessory minerals o f this rock are apatite, magnetite and probably some ilmenite. The fine-grained iron ores comprise less than 5 per cent of the mineral assemblage. They are scattered d i f f u s e l y throughout the fine-grained matrix but are not common i n the coarse mineral aggregates. Some of the Ilmen-i t e (?) grains are surrounded by narrow borders of a fine-grained colorless or opaque white substance which may be leucoxene. A few small grains of id i o b l a s t l e grains of apatite are scattered through the rock, The mineral assemblage of quarts, plagioclase f e l d -spar, c o r d i e r i t e , anthophylllte, b i o t l t e , and garnet can be c l a s s i f i e d i n the cordierite-anthoi h y l l l t e subfacies of the amphibolite facies. The white mica and chlorite present i n this rock type are of a secondary o r i g i n and therefore are not to be considered i n the equilibrium assemblages. The paragenesis of the above anser biage Is represented in f i e l d 3 of the ACF diagram (Figure 4) representing rocks with excess S i 0 2 and deficient in K^O. A mineral assemblage somewhat similar to th© on® above has been described by Eskola (1914) from the O r i j a r v i region of Finland. Here, the garnet iferous c o r d i e r i t e -anthophyllite hornfels gradually grades Into a coarse-grained almandine-blotite rock type. Cor d i e r11 e-Ant hop hy11i t e-C uaasln g t oni t e-Horn f e1s Hornfelses containing both anthophyllite (Mg,Fe)y ( O H ) 2 S i g 0 2 2 , and cummingtonlt© (Fe,Mg)y S l g 0 2 2 (OH)g , i n addition to quartz, plagioclase feldspar, and b i o t i t e were collected near the main contact on the south slope of the ridge. The rocks studied consist of a compact, f i n e -grained, greyish aggregate of microscopically indeterminable minerals. The rock also contains l e n t i c u l a r zones of coarser reddish materials. Harrow fractures f i l l e d with reddish-brown iron oxides traverse th© rocks i r r e g u l a r l y at oddly spaced intervals. Under the microscope this rock type consists largely of a very fine-grained granoblastic mosaic of quartz, plagio-clase feldspar, and c o r d l e r i t e . The average grain size of these minerals i s about 0.03 ma i n diameter. Poeclloblastic grains of short columnar anthophyllite and cummingtonlte are scattered evenly through the fine matrix. In this occurrence the amphiboles average 0*2 mm i n length. Small amounts of tiny brown b i o t i t e flakes are also present In the -roundmass. Within the fine-grained matrix coarse-grained phases of quartz, (with) anthophyllite and cummlngtonit© are present. In these the quartz grains are t i g h t l y packed i n ir r e g u l a r l y shaped lenses or long narrow vein-like bodies completely traversing the thin-sactions. fhe zones of anthophyllite and cummlngtonite are also l e n t i c u l a r or occur i n vein-like bodies extending through the quartzofeldspathic matrix (Plat© 11,b). As has been b r i e f l y mentioned the groundraass of the hornfels Is very uniform In grain size and texture, Th® quartz and plagioclase feldspar are generally clear and colorless and show l i t t l e signs of deformation. The compo-s i t i o n of the plagioclase was d i f f i c u l t to determine because of the very fin@-graln@d nature of th© mineral but judging from i t s refr a c t i v e indices It appears to be roughly An^Q or andesine. The tiny cordlerite grains found i n the matrix show an unusually well developed sieve texture for their s i z e . Coarser grains, up to 0.1 ma of p o i k l l i t i c cordlerite are present in the coarse lenticules of quartz along with minor plagioclase, and secondary white mica and ch l o r i t e . The relationships and occurrences of the antho-p h y l l i t e and cummlngtonit® are of particular interest. Anthophyllite and ewmmingtonlte were found to occur In homo-axial intergrovths which because of the difference in ex-tinction conditions present an appearance of twinning. This can b© best observed in sections of the large grains i-(010), in which the anthophyllite lamellae has parallel extinction while th© extinction of the cummingtonlt© is oblique. fhe cummingtonite was identified in thin section on the basis of the following propertiess its aaphibole type cleavage, an extinction angle of 2AC * 16-18°, 2V about 80 - 8 5°, (*•) ve, moderate birefringence, and i t s fibrous lamellar habit. The lenses and veinlets previously mentioned are composed largely of coarse euBaaingtonit®. In these occurrences the grains of this mineral attain lengths of 1*5 to 2*0 mm and widths of 0 .5 mm. Small grains of cummlngtonita associated with anthophyllite also occur in the fine-grained matrix. The euraaingtonite often shows a wavy extinction and an irregular, coarsely developed twinning i n both basal and longitudinal sections. Th® anthophyllite lacks these features entirely. From these observations the writer concluded that camaingtonitt and anthophyllite are both present. This association Is by no moans rare. The Orljarvi region described by Eskola (1914) is noted for its meta-morphia assemblages containing anthophyllite and cummlngtonit® i n addition to cordierite, b i o t i t e and plagioclase. T i ney and Plett (1929) have described ant hot hy 11 i t e-c urrm in «rt oni t e assemblages i n which the two minerals occur as Independent crystals in p a r a l l e l growths. They also described v e l r l e t s and stringers of cummingtonite cutting Irregularly through the rocks. T i l l e y (1937) has also mentioned this mineral association i n his descriptions of the granulites of the Lizard. The presence of both these minerals i n the same rock presents a problem with regard to i t s facies c l a s s i -f i c a t i o n i n terms of the mlneralogieal phase rule. This occurrence may b© regarded as an instance of simultaneous c r y s t a l l i z a t i o n of the two forms of Mg^ Si,. 0 ^ (0H) 2 often in an intergrown habit (T and V, 1951). Turner and Verhoogen state as an alternative that the coexistence of these two minerals may possibly indicate the f a i l u r e of the rock constituents to reach equilibrium. There are also minor amounts of white mica and chlorite present i n the rock but their relationship to cor-d i e r i t e and b i o t i t e suggests a secondary o r i g i n . Magnetite and probably some ilmenite are present as accessories i n small quantities. Th© grains are irregular or rounded and they are generally-less than 0,02 mm i n diameter. 56 The occurrence of both anthophylllte and cumming-tonite in the same rock suggests non-equilibrium conditions and as a result th© hornfsls cannot he c l a s s i f i e d hy the facies p r i n c i p l e . Cordierite-Muscovite-Plotite Hornfels The cordierite-xjuscotive-biotite mineral assemblage represents a hornfels with excess SiOg and deficient K 2 0 . In addition to the three phases mentioned, quartz and plagio-clase are extra n o n - c r i t i c a l members• This rock type was collected approximately 200 yards wast of the main contact. In the f i e l d this rock i s characterized by the large amount of 'coarse' brown b i o t i t e . As a whole the rock is a hard,compact, r e l a t i v e l y coarse-grained rock composed of macroscopically v i s i b l e individual mineral grains. Bedding is marked by the lineation of b i o t i t e flakes and by compos-i t i o n a l variations. On the fresh surface the rock Is greenish-black but on the weathered portions i t is a reddish-brown* In thin-section this reck type consists of a compact granular mosaic of unfractured and unaltered quartz and plagioclase feldspar, cordierite, b i o t i t e and rruscovite. These minerals, with the exception'of the micas, are xeno-b l a s t i c towards one another and commonly poeciloblastlc. The average grain size of the minerals is about 0,2 mm. 57 The composition of the rock Is listed as follows: quartz 30g plagioclase 25% cordierite 1% biotite muscovite % chlorite accessories l-2£ The composition of the plagioclase was calculated from sections i n th© gone-La, showing A l b i t e twinning and basa;l. cleavage. The maximum extinction angle was 2 9°, which corresponds to Ab^Q An^Q. Zoning In the plagioclase of this rock is especially well developed. The variation from core to rim i n a few of the grains studied is from about Ab^ 0 An<-0 to Abg2 An^g. Some of the quartz and feldspar grains in this rock a t t a i n diameters of 1.5 to 2,0 mm but the distinct size variation described in other hornfelses is absent. The variation in grain size is gradational rather than abrupt. The cordierite is xenoblastlc and commonly shows a well developed sieve texture in both large and small grains. I t Is generally partially altered along tiny fractures or grain boundaries to a featureless,isotropic,yellow material. This substance was not positively identified but It may be a chlorite mineral. This mineral i s^entirely confined to the cordierite grains and does not encroach upon adjacent minerals. The auseovite is present In" sheaf-like forms which are randomly scattered throughout the mineral assemblage. 58 It i s i d i o b l a s t i c but aaver posclloblastie. The b i o t i t e la a dark brown variety, Idioblastic i n habit, and does not generally exhibit sieve texture. It appears to be concentrated Into elongate p a r a l l e l zonos probably representing the o r i g i n a l plane of sedimentary bedding. The only accessory mineral of note i n th® assemblage i s a few grains of i d i o b l a s t i c apatite. The five-phase assemblage cordierits-muscovite-blotite-quartz-plagioclase, can be f i t t e d readily into the facies c l a s s i f i c a t i o n of the cordierite-anthophyllite sub-faeies. The assemblage is represented i n Figure 5» by f i e l d 2 of the AKF diagram representing a rock with excess S10 2 and deficient i n K^O. In this diagram the KgO is used as a component to indicate i t s deficiency In the bulk composition of the system, Cordlerite-Muscovlte-BIotltQ-Garnet Hornfels This rock type is somewhat similar to the preceding hornfels except for the addition of an extra phase, garnet. This rock was collected a short distance to the west of the cordierlte-muscovite-biotite hornfels. The surface exposures of this rock hav® been d i f f e r -e n t i a l l y weathered, yleldlng^li'regular surface. The fine-59 grained groundmass of the hornfels has been decomposed and carried away leaving scattered knobs of ss»all subhedral red garnets, 'fhe rock exhibits an irregular mineral banding which Is int e n s i f i e d by p a r a l l a l limonitic zones developed along bedding fractures. Tiny b i o t i t e flakes are scattered throughout the rock. On the weathered surfaces the rock i s a mousey-grey but on fresh surfaces i t is bluish-black, Under th® microscope the hornfels consists of a granoblastic fine grainad mosaic of quartz, plagioclase feldspar, c o r d i e r i t e . Porphyroblasts of garnet, eordiorlte, quartz and b i o t i t e occur within th© clear generally unaltered groundmass. The mineral composition of the rock is approx-imately as follows i Quartz 20% plagioclase 35-40! b i o t l t e 10$ cordierite 100 garnet 15'* Muscovite % c h l o r i t e 1-2?-accessories 1-2! Plagioclase which forms a large part of the fi n e -grained groundmass generally shows colysyntbetie twinning. The maximum extinction angle found In sections of the f e l d -spar a, showing basal cleavage was X A{010} » 18°, which corresponds to Jkh&y A n ^ or ollgoclase-andesine. Zoned plagioclase i s common in the rock, "The feldspar has an i d i o -bl&stie habit, and on the whole tends to exhibit a preferred orientation. These grains appear to be roughly aligned p a r a l l e l to the b i o t l t e stringers and the bedding fractures, Th© grain size of the minerals in the groundmass, including quartz, feldspar and. biotite is about 0.2 mm ia diameter. The quartz is xenoblastic and ranges in size from the average 0.2 am in the matrix to about 1.0 mm in the porphyro-blastic grains. The quartz is rarely poeciloblastic and shows no sign of deformation such as wavy extinction. Th© biotite is of the common dark brown variety and i t is always idioblastic towards the other minerals. The biotite flakes 11© in parallel linear bands traversing the rock. This feature is not visible macroscopieally because of the small grain size of the biotite, however these linear aggregates appear to follow th© coarse bedding features. Most of the biotite is clear and fresh, but some grains have been partially altered to fibrous chlorite. The cordlerite occurs in the matrix as large xeno-.blasts up to 3.0 mm in diameter in which numberous small grains of quartz and biotite are poeciloblastically enclosed (Plate III,fe). Most of the cordlerite grains appear to be simple crystal®, but many of the large porphyroblasts commonly show an irregular, complex,multiple twinning. In thin-section the cordlerite is seen to be colorless, non-pleoehroic, and rarely exhibits the often characteristic yellow halos. Its birefringence and relief a r e generally higher than those of the quartz or feldspar allowing-it to be readily ident-ified. Many of the cordierit© grains a r e wholly or in part 61 a l t e r e d to a reddish-brown isotropic substance which appears to be f i b r o u s . This a l t e r a t i o n product extends along wavy-f r a c t u r e s i n the cordierite and iaa.y r e p l a c e a l l or part of the g r a i n s , but t h i s m a t e r i a l i s r a r e l y found i n adjacent m i n e r a l s , The garnet commonly forms l a r g e i r r e g u l a r p o i k i l i t i c grains attaining diameters of 2 to 5 ma. The r e f r a c t i v e index .is greater than l.Eo and the s p e c i f i c gravity i s approximately 3.9* These data suggest that the composition Is almandine. Th© large grains of* garnet contain up to 30 per cent or sore by volume of q u a r t z , b i o t l t e , and magnetite. These large grains of garnet have a h e l i c i t l c s t r u c t u r e which Is i n d i c a t e d by the c o n t i n u i t y of magnetite grains both Included i n the garnet and i n the quarts ofeldsphathic ground-mass, Th® l i n e a l ass ©sib l a g ss of b i o t i t e f l a k e s commonly curve outwards and around the garnet porphyroblasts. The subparallel o r i e n t a t i o n of the p l a g i o c l a s e f e l d -spars and the well developed f o l i a t i o n of the b i o t i t e may have resulted froa; the r o t a t i o n and deformation of the con-s t i t u e n t s during r e c r y s t a l l i z a t i o n . The flow l i n e s of b i o t l t e c a r v i n g around the garnet at f i r s t suggest the garnet exerted a s t r e s s ( f o r c e of c r y s t a l l i s a t i o n ) upon the surround-ing rook c o n s t i t u e n t s . However the presence of the h e l e c l t l c i n c l u s i o n s of magnetite i n d i c a t e s that the force of c r y s t a l l -i z a t i o n was not s u f f i c i e n t to d i s r u p t their o r i e n t a t i o n 62 (which i s p a r a l l e l to the b i o t i t e lamellae). This apparent flowage of b i o t i t e may not have been caused by the growth of garnet but may be an example of compaction around a hard core such as i s often found i n sediments. Another explan-ation i s also possible. The garnet i s surrounded by a b i o t i t e free zone. This may represent an area In which the b i o t i t e Is being absorbed to promote the growth of garnet thereby giving the appearance of forceful c r y s t a l l i z a t i o n . The r e a l answer to th© problem Is not readily apparent but i t may l i e i n one of the abov© hypotheses. Th© mineral phases of importance In this rock type ares c o r d l e r i t e , muscovite, b i o t i t e , garnet, quartz, plagio-clase feldspar. The rock evidently contains excess s i l i c a and i s deficient i n potash as indicated by the absence of orthoclase feldspar. I f the assumption is again mad© that the almandine garnet c r y s t a l l i z e d simultaneously with cord-l e r i t e , then garnet does not have to be considered as an extra phase. The chl o r i t e In the rocks i s present both as a l t e r -ation borders on b i o t i t e flakes and as discreet masses assoc-iated with th© porphyr©blasts of cordlerite and garnet. The muscovite i s present i n small quantities usually scattered d i f f u s e l y through the rock although i n some cases i t i s adjacent to altered c o r d l e r i t e . Whether the majority of the whit© mica i s primary or secondary could not be determined,. The minerals Indicated may be a non-equilibrium assemblage. On the other hand i f the muscovite is assumed primary, and the formation of garnet i s due to an excess of PeO over MgO then the rock appears to f i t into f i e l d 2 of the M F diagram figure 5f for the cordierite-anthophyllite subfaeies. Cordierite-Orthoclape-Biotlte Hornfelses In outward appearance this rock type closely resem-bles -the majority of hornfelses of the inner contact zone. The rock has a bluish-grey color on the fresh surface. Irregular lenses of yellowish-grey materials distributed through the hornfels give i t a banded appearance* This color banding i s not apparent however on the weathered surface of the rock which i s commonly coated with a crust of reddish-brown iron oxides. This rock i s also s l i g h t l y vuggy. The vugs are small and are commonly f i l l e d with a porous aggre-gate of limonitic materials, The rock i s fine-grained, and massive. Apart from the small b i o t i t e flakes the majority of minerals i n the hornfels are too fine-grained for iden-t i f i c a t i o n . Under the microscope th® roek i s seen to be composed of a mosaic of fine-grained quartz, plagioclase feldspar, c o r d l e r i t e , and minor b i o t i t e . Large psrphyroblasts of eordiei"lte and orthoclase feldspar..occur within th® fine matrix.. The mlneralogieal composition of this hornfels as determined from thin-Sactions i s as follows 5 64 plagioclase cordierite b i o t i t e quartz orthoelase white mica accessories 10?g i i 3% The average grain size of the Minerals forming the ground-mass of the hornfels is about 0.2 ram. The quarts is color-less and xenoblastic towards the other minerals. The plagio-clase of this rock, unlike that of the others described, i s generally twinned by the Carlsbad law and less frequently Alb i t e twinning i s present. The composition of the plagio-clase i s approximately A b ^ Aa^tj* At least 10 per cent of the plagioclase has siapla zoning very similar to that found i n the other hornfelses described. The cordierite which makes up a large portion of this rock i s generally porphyroblastlc and xenomorphic. The major-i t y of the grains exhibit coarse irregular twinning resembling that of feldspar. The large porphyroblasts attain dimensions of about 1.0 IBIS. A l l of th© cordierite grains are altered to a s l i g h t extent along wavy fractures or grain boundaries either to fine-grained white micas or c h l o r i t e . The reaction can be expressed as follows: c o r d i e r i t e * H 20 * muscovlte * chlo r i t e Cf) Or I f the reaction i s mutual between b i o t i t e and cordierite to for® Muscovite and chlorite the following reaction w i l l holds b i o t i t e <- cordierite * water • Muscovite + pennlnite * quartz ( 2 ) The alteration of cordlerite to muscovite and c h l o r i t e may have been f a c i l i t a t e d in (1) by the metasomatie addition of potassium, or i t may have been supplied In the mutual reaction involving b i o t i t e , (2), In either case the fact that th® muscovite (or s e r i c i t e ) is almost entire l y limited to c o r d l e r i t e grains c l e a r l y indicates i t s secondary o r i g i n . This of course simplifies th© c l a s s i f i c a t i o n of the hornfels i n terms of the facies p r i n c i p l e . The b i o t i t e i n this rock type i s commonly idloraorphic and poeclloblastic. It Is a dark brown variety and i s often altered along i t s borders to a green c h l o r i t e . Orthoclase feldspar occurs in this rock type entire* l y as large porphyroblastic grains Irregularly distributed throughout th© fine-grained quartzofeldsphathic groundmass. These purphyroblasts have an average diameter of 3 mm but i n their greatest dimensions they attain lengths of 5 am# The grains are colorless and clear and commonly exhibit Carls-bad twinning. The orthoclase i s readily distinguishable in thin-sections because of i t s low negative r e l i e f as compared to Canada balsam, i t s negative optic sign and large optic angle. The orthoclase contains numerous tiny rounded i n -clusions of quartz and plagioclase feldspar. In almost every grain studied the central portion of the orthoclase Is re-l a t i v e l y clear of inclusions while towards the edges the proportion rapidly increases (Plat© IV,a). At th® outer 66 extremities of the pornhyroblasts the orthoelase becomes diffused through the interstices of the quartz-plagioclase groundmass. The outer l i m i t s of the grains are therefore very i n d i s t i n c t but they can be distinguished under crossed nlcols as a l l portions of the Individual orthoelase grains extinguish simultaneously. The Iron ores i n the hornfels consist largely of small rounded grains of magnetite and some rod-Ilk© grains of ilmenlte. This material i s scattered Irregularly through the main portions of the rock but i t i s not as concentrated i n the yellowish-grey zones mentioned i n the macroscopic description, A few grains of idiomorphic apatite were seen but i t is rare i n these rocks. The essential mineral assemblage of this rock type Includes c o r d i e r i t e , b i o t i t e , quartz, plagioclase, and ortho-elase. The association of cordierite and orthoelase suggests that this hornfels belongs to the pyroxene-hornf els f a d e s as potash feldspar i s unstable i n the presence of cordierite in the amphibole facies. The paragenesls of the pyroxene-hornf els f a d e s Is Illustrated i n Figure 1, In which the possible equilibrium assemblages for a s i l i c a - r i c h rock with excess K 20 are shown.' This assemblage described probably belongs to class 4 of the pyroxene-hornf els f a d e s . In these rocks b i o t i t e i s a possible member of a l l but th© most cal c i c rocks Illustrated by classes 8 to 10. In these hornfelses Muscovite cannot coexist i n squllibrium with cordlerite, but i t may develop as a product of retrogressive metamorphism. In the rocks described the muscovite appears as an a l t e r -ation product of c o r d l e r i t e . Dlopside-Plagicclase Hornfels Only one axaaple of this rock type from the Harrison contact aureole was studied, Macroscopically this rook i s greenish-grey, very fine-grained, and has a horny texture, fhe weathered surface i s smooth and coated with a f i l m of reddish-brown Iron oxide. Structure Is v i s i b l e neither in the outcrops or in freshly broken rock, hut i n saw cut sections a d i s t i n c t lamination i s apparent. The minerals comprising this rock type ar© not i d e n t i f i a b l e aag&scopieally. In thin-section the rock can be seen to consist of a very fine-grained aggregate of pyroxene and plagioclase. The banding In the rock i s formed by a gradual gradation In th© grain size of th© pyroxene from layer to layer (Plate IV b). The pyroxene was i d e n t i f i e d i n thin-section as probably dio-pside or diopsldlc-auglte, It has two cleavagas at right angles (approximate), a moderate positive r e l i e f , 2AC * 4 8 ° , a positive optic sign, and a 2V of about 60°. The pyroxene forms about 80 per cent of th© rock and the grains range In. size from about 0,01 mm to 0.1 mm In diameter. The banding resembles the graded bedding commonly found In sediments* 68 Although the composition of the plagioclase i n this rock was not determined accurately because of the f i n e -grained nature of the material, i t s refractive Indices suggest i t i s about andesins. The feldspar completely lacks twinning or zoning and i t i s clear and colorless, i t i s always i n t e r s t i t i a l to the pyroxene and the grain size seldom exceeds 0,01 mm i n diameter. Quartz i s rare i n the rock but sose grains may be present with the i n t e r s t i t i a l feldspar. Potash feldspar, micas, and accessory minerals are t o t a l l y lacking i n th© rock. Th© c l a s s i f i c a t i o n of this hornfels is somewhat doubtful because of i t s simple assemblage, but i t can be correlated to the pyroxene hornfels facies. The paragsnesis of such a rock may be i l l u s t r a t e d in Figure 1, class No. 7, which represents the equilibrium assemblages derived from dolomitic limestone, Quartzite The quartzite is characterized by the predoiuinar.ce of coarse quartz. Outwardly the rock has the appearance of a typ i c a l passive quartzite. The fresh rock has a golden-yellow tinge and the weathered surface i s reddish-brown, due probably to a f i l l s of iron oxides. This rock type does not appear to be very common i a the Agasslz Group. Under the microscope th© rock i s seen to consist of an aggregate of coarse-grained quartz and ainor I n t e r s t i t i a l materials, fhe average diameter of the quartz grains Is about 1 mm, but grains 2 to 3 mm. in size are common. The boundaries between the quartz grains are sutured and probably represent amoeboid extension of quartz from o r i g i n a l l y c l a s t i c materials. These i n t r i c a t e Interlocking grains have an implication f a b r i c . The quartz i s clear and colorless and contains about 2 per cent of small rounded Inclusions. The quartz comprises 90 per cent or more of the rock. The minor accessory minerals occurring i n the rock are hornblende, c h l o r i t e , plagioclase feldspar and very minor amounts of apatite and iron ores. These minerals generally form vein-like bodies between quartz grain© or are scattered i r r e g u l a r l y through the quartz grains. They probably re-present the impurities contained in the ori g i n a l sandstone, 'Pneunatolytic' Kotamoryhism The rocks which have been affected by, or appear to have been affected by pneutaatolytle metamorphism are of limited occurrence i n the Harrison aureole. The pneumato-l y t l c rocks to be described hare are a l l close to the granitic intrusIons, Igneous intrusion i s generally accompanied by a Mag-netic f l u i d phase of high temperature which not only supplies th® surrounding rocks with heat but-also with water and other v o l a t i l e compounds, resulting in pneumatolytle or hydrotharmal isetaiaorphism. 70 Although minor constituents such as ?20~ nay have been introduced i n t o the system* I t s presence has been neglected i n the c l a s s i f i c a t i o n * The commonest pneumatolytic minerals contain such metals as i r o n , and metalloids such as f l u o r i n e , c h l o r i n e , sulphur boron and phosphorous. Limestones or limy roeks are especially susceptible t o pneuaatolytic contact eieta-morphlsm w i t h the formation of skarn. S i l i c a t e rocks are commonly not as i n t e n s e l y a l t e r e d , but by the a d d i t i o n of f l u o r i n e or lithium, feldspars may ba changed to micas. Other minerals introduced In a r g i l l a c e o u s rocks are a p a t i t e , magnetite and pyrite. When the agents of metasomatism or pneumatolysis are of magmatic o r i g i n the elements Introduced i n t o adjacent rocks are often those i n which the magna i s i t s e l f d e f i c i e n t . This has been r e f e r r e d to by Ledochmlkow as the principle of p o l a r i t y ( I * and V, 1951). This p r i n c i p l e r e f l e c t s the eon* t r u s t i n g compositions of th© d i l u t e s o l u t i o n s and th© s i l i c e o u s malts froa which th«y separate. Several d i f f i c u l t l a s are involved i a the demonstration of pneuaat©lytic metaaorpMsm, The heavy s o l i cover on Harrison Ridge has sad* i t d i f f i c u l t to f o l l o w i n d i v i d u a l horizons In which, an a d d i t i o n of materials has been suspected. Minerals which are generally considered t y p i c a l of pneuaato-l y t i c or metasomatlc a c t i o n are absent, A lack of chemical 71 analyses has also prevented the determination of whether certain minerals carry the characteristic elements of pnouaatolysis. Thus no rocks rich in fluorine-chlorine -or boron-bearing minerals have been found in the locality. In general the lack of evidence indicates that meta-somatism was not significantly active during metaraorphlsau The granitic rocks are deficient In potash as are the majority of hornfelses from the contact zone. The ortho-class-bearing hornfelses described were not found near the contact of the diorlte but about 100 yards distant. The general absence of ortboclas© suggests that the rocks entering the contact aureole are deficient In potash, Muscovite Hocks The Muscovite rock types appears to bo restricted entirely to the contacts of the fine-grained, dark, border phases of the small diorlte platan. The zone of pneumato-l y t i c alteration extends at least 150 feet outward froa the contact, but the extent along th® contact was not determined. These altered rocks have a bleached baked appearance. They are buff colored on the fresh surface and about 20 or 30 per cent of the rock Is vuggy. Lisonitic minerals partially f i l l these vugs. Th® rock is completely anisotropic and very fine-grained although Muscovite is eoaaaonly coarse enough to be identified atacroseopic&lly. 72 In thin section the rock is seen to consist of Muscovite, quartz, plagioclase, chlorite, clay minerals, and liaionlte, fhe average grain aiz& of the minora l a is about 0,01 mm although at the diorite contact the constit-uents are somewhat coarser. This rock: probably represents an intermediate stags In thermal metaraorphlsm upon which pneumatolysis has been superimposed. The composition of the rock Is roughly as followst Quartz 20% plagioclase 20% muscovite 40-50$ chlorite 5*10;l iimonito % clay (Kaolin?) 2-3,1 The quartz and plagioclase are clear and do not appear to have been extensively altar>d. The white mica i s commonly fiat-grained but the flakes range i n size frora 0,5 m, to 0.01 mm* It Is i d i o b l a s t i c towards the other constit-uents and has a random distribution. Barker (1932) considers that an abundance of white mica In metamorphic rocks near igneous contacts is evidence for pneumatolytie metamorphism. The production of white mica Is at the expense of such minerals as feldspar, andalusit* and cordlerite. Iron eras are s t i l l present in th© rock In small quantities but they have been largely altered to amorphous aggregates of limonite. In some cases grains of magnetite ate rlmaed by borders of liaionite, 73 The bleached| baked appearance, the vuggy nature, the presence of limonite and the abundance of museovitt clearly indicate pneuraatolytic raetamorphista. Skarn Hocks Skarn occurs in the Harrison aureole near the main contact on the lover south slope of Harrison Ridge. The rock consists of a coarse-grained aggregate of garnet, magne-t i t e , ealcite, quartz, pyroxene and epldote. The garnet is reddish in thin-section and is present as subhedral crystals with an average diameter of 3naa* In some specimens the garnet is finely zoned* Th© index of refraction Is greater than 1.&0 and the s p e c i f i c gravity is about 3.9. These properties and an X-ray powder photograph show the garnet to be andradite. The garnet also occurs in coarse spongy-looking masses containing inclusions of quartz, epldote, and pyroxene. In general the last mentioned minerals form the i n t e r s t i t i a l matrix between garnet crystals, Th© pyroxene i s l i g h t green in color, non-pleochroic, and is short prismatic In habit. The optical properties (+•) 2V « 60°, ZAC S 43°, indicate the mineral i s probably dlopside. The epldote Is grass green-, strongly pleochroic and is xenoblastic towards th© other minerals. It commonly occurs as irregular patches and veinlets within the andradite. The 74 The properties of the epldote are as follows: (-) 2f « 80° Pleochrolsm X ** colorless If * yellow Z a lemon yellow In addition the Identification was checked by an X-ray powder photograph. The accessory minerals in tho skarn are magnetite, c a l c i t e , and minor apatite. The origin of the rock Is unknown, but its mineralogy and texture suggest a pnemaatolytic origin. However, i t may represent an unusual product of normal contact metasiorphlsm of a limy rock. CHAPTER IV DISCUSS101 GF METAMORPHISM AT HARBISON 1IDGE Physical Conditions Influence of Stress The fact that stress has had l i t t l e influence on th© development of hornfelses i n the Harrison aureole is i n d i -cated by the general absence of schistosity. There are i n addition abundant evidences to support th© contention that at no time was th© effect of stress s u f f i c i e n t to cause wide-spread shearing or crushing of th© contact rocks, (1) The preservation of the quartz and feldspar grains in the c l a s t i c rock, ( 2 ) The preservation of primary sedimentary features. (3) The absence of such stress minerals as stau r o l i t e , fcyanite, or ©hloritold. ( 4 ) The presence of such antistress minerals as andaluslte, co r d l e r i t e , and c a l c i c plagioclase, (5) The unstretehed character of th© pebbles i n the conglomerate, (6) Absence of textures indicating r o l l i n g of garnet. Source of Heat Th© r i s e of temperature required during contact or thermal metamorphism is generally attributed to Igneous intrusion. In addition several other processes may have supplied heat. Those commonly considered include deforma* tion during orogeny, and a rise In temperature due to a depression of th© rocks to a deeper and hotter part of th© earth's crust. It can hardly be doubted that some heat would be produced during the folding of the sediments but the structural and mineralogical evidence indicates this source played a negligible role. The magnitude of the temperature rise that can be attributed to the burial of the rocks at depth is unknown, However, the fact that the sediments outside the inner zone of hornfelses are unmeta-morphosed indicates this process was not important. The spatial relationships of the hornfelses to the diorite contacts is clearly direct evidence for the emission of heat from the Intrusives, In addition the composition of the plagioclase shows a small but measurable increase in anorthlte molecule content towards the diorite contact, Zones of Contact Metamorphls® As previously indicated th© sediments have all taken on an indurated aspect. Two types of changes can be observed as the diorite is approached. These arei (1) a change in grain size, and (2) a change In mineralogy. Although sediments of al l types and grain sizes af© included in the mass,thin-section studies show there is 78 i n general a gradual increase In grain size towards the contact. Here the grain size has increased measurably-resulting i n a 'coarse-grained' hornfels with a granoblastle texture. The details of this increase of grain size w i l l be reserved for treatment later in this paper i n which the more important rock types and minerals w i l l be discussed i n d e t a i l . The mineralogical changes observed In approaching th® d i o r l t e contact are* (1) zone of b i o t i t e development, and ( 2 ) zone of cordierite development. This d i v i s i o n i s not intended to give the Impression that b i o t i t e or cordier-i t e are predominant constituents i n the hornfelses or that they are universally present In the rocks. It i s merely a c l a s s i f i c a t i o n adopted to comply with descriptions from ©ther l o c a l i t i e s , and because these two minerals make ideal indieles of the thermal metamorphism i s this area. B i o t l t e i s one of the f i r s t and most prominent minerals Involved In th® thermal rsconstltution of th© sediments, and cord-i e r i t e appears to be generally limited to the inner zone of coarse hornfelses, ( 1 ) Zone of B i o t i t e Development: B i o t i t e i s commonly present i n the indurated sediments of the outer aureole. The formation ©f this mineral i s one of the most notable features of this zone but not the only mineral heralding reconstitution. Other Indications of reconstruction i n the outer zone are 79 th® development or r e c r y s t a l l i z a t i o n of quartz, andalusite, and anthophyllite, B i o t i t e has been chosen as the index mineral for this zone because i t is easily recognizable both i n th© f i e l d and under the microscope, and also because of Its abundance i n the rocks. At i t s f i r s t appearance the newly formed b i o t i t e consists of tiny brown flakes often assembled i n irregular aggregates. The flakes gradually increase i n size as the contact is approached, and the e a r l i e r aggreg-ates become lost In the formation of coarse flakes. Commonly, however, the coarse flakes ar® found in coarse clusters scattered i r r e g u l a r l y through th® hornfelses, Andalusite crystals (variety ch i a s t o l i t e ) were found i n a specimen of black a r g i l l i t e collected from a bed, intercalated with th® basal Agasslz, In thin-section this rock consists of an extremely fine-grained matrix of black carbonaceous material with small patches of s s r i c l t e , quartz grains and other unidentified materials which are possibly clay minerals, fhe andalusite crystals ar® scattered singly and In groups i n the matrix. They contain large amounts of inclusions and attain a length of at least 3 mm. The zones or groups of andalusite grains appear to be related to and coincide with thin, dark bands i n the rock which probably possess a large amount of carbonaceous material. The quartz or chert grains contained in this rock are coarse-grained In comparison with th® matrix. Many of 80 th® large grains are i n part or wholly r©crystallised into fine-grained mosaics which have retained the or i g i n a l grain outline. This effect Is especially noticeable i n the pebbles of the Agassis conglomerate where chert ranging from sand to pebble size has been r e c r y s t a l i l z e d to fine-grained mosaics, Anthophylllte appears f i r s t i n the outer zone along with the b i o t l t e . fhe f i r s t indication of Its development into recognizable particles Is i n the matrix of the Agasslz conglomerate as radiating clusters of tiny aclcular crystals and as separate Individuals. It Is not entirely confined to the matrix but commonly penetrates or forms within quartz masses. The anthophylllte also increases i n size gradually towards the contact where i t attains i t s maximum development both In size and quantity. The westerly l i m i t of the b i o t l t e zone was not d e f i n i t e l y established. In the portion of the area examined the tiny mica flakes were f i r s t discovered i n specimens obtained at a distance of 1-1/4 miles from the 'visible sur-face contacts. Indeed, the b i o t i t e zone may include most of the sediments outside the narrow inner aureole. (2) Zom of cordierite development? Cordierite was Identified In hornfelses up to a distance of 175 yards from the d i o r l t e contact. In this zone the sediments have on the whole, become entirely re-constituted Into 1 coarse-grained' granofelastle hornfelses, This i s the zone of true hornfelses. Essentially the rocks i n this zone consist of quartz-rich hornfelses made up of plagioclase feldspar, amphibole, b i o t i t e , c o r d i e r i t e , garnet, and other accessory minerals, Zoning i n Plagioclase This discussion is primarily a description of zoning i n plagioclase feldspars of the inner zone of hornfelses* Oscillatory zoning of feldspars i s most common In rocks of magmatic ori g i n , although normal and reverse zoning in plagioclase i s commonly observed In rocks formed by assimilation, Zoning i s much less common i n the meta-morphic feldspars. Frequent references are made to the r a r i t y of zoned plagioclase i n metamorphic rocks* The general rule In Igneous rocks i s that In the zoned plagioclase crystals the soda content Increases outwards. This is also the type of plagioclase that predominates in phenoerysts of daeltes and rhyolites of undoubted magaatlc o r i g i n . Turner and ferhoogen (1951) have noted by contrast that i n metamorphic rocks -products of dif f u s i o n and reaction In an essentially s o l i d medium - zoned structure In plagioclase and the tendency towards idiomorphism such as Is normally displayed by f e l d -spars of granitic rocks are comparativelymre; and metamor-phic plagioclase, moreover, di f f e r s markedly from plagioclase 82 of H?ost granites as regards th© type of twinning developed, T i l l e y (1924) has described a Class 4 pyroxene-hornfels i n which zoned plagioclase i s present. The central part of the feldspar i s andeslne and i s always more c a l c i c than the rims which are oligoclase. Eskola (1914) has de-scribed granulltes i n which the plagioclase of the ground-mass shows a "characteristic and peculiar structure". The composition of the cores and rims i s A b ^ An^ 4 and Ah^o, An^, respectively. He states that the largest plagioclase grains i n the rocks are idlomorphlc and commonly have alternating zones, the most c a l c i c forming narrow rings, Se also de-scribed a curaingtonite-ajphibolite i n which the plagioclase shows a marked inverse zonal structure i n which th© kernals, or cores are markedly more sodic than the s h e l l s , Zoned plagioclase feldspar was observed i n the majority of th® hornfelses from the Harrison contact-aureole. Of the many grains i n which i t was found, a l l showed a remark-ably similar zonal pattern. On the whole, about 5 to 10 per cent of the plagioclase feldspar is c l e a r l y zoned, In each specimen the kernal or cor® was taore c a l c i c than the rims. The zonal arrangement was simple and generally i n -volved from f i v e to eight concentric rings. These can be c l e a r l y seen by using either the Becke l i n e or Inclined l i g h t method. The more cal c i c zones stand out as narrow ridges separated by sodic material (Figure 9). Th© general lack of zoning In minerals i n Isomor* phous series is taken as evidence of the e f f i c i e n c y of d i f f u s i o n In c r y s t a l l a t t i c e s at isetaEorphlc temperatures i f th© energy of i o n i c s u b s t i t u t i o n i s low - i t Is about zero in p l a g i o c l a s e (T, and Y, 1951), The zoning i n the p l a g i o c l a s e of the hornfels i s b a s i c a l l y s i m i l a r to the o s c i l l a t o r y - n o r m a l zoning found in magmatic f e l d s p a r . I n the l a t t e r , the zoning i s produced during th© complex c r y s t a l l i s a t i o n of the fe l d s p a r from the Magma, I f I t i s assumed that the reactions producing the isetaatorphic p l a g i o c l a s e s are e s s e n t i a l l y du© to thermal d i f f u s i o n , the im p l i c a t i o n s are; (1) the r e e r y s t a l l i z a t l o n process proceeded with decreasing temperature, and (2) the radius of i o n i c d i f f u s i o n was l i m i t e d . Many Btetamorphle minerals are considered metastable, f o r high temperature minerals p e r s i s t to low temperatures. In l i m i t i n g the-sphere of s o l i d d i f f u s i o n a condition Is invoked which a l s o l i m i t s the amount of ma t e r i a l a v a i l a b l e f o r c r y s t a l growth. Pore f l u i d s ar© probably e s s e n t i a l to the process, both as a medium of c i r c u l a t i o n and as a c a t a l y s t , The formation of zoned p l a g i o c l a s e i s probably dependent upon the factors governing d i f f u s i o n r a t e s , such as the s i z e of the ions or molecules, the valences, and the concentrations. I f we assume that th© core or kernal of the zoned metamorphlc p l a g i o c l a s e represents a nucleus of c r y -s t a l l i z a t i o n the composition of the zones formed about t h i s 84 nucleus is dependent on the above factors governing meta-morphic diffusion. The alternation of calcic and less cal-cic zones may b® formed by repeated supersaturatlon and depletion of the feldspar constituents within the sphere of diffusion. Original Composition and Distribution of Metamorphic Types s The sediments of the Agasslz Group grade upwards from a coarse basal conglomerate through arkoses Into a r g i l l i t e s . Andaluoite is rare within the contact aureole but i t was Identified in argillites irterbedded vith the members of the basal conglomerates. The presence of anda1us ite and the absence of biotlte in this rock may indicate the presence of significant amounts of kaolin or similar minerals. The formation of andalusite can. be represented simply by the decomposition of kaolin: I 4 A l 2 S i 2 • A l 2 SIO^ * 2Hg0 The f i r s t mineral to form In the fine-grained matrix of the conglomerates and arkoses is biotlte. The material required for the formation of biotite would Include chlorite, serlcite, and Iron ores, and perhaps free s i l i c a . The origin of the sandstone hornfels is probably simple. It represents a thermally metamorphosed slightly Impure quartz sandstone. The rock has completely lost its o r i g i n a l elastic texture and has asmuaod an implication fabric. On the basis of the microscopic examination of the cordierite-ortnoclase-biocite hornfels some conclusions can be drawn as to the total composition, fhe presence of excess KgO and SiQg is Indicated by th© presence of ortho-clase, quartz, and blotIt®, As andalusite is absent, and cordlerite present the presence of original chlorite Is suggested ( T i l l e y , 1924), The original sediments were probably siliceous chlor i t e - s e r 1c11e sediments low In kaolin Minerals* The majority of th® metatsorphie rocks described have been classified as amphibolite hornfelses* In general the assemblages include quartz, plagioclase, cordlerite, anthophyllite, cummlngtonit© and biotite, Muscovite and garnet are also present* They a l l contain the accessory minerals apatite, pyrite and magnetite. The relative amounts of th© constituents vary from rock to rock but there appears to be a tendency to develop the maximum amount of quartz and plagioclase, Th© occurrence of cordlerite is widespread in these hornfelsas* Magn«sia could be derived from biotite, but this Involves th© formation -of orthoclase which is generally lacking i n the hornfelses, 2 biotite * 3 muscovite *> 24 quartz * 2 cordlerite *~ 5 orthoclase •> 6 water.. t h i s reaction would involve the concomittant removal of potash from th© system i n order to produce the required hornfels. The presence of excess white micas i n the o r i g -i n a l sediments would also lead to the formation of ortho-elase, HgK A13(S104)3 * Si02 » I A1S1308 * A l 2 S10$ + H20 Unless magnesia was introduced from outside the system the best explanation for the development of cordierite appears to be the presence of abundant chlorite i n the ori g i n a l sediment. Some conclusions concerning the bulk composition of the cordler11e-anthophyllite hornfelses can also be drawn. Those rocks are deficient In KgO but they contain s u f f i c i e n t SiOg| Ka^O and OaO for the formation of abundant quartz and feldspar. In addition the magnesia content i s s u f f i c i e n t to have allowed the production of both cordierite and antho-p h y l l l t e (or cummlngtonite). It appears therefore the majority of the hornfelses In the Harrison aureole were de-rived fro® aluminous c h l o r i t l c sediments. Such materials may have been derived by the erosion of basic rocks. T i l l e y and F l e t t (1929) have Investigated the or i g i n of the eordlerite-anthophylllte and related hornfelses from lenidjaek, Cornwall. Chemical analyses of the rocks re-vealed that they were abnormally r i c h i n magnesia, Iron oxides, and alumina, and low In alkalies and lime. Compari-sons were made with normal marine sediments, but the results indicated the hornfelses were not equivalents. It may be noted here that th® composition of the Q r l j a r v i hornfelses ^re: remarkably similar to the Kenidjack rocks, fhe analyses of rocks from these regions have been plotted on the ACF diagram for th© cordierite-anthophyllite subfaeies. (Figure 10). The analyses of the hornfelses are compared to plateau basalts, greenstones, and hornblende schists, T i l l e y and F l e t t stated that th© chemical character of the plagioclase r i c h hornfelses was closely a l l i e d to d o l e r i t i c rocks. They have considered four hypotheses for the meta-morphic o r i g i n of the hornfelses* (a) Sediments abnormally r i c h i n c h l o r i t i c material. (b) Sediments metasomatically affected by introduction of magnesia and iron oxides. (c) Mixed rocks formed by assimilation of aluminous sediments by d o l e r i t i c intrusions, (d) The o r i g i n a l d o l e r i t i c intrusions which had been sub-ject to intensive decomposition by atmospheric weather-ing i n which lime and a large part of the a l k a l i e s , especially soda, had been removed, The f i r s t view was discarded because of the intimate f i e l d relations of the hornfelses with greenstones, and the presence of much feldspar. As for the second, T i l l e y and F l e t t found no evidence for metasomatism i n th® Kenidjack area. The third hypothesis was also discarded on the basis of f i e l d and microscopic studies. They believe the fourth F i g . 10. Chemical C h a r a c t e r i s t i c s of D o l e r i t e s and Greenstones. P l o t of analyses to show the r e l a t i o n s h i p of the anthophyllite-cummingtonite rocks of Kenidjack to s i m i l a r rocks i n the O r i j a r v i region, s.nd to normal b a s a l t i c or d o l e r i t i c rocks and t h e i r metamorphosed equivelents. 1-3. Anthophyllite-cummingtonite rocks of Kenidjack 4-5. Anthophyllite rocks of the O r i j a r v i region 6 -7 . Cummingtonite amphibolites, O r i j a r v i region 8-9. Greenstones, Newlyn Quarries, Cornwall 10. Hornblende s c h i s t , leneage, Cornwall 10. Average plateau basalt A= A1 20 3 -K20 + Na20 , 0= CaO, F= FeO + MgO - T i 0 2 An= a n o r t h i t e , Cd= c o r d i e r i t e , Ap= anthophyllite Ac= a c t i n o l i t e Hb= f i e l d of hornblende Cu= analysed cummingtonite ( a f t e r T i l l e y and F l e t t , 1929) 88 process was responsible f o r the nature and r e l a t i o n s h i p s of the h o r n f e l s e s . B r i e f l y they considered that o r i g i n a l d o l e r l t i c Intrusions were subjected In part to Intense weathering whereby c a l c l t e , c h l o r i t e , serpentine and Iron oxides were developed from the p l a g i o c l a s e , pyroxenes, and p o s s i b l y o l i v i n e , The formation of c a l c l t e from basic p l a g i o c l a s e r e s u l t e d from the f r e e i n g of alumina as a hydrous phase. During these processes, lime and a l k a l i e s were I n t e n s i v e l y leached from the d o l e r i t e s , Later contact metamorphism of aluminous and e h l o r i t i c residues produced the e o r d l e r l t e -a n t h o p h y l l i t e h o r n f e l s e s , l o c k s r i c h In p l a g i o c l a s e would be derived from p a r t l y leached rocks. The w r i t e r believes that t h i s process cannot be applied to the fine-grained sediments of Harrison Bldge which have been metamorphosed to s i m i l a r hornfelses of the e o r d i e r i t e - a n t h o p h y l l l t e subfaeies. Metasomatism was a l s o ruled out as the o r i g i n f o r the constituents of the hornfelses i n the Harrison aureole. Anthophyllite-cummlngtonite bearing rocks appear to be s t r i c t l y confined to a zone at th© contact. Outwards from the contact the rocks become e o r d i e r i t e - a n t h o p h y l l i t e horn-f e l s e s , c o r d i e r l t e - b i o t l t e r o c k s , and then c o r d i e r i t e -blotite-muscovite assemblages. In places the rocks contain porphyroblastic garnet and r a r e l y orthoelase. The d i s t r i -bution of the rock types as presented here i s only approximate as i n s u f f i c i e n t f i e l d and microscope work has been done to outline the rock types i n d e t a i l . The outer l i m i t of the inner hornfels zone Is d i f f i c u l t to define, but In general the mlneralogieal and taxtural features are s u f f i c i e n t l y marked in the f i e l d to define the contact within several hundred feet. Metamorphic History; Summary To summarize the preceding discussion, the contact metamorphlsm occurred after the folding and during the I n -trusions of the sediments and volcanics by the granitic magma. The contact metamorphic rocks were r e c r y s t a l l l z e d under low stress conditions, and are characterized by the presence of such antistress minerals as c o r d i e r i t e , c a l c i c plagioclase and some andalusite. It Is apparent that the folding and t i l t i n g of the sediments must have preceded th® igneous intrusion for it-was stated e a r l i e r that peripheral schlstosity i s absent and that the Intrusive© exerted no large deforming stresses on their w a i l rocks, A passive or quiet emplacement of the magma consequently raised the temperature of the country rock resulting i n r©c r y s t a l l i z a t i o n , The temperatures attained In th© wall rocks were In part dependent upon the distance from the contact so that the ©aximta temperature conditions probably occurred at the contact. Th© extent of r e c r y s t a l l l z a t i o n of the wall rock® closely paralleled the thermal gradient which decreased at distance from the intrusives. CHAPTER V The igasslz Group sediments exposed on Harrison Ridge have been intruded by a r e l a t i v e l y homogenous b i o t i t e * hornblende d i o r i t e . The intrusion of the magma was pro-bably a passive or quiet process, as indicated by the absence of peripheral schlstosity, Th® presence of a few scattered xenoliths of metamorphosed sediments in th© d i o r i t e indicates that piecemeal stoping was effective to some extent i n the emplacement of th© magma. As a direct result of the i n t r u -sion the o r i g i n a l l y unmetamorphosed sediments adjacent to the pluton were thermally metamorphosed to rocks of th© amphibolite and pyroxene hornfels facies, Th© l a t t e r rock types ar® of minor Importance and are restricted to the lower part of the contact at Harrison Ridge, The amphibolite hornfelses of the Inner contact zone are essentially fine-grained rocks with excess s i l i c a and deficient i n potash. These hornfelses are nearly a l l possible members of the cordierite-anthophyllite subfaeies and include the following assemblagess 92 1, Cordlerite-anthophylllte-blotlte-plagioelase-quartz 2, Cordierite-anthophyllite-blotite-almandine-plagloclase-quartz 3, Cordi@rite-anthopliyllite-cuamlngtoriito-.plagioclase-. quartz 4, Cordierlte-muscovlte-blotlte-plagloclass-qttarts 5, Cordierite-muscovite-biotite-almandine-plssicclase-quartz. The pyroxer: a-hornf elsss Include the following assemblages: 1. Cordierite-orthocl&se-biotite-plagloclase-quartz 2. Dlopslda-plaglocla.se. The pyroxene-hornfels facies Is generally con-sidered to Include the products of high-temperature contact metamorphlsm, c h a r a c t e r i s t i c a l l y developed in the innermost metamorphic aureoles surrounding plutonlc i ntrusIves. Hornfelses of the amphibolite facias, corresponding to a lower temperature range commonly occupy the outer zones of such aureoles and may extend to the Igneous contact without the Intervening pyroxene-hornfels zone. In the inner zone of the Oslo aureole GoldSchmidt suggested 1000° to 1200° C. as the probable range of temperature for contact metaniorphism. Turner and Verhoogen (195D consider that 700° to 750° C. i s a reasonable l i m i t for the temperature of transition between the pyroxene-hornfels and amphibolite facies. Barth (1952) has suggested that the maximum temperatures attained in the amphibolite facies is about 500° C. However, as yet, Insufficient data nave been collected to allow the selection of suitable critical mineral assemblages to delimit meta-morphic temperatures for quartzofeldspathic rocks. Apart from the anthcphyllite-cumnlngtonite hornfels adjacent to the diorite contact, a strong Indication of equilibrium in the various mineral assemblages is afforded by.the similarity of rocks In the Harrison Bidga Aureole to those in geographically distant localities. In this connect-ion a brief reference can be male to th© lenlctjaek district of Cornwall and the Orljarvl raglon of Sweden. Both Kenid-jack and O r i j a r v i are noted for their occurrences of met a-morphic assemblages containing various associations of antho-phyllite, eummingtonite, cordl@rit#, plagioclase, biotite, and quartz. In the Kenldjack ar©a there has baen extensive removal of lime fros, and addition of potash and silica to, initial basic igneous rocks $ the Orijarvi region is an ex-ample of magnesia-Iron metasomatism of o r i g i n a l l y highly siliceous l e p t i t e s , the assemblages at Harrison Ridge are presumed to have been, derived from aluminous-chloritic sediments without substantial matasomatie additions, fhe anthophyllite bearing rocks of Kenidjack, Orijarvi and Harrison Ridge present an instance of chemical and miner&log-ical convergence from different parent rocks under similar pressure-temperature conditions, • Honing in metaoiorphic plagioclase feldspar has been b r i e f l y discussed. Although the mechanics of such zoning have not bean f u l l y explained i t is suggested that zoning i n metamorphic plagioclase may not be such a rare occurrence as commonly suggested. 95 Plato 1 B. Specimen of leucocratic d i o r l t e and dark, f i n e -grained inclusion, X 2/3. 96 Plate 11 A. Thin-section of cordierite-anthophyllite hornfels, showing coarse development of anthophylllte in a matrix of quartz and plagioclase feldspar, X 5 5. B . Coarse intergrov>th of cumtrdngtonitt; and anthophylllte i n a fine-grained quartzofeldspathic groundmass. Crossed polarized l i g h t , X 55. ^ 97 Plate Thin-section of hornfels showing a porphyroblast of almandine garnet containing inclusions of quartz and feldspar. Crossed polarized l i g h t , X55. Porphyroblast of poeciloblastic cordierite showing coarse multiple twinning. Crossed polarized l i g h t , X55. 98 Plate IV . Porphyroblast of orthoclase showing numberous inclusions at outer grain boundaries. Grossed polarized l i g h t , X25. B. Thin-section of pyroxene-plagioclase hornfels showing lamination and grading of the mineral components. Crossed polarized l i g h t , X?5. 99 Barth, T.F.W., (1952)J Theoretical Petrology, John Vlltr ft $QQ* Inc., 1 « fork, 1952* Bauerman, I., (1085)* Report on th® Geology of th® Country mmv the F©Tty*niBth P a r a l l e l of f o r t h Latitvd* West of tha Rocky Mountains, Geological Survey of Canada, Bsport of P?ogf#«|: 1882-4. Buerger, H.J,, and v.ashken, II., (194?): MetamorphisE of Minerals, Am. Mla M Vol, 32, pp, 296*308. Burl ay, B.J. (1954): Soma S p i l l l t e s and. Iwatophyres froft Harrison Mills, B.C., unpjblishai M.So. Thesis, University of B r i t i s h Columbia, Cairnes, C.E., (1942): Hops, Geol. Sorv., Canada, Map 737A. Crickaay, CH., (1930): Th® Structural Connection bttwaen th© Coast Range of B r i t i s h Colwhia and tha Cascade Eaaga of Washington, 0«ol« Mag,, v o l , 67, pp* 462*491, DalTt U912)s Otology of th® fforth Ju*#*i«an C o r d i l l e r a at the Forty - n i n t h Parallel, Geol. S u n r . f Canada, Mem, 36. Eskola, P,, (1914): On the petrology of the Orijarvi legion in southw#it#tn Finland, Gonna, (tool, finlands, B u l l , 40, Folllmtbta, i , E , , (1941): Opti© Prop«rtl** of eo*di«rit« i n Relation to Alkalies i n the Cordlerita-Beryl Stru-cture, Aat, Min,| v o l , 26f pp, 485-500, Barker, A,, (1932)i Mat«wwrphlt»» Methuen, London, Holmes, A., (1920): The Fomenclatura of M e t r o l o g y , Thomas Murby and Co,, tondon, Shand, S.J., (1951): The Study of Socks, 3rd ed. Thomas Murby and Co,, t*o&don, Smith, ©•©.. and Calkins, F.C, (1904): A Geological Raoonnalssanee across the Forty-ninth ParaHal? tJ.S. GMl. &u*vM B u l l 23£, 100 Tilley, CF. (1924) s Contact Hetamorphism i n the Comrl© Area, Journ. Geol., vol, 80, pp. 22 - 71 (1937)* Anthophyllite-Cordierite Granulltes of the Lizard, Geol. Mag., v o l . 7 4 . Tilley, C.E.. and Flett, J.S. (1929): Hornfelses from lendijaek, Cornwall, Geol. Surv. 01*. Brit», Summary of Progress, pt. 2, pp 24-41. turner, P.J., and Verhoogen, J . , (195D* Igneous and Metamorphic Petrology, McGraw-Hill, Kew fork, Walton, M., (1955)* The Emplacement of ^ Granite", Am. Jour. S c i . , v o l , 253, VP 1-18« Vinc h e l i , A.N,, (1951)* Elements of Optical Mineralogy, John Wiley and Sons, Inc.-, Hew York, Yoder, H.S. (1952): The Mg0-Al20^ - S10 2- H«0 system and related Metamorphic Facies, Am. Jour. Sci. Bowen Volume, pp. 569-^27. 

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