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A sheared and altered pendant in the Cassiar batholith, headwaters of the Stikine River, B.C. Taplin, Arthur Cyril 1951

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A SHEARED AND ALTERED PENDANT IN THE CASSIAR BATHOLITH, HEADWATERS OF THE STIKINE RIVER, B.C. by ARTHUR CYRIL TAPLIN T A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF ARTS in the department of GEOLOGY AND GEOGRAPHY We accept this thesis as conforming to the standard required from candidates for the degree of MASTER OF ARTS Members of the Department of Geology and Geography THE UNIVERSITY OF BRITISH COLUMBIA Apr i l , 1951 ABSTRACT Thi s t h e s i s represen ts a f i e l d and pe t rog raph ic s tudy o f an e l o n g a t e , t a b u l a r ' r o o f pendant ' w i t h i n the C a s s i a r B a t h o l i t h . The g e n e r a l area, l i e s between the headwaters o f the S t i k i n e and F i n l a y r i v e r s i n n o r t h - c e n t r a l B r i t i s h Columbia . S ince no p rev ious g e o l o g i c a l mapping had been c a r r i e d out i n t h i s a r e a , some d e t a i l s on the e x t e n t , l i t h o l o g y , and s t r u c t u r e o f the va r ious format ions are p r e sen t ed . These c o n s i s t o f the Tak la Group o f v o l c a n i c s and marine sediments r ang ing from upper T r i a s s i c to upper J u r a s s i c i n age; the C a s s i a r I n t r u s i o n s r e p r e s e n t i n g one gene ra l p e r i o d o f b a t h -o l i t h i c i n v a s i o n ; and the Sus tu t Group, composed o f consp icuous ly bedded c o n t i n e n t a l s t r a t a o f upper Cretaceous and Paleocene age. Minor b a s i c i n t r u s i v e s o f probable T e r t i a r y age are a l s o p r e s e n t . The pendant occurs w i t h i n i n t r u s i v e s o f gabbro ic compos-i t i o n . A wedge-shaped body a long the wes tern border c o n s i s t s o f quar tz gabbro. B l u i s h green hornblende , compr is ing some 2$% o f the r o c k , shows a l i n e a t i o n which i s a l s o v i s i b l e i n the o r i e n t a t i o n o f the c a l c i c p l a g i o c l a s e . Hornblende i s the main mafic c o n s t i t -uent , qua r t z comprises l e s s than 10$ o f the t o t a l c o m p o s i t i o n . The l i n e a r s t r u c t u r e s p l u s the presence o f numerous f o r e i g n x e n o l i t h s w i t h i n t h i s more b a s i c border phase, l e a d to the c o n c l u s i o n t ha t the e a r l y i n t r u s i o n was f o r c e f u l , t h a t emplacement was by p iecemeal s t o p i n g , and t ha t flowage o f the magma occur red dur ing c r y s t a l l i z a t i o n . The main intrusive east of the pendant consists of massive grano-gabbro, composed of some $0% calcic plagioclase, 20% microcline, 20% quartz and 5% biotite. The emplacement of this intrusive was more passive. Phases intermediate to these two extremes are pre-sent, mainly along the eastern border of the pendant. These are of hornblende bearing granogabbro. The intrusives represent d i f f -erentiated phases of a common magma. The pendant i s composed of a conformable sequence of pyro-clastics and calcareous sediments. The pyroclastics (tuffs) are predominantly of basaltic composition and form the flanks of the tabular structure. The calcareous sediments represent alternating deposition of tuffaceous and sedimentary material. The succession at three l o c a l i t i e s i s presented'in some detail. It is showm that thermal metamorphism of the basaltic tuffs has been negligible and that these beds have insulated the central portion of the pendant from la t e r a l l y spreading thermal and metasomatic effects. The ther-mal metamorphism of xenoliths of basaltic tuff i n the quartz gabbro is shown to be insignificant, whereas sedimentary xenoliths have been converted to an amphibole hornfels assemblage i n equilibrium with the magma. Dynamic metamorphism has been active within the intrusive and older rocks, rendering those within the pendant to a schistose condition. Effects of earlier, ascending thermal metamorphism have * been largely obscured by dynamic metamorphism. Hydrothermal alter-ation of the fault and shear zones to aggregates of quartz-ankerite and chromian muscovite, represents the f i n a l stages of the crystal-l i z a t i o n of a granitic magma. Fie l d and petrographic c r i t e r i a of 1granitization' are presented and c r i t i c a l l y examined. The coarse grained gabbros are shown to represent crystallization from a true magma. Maps and plates i l l u s t r a t e the main features . The appen-dix includes determinative mineralogy and petrographic descriptions of the intrusives. TABLE OF CONTENTS Page INTRODUCTION Location and Access 1 Physiography and Glaciation 2 F i e l d Work 3 Laboratory Work 5 Acknowledgements. 6 GENERAL GEOLOGY Table of Formations 7 Description of Formations 8 Takla Group . 8 Cassiar Intrusions 13 Sus tut. Group If? Tertiary (?) If? DETAILED GEOLOGY Definition of Roof Pendant 17 The Flanking Area of Volcanic Rocks.... 17 Intrusive Rocks General Statement 19 Granogabbro 20 Quartz Gabbro 22 The Sheared and Altered Pendant General Statement 27 Along "A" Ridge 27 Along "B" Ridge 33 Along "C" Ridge 38 GENESIS OF THE PENDANT C r i t i c a l Features Uf? Igneous Intrusion and Thermal Metamorphism U? Dynamic Metamorphism TABLE OF CONTENTS (continued) CARBONATIZATION AND SILICLFICATION Page .57 MAGMATIC DITRUSION versus GRANITLZATION 62 CONCLUSIONS 66 APPENDIX - Petrography of Intrusive Rocks 69 ILLUSTRATIONS Figure 1 - Generalized Geological Map of Stikine-Finlay headwaters.8 Figure 2 - Geological Map of the Chukachida Lake Area - In rear pocket Figure 3 - Structural Section along A-A of Figure 2 - " " " Figure k - Geological Sketch Map of "A" Ridge Figure 5 - Geological Sketch Map of "C" Ridge Plate I - A. Meanders in the broad valley of the Chukachida River....following page 2 B. View westerly across "A" ridge " 11 » Plates II - VI i n succession following Appendix. Plates A SHEARED AND ALTERED PENDANT IN THE CASSIAR BATHOLITH,  HEADWATERS OF THE STIKINE RIVER, B.C. INTRODUCTION Location and Access The general area is midway between the headwaters of the Stikine and Finlay Rivers, in north-central British Columbia. It lies mainly to the west of longitude 1 2 9 degrees and between latitudes 5 7 and 5 8 degrees. Access to the area is possible by several routes involving either travel by air or by a combination of rivers and trails. Pon-toon equipped aircraft can be used successfully after late May, as Fishing Lakes, Toodagonne Lake, and Chukachida Lake provide ample landing space for planes with a payload of from 1 2 0 0 - 1 7 0 0 pounds. Planes are available for charter at Stewart on Portland Canal or at Fort St. James on Stuart Lake. Access to the region is also gained by trails from Telegraph Creek at the head of navigation on the Stikine, or from Fort Ware at the head of freight navigation on the Finlay River. Pack horses can normally be obtained at Telegraph Creek, at Coldfish Lake on the edge of Spatsizi Plateau, or at Terminus Mountain near the junction of the Gataga and Kechika Rivers, in the northern part of the Rocky Mountain Trench. Fair trails lead from these mentioned outposts to various parts of this area. - 2 Physiography and Glaciation Physiographically, the region straddles the theoretical division between the Stikine Ranges of the Cassiar Mountains on the north, and the Swannell Ranges of the Omineca Mountains on the south (Bostock, 19U8 pp.U5.)« These physiographic sub-divisions are actu-a l l y one, as together they constitute a more or less continuous mount-ain belt. To the west, the mountains are bordered by the gently undulating surface of Spatsizi Plateau. On the east are the nearly parallel ridges and moderately uniform summits of the Russel and Kechika Ranges. Within the rugged mountain belt the r e l i e f varies from 2500 - 3000 feet. Most of the wide main valleys are at an elevation of around 4,000 feet, with the higher mountains rising to 7,000 feet and occasionally to well over 8,000 feet. The highest peaks are those carved from the main batholithic core extending through the mountain belt west of Fishing Lakes. Further west, mountains sculp-tured from the volcanics of the Takla Group are s l i g h t l y lower but equally as rugged, with knife-edged ridges and abrupt cliffs.surround-ed by steep talus slopes. Drainage within the region i s westerly to the Pacific via Chukachida and Stikine Rivers,or easterly and northerly to the Arctic via Toodagonne and Finlay Rivers. These main valleys have stretches of gentle gradient, i n which the rivers contain numerous meander Plate I Meanders i n the broad v a l l e y of the Chukachida Rive To the north, i n t e r l o c k i n g mountains of the Stikine Ranges, carved from the b a t h o l i t h i c core. View easterly across "A" ridge, with the northwest arm of Chukachida Lake i n right centre. Mountains of the "granite ranges" beyond r i s e to elevations of 8,000 feet. Note the dissection of the ridge by f a u l t s . patterns, interspersed with short sections of steep gradient with corresponding falls and rapids in the rivers. Timberline is around the 5*000 foot elevation. Many of the main and tributary valleys are quite swampy, with much muskeg and "buckbrush" hindering travel. The main valleys are characteristically U-shaped, while the smaller transverse tributaries are more typically V-shaped and hanging with respect to the larger valleys. Glacial striae and erra-tics can be noted at elevations up to 7,000 feet. In the northeast part of the McConnel Creek map area, striae and alignment of drumlins indicate that the ice sheet moved southerly and southeasterly from the vicinity of Thutade Lake (Lord, 1948, p.5.). Several of the peaks in the "granite" ranges contain rather small alpine glaciers which appear to be stagnant at the present time, for streams issuing from these are quite clear. Numerous abandoned cirques testify to the former prevalence of alpine glaciers in the Cassiar and Stikine Mountains. Both north-south and east-west trending valleys contain much moraine in which the rivers have entrenched themselves in recent times. Stream cut terraces are present in Toodagonne and McClair valleys. Field Work Specimens and data relative to the thesis were collected during the 1950 season while the writer was engaged in prospecting - h -for Northwestern Explorations Limited, a Canadian subsidiary of Kennecott Copper Corporation of the U. S. A. The prospecting pro-gram involved several three man parties, of which one man in each party served as packer and cook, while the other two investigated the assigned area for evidence of promising mineralization. In 1950 the areas to be covered had not been geologically mapped. Reconn-aissance-type mapping of the general distribution and structure of the various distinguishable formations and batholithic rocks was carried out by the party, using an 8 miles to the inch topographic map as a base. In addition, large scale sketch maps and descriptions resulting from detailed surface examination were prepared for a l l significant mineralization discovered. Each party was equipped with five or six pack horses and moved camp every three or four days. Approximately 500 square miles was covered in reconnaissance fashion during a four month season. While engaged in such work the writer noticed the pendant of highly sheared rocks within the batholith. As the pendant trends northwesterly, sub-paralleir to the Chukachida River, i t was easily examined along tributary valleys extending transverse to this trend. Three separate traverses were carried out along the f i r s t three interdrainage ridges northwest of Chukachida Lake (A-A, B-B, C-C, on f i g . 2). In addition, the component bands within the pendant were examined in the sections exposed by small cirque basins on the northwest side of each ridge. Representative samples showing the - 5 -main lateral variations in composition and structures were collected along the ridges designated. The pendant was traced along strike from the arm of Chukachida Lake to beyond the basin at C, with particular attention being paid to vertical continuity of the sheared pendant as shown by outcrops near the valley bottoms. Downward changes in mineralogy, structures and textures were also sampled and noted wherever present. Rough sketch maps of the three separate ridges and basins were prepared in the field to show the distribution of the volcanic and intrusive rocks. As the prospecting party was constantly moving throughout the area, i t was impossible to spend more than two days in examining the pendant when i t was f i r s t noted. However, two subsequent side trips were made from base camp at the mouth of McClair Creek, for the express purpose of collecting further data. In a l l , some six days work was expended by the writer in collecting material. Snow and rain hindered the work in August and prevented close examination along the upper limits of the talus slides in the respective basins. Laboratory Work The laboratory work involved the preparation and study of 35 thin sections of the sheared volcanics and sediments within the pendant, and of the enclosing intrusives. Particular micas, chlorites and carbonates were identified by the use of index oils - 6 -and immersion methods. Spectographic analyses of two.representative carbonate rocks were performed by Mr. J. Gower of the University's Geology Department. A topographic map of the area on a scale of 1 inch to 2500 feet was prepared from aerial photos with the invaluable assistance of Mr. R.K. Vivian, a senior Forestry student of this University. Geology was plotted on this map, using f i e l d sketches and notes, as well as the excellent aerial photos of the area. ACKNOWLEDGEMENTS The writer wishes to express his gratitude to Mr. W.P. Hammond of Northwestern Explorations Limited, for the permission to collect the material for the thesis while engaged by this company. The help-f u l suggestions and criticisms offered by Dr. H.C. Gunning, Dr. K.C McTaggart, and Dr. L. Dolar-Mantuani are greatly appreciated. The cooperation of fellow graduate students was invaluable throughout the preparation of this paper. The writer i s deeply indebted to Mr. R.K. Vivian for essential aid i n the preparation of the topographic map, and to Mr. J. Gower for his willing performance of two spectographic analyses. The preparation of the numerous thin sections would have been impossible without the unfailing assistance of Mr. J. Donnan, n techician in the Department of Geology. - 7 -TABLE OF FORMATIONS RECENT and Basalt dykes and lavas TERTIARY OUP PALEOCENE PS o and Conglomerates, sandstones and shales. SUSTUI UPPER CRETACEOUS CASSIAR INTRUSIONS (Upper Jurassic and/or . Lower Cretaceous) Granodiorite, quartz-diorite and related rocks. UPPER DIVISION + 5,000 feet, (middle U. Jurassic to Mainly sediments: Pebble conglomerate, Carbonaceous shale, a r g i l l i t e , limestone. Minor interbedded pyroclastics and lavas. P o Early L. Jurassic + 18,000 feet. Mainly volcanics: purplish red, green and gray pyroclastics, andesitic and basalitic lavas with numerous white plagioclase phenocrysts. Minor sediments - a r g i l l i t e and limestone. PS LOWER DIVISION EH Jurassic? and U. Triassic + 10,000 feet. Mainly greenish pyroclastics and lavas with numerous blocky, black pyroxene grains and phenocrysts - not recognized i n this area. Light green to gray, meta-andesites, meta-basalt and greenstones. Considerable thicknesses of limestone and a r g i l l i t e . - 8 -OENERRLIZED GEOL OGICRL MRP OF STIKINE - FINLRY HERDWRTERS. LEGEND - C any / ,t.s.,sJ> CRSSIRR In/rnuctvES. TftXil 6Hourt MRINLY SEiinenTS. - / t. arf//;ft_ t i l / ' . Ctmf/., tuff. TfiKLR 6<*0UP f URINLV VOLCHHICS • flPPnOXIMhiTC CONTHCT of SA eared fend int. C i t i r O U * 1NTERVRL — 2.000 frtt. Figure 1 . - 9 -GENERAL GEOLOGY Description of Formations The Takla Group, named from its widespread occurrence in the Takla Map-Area to the south (Armstrong, 1946.)? consists of 20,000 -30,000 feet of essentially conformable volcanic and sedimentary strata. Fossil evidence indicates the formations range, in age from Upper Tri-assic to Upper Jurassic (Armstrong, 1949 j Lord 1948). The group has been divided in the McConnell Creek Map-Area to the south (Lord, 1948, p. 15), into a lower division' (Upper Triassic) of mainly greenish pyro-elastics and lavas, and an upper division (Jurassic) containing some 20,000 feet of mainly reddish lavas and pyroclastics, plus several thousand feet of shallow water marine sediments and minor intercalated volcanics, carbonaceous strata and coal. These divisions were recog-nized by the writer's party in the area between Thutate and Chukachida Lake. The small scale of map used, however, did not permit the separate mapping of these divisions. • The lower division in this area contains considerable thick-nesses (750 - 1000 feet in individual lenses) of dense blue grey lime-stone, dark grey tremolitic limestones, and minor argillite. The lime-stone contain numerous non-diagnostic crinoid stems up to 1 inch in diameter. The argillite contains fossils tentatively identified by Mr. G. L. Bell of this University as Halobia and Monotis:, both indicat-ing an Upper Triassic age. The lavas of the lower division are mainly - 10 -meta-andesites, meta-basalts and green-stones. They are dominantly light green to greenish grey in colour and considerably more altered in field appearance than the flows of the upper division. The upper division consists mainly of flows and pyroclastics forming the lower members, with sediments predominant in the upper parts. The flows are commonly purplish red or brownish red, some green or grey, and range in composition from andesites to basalts. The pyro-clastics are of the same hues, and range from massive agglomerates to thin bedded tuffs. The upper sedimentary members were outlined roughly on the small scale map, since they contain appreciable thicknesses of limestone which were closely investigated for mineral deposits. The sediments include black carbonaceous shale, banded argillites, tuff-aceous sandstone, calcareous tuffs, coarse agglomerates,and limestone. These sediments are characteristically thin bedded with the exception of the agglomerates and limestones. The agglomerate contains sub-ang-ular boulders and fragments of various volcanic flow rocks, with a good proportion of dense grey limestone fragments• The boulders are up to 10 feet in diameter. The limestone is commonly massively bedded and up to 300' feet in thickness where interbedded with argillites and shales, but averaging only 20 feet within single strata, where interbedded with calcareous tuffs and massive agglomerates. The chert pebble conglom-erates and coal seams noted among the strata of the upper division in the McConnell Creek Map-Area (Lord, 1948, p. 22) were not distinguish-ed in this area. Wood fragments were noted in the tuffaceous sandstone - 11 -at one locality. The youngest Takla beds occupy the central portion of a major syncline trending northwesterly from east of Belle Lake to beyond the upper Moosehorn Creek valley. The probable continuation of this syn-cline lies south of Chukachida River near its junction with the north-erly flowing branch of the Stikine. A major anticline lies to the north of this syncline and just south of Mt. McNamarra. Minor warps occur on the limbs of these major foldsj two small anticlines and an intervening syncline were noted within 3/4 mile in a section exposed in a tributary valley south of Chukachida River near its junction with the Stikine. Dips of the Takla Group strata are generally moderate in this area, averaging 45 degrees, except in the vicinity of the batholith where dips of 65 degrees or more are prevalent. It has been indicated by Lord (1948, p. 53) that two periods of orogeny have affected Takla rocks in the McConnell Creek Map-Area. These can be clearly recogniz-ed in the adjoining area. The f i r s t period of folding, possibly relat-ed to the emplacement of the Omineca Intrusions, affected Takla form-ations prior to the accumulation of the Sustut Group. The second, inter-val of folding followed the deposition of Sustut strata. Northwest of Thutade Lake the Sustut rocks are nearly horizontal while to the - 1 2 -southeast they exhibit progressively tighter folds and steeper dips on approaching the Omineca Fault,which is believed to be the northward continuation of the Pinchi Fault (Lord, 19U8, p. f>0). The two separate orogenies seem, however, to have imparted a single fold pattern on the Takla,Group strata. Faults and shear zones are numerous in this area and are particularly marked in the volcanic members of the Takla Group, by altered schistose zones along which replacement by rusty weathering ankeritic carbonate is characteristic. Not a l l carbonate zo(nes mark major faults, but many of such altered zones consist of widths up to 1000 feet of visibly schistose or fractured rocks. Many of the faults so indicated were localized along intrusive contacts where contact metamorphic effects in the volcanics of the Takla Group had rendered them highly susceptible to fracturing and faulting. In the McConnell Creek area to the south, the greatest displacements evidently occurred in post-Paleocene time, but the major faults are older than the Tert-iary and Recent basaltic rocks. Movements within the Chukachida Lake pendant may have occurred since the intrusion of basaltic dykes of sim-ilar composition to those of the more southerly area. The relative vertical movement on those northwesterly trend-ing faults has been considerable, but in various directions. The Omineca Fault is apparently a steeply dipping thrust fault from the southwest, this side having moved upwards relative to the northeast side. The - 13 -apparent vertical displacement may be as much as 10,000 feet (Lord, I9I48, p. I 4 . 8 ) . On other similar trending faults the relative movement has been in the opposite direction. The Cassiar Intrusions in this area form a batholithic mass extending from Mt. McNamarra at least 10 miles to the east, and trend-ing northwesterly along the eastern boundary of the area. West of Fishing Lakes the rocks are apparently granitic types extending unin-terrupted as far as Jack Lee Creek. Between this and McClair Creek large areas of Takla Group rocks are probably underlain at no great depth by intrusives. Along McClair Creek valley granitic rocks under-li e the lower parts of the valley and extend one third to one half way up the bordering mountains. South of Chukachida River in the f i r s t ten miles below its source the mountains are composed of massive gran-i t i c rocks capped by 100-200 feet of variously altered volcanics. A highly sheared pendant of volcanic rocks occupies the lower part of McClair Creek valley. The pendant extending northwesterly from Chuk-achida Lake lies several miles within the main batholith contact. Minor intrusive bodies, as indicated on the geological map of the area (Fig. 1 ) , occur far to the west of the main batholith. The dominant rocks of the Cassiar Intrusions are grey and pink granodiorites and quartz diorites. They range from medium-grained, equigranular rocks to coarse grained porphyritic types containing large - 1 U -pink orthoclase phenocrysts. With the exception of scattered dykes of brownish pink quartz porphyry, pink quartz-feldspar porphyry and creamy f e l s i t e , no contrasting age relationships were exhibited by any of the phases. Local gneissose phases or those of slightly varying col-our j, texture and composition were observed to grade into one another. None of the three types of dykes indicated above wa-s ? observed cutting other dykes. The Cassiar Intrusions cut both upper and lower divisions of the Takla Group i n this area. Northwest of Thutade Lake nearly hori-zontal strata of the Sustut Group unconformably overly the eroded sur-face of the batholithic rocks. Since one third to one half of the boulders and pebbles of the Sustut conglomerate are of granitic rocks, the Cassiar Intrusions are post-Upper Jurassic and pre-Upper Cretaceous in age. Theywere^probably intruded i n late Jurassic or early Creta-ceous time. Wherever observed the contacts between bodies of Cassiar Intrusions and the invaded Takla strata are steep and well defined. At three separate l o c a l i t i e s Takla Group rocks were noted dipping steeply towards the contact. Contact metamorphic effects are prominent ad-jacent to the intrusive contacts, but rarely over widths greater than 50 feet. The most common type of alteration of the volcanics has been to propylite. Where the granitic intrusions have invaded calcareous ' strata, minor skarn assemblages and small lenses of specularite, mag-netite, sphalerite or chalcopyrite mineralization have been developed. - 15 -Sustut Group In the McConnell Creek map-area the Sustut group comprises more than 3,000 feet of conspicuously bedded and banded continental strata of Upper Cretaceous and Paleocene ages. These consist mainly of interbedded conglomerates, sandstones and shales, with minor tuffs and coal seams. Nearly 2,000 feet of Sustut sediments are exposed in the plateau country northwest and southeast of Metsantan Lake. The conglomerate contains pebbles and cobbles derived from the Cassiar Intrusions, as well as from the Mesozoic and late Paleozoic sediment-ary and volcanic strata of the area. The matrix is the sand-sized equivalent derived from erosion of similar rocks. Individual beds are usually less than 50 feet thick. The sandstones are often cross-bedded and contain numerous ripple marks. From the Stikine River to the southeast for a distance of thirty miles, the Spatsizi Plateau is underlain by nearly flat-lying conglomerates and sandstones, the regional dip being only about 5 degrees to the southwest. Further to the southeast the Sustut strata acquire progressively steeper dips on approaching the Omineca Fault. Table mountains give way to sharply sculptured fold mountains. Tertiary (?) The youngest consolidated rocks of the area are minor in-trusions of basalt which cut both Takla Group strata and the Cassiar Intrusions. Dykes are rarely more than a few feet in width and even in the light coloured granitic surroundings cannot be traced more - 16 -than a half mile. In two localities these basalt dykes were very numerous and could be traced downward on cirque walls into peculiar "Chinese porphyry" stock-like masses, which intrude the granitic rocks. This parent of the basalt dykes would be termed a basalt porphyry in the field. The plagioclose phenocrysts comprise nearly $0% of the rock in many cases. No specimens of these porphyries were avail-able for petrographic study. A thin section of one of the related basic dykes shows this to be composed mainly of augite and Labrador-ite (An 6$). These basic intrusives are lithologically distinct from both the Cassiar Intrusions of this area and the Kastberg Intrusions (Tertiary) of the adjoining McConnell Creek map-area. The basalt dykes alone would be correlated with the abundant late Tertiary basalt-ic intrusives and extrusives in north-central British Columbia. It is also possible that the basic intrusives in this area are intrusive phases of the younger Takla volcanics. In the southern part of the area greenstone plugs were noted intruding limestone interbedded with similar greenstone. - 17 -DETAILED GEOLOGY Definition of Roof Pendant The usage of the term roof pendant i s essentially that defined by Daly (1906, p. 336) as follows, a "block of roof rock almost completely surrounded and probably underlain by granite. Such a block, once a downward projecting part of a roof i n stock or batholith may be named a roof pendant". Another essential feature i n the definition i s "often perfect preservation of the regional strike". This paper deals with a roof pendant defined as above. The term has been shortened to "pendant" because the body of rock under consideration has originally formed a downward projecting part of a flanking area of volcanic rocks, which i s -also*' almost com-pletely surrounded and probably underlain by granitic rocks, and thus i n i t s e l f a roof pendant. Flanking Area of Volcanic Rocks Bordering Chukachida River on i t s north side i s a consider-able thickness of flows and tuffs forming a narrow strip that was probably originally connected with the larger area of Takla Group volcanics immediately to the south. The volcanics north of the river are dark grey andesites and basalts, with interbedded purple tuffs. Contacts between the flows and the tuffs reveal that the dips average - 18 -6$ degrees towards the batholith on the north. The purple tuffs have a spotted appearance due to development of aggregates of green-ish epidote. No vi s i b l e bedding i s present i n these tuffs. The andesites also contain much epidote and carbonate stringers are num-erous throughout both types of volcanics. Adjacent to the intrusive contact on A ridge , faulting and brecciation has occurred along the flow-tuff contacts. Carbonate stringers are more numerous here and much of the brecciated rocks are altered to buff weathering car-bonates. The altered brecciated zones weather more rapidly than their surroundings and form steep-sided gulleys trending down the mountain towards Chukachida Lake. The volcanic rocks are intruded along^A*ridge by a tongue of the batholith which i s only about 3f>0 feet wide on the ridge but widens downwards on either side. To the northwest considerable ex-panses of plutonic rocks separate the. volcanics on the west from the enclosed pendant. The intrusive contact has been the locus of fault-ing along one third of i t s length northwest of the lake. This fault, dipping 60 degrees westerly, has several' steep, easterly dipping branches which form the carbonated breccia zones on^A"ridge. The brecciation along the main fault i s mainly i n the altered volcanics adjacent to the contact, although the intrusive i t s e l f has suffered . 1 The ridges and adjoining basins are designated A , B, C; the letters coinciding with the detailed traverses A-A, etc. - 19 -cataclastic effects. The volcanics immediately adjoining the contact are sliced into large lenticles of sheared and brecciated rock. A zone extending some 2l0 feet from the contact has suffered this breccia-tion plus further alteration to rusty weathering carbonate within 15 feet of the contact. This later alteration along the main and branch faults consists of ankeritic carbonate containing purplish veinlets of quartz and iron oxides, the whole replacing the finer matrix i n the brecciated zones and forming narrow veins i n the intrusive rock. The faulting has obscured many detailed irregularities i n the intrusive contact, which now i s exposed on the south slope of vA"ridge as a gully extending obliquely down the mountain. Where l a i d bare by running water, the brecciated, altered contact i s clearly v i s i b l e . From across the lake to the south, the pattern of the main fault and i t s branches i s evident, as on the aerial photos. Intrusive Rocks General Statement The sheared and altered pendant i s separated from the flank-ing area of volcanic rocks by a tongue of coarse grained gabbroic intrusives. In the f i e l d i t i s apparent that the plutonic rocks bounding the pendant on the southwest possess strikingly different structures and mineralogical composition to those on the northeast. Thus one intrusive would be termed a gneissic quartz-diorite, the other a massive, coarse grained biotite granite. Their sheared and -. 20 ,-altered equivalents, however, are quite similar i n appearance and composition. Petrographic study of some 10 representative specimens shows a gradation exists between the two main types. Granogabbro The intrusive to the northeast i s a coarse grained plutonic type, cream coloured and readily susceptible to weathering. Because of i t s low mafic content and apparent predominance of quartz and al k a l i feldspar, i t would be termed a granite in the f i e l d . This distinctive rock forms an extensive area of the Cassiar Batholith. It occurs paralleling the pendant for some 8 miles and extends at least 3 miles to the east. It was not observed intruding any other phases of the batholith. Creamy f e l s i t e dykes are associated with this coarse grained intrusive. These trend northwesterly and do not evidently cut the gneissic rocks to the west. Petrographic study of a representative specimen of this coarse grained intrusive reveals some rather startling features. A Rosiwal analysis indicates the mode to be $2% plagioclase (An^-An^), 21% microcline (microperthitic), 20% quartz and 5% biotite, with minor chlorite, apatite, muscovite and magnetite. The rock i s best class-i f i e d according to the classification of Johannsen (1939, vol. 1, pp. llj.0-158), by which i t belongs to Class 2, Order 3, Family 7. The rock name granogabbro signifies that i t contains appreciable amounts of a l k a l i feldspar i n addition to the basic plagioclase. - 2 1 -Adjacent to the pendant for some 1,000 feet the granograbbro is sheared and altered. Microscopic study of the sheared border phases of the granogabbro body shows that i t varies considerably in composition as the pendant is approached.. Whereas biotite is the only original mafic mineral in the main mass, within U00 feet of the pendant hornblende appears with biotite, together constituting 10-20$ of the rock. This hornblende is of the variety Hastingsite and a primary constituent of the rock, for i t has suffered deforma-tion with the plagioclase and has been replaced by microcline in a similar manner. With increase in the proportion of hornblende, bio-tite disappears and the granogabbro more nearly approaches the quartz gabbro in composition. It is proposed that the variations i n the intrusives are spatially related to the contact of the flanking area of volcanics rather than to the enclosed pendant for the sheared intrusive;, on the west side of the pendant along "C" ridge contains abundant microcline and is nearly identical with the sheared grano-gabbro to the east. This visibly grades into the gneissic quartz gabbro further west. Elsewhere along the west side of the pendant, the sheared intrusives represent the quartz gabbro. The appearance of typical massive though .• sheared granogabbro is shown on Plate IV A. The shear fractures are marked by carbonate and epidote veinlets. At no place was primary lineation noted in the coarse grained granogabbro. Shearing has greatly reduced the grain size within 250 feet of the pendant, but this may be due, in part, - 22 -to the chilled border. Four separate east dipping faults are present within the above distance, but shearing has occurred along many closely spaced fractures between these main structures. While the main shears have the same attitude, the smaller structures dip at various angles to both the east and west, and shown overlapping age relationships. Quartz Gabbro This forms the body of intrusive rock bounding the west side of the pendant for most of its known length. Some idea of its megascopic fabric is shown by Plate V A . It is characteristically fresh in appearance, with a prominent lineation due to sub-parallel alignment of hornblende prisms. Within a zone some J>00 feet wide adjacent to the pendant this original lineation is largely obscured by a secondary fabric imposed by dynamic metamorphism, as illustrated by Plate TV B # The cataclastic effects to which the quartz gabbro has been subjected gradually becomes less apparent at greater dis-tances west from the fault zones and the pendant. At a distance of one half mile the gneissic intrusive s t i l l shows evidence of subsidiary shear fractures. Under the microscope these are displayed as crushed zones transecting the specimens at 80-90 degrees to the lineation. Drag effects on the twin lamellae of plagioclase are prominent. The attitude of the lineation at two localities is indicated on the large map, Figure 2. - 23 -Rosiwal analysis of a typical thin section of the quartz gabbro indicates the mode to be 60.5$ plagioclase (An^), 8.0$ quartz, 22.$% bluish green hornblende, 2.0$ sphene, 1.5$ apatite and mag-netite, plus 3.5$ epidote and 2.1$ chlorite as alternation products. Following Johannsen's classification this rock belongs to Class 2, Order 3, Family 8. The plagioclase is only slightly more basic than the most calcic zones of that contained in the granogabbro. The greater proportion of mafics and lower quartz content shows that the quartz gabbro is readily conceived as the basic border phase of the main body of granogabbro. The smaller aereal extent and striking lineation would further indicate the validity of this con-ception. The hornblende bearing granogabbro is undoubtedly a type intermediate to these two extremes. The lineation in the quartz-gabbro is considered to be primary for several reasons. Its trend is sub-parallel to the con-tact of the bordering volcanics, and transverse to the major fault and shear zones. The lineation is primarily due to alignment of hornblende prisms which are themselves considered to be original constituents of the intrusive rocks. The hornblende was determined as Hastingsite, (2V = 65-70°, CAZ = 30°) which is listed as having these bluish green pleochroic colours (Larsen and Berman, 1934). The crystallization of this bluish green hornblende is pretectonic. The abundance of twinned grains and the general absence of sieve - 2k -texture support its primary origin* No evidence was noted in any of the thin sections that would indicate that this variety of horn-blende originated by deuteric alteration of an originally brown variety. The possibility of total alteration cannot be eliminated, for under these circumstances there would not exist any evidence of the original status of the hornblende. The linear structure..: in the gneissic quartz gabbro is also shown by the sub-parallelism of large plagioclase grains. Under the microscope i t is evident that within various particular fields of view the plagioclase and the hornblende have crystallized nearly simultaneously, and that both have the same linear trend. In the section as a whole both these minerals deviate by some 10-20 degrees from the mean trend which is north, 60 degrees west. Line-ation of this type in intrusive rocks has been attributed by many structural petrologists to flowage of a partially crystallized magma. The importance of such a process at this locality should not be fully assessed without detailed study of a l l linear structures in the quartz gabbro. The origin of this primary lineation is undoubt-edly related to the mechanics of the igneous intrusion. The pre-sence of numerous xenoliths of country rock within the gneissic border phase, their complete absence in the more massive intrusive, and the apparent conformity between the lineation and one of the intrusive contacts, are features which indicate a forceful intrusion - 25 -accomplished by piecemeal stoping. Xenoliths of volcanic rocks are present i n the gneissic intrusive over a ver t i c a l range of some 2,000 feet. Those at lower horizons are of basalt, which would not conceivably sink in the gabbroic magma. However, since the plunge of the observed lineation i s some 35 degrees to the northwest, i t i s possible that the magma, while s t i l l only p a r t i a l l y crystallized, rose upwards from this direction. Thus the basic xenoliths may have been carried up from a source below. The gabbroic magma has not been able to dissolve these inclusions which are sli g h t l y more basic i n composition. It has however, reacted with others: to convert them to phases more nearly i n equilibrium with i t s e l f . In considering piecemeal stoping as the process by which the quartz gabbro was emplaced, one cannot ignore the relatively steeper dips of the volcanics adjacent to batholith contacts. The low content of volatiles and high prevailing temperatures would impart a high degree of f l u i d i t y to the gabbroic magma. Thus although l i t t l e '-'external;.c pressure would be required to i n i t i a t e flow, the steep transgressive nature of the intrusive contacts would seem to indicate that the emplacement was forceful rather than permissive, and that crowding aside of the walls has occurred. The steeper dips of Takla Group strata adjacent to the batholiths may be due in part to the buttress effect of these massifs during orogeny. The elongate, northwesterly trending batholiths of the Cordillera are - 26 -commonly considered to be syntectonic in general time of intrusion. The l a t e r a l l y directed orogenic forces would thus be a major factor i n the formation of relatively small "injected" bodies, such as that of the gneissic quartz gabbro. Any attempt to propose the mode of intrusion of the mass-ive granogabbro would be mere speculation. It contains no xenoliths of any of i t s walls or roof and the only observed contact i s that forming the east boundary of the pendant. This i s obscured largely by faulting, but seems to be gradational. Since the quartz gabbro was emplaced into a series of volcanic rocks as a transgressive wedge, the granogabbro must have been intruded more permissively, following the bedding planes of the original roof. That none of the former roof i s now evident may be due simply to erosion. On the other hand the large body of granogabbro may represent the crystal-l i z a t i o n i n situ of an only s l i g h t l y differentiated magma. Apparently contradictory to the conception of a flowing viscous magma are numerous hornblende grains developed nearly normal to the plunge of the lineation. These are visible i n the specimen shown by Plate V-A. These grains are diversely oriented i n the plane of the lineation, which here shows minor development of platy flow structure. These may represent solid crystals enclosed i n the flowing laminae, the turbulent, rotational movement of the latter having imparted a dimensional orientation transverse to the main - 27 -direction of movement. The diversely oriented hornblende prisms do not exhibit any relation to the post-magmatic deformation. It has been considered sufficient i n this paper to indicate the features visible i n the f i e l d and under the microscope, which seem to indicate that the lineation i s primary i n origin and can readily be attributed to flowage of a pa r t i a l l y crystalline magma. THE SHEARED' AND ALTERED PENDANT  General Statement The general nature of the pendant as indicated by the definition on page 17, w i l l be better appreciated now that the feat-ures of the plutonic rocks and their respective modes of intrusion have been indicated. The pendant i s composed of a conformable se-quence of pyroclastics and clastic sediments, with at least one thin interbed of limestone. These rocks have been subjected to both thermal and dynamic metamorphism. Metasomatism has been active mainly during deformation. The general lithology at two lo c a l i t i e s i s indicated on the maps of Figure It, 5 , and 6 . Some details of the metamorphosed intrusive and pendant rocks are presented below. Along "A" ridge A - l represents the sheared quartz-gabbro intrusive, as shown on Plate T7B. The original linear orientation of hornblende grains i s s t i l l v i s i b l e i n hand specimens. Shearing has resulted - 28 -in some reduction of grain size. Development of much chlorite, epidote and sphene has been aided by the shearing. Purplish car-bonate veinlets are numerous in the altered rock. The contact of the intrusive with A-3 is gradational and is not sheared. A-2, a narrow (1 foot) quartz-carbonate vein-filling around fault breccia in the quartz gabbro. The vein contains augens of crushed quartz and plagioclase. This f i l l i n g is similar to that introduced along the main fault zone. Though A-2 vein is very narrow, carbonate alteration evidently extends sufficiently within the walls on either side to form a weathered zone which appears as the line-ament shown on Figure 2. A-3 is a dark grey volcanic, quite fresh in field appear-ance but exhibiting considerable alterations under the microscope. The rock shows no cataclastic effects and relatively minor contact metamorphic effects from the nearby intrusive. These are mainly the development of chlorite and sericite in the matrix. In some areas the aphanitic groundmass is more or less welded together. Most of the large plagioclase grains are of labradorite. They are altered slightly to sericite, some wholly to carbonate. There are no indications of neomineralization as a result of the intrusion. Any original ferromagnesian minerals, have been replaced by car-bonate i Aggegrates and halos of magnetite surround a l l the larger grains and crystal fragments. Though this rock ressembles a por-phyritic basic flow, the presence of some obvious crystal fragments - 29 -together with the contrast between a felted matrix in some local-ities and a welded aggregate in others are stongly suggestive of a basic tuff. The absence of even minor relicts of ferromagnessian minerals supports this proposal. A-k, a rusty weathering quartz-carbonate "vein". The car-bonate occurs as large, irregular and interlocking grains, often plume shaped. The quartz is in lamellar and fibrous forms. Sutured bor-ders and undulose extinction are also characteristic. Networks of opaque black metallics are prominent. Present in minor amounts are sericite and chlorite. Post crystalline shearing has occurred through-out, resulting in granulation of the minerals along narrow zones. A-5, consists of apparently uniform chlorite schist. Breaking open of the dark grey chlorite folia reveals large augens, which under the microscope are seen to consist of recrystallized quartz and basic plagioclase. The quartz is present as elongate fibrous grains showing a prominent lineation parallel to the shear-ing directions. Sutured borders are present on a l l these grains. Undulose extinction in two directions is also marked. The recrystal-lized plagioclase (An^) occurs as generally elongate, curved grains. The twin lamellae are very sharp and there are no inclusions within the irregular grains. These are inclined at various angles up to 90 degrees to the shear planes. The curved, inclined form of most - 30 -of the plagioclase grains indicates that stress was s t i l l an active factor during the recrystallization of these minerals. The augens are up to an inch in diameter. A l l of the material has not recrysta-llized for zones of finely crushed quartz and plagioclase alternate with zones of recrystallization. Minor carbonate and sericite are present in the augens. The chlorite of the schist is clinochlore. A-6, consists mainly of rusty weathering quartz-sericite schist containing numerous small (l-2mm.) augens of quartz and plag-ioclase. The quartz in the augens is severely strained and shattered. The plagioclase shows the indistinct, patchy twinning of incipient recrystallization. The rusty appearance is due to weathering of fine folia of chlorite between the sericite. Some lens shaped augens of quartz are unstrained while others show various stages of rotation and resulting strain effects. The recrystallized plagioclasesare often enclosed in a murky mass of feldspar showing the same relief and absence of albite twinning. The recrystallized plagioclase appear to be of uniform composition (An^). Some dense, relatively unsheared lenses occur in these schistose rocks. The massive material is clearly a tuffaceous sand-stone, composed of sand-sized angular fragments of quartz and basic plagioclase in a mainly calcareous matrix. Argillacious partings - 31 -in this have been converted to chlorite and sericite. The shearing has been localized along these planes of weakness. Some recrystal-lization of the calcareous matrix has occurred. The quartz is much strained and fractured. ' Boehm lamellae are also distinct in the quartz, which constitutes two thirds of the fragments. Rims of short stubby recrystallized plagioclase grains surround some of the original labradorite. Some rock fragments are present and recrystal-lization of the feldspars in these has occurred. The main size range of the particles is from 0.5 - 1.0 mm. The irregular shape of most fragments, together with the sub-rounded form of other grains, indicates a mixed origin. Tuffaceous sandstone is considered a more appropriate name than lith i c tuff. A-7, consists of sheared and altered volcanic rock, the original composition of which is doubtful. It was termed greenstone in the field. This forms the northeastern boundary of the pendant as exposed along "A" ridge. The contact with the granogabbro is gradational over a width of 25 feet. Considerable shearing has occurred over a width of 300-U00 feet. The greenstone may have been originally a porphyritic basic flow or tuff. The equivalent formation on "C" ridge is a strongly sheared tuff. Porphyroclastic grains of tan hornblende and of basic plagioclase may be original constituents, although the possibility that they were introduced from the adjacent intrusive - 32 -i cannot be totally disregarded. The contact metamorphic and meta-somatic effects that can be attributed to the quartz-gabbro alone have not been indicated as being very prominent. In thin section the original aphanitic groundmass can occasionally be distinguished as a felt of fine plagioclase laths. Replacement of the plagioclase phenocrysts by epidote has been nearly complete. In addition epidote veinlets occur parallel to the shear fractures. Much chlorite occurs throughout the streaked matrix. Late carbonate veinlets transect a l l minerals of the greenstone. A-8, is the contact phase of the coarse grained granogabbro. It shows the effects of strong shearing and alteration. Mortar structure is characteristic of the deformed plagioclase grains. The quartz shows strongly undulose extinction, often in two direct-ions within single grains. Alteration of the plagioclases and the microcline to sericite has been considerable, and of the original biotite to penninite has been complete. Much epidote and clinozo-isite are developed in the most severely crushed zones. In slightly less altered granogabbro beyond this, (A^) some original biotite and hornblende are present. Sphene has been developed as an alter-ation product of the minor hornblende present. The plagioclases are zoned but alteration has considerably obscured this. The approximate composition of the granogabbro is 50% plagioclase (An^ Q - An^0), 15-20$ microcline and microperthite, 10-15$ quartz, - 33 -10-15$ epidote and chlorite as alteration products of the original mafics. Sphene and apatite are minor constituents, the former being mainly secondary. Earlier deuteric alterations i n the intrusive rocks are described under petrography. Along "B" Ridge Although the succession along "B" ridge i s essentially similar to that just described, a resume of the general sequence and some detail on a large xenolith i n the quartz gabbro are given below. Only five thin sections of rocks from this l o c a l i t y were studied. B-ly i s a relatively large (50 X 150 feet) xenolith i n the gneissic quartz gabbro. The sharp contacts are irregularly embayed in detail, but the general shape of the included mass i s that of a lens, par a l l e l to the trend of the pendant. What was considered to be probable fine bedding was megascopically visible i n the xeno-l i t h . In thin section the rock i s a quartz-hornblende - plagioclase hornfels, with the hornblende and minor biotite occurring along sub-parallel bands which appear asorelict-i bedding. The hornblende i s the bluish green variety as present in the intrusive rocks. The quartz and plagioclase form a uniform mosaic of grains ranging from .01; - 0.1 mm i n size. Scattered throughout this matrix are larger grains of murky plagioclase showing patchy albite twinning and con-taining numerous inclusions of the fine grained matrix and of the hornblende and chlorite. The f o l i a t i o n expressed by the hornblende grains diverges around these large grains. They are considered to - 3k -be original constituents of the sediment that have pa r t i a l l y recrys-t a l l i z e d . Minor constituents of the hornfels are epidote and penn-ini t e as alteration products of the hornblende and biotite respect-ively, also a few grains of apatite and sphene. One elongate but rounded grain of apatite i s O.ii mm in diameter. The sphene occurs i n irregular aggregates and thus i s a secondary constituent. The small, sharply twinned plagioclase grains within the mosaic were determined as labradorite. Orthoclase i s also present i n considerable amounts. Although a large number of grains were examined i t i s impossible to say that recrystallization has been complete and that the composition of the plagioclase i s uniform. One of the large murky plagioclase grains i s evidently andesine. Another large grain containing inclusions of the matrix was deter-mined as Labradorite (An^ Q). The hornfels i s considered to represent a thermally met-amorphosed s i l t y sediment. The large murky feldspars may represent scattered detrital or pyroclastic feldspar fragments. Recrystalli-zation has obscured their original boundaries. The sediment prob-ably contained a large percentage of siliceous material as compared to argillaceous, for quartz i s a major product of the thermal met-amorphism (Turner, 19U8, p. 79). Some AI2O3 was undoubtedly pre-sent to account for some of the components of hornblende and biotite. The main additions were probably CaO, MgO and FeO. Without further - 35 -knowledge of the original composition i t i s impossible to consider removals from the sediment. By comparison, inclusions of slate i n the Duluth gabbro have been converted to hornfels consisting chief-l y of labradorite, pyroxene, biotite, cordierite, and l o c a l l y quartz, magnetite, garnet, epidote, etc. The gains i n the slate inclusions . were mainly CaO and MgO, plus some FeO and Ti02, with correspond-ing losses i n 1^0, Si02 and H2O. The development of cordierite i s augmented i n the zone of higher temperatures within 5 feet of the intrusive contact (Grout, 1933). The amphibolite hornfels facies as represented i n this xenolith on "B" ridge i s indicative of a lower temperature than that required by the pyroxene hornfels discussed by Grout. Turner (19U8, p. 78) states that where K2O i s present i n excess the appearance of andalusite, cordierite or anthophyllite i s prevented. A stable assemblage l i s t e d by Turner i s hornblende - biotite - plagioclase -microcline - quartz. The contact between intrusive rocks and the schistose older metamorphic rocks i s sharply defined by faulting. A 50 foot zone of v i s i b l y sheared gabbro appears in sharp contrast to 30 feet of schistose quartz-sericite-carbonate rocks containing several narrow quartz-carbonate veins. In the f i e l d these schistose rocks were considered to represent sheared sediments or pyroclastics, for the texture becomes gradually coarser to the northeast and the rock - 3 6 -becomes a coarsely foliated chlorite schist containing augens of quartz. No specimens of the quartz-sericite schist were obtained because of i t s extremely friable nature. The whole zone represents one of numerous shear zones i n the intrusive and older rocks over a width of some 800 feet. These evidently are subsidiary faults re-lated to the continuation of the carbonated breccia zone along the intrusive contact on "A" ridge. In the section shown on Plate II A, one of these steep subsidiary faults i s visible i n the quartz gabbro. A carbonated breccia zone occurs further to the west on the abrupt northeast slope beyond "B" ridge. It was not observed on "B" ridge proper, for large gl a c i a l boulders cover much of a rounded nose on the west end of this mountain. The sheared intrusive rocks at the contact with the quartz-sericite schist are represented by specimen B-2. Intense cataclas-t i c effects and considerable alteration are present. Deformation and crushing of the basic plagioclase i s followed by development of epidote i n the mylonitic material. Some of the plagioclase has recrystallized i n l o c i between the crushed zones. These plagioclases are characterized by sharp, curved twin lamellae in contrast to the bent and displaced lamellae of the original feldspars. These sec-ondary feldspars are of the same composition, but i n addition to the above features, show only minor replacement by late carbonate, whereas the original feldspars contain much chlorite and carbonate. Carbonate veinlets are numerous throughout the specimen. - 37 -B-3, i s a quartz - carbonate - mariposite gneiss. The quartz occurs as lamellar and fibrous grains, evidently recrystal-liz e d , which constitute large augens i n the carbonate. The carbon-ate i s i n plumose grains with f o l i a of the green chromian muscovite separating adjacent laminae of carbonate and fibrous quartz. The f. ankeritic carbonate replaces the quartz and euhedral rhombs of the former are numerous i n the bands of fibrous quartz. Some of the carbonate is reduced to a fine grained condition by late shearing. The contact with B-h i s sharp. B-J|, was termed a sheared p h y l l i t i c sediment in the f i e l d . It exhibits fine banding of alternating lig h t and dark gray layers. The surfaces of these contain numerous slickensides. Rusty weather-ing i s characteristic. Since both l i g h t and dark layers effervesce vigorously i n cold dilute HC1, the rock is evidently a sheared ar-gillaceous limestone. This i s some 60 feet i n thickness and i s followed by 20 feet of chlorite schist, which was so similar i n f i e l d appearance to that along "A" ridge that no specimen was taken. B-5 represents a 1 foot zone of quartz-sericite-carbonate schist, indistinguishable i n hand specimens from A5. The matrix i s markedly calcareous. B-6, consists of 2 feet of dense, gray limestone. A thin section shows the fine bedding and the presence of st y l / o l i t e s . - 38 -Very minor recrystallization of this limestone has occurred. It i s in sharp contact with i t s surroundings. B-7, consists of 225 feet of p h y l l i t i c sediments (as B-4) grading into a dense, gritty, green schist, which is very similar i n appearance to C-9« The gr i t t y matrix is calcareous, but much argillaceous and chlor i t i c material are evident i n hand specimens. This schist gradually becomes coarser textured to the northeast as the sheared pyritized contact zone i s approached. The rocks at the contact are pyritized greenstones much like C-X i n f i e l d appearance. Four shear zones are present here as along "C" ridge. These are characterized by several feet of rusty weathering s e r i c i t i c and chlori t i c schists. Large amounts of coarse grained pyrite are pre-sent i n the intervening rocks. Much quartz has also been introduced. The sheared volcanics and intrusives are indistinguishable over a width of 10-15 feet at the contact. Along "C" ridge C-l, i s the sheared intrusive adjacent to the pendant. Whereas the sheared plutonics west of the pendant at "A" and "B" are of the quartz diorite, C-l i s equivalent i n composition to the hornblende bearing granogabbro along the eastern contact. It con-tains 10-15$ microcline, 6$ quartz, 18$ hornblende, 50$ plagioclase ^62)* w i' t l 1 2 ^ apatite and minor magnetite as accessories. As alteration products clinozoisite, epidote, sphene, and penninite compose some 10$ of the section studied. Cataclastic effects are - 3 9 -prominent throughout , bu t the p a r a l l e l s t r u c t u r e o f hornblende and p l a g i o c l a s e i s s t i l l p r e s e r v e d . The i n t r u s i v e here con ta ins a l a r g e x e n o l i t h , and pene t ra tes between C - 3 and the quar tz -carbonate band. Th i s narrow tongue o f the i n t r u s i v e con ta ins numerous s m a l l xeno-l i t h s o f the same appearance as C - 2 . These are darker i n c o l o u r than the p i n k and grey i n t r u s i v e . The con tac t s between the i n t r u -s i v e and C - 3 are sharp and v i s i b l y c h i l l e d , even though s l i c k e n s i d e s show tha t f a u l t i n g has occu r red a long them. C - 2 , i s cons ide red t o be p robab ly a cognate x e n o l i t h r e -p r e s e n t i n g the e a r l i e r qua r t z gabbro. The compos i t ion and t e x t u r e are t ha t o f the sheared q u a r t z gabbro, as B - 2 . There i s s l i g h t l y more c l i n o z o i s i t e accompanying the ep idote a l t e r a t i o n o f crushed zones and the f e l d s p a r s are co lou red l i g h t brown by f i n e l y d i s s -eminated c l a y m i n e r a l s . R e c r y s t a l l i z a t i o n o f the p l a g i o c l a s e i s not as e v i d e n t . There i s no m i c r o l i n e p resen t and much o f the f i n e r g r a i n e d appearance i s due to shear ing which has a f f e c t e d the xeno-l i t h t o a g r e a t e r degree than the e n c l o s i n g granogabbro. The s i m i -l a r i t y o f the t e x t u r e and compos i t ion to B - 2 i n d i c a t e s t ha t the x e n o l i t h i s o f the e a r l i e r i n t r u s i v e . Some 1 0 0 yards west of t h i s x e n o l i t h the p i n k and g rey phase was observed to grade i n t o the g rey , more s t r o n g l y g n e i s s i c phase . C - 3 , c o n s i s t s of a r e d d i s h brown t u f f w i t h one s m a l l i n -t e r c a l a t e d l e n s o f coarse r p y r o c l a s t i c s . The megascopic appearance - Uo -and f i e l d association are of a pyroclastic, but the microscopic tex-ture and composition show no more of the often cited c r i t e r i a of pyroclastic origin than some of the more doubtful tuffs previously described. This tuff consists of large phenocrysts and crystal fragments of basic plagioclase i n an aphanitic reddish brown matrix. Some large mosaic aggregates of clear subhedral quartz are also present. The brownish matrix appears as a welded mass of small irregular quartz and feldspar grains. This tuff occupies, the same relative position as A-3, but i s more siliceous. The intrusive contact i s also sharp rather than gradational. The brown colour-ation i s not original, but seems to be due to finely divided clay minerals. Some residual laths of basic plagioclase can be recog-nized i n this. Considerable amounts of green chromian muscovite form irregular fibrous f o l i a throughout the brownish matrix. Some replaces plagioclase grains, as does carbonate. No original mafic minerals are present. A mineral occurring as an alteration product of the feldspars, i n vermiform aggregates of radiating fibers with very high birefringence was tentatively identified as pyrophylite. It i s closely associated with the clear quartz aggregates. C-$, i s a narrow, sinuous basalt dyke that trends north So degrees east, and thus cuts traversely through the brownish tu f f . It terminates abruptly at both the contacts of the tuff. In thin section the basic rock i s considerably altered, but consists of large zoned phenocrysts of basic plagioclase (An' - An^Q) i n a fine grained - hi -matrix of plagioclase laths (An6o) and altered augite grains. The augite i s largely replaced by brown hornblende and epidote. Other secondary products are chlorite replacing the plagioclase, plus dis-seminated grains of magnetite, and minor carbonate. This porphyrit-ic dyke i s of similar composition to the other basalt dykes of the area. It has evidently followed a northeasterly trending fracture, one of a joint system that i s well developed i n the batholithic rocks and has been responsible for the location of the transverse tribu-taries of the Chukachida River. The dyke may thus be post-batholith, and of Tertiary age. It has been displaced by faulting along the intrusive contact and within the pendant. No continuation of the dyke was located. C-6, consists of a buff weathering quartz-carbonate band, somewhat foliated and contorted. No thin section of this was ex-amined. The contacts of this on either side are sharp. C-7, consists of sheared p h y l l i t i c sediment, evidently equivalent to B-I4. It contains alternating argillaceous and tuff-aceous laminae i n a calcareous matrix. A thin section of the more schistose phases shows alternation of bands of crushed quartz and feldspars with f o l i a of chlorite and sericite. Quartz veinlets trend across the sheared sediment. C-8, i s a narrow lens of banded quartz-carbonate gneiss, some 3-5 feet wide and only 100 feet i n length. In megascopic appear-ance as shown by Plate VI B i t greatly res^embles a siliceous dolo-mite. However, the refractive indices reveal the carbonate to be - uz -ankerite. Folia of green chromian muscovite occur between the plu-mose grains of quartz. In some areas of a typical thin section the plumose quartz grains assume a spherulitic habit. The carbonate i s also i n large ragged, feathery grains. Some pyrite and sphalerite occur i n this gneiss. Another thin section of this rock contains no chrome mica, but much fine chlorite associated with fibrous, chalcedonic quartz. C-S, is a serpentinized alteration zone extending between C-8 and C-6. This i s considered to represent an early stage i n the alteration leading to the formation of the quartz-ankerite-maripo-site gneiss. Complete discussion of this alteration w i l l be pre-sented on later pages. A thin section of the serpentinized rock shows typical meshes of fibrous antigorite, i n part replaced by talc and magnetite, as pseudomorphs. Minor carbonate i s present between the antigorite fibers and as veinlets following the individ-ual boundaries of the mesh structures. The serpentinization has affected the calcareous tuff between the two bands of quartz-ank-erite gneiss. I t i s weakly developed below and above the termin-ations of the relatively short lens C-8. C-9, (not shown on Figure 6) consists of a thin (1 foot) lens of dense sedimentary material i n schistose calcareous tuff (C -7 ) . It has retained much of the original texture, for the shearing has mainly been directed around thin lenses such as this and the dense - U3 -grey limestone B-6. The thin lens here consists of a fine grained tuff i n a mainly calcareous matrix. The crystal fragments are relatively unaltered. These are almost exclusively of basic plagioclase, a few are of more acidic feldspars and of quartz. Only minor sericite has been developed i n the feldspars. The angular fragments range i n size from ,0k mm. to .5 mm., averaging .2 mm. The matrix i n part consists of finely fibrous chlorite. Some magnetite grains are present but no original ferromagnesian minerals. Intersecting shears are vis i b l e i n the thin section. In these the feldspar and quartz grains are finely crushed. C - l l and C-X, represents a highly sheared and altered band of which the specimens would be termed sheared^ propylitized volcanics. Even under the microscope, their original composition i s uncertain. The intrusive rocks beyond have been highly sheared and altered. They are distinguished by their speckled grey appear-ance i n contrast to the greenish volcanics. The alteration immed-iately adjacent to the contact consists of bleaching and s i l i c i f i -cation of the volcanics, plus introduction of disseminated coarse grained pyrite. Four separate east dipping faults cut the altered volcanics and the intrusive adjacent to the contact. Along "C" ridge these are marked by narrow schistose zones i n the gabbroic intrusive and by slickensided c h l o r i t i c shear zones i n the altered - hh -volcanics. At the immediate surface the latter are also disting-uished by rusty zones i n which iron oxides derived from the weather-ing of the s i l i c i f i e d and pyritized contact rocks have been depos-ited. The faulting has been distributed over a width of at least 250 feet. Thin sections of- some of the least altered volcanics near the contact reveal some textures nearly identical with that of other tuffs of the pendant. Large grains of basic plagioclase have been altered almost completely to radiating aggregates of epidote. Since the radiating epidote grains have not suffered brecciation they are either post shearing or the section represents an isolated lens between shear fractures. The latter i s evidently the case, for as illustrated by Plate III B , lenses of sheared and unsheared material alternate throughout these altered volcanics which probably repre-sent propylitized tuffs. Considerable chlorite i s present i n the aphanitic groundmass, but the jumbled texture of irregular quartz and plagioclase grains i s s t i l l evident. Large crystal fragments and phenocrysts are indicated mainly by the outlines of the epidote aggregates. C-X i s much the same as C - l l , but shows more marked zones of granulation and streaking i n the tuffaceous matrix. C-10, i s the finer grained, altered intrusive adjacent to the contact. It i s medium grained and speckled grey and black. - 16 -The grain size v i s i b l y coarsens away from the altered contact zone. In thin section this rock shows considerable alteration but minor shearing effects. Quartz i s the only fresh mineral and appears to be deuteric for i t replaces large areas of the plagioclases, forming myrmekitic intergrowths. Mo microcline i s present i n this contact phase so the rock i s actually quartz gabbro rather than granogabbro. The plagioclase i s much altered to sericite and epidote. Hornblende is altered completely to penninite, sphene and carbonate. The fact that quartz gabbro appears on this side of the pendant and the granogabbro on the opposite side, illustrates that differentiation i n the magma was not controlled by the pendant. As indicated previously these phases are reversed i n their relative positions at the other two l o c a l i t i e s studied. In addition frag-ments of the earlier quartz gabbro are enclosed i n the granogabbro. This i s the only direct evidence of their relative ages. GENESIS OF THE PENDANT C r i t i c a l Features Now that the lithology of the rocks composing the pend-ant has been discussed, i t i s feasible to consider i t s origin. Roof pendants i n general are often separated into two main types. Of the extensive areas of older metamorphic rocks occurring within the var-ious phases of the enormous Coast Range Batholith, many are large, - 46 -relatively shallow saucers, representing irregularities of the o r i -ginal roof. Their preservation i s relative to the amount of ero-sion that has ensued since the highest cupolas of the batholith were f i r s t exposed. Such areas of variously metamorphosed rocks are often referred to as 1 roof remnants1 rather than roof pendants. Mineralization within these is often of contact metamorphic type. Sharply contrasted to this variety are other masses of older vol-canics and sediments which have apparently been deeply infolded with-i n the batholith or have formed near vertical partings between sep-arate intrusives (Gunning, 1950). Many of this type, such as the Brit/annia mine pendant, have been shown to persist without appar-ent change through vertical ranges of several thousand feet. The rocks are often highly deformed gneisses and schists. Shear zones are particularly numerous. Any mineralization present i s hydrother-mal rather than contact metamorphic. A deeply infolded parting of such metamorphic rocks within the Sierra Nevada pluton has been aptly termed a 'roof septum' (Mayo, 1935). The pendant discussed i n this paper possesses character-i s t i c s of each of these types outlined above. It i s shown at two l o c a l i t i e s to persist without apparent changes i n lithology or struct-ure through a vertical range of 2,000 feet. The enclosing intru-sive rocks are considered to represent differentiated phases of a common magma. The rocks composing the pendant have been highly deformed. In contrast to these features,. the only metallic - U7 -mineralization i s of the contact metamorphic type. Other character-i s t i c s that have a bearing on the possible genesis of the pendant are now presented. Igneous Intrusion and Thermal Metamorphism The mechanics of igneous intrusion have already been pro-posed, particularly for the gneissic quartz gabbro. It should be emphasized that these are highly theoretical, for they are based mainly on two measured attitudes of the lineation and the presence of numerous xenoliths i n the gneissic intrusive west of the pendant. The contact between this body and the adjoining volcanics was only examined with any detail on the south slope of "A" ridge. The con-tacts between the pendant and intrusives were noted more carefully, and as previously indicated, are mainly sharp along the western bor-der and are obscured by alteration along the opposite side. It i s probable that the quartz gabbro while s t i l l highly f l u i d was injected upwards along the most obvious planes of weak-ness i n i t s cover of volcanic and sedimentary rocks. Contacts be-tween strata of highly contrasted composition and texture may have offered more favorable zones. The pyroclastics and impure sediments within the pendant, should have been highly permeable. Yet their ab-normal composition seems to have prevented extensive injection and plutonic metamorphism by the magma. This early intrusive was pre-sumably chilled along the borders of the tabular body of older rocks - J U S -at an early stage i n the sequence. The paral l e l attitudes of the strata within the pendant and those of the adjacent area of volcan-ics i s considered to be accidental, for the faulting along the i n -trusive contacts would have undoubtedly disturbed the attitudes present i n the flanking area of volcanics. That the tabular pen-dant was detached from i t s surroundings by the f i r s t surge of gab-broic magma i s suggested by the indirect evidence that flow was later i n i t i a t e d towards a locus of reduced pressure. Under the i n -fluence of tangential orogenic forces from a westerly direction, upward flow of the f l u i d magma was stimulated i n the confined area between the tabular downward projecting pendant and the flanking volcanics. During the late stages of crystallization and dif f e r -entiation, minor resurgence of a more alkaline residual magma occurr-ed. This magma lo c a l l y invaded the chilled contacts of the earlier intrusive and the pendant , and at further distances where crystal-l i z a t i o n was not complete, was able to merge with this earlier phase. The events were not widely separated i n time although the quartz gabbro and granogabbro are strongly contrasted i n composition. The more alkaline nature of the massive granogabbro is due to replace-ment of early formed crystals by a deuteric residue charged with SiC>2, K 2 0 and Na20. The main differentiation and crystallization was of a phase considerably different to the quartz gabbro in com-position, for i t contained less than 10$ of ferromagnesian constit-uents . Phases intermediate;. i n composition between the biotite- j granogabbro and the hornblende-quartz gabbro are present mainly - k9 -along the eastern border of the tabular pendant. Quartz and micro-perthitic microcline were introduced at a late stage i n the crystal-l i z a t i o n of the granogabbro and intermediate phases. The thermal metamorphism that can be attributed to the gabbroic intrusives i s of low grade. A summary of observed effects w i l l serve to il l u s t r a t e this. Along "A" ridge the quartz gabbro grades into a band of porphyritic basic tuff. Away from the contact this tuff contains crystal fragments and phenocrysts of basic plag-ioclase i n a chl o r i t i c matrix. At the gradational contact, develop-ment of hornblende and quartz must have occurred over a relatively narrow range i n the tuff, to account for the gradation. Away from this narrow gradational zone the only evident effects are partial recrystallization of the aphanitic matrix and development of mag-netite coronas around crystal fragments. Development of sericite and chlorite are also attributable to the adjacent intrusive. Along "B" ridge the quartz gabbro contains a xenolith of recrystallized sedimentary and/or tuffaceous rock, now represented by a hornblende-plagioclase-orthoclase-quartz hornfels. In accord-ance with the behaviour of inclusions i n magma as determined by Bowen (1922, pp. 513-570), the original sediment must have contained mineral aggregates of such bulk composition that they would repre-sent a member of the Reaction Series with which the gabbroic magma was effectively supersaturated. Yet elsewhere xenoliths of more - $0 -basic composition than the magma have been essentially stable i n their original form. The sediment has been converted by reaction with the magma to a hornfels more nearly in equilibrium with the magma. Possible additions from the magma were FeO, MgO and CaO, plus minor Ti02» These may have been wholly recomposed constit-uents of the original sediment. Apatite appears to be a stable r e l i c t i n this hornfels. Along 11C" ridge a band of silic i o u s tuffs has , suffered two successions of igneous intrusion and thermal metamorphism. The contact of the latest intrusive and the tuff i s sharp. This brown-ish pyroclastic consists of crystal fragments and phenocrysts of calcic plagioclase i n a chlor i t i c and siliceous, aphanitic matrix. While some relatively large grains of quartz have been introduced, the irregular quartz i n the matrix has the appearance of recrystal-l i z a t i o n . The bulk composition consists of CaO, K^ O, FeO and MgO, combined with AI2O3 and Si02> plus some free s i l i c a . This i s equiv-alent to the bulk composition of the hornfels (B-2) which evidently represents an assemblage in equilibrium with the magma. There has been only minor introduction of Si0 2, plus considerable development of sericite and clay minerals. Chlorite may have formed from o r i -ginal ferromagnesian constituents. Approximately one-half mile west of C-2 and at some 2,000 feet lower elevation, numerous basic inclusions occur i n the gneissic . 51 -quartz gabbro. These are up to £0 feet across and irregularly em-bayed by the intrusive, which shows a slig h t l y finer grained chilled border adjacent to these. A thin section (C-lU) of one of these xenoliths shows the rock to be a basaltic tuff. Crystal fragments comprise two thirds of the rock and average 0.2-0.3 mm. in size. These are mainly of calcic plagioclase and brown hornblende. The matrix consists largely of secondary biotite and chlorite. Magne-t i t e halos surround a l l of the crystal fragments. Many of the plag-ioclases are fa i n t l y zoned, the composition being as calcic as An^Q. Slight replacement of the plagioclase grains by biotite i s evident. This basic tuff has suffered but minor alteration. The magma may have introduced some K^ O to aid in the formation of biotite, ^ though this i s a constituent i n which the quartz gabbro i s i t s e l f lacking. The assemblage represented by the basaltic tuff i s essential-l y i n equilibrium with the magma, even though free quartz i s not a constituent. Along the eastern edge of the pendant the thermal metam-orphism of basic tuffs has been already discussed i n some detail. Alteration of these tuffs to propylitic aggregates of quartz-epi-dote-chlorite with quartz and pyrite has occurred over a width of some £0 feet. As considerable epidote and chlorite are also pre-sent i n the intrusive, the contact has been considerably obscured, although along "C" ridge there i s a visible decrease i n grain size i n the intrusive as the pendant i s approached from the east. The - 52 -visible effects of the alteration of the basic volcanics are the bleached greenish colour of the contact rocks. Addition of quartz and pyrite seems to be the main metasomatic effect. The other con-stituents represent exchanges rather than additions. CaO displaced from the calcic plagioclases by reactions i n the late stages of crystallization i n the magma could combine with mafic constituents to form epidote. Within the more central portions of the pendant calcareous strata have experienced but minor recrystallization and no formation of metasomatic s i l i c a t e s . In the basaltic tuffs along the borders of the pendant there i s no evidence that grains of plagioclase have been recrystallized to their present stable state. Furthermore no growth of plagioclase porphyroblasts i s evident. That the gabbroic intrusives have achieved only minor thermal (contact) metamorphic effects upon the strata within this pendant is considered to be the result of several factors l i s t e d below. 1 . The intrusives are of basic types, high i n CaO, FeO and MgO combined with AI2O3 and S i 0 2 , representing crystals slightly more basic than the magma with which they were in equilibrium. 2. It i s generally agreed that basic magmas represent high temperatures of crystallization and corresponding low concentrations of volatile constituents. The gabbroic magmas under consideration - 53 -are probably more equivalent i n these features to intermediate mag-mas, for f e l s i c constituents are present i n larger amounts than the mafics. They are nevertheless relatively "dry". 3. The tabular pendant i s bounded by basic tuffs whose chem-i c a l and mineralogical composition are so markedly similar to the . crystalline phases i n equilibrium with the magma,that with the ex-ception of some migration of FeO and MgO at the immediate contact, the thermal metamorphic effects have been insignificant. The basic tuffs have further insulated the rest of the narrow pendant from la t e r a l l y spreading thermal and metasomatic effects. Xenoliths of basic tuff i n the quartz gabbro have suffered only the development of biotite, the constituents of which may have originally been pre-sent. In contrast to this, a sedimentary xenolith within the quartz gabbro has been converted to a hornfels containing a mineral assem-blage i n equilibrium with this magma. The apparently greater contact metamorphic effects along the eastern margin of the pendant may be attributed at least i n part to the gre ater activity of residual magmatic solutions within the granogabbro intrusive. Metamorphism of the strata within the pendant that can be related to rising magmatic emanations must also have been at a minimum, for limestones and calcareous tuffs adjacent to the var-ious schists have not experienced recrystallization or formation - 5U -of secondary s i l i c a t e s . This may appear to be discounted by insul-ating effects of the basaltic tuffs. However, the composition of recrystallized plagioclase in the schists A-f? and A - 6 , indicates that while equilibrium may be in effect within each deformed band, there is no common stable composition. Deformation has undoubtedly played a major role i n the recrystallization of quartz and feldspars within the schistose rocks of the pendant. Since this has been post-intrusion, early effects by rising emanations during the main crystallization of the magma have been obscured. The mineral assemblages comprising the metamorphic rocks of the pendant and of various xenoliths, a l l enclosed i n gabbroic intrusives, seem to represent a general equilibrium with the gabb-roic intrusives. One i s forced to speculate on the disposition of the sediments and volcanics that have been displaced or incorporated by the magma. This paper does not attempt to solve the controver-s i a l 'room problem'. Some discussion of granitization w i l l be dealt with on subseqtient pages. Dynamic Metamorphism • Dynamic metamorphism has been extremely active within the intrusive and pendant rocks alike. The resultant effects produced depend largely upon the type of rock affected. Thus the westerly dipping fault along the intrusive contact is a relatively wide brec-cia zone on "A" ridge suggesting one main interval of faulting that - % -attained r e l i e f of pressure mainly upwards. The imbricate struct-ure of the branch faults here and the westerly dip of the main brecciated zone seem to indicate a thrust-fault from the west or southwest. Further to the north this main fault diverges into the intrusive rocks, and no faulting i s visible along the volcanics-intrusive contact on "B" ridge. The carbonated breccia zone is considerably narrower where observed on the northerly slope of "B" ridge. Its dip i s near vert i c a l , as are dips of three or four sub-sidiary faults observed here. One of these steep faults i s evident on Plate II B. The faulting has been distributed over a width of some 300 yards. Narrow chlor i t i c shear zones mark the subsidiary faults i n the intrusive. In individual specimens carbonate and epidote veinlets indicate intersecting shear fractures. Faulting within the pendant and the intrusives to the east is particularly prominent.' The differential movements have been shearing along bedding i n the sediments and pyroclastics, and along planes of weakness i n the intrusives, these latter planes obviously being related to the attitude of the tabular, pendant. Prominent "S" planes within the metamorphic rocks have a general dip averaging 70 degrees eastward. The f o l i a t i o n shows minor crenulations and local r o l l s with reversals of dip. Lenticles of highly schistose rock often alternate with unsheared l e n t i c u l a r i r e l i c s . Augens of quartz and plagioclase i n the schistose rocks are indications of the rotational movements i n effect. Some of the calcareous matrix i n - 56 -the tuffs has flowed under the stress imposed, to f i l l fractures in adjacent argillaceous laminae and i n individual augens. Some recrystallization of the quartz and plagioclase has occurred within augen structures of the various schists. In the most highly deform-ed rocks, such as the chlorite schist on "A" ridge, the recrystal-li z a t i o n has been i n directions to offer the maximum resistance to deformation. "Throughout the deformed rocks, quartz shows a greater tendency than feldspars to develop preferred orientation, at least of such a nature as to be evident with normal petrographic micros-copes. The most highly sheared rocks at one lo c a l i t y are not nec-essarily the stratigraphic equivalents of those with such charac-te r i s t i c s further along the strike of the pendant. Shearing has been mainly concentrated along the most argillaceous laminae of i n -dividual strata. Variations i n the original lithology were undoubt-edly present. That the shearing movements occurred while the rocks were essentially at their present position i n the earth's crust, i s shown by the local nature of the deformation and the absence of mineral assemblages c r i t i c a l for rocks deformed at deeper levels. Parts of the pendant may have been downfaulted into their present position. However, the gradational contact between quartz gabbro and basic tuff on."A" ridge, as well as the absence of shearing within this tuff serve to contradict this concept as including the whole pen-dant. No direct evidence of directions of relative movement was noted. - 57 -Coarse grooves on the northeast contact of the quartz-carbonate band (A-li) plunge 25-35 degrees to the north. These may indicate the main direction of the latest movement at this l o c a l i t y . In the absence of further evidence, the faulting within the pendant and adjacent granogabbro i s tentatively attributed to marginal thrusts related to elongation of the batholithic mass. If such i s the case the central portions of the pendant have been downfaulted relative to i t s flanks. The preservation of the pendant may thus be direct-l y related to post-intrusive faulting, though i t s origin i s related to various factors involving the mechanics of intrusion and the composition of the intrusive and older rocks. CARBONATIZATION AND SILICIFICATION Many of the faults and shear zones discussed previously have been altered to buff or rusty weathering aggregates of anker-i t i c carbonate and quartz, with minor amounts of chromian Muscovite and chlorite. Such carbonatized zones are prominent features through-out north-central British Columbia. In the Fort St. James map-area (Armstrong, 19U9) carbonatization i s widespread along the Pinchi and Manson fault zones, and numerous, apparently related subsidiary breaks. Such alteration i s also characteristically developed i n the Aiken Lake and McConnell Creek map-areas (Roots and Armstrong, 1948; Lord, 19U8). Though successive stages of faulting and alter-ation have occurred along these zones, the carbonatization and - 58 -s i l i c i f i c a t i o n are attributed to the same ascending hydrothermal solutions that formed the abundant quartz-carbonate veins i n these areas. A notable feature of such alterations that have been reported in various parts of the world, i s that sediments, volcanics and intrusives alike are highly susceptible. The end product i n many cases shows l i t t l e or no evidence of the rock from which i t was formed. The quartz-ankeritic carbonate alteration appears i n two distinct forms i n this area. One type occurs as f i l l i n g of brecciat-ed fault zones with considerable replacement of the breccia par t i -cles and the surrounding wall rocks. The texture i s an irregular mesh of fine to coarse grained quartz, with carbonate f i l l i n g the interspaces. Minor chlorite and sericite are present but chromian muscovite i s absent. Much of the quartz i s later than the carbon-ate. This type i s represented by specimen A-2, as alteration and f i l l i n g of a breccia zone i n the intrusive rocks. The fresh colour of quartz-ankerite veinlets and replacements i n these brecciated zones i s often mauve to purplish brown. A more distinctive type consists of resistant bands of quartz, carbonate and 'mariposite', which possess a coarse f o l i a -tion. These are restricted i n occurrence within the pendant and adjacent to one of the intrusive contacts. The banded rock i s a crystalline gneiss composed of alternating f o l i a of fibrous or lamellar quartz and ankeritic carbonate. Preferred orientation - 59 -of these minerals is characteristic. Chromian muscovite, sericite and chlorite are minor constituents. The observed succession is often quartz earlier than the carbonate. Though these foliated rocks are rusty weathering the fresh specimens greatly res/emble grey siliceous marbles^ A spectographic analysis of carefully selected carbonate from one of these foliated rocks was compared to a similar analysis of the type i n i t i a l l y described. Both analyses showed strong lines for Fe, Mg, Ca. The refractive indices had previously i n d i -cated ankerite as being the carbonate occurring in both types. The nature of the solutions responsible for this common and widespread type of alteration has been variously interpreted by previous workers i n diverse regions. At numerous lo c a l i t i e s carbonatization, accompanied by s i l i c i f i c a t i o n has been related to serpentization and steatitization of ultramafic intrusives. Many of these metasomatic processes are genetically related to each other. They often occur at relatively low temperatures by deuteric react-ions i n such ultramafic rocks. In the Motherlode System of C a l i -fornia (Knopf, 1929), the carbonatization accompanying gold-quartz veins has been attributed to a deep-seated consolidating granitic magma. Exhalations of C02 that later condensed to a liquid, have displaced enormous quantities of s i l i c a from the wall rocks, regard-less of their composition. Here, serpentini zation of greenstones preceded the ankeritic alteration. The absence of ultramafic intrusives i n this l o c a l i t y lends support to the conception that the pervasive hydrothermal - 60 -solutions originated from a consolidating granitic magma. Ultra-mafic- bodies are , however, numerous in adjacent areas but i t is «• improbable that such a deep-seated magma consolidated beneath the gabbroic intrusives. The carbonatizing solutions evidently o r i g i -nated by a late stage concentration of CaO, FeO, MgO, Si02, KgO, C02 and HgO in the magma. This concentration illustrates the prin-ciple of polarity, for the late hydrothermal solutions would con-tain numerous constituents i n which the magma at this stage should be deficient (Turner, 1948) . The early phases were also enriched in most of these components. The steeply dipping shear zones i n the pendant and intru-sive rocks offered permeable channels for these solutions. No def-inite paragenesis can be determined, for stress was an active fact-or during and after crystallization. Serpentinization of the wall rocks is often l o c a l l y prominent, and particularly along "C" ridge between two zones of the quartz-ankerite alteration. No r e l i c t minerals are present i n any of the specimens of serpentine or car-bonate rocks to suggest the stages of serpentinization. In the ser-pentinized zones, talc and carbonate appear later than the antigorite. Strings of magnetite grains are common in the talc-serpentine rocks and are also predominant constituents of the carbonate alteration zones. Minor amounts of reddish brown sphalerite occur;, in both types of alteration, so that i t is further evident that a close relationship exists between serpentinization and later carbonati-zation and s i l i c i f i c a t i o n . The original rock on "C" ridge may have - 61 -been essentially greenstone. Serpentinization was the f i r s t process performed by the hydrothermal solutions. Changes probably involved additions of H 2 0 and MgO, with exchange of AI2O3 , K 2 0 , CaO and FeO to form some magnetite, sericite and carbonate. Alteration of ser-pentine to talc could be accomplished by addition of C 0 2 . 2H|+Mg^  S i 2 0 O + 3 G 0 2 > H2Mg3 Si^O^ + MgCC^ + 3 H 2 0 In the presence of excess C 0 2 , magnesite and quartz could be produced (Turner, 19U8). With excess iron the f i n a l products can be conceived as quartz and ankerite plus minor sericite and chlorite. Chromite i s not necessarily present unless the serpentinization process involves ultramafic rocks. The chromian muscovite obtains i t s chrome content from the hydrothermal solutions. As indicated by Whitmore and associates (1946, p. 1 9 ) , the occurrence of these chrome micas i s typical with ankerite. A derivation in part from hydrothermal solutions originating from granitic magmas i s strong-l y suggested i n most occurrences. Away from the more strongly altered zones, carbonatization has exhibited a strong preference for ferromagnesian minerals, with feldspars next i n order. Replacement of quartz i s evident i n many of the foliated quartz-ankerite bands. In the carbonatized and s i l i c i f i e d fault breccia, r e l i c t augens of crushed plagioclase are present. - 62 -MAGMATIC INTRUSION VERSUS GRANITIZATION In recent years numerous geologists have proposed that many rocks of granitic character have been derived by metasomatic replacement of older rocks, without having passed through a magma-t i c stage. This process has been termed 'granitization* (Read, 1948. p. 5). Many bodies of granitic rock which were originally considered to be magmatic intrusives have more recently been shown to have originated wholly or i n part by processes of granitization. These processes involve progressive metasomatism, with perhaps re-lative l y small quantities of rock undergoing changes at any one time. Four mechanisms l i s t e d by Goodspeed (1948, p. 72) are " (a) gradual permeation of activating solutions, (b) recurrent fracturing per-mitting solutions to act progressively upon the wall rocks, (c) cry-st a l l i z a t i o n replacement, (d) solid diffusion". One or more of these mechanisms may be dominant in any particular area. There i s general disagreement between various workers as to the nature of these sol-utions. By some they are considered to be true magmas, whereas others propose that the emanations or juices, are residual aqueous extracts from magmas. The effectiveness of the processes are also much disputed. The writer considers i t essential that present workers in extensive areas of granitoid rocks, assess the f i e l d and petrographic evidence relative to the origin of such rocks. By recognizing the - 63 -p o s s i b i l i t y of granitization as a major geologic process, more complete c r i t e r i a for both magmatic and metasomatic granites should accumulate. Most of the detailed explanations so far offered for granitization, envisage light coloured granular rocks consisting mainly of quartz and alkaline feldspar as the ultimate products. The more intermediate and basic rocks are represented as the basic fronts of the emanations. The sequence of geochemical changes involved i n the transformation of various rock types are summarized by Reynolds (1946, p. 390). The stages for basic igneous rocks may be appropriately presented. They involve an i n i t i a l desilication and basification by addition and geo-chemical culmination of Ca-Fe constituents plus minor Ti02, P2O5, and MnO. With later alteration, Si02> alkalis and often AI2O3 are added while a l l other constituents decrease. The migration of Fe, Mg con-stituents forms basic aureoles surrounding granitic rock. With consideration of the relatively minor amount of de-tai l e d geological and geochemical investigations normally undertaken i n areas of granitic rocks, i t is apparent that c r i t e r i a for the rec-ognition of true magmatic and metasomatic granites w i l l be particular-l y valuable. If the rocks i n question should appear to f i t the f i e l d and petrographic c r i t e r i a for granitization, further detailed invest-igations would then be warranted. The following c r i t e r i a of graniti-zation have been collected from articles by authorities i n the gen-eral f i e l d (Grout, 1949: Wahlstrom, 1947). Some comments are inserted where these apply to the gabbroic intrusives of this area. - 6h -Field Criteria 1. Areal replacement with no evidence that the country rocks have been pushed aside. (Relations i n this area suggest the opposite). 2. Continuation of regional structures such as foliation into the granitic mass. (The bulk of the gabbros are massive, the lineation of the gneissic quartz gabbro shows no re-lation to regional or local attitudes of invaded strata). 3. Parallel orientation of inclusions and structures i n i n -clusions, with structures i n country rock. (Trend of l i n -eation not parallel to structures of country rock; trend of the pendant i s para l l e l but this i s considered to be accidental and due to late faulting). 4. Lithological similarity between country rock and inclusions. (Such i s definitely the case i n this area). 5%. Gradational contacts, especially along the strike of bedd-ing i n the invaded rocks. (Such a contact is present at one l o c a l i t y ) . 6 . Relict structures (ghosts) indicating inclusions which have been almost entirely replaced or recrystallized. (While inclusions have been recrystallized, they are s t i l l separate entities i n the gneissic gabbro). - 65 -Microscopic Criteria 1. Direct replacement of one mineral by another . (Present in many igneous rocks, so of l i t t l e value). 2. Segregation of minerals i n parallel layers, suggesting stra t i f i c a t i o n i n original rock. (Linear arrangement of plagioclase and hornblende grains i s due to flowage of a magma during crystallization). 3. Unzoned porphyroblasts of feldspar with ragged contacts. (Feldspars i n the quartz gabbro are unzoned, but show no porphyroblastic tendencies. Absence of feldspar porphyro-blasts i n contact rocks). It. More than one variety of plagioclase in separate grains (Microcline microperthite associated with calcic plagio-clase i s attributed to deuteric reactions by residual mag-matic solutions). 5. Reversal of normal order of crystallization as proposed for rocks crystallizing from a magma. (No indications of such reversals i n these gabbros). 6. Presence of minerals such as sillimanite, chlorite, etc. that are common in metamorphic rocks. (Absence of such aluminous silicates here. Chlorite i s so common i n igne-ous rocks that i t s presence i s not diagnostic). The above c r i t e r i a w i l l serve to indicate mainly that those relating to petrographic evidence are far too vague. It - 66' -should also be apparent that the gabbroic rocks do not satisfy to any appreciable extent either the f i e l d or petrographic c r i t e r i a for granitization. On the other hand, the presence of chilled bor-ders, probable flow structures, uniform pattern i n overfall texture and yet common minor accessories i n diverse rock types, serve to indicate that these gabbros represent intrusives that have crystal-l i z e d from a true magma. CONCLUSIONS A summary of conclusions regarding the lithology and gen-esis of the pendant^ana of the gabbroic intrusives i s presented below. 1. The quartz gabbro and granogabbro represent differentiates from a common magma. 2. The differentiation was not controlled by the tabular pendant. 3. .'The quartz gabbro was intruded prior to the granogabbro. The relatively high temperatures and high content of the basic com-ponents i n the magma enabled i t to retain a high degree of fl u i d i t y . Under the influence of external pressure, flow was initiat e d i n the quartz gabbro as i t was injected into i t s con-fined position between the pendant and the adjacent wall rocks. k» The early phase of intrusion was forceful and the magma advanced by piecemeal stoping, whereas the later stages seem to have - 67 -been more passive, with the magma crystallizing more or less i n place. '5» The tabular pendant may have been detached from i t s surround-ings by the early injections, as flow of the early phase was maintained towards a locus of released pressure. 6. The pendant comprises a conformable sequence of pyroclastic and sedimentary rocks. The whole succession, and also individ-ual strata; represent alternating deposition of marine sediments and pyroclastic material. 7. The mineralogical and chemical composition of the pyroclastic beds forming the flanks of the pendant are such that thermal metamorphic effects from the intrusives are relatively insig- -nificant. The basic tuffs further insulated the central portions of the pendant, so that calcareous strata occurring here were not recrystallized. Laterally spreading metasomatic effects are also negligible. 8. Some metamorphism and metasomatism may have been accomplished i n the pendant by ascending thermal solutions originating during the main period of intrusion and crystallization. The presence of recrystallized calcic plagioclase in augen structures suggest that indirect componental movements were involved as well as the direct deformational movements. 9. Dynamic metamorphism has played an important role i n reducing" many of the strata within the pendant to a schistose condition. - 68 -Deformation has been localized within certain incompetent zones, which may comprise an entire stratum or individual laminae. Relict lenses of unsheared rock aid i n the interpretation of schistose zones. 1 0 . Faulting has also been prominent within the intrusives and along intrusive contacts. The main fault to the west of the intrusive may be the near-surface expression cf a thrust from the west. The easterly dipping faults may be related to a marginal thrust developed by elongation of the batholithic mass. 11 . The central portions of the pendant may have been downfaulted relative to the borders. 1 2 . Carbonatization and s i l i c i f i c a t i o n have occurred along some of the fault and shear zones. These alterations are due to hydrothermal solutions which originated during the late stages of a crystallizing granitic magma. The quartz-carbonate bands within the pendant show fol i a t i o n and were formed mainly syn-chronous with deformation. Those along the intrusive contact and i n the intrusives show a disordered arrangement and are thus post deformation. Minor late movements have affected both types of altered zones. 1 3 . This elongate, tabular pendant represents a type intermediate to the two main types commonly recognized. Though the fault zone extends beyond the limits of the area studied, i t is uncertain i f the same i s true for the roof pendant. - 69 -APPENDIX A. REFRACTIVE INDICES 1. Ankerite n e 1.526 - 1.530 1.700 - 1.717 2. Chrome Mica (Fuchsite) n min. 1.599 n max. 1.609 2V 30 - 1|0° i n thin sections. Indices for fuchsite l i s t e d by Ihitmore (1946, p. 1) are a 1.559 - 1.569 b 1.593 - I.60U g 1.597 - 1.611 The indices increase with the chrome content. B. DESCRIPTIVE PETROGRAPHY 1. Biotite Granogabbro (C-13) Structure - massive; myrmekitic and sutured structures. Texture - coarse grained, not granitic. P o i k i l i t i c , micro-perthitic. Composition - $2% Labradorite (An^ 2 - An^ 0), 21% microcline, 20$ quartz, $% biotite. Minor accessories are apatite and magnetite. Alterations Alteration of biotite to penninite and muscovite. Minor development of sericite i n the feldspars, some epidote also present. Reaction Structures Late stage replacement of the plagioclases by - 70 -quartz and microcline i s evident throughout. Large masses • of these minerals enclose the earlier ones. Myrmekite struc-tures are developed where the potash feldspar sends replace-ment fingers into labradorite. Sutured borders are present on a l l quartz grains. The microperthitic texture in micro-cline consists of irregularly branching veinlets. The min-eral involved i s not l i k e l y to be calcic plagioclase. No albite twin lamellae are vis i b l e , but the index and bire-fringence are slightly greater than those of microcline. 2. Quartz Gabbro Structure - gneissic, some cataclastic structures. Texture - nearly coarse grained, gabbroic texture. Composition - 60.5$ Labradorite (An^), 8.0$ quartz, 22.5$ bluish green hornblende. Accessories are apatite, magnetite and sphene. Alteration products, mainly chlorite and epidote, comprise some 5$ of the total. Alterations - Considerable development of epidote in the plagioclase along microscopic shear fractures. Cataclastic effects are lo c a l l y very prominent. These con-s i s t of crushed zones with clearly visible drag and displace-ment of twin lamellae. The hornblende i s slightly altered to epidote, penninite and clinozoisite. Sphene occurs i n the amphibole along cleavage and twinning planes. Other Features Resolution of early plagioclase grains by the magma is evident. The f i r s t and second generations seem to - 71 -represent the same general composition. Quartz replaces the plagioclase throughout. In the more strong sheared and altered represent-atives of the quartz gabbro, irregular aggregates of sphene are apparently alteration products. Where associated closely with apatite and hornblende i t i s probably an original con-stituent . 3. Intermediate Types Hornblende granogabbro occurs mainly along the eastern flank of the pendant. It is equivalent to the quartz gabbro plus microcline. Much' of the intrusive of this i n -termediate composition i s merely quartz gabbro that has experienced introduction of potash feldspar from late mag-matic residues. However, the inclusion of quartz gabbro i n hornblende granogabbro along "C" ridge, shows that intrusion of phases with this composition did occur. The textures and alterations are as the quartz gabbro. The approximate composition i s 50% plagioclase (An^Q - An£ 0), 15% microcline, 10-15$ quartz and 15% horn-blende. Accessories are normal apatite, sphene and magne-t i t e . Minor biotite occasionally i s present. 3. Deuteric Replacements The late crystallization of microcline and quartz are probably due to' permeation of the pa r t i a l l y crystalline - 72 -material by a residual magma, containing relatively high concentrations of K^ O, NagO, B^ O and SiC^. This deuteric replacement of the early formed minerals occurred before the hydrothermal stage causing carbonatization and s i l i c i -f i c ation. The microperthite appears to have originated by replacement processes after the microcline had crystallized. It may have been aided by early deformation, for such is indicated by the irregular branching form. Plate I I •View of "A" ridge and basin. Approximate position of pendant (P) and of some of main and branch fault (P) are indicated. View of section exposed on N . E . side of " B " ridge. Approximate position of pendant and of intrusives indicated. Severe faulting of a l l rocks Is notice-able. The pendant extends to the valley floor. P l a t e I I I A p o r t i o n o f the sheared pendant on "A" r i d g e . The r o c k s are c h l o r i t e s c h i s t and sheared greenstone. Note t r a n s v e r s e f r a c t u r e s , i n c l i n e d t o f o l i a t i o n o f the c h l o r i t e s c h i s t . Sheared v o l c a n i c t u f f i n h e a d w a l l o f 1 1C" b a s i n . T h i s n e a r v e r t i c a l c l i f f f a c e shows b e g i n n i n g o f metamorphi d i f f e r e n t i a t i o n , a c c e n t u a t e d by s u n l i g h t on wet r o c k . Plate IV Sheared granogabbro, showing int e r s e c t i n g s planes and megascopic f a b r i c . (X 1.5) Sheared quartz gabbro showing numerous intersecting s planes. The dark v e i n l e t i s of quartz carbonate. (X 1 . 5 ) Plate V, Rusty weathering quartz carbonate replace-ment i n a f a u l t zone. Coarser breccia p a r t i c l e s of gabbroic rock are unreplaeed. (x 1.4) Plate VI A_ Typical appearance of crenulated bands of quartz-ankerite-iaariposite gneiss. The white i s quartz. F o l i a of chrome mica are v i s i b l e i n upper l e f t . (X 1.3) B_ Crenulated bands of grey quartz-ankerite. gneiss. Thin dark bands are mainly chal-cedonic quartz and c h l o r i t e . BIBLIOGRAPHY Armstrong, J.E.; Roots, F.E. (1948) : Aiken Lake map-area, B.C.; Geol. Surv. Canada, paper 48-5• Armstrong, J.E. (1949) : Fort St. James Map-Area, Cassiar and Coast Districts, B.C. Geol. Surv. Canada, Memoir 252. Balk, R. (1926) : Primary structures of granite massives, Bull. Geol. Soc. Am., Vol. 9b, pp. 679-696. Batholith Committee (1933) : Problems of the batholiths, Mat. Research Council, Div. Geol. Geog., Washington, D.C. (1935a): Annotations of selected papers on the mechanics of ign- eous invasion. (1935b): Comments on magmatic stoping. Bil l i n g s , M.P. (1942) : Structural Geology, Prentice-Hall, New York. Bostock, H.S. (1948) : Physiography of the Canadian Cordillera, with special reference to the area north of the 55th p a r a l l e l . Geol. Surv. Canada, Memoir 247. . Bowen, N.L. (1922) : The behaviour of inclusions i n igneous magmas, Jour. Geol., Vol. 30, pp. 513-570. Charlewood, G.H. (1935) : The nature and occurrence of carbonates i n veins, Econ. Geol., Vol. 30, no. 5, pp. 502-517. Cloos, E. (1946) : Lineation, a c r i t i c a l review and annotated bibliography, Geol. Soc. Am., Memoir 18. Cooke, H.C. (1948) : Back to Logan, a discussion on granitization, Trans. Roy. Soc. Canada, Vol. 42, sec. IV, pp. 29-40. Daly, R.A. (1906) : The Okanagan composite batholith of the Cascade Mount-ain System, Bull. Geol. Soc. Am., Vol. 17, pp. 329-376. 7* Daly, R.A (19ll| (1933 Goodspeed (1950 Grout, F.: (1932 (1933 (1937 (19itl (1950 Gunning, ] (1950 Harker, A (1932 Ingerson, (1945 Johannsen (1939 Knopf, A. (1929 Larsen, E (1934 Lord, C.S (1948 Mayo, E.B (1935 : Igneous rocks and their origin, McGraw-Hill, New York. : Igneous rocks and the depths of the earth, McGraw-Hill, New York. G.E. : Origin of Granites, Geol. Soc. Am., Memoir 28, pp. 55-77* : Petrography and Petrology, McGraw-Hill, New York. : Contact metamorphism of the slates of Minnesota by granite and by gabbro magmas, Geol. Soc. Am., Bull., Vol. 94, pp. 989-1040. : Criter i a of origin of inclusions in plutonic rocks, Geol. Soc. Am., Bull. 48, p. 1521. : Formation of igneous-looking rocks by metasomatism, Geol. Soc. Am., Bull., Vol. 52, pp. 1525-1576. : Origin of granite, Geol. Soc. Am., Memoir 28, pp. 45-53« .C. : The Cordilleran region, discussion of C.I.M.M. Jubilee volume, Can. Min. Met. Bull., Feb, 1950. : Metamorphism, Methuen and Co., London. E., Tuttle, O.F. : Relations of lamellae and crystallography of quartz  and fabric directions i n some deformed rocks, Am. Geophys. Union, Trans., Vol. 26, pp. 95-105. A. : A descriptive petrography of the igneous rocks, Vol. I, 2nd. ed., Univ. of Chicago Press. : The Motherlode System of California, U.S. Geol. Surv. Prof. Paper 157. S., Berman, H. : The microscopic Determination of the Nonopaque Minerals, U.S. Geol. Surv., Bull. 848. : McConnell Creek Map-area, Cassiar Di s t r i c t , B.C., Geol. Surv. Canada, Memoir 351. Some intrusions and their wall rocks i n the Sierra Nevada, Jour; Geol., Vol. 93, no. 7, pp. 673-689. Read, H.H. (1950) : Granites and granites, Geol. Soc. Am., Memoir 28, pp. 1-19. Reynolds (I9U6) : The sequence of geochemical changes leading to graniti- zation, Geol. Soc. London, Quart. Journal, Vol. 99> pp. 205-246. Pettijohn, F.J. (19U9) • Sedimentary Rocks, Harper and Bros., New York. Pirsson, L.V. (1915) : The microscopic character of volcanic tuffs, Amer. Jour. Sci., Vol. UO, pp. 191-211. Wahlstrom, E. (19^0) : Introduction to theoretical igneous petrology, Wiley and sons, New York. Walker, J.W.J Bancroft, M.F., and Gunning, H.C. (1929) : Lardeau Map-Area, B.C., Mineral Deposits by H.C. Gunning, Geol. Surv. Canada, Memoir 1 6 1 . Whitmore, D.R.E. et a l (19U6) : Chrome Micas, Amer. Mineral. Vol. 31, pp. 1-21. Turner, F.J. (19U8) : Mineralogical and Structural Evolution of the Metamorphic  Rocks, Geol. Soc. Am., Memoir 30. ill 2. G E O L O G I C A L MAP OF T H E C H U K A C H I D A LAKE A R E A , C A S S I A R DISTRICT , B.C. S C A L E -1 inch t o 4 mile C o n t o u r I n t e r v a l Soofect. TOPOOf\f\?Hi B y P H O T O G R M M E T R V . GEOLOGY BY fi.C.TRPLIN /9SO. LEGEND SHEARED y ALTERED EQUIVALENTS or [2] /"vi) GE CORRSE GR.R1NE* GRRNOGA&bRO GNEISSIC QURRTZ GRBbRO SCHISTOSE, FILTERED VOLCRNiCS. TRKLR VOLCRNICS - Andesit.c and basaltic flows,tuff«. FAULT - observed * assumed or not ca1 on aerial photos " V . CONTACT - observed. Approximate . LIN EAtlENT , Observed only on aerial photos. N.E. LIMIT OF CLOSELY SPACED SHEARING INTFRbRAlNRGE RlbGE N«^Si LINEATION - STfl/K£ , 2>/P v e r t i c a l , inclined fcixX STRIKE , DIP OF FORrlRTIONS , FRULTS. i . bETRILED TRAVERSE o CENTRES OF AERIAL PHOTOS R.L.T. - Wdrch 195/ STRUCTURAL SECTION ALONG A-A of FIGURE 2. me. —t i.-*\J - 4 i. 1 \A-> I Sheared Quartz Gabbro. \R-L j Quartz-carbonate breccia f i l l i n g . /?-3 Paeic cr y s t a l t u f f , altered but not sheared. /?-+ Rusty weathering quartz-carbonate gneiss. fi s Chlorite schist containing r e c r y s t a l l i z e d augen of Labradorite and quartz. Quartz-sericite schist, minor lenses of tuffaceous sand stone. R-I Sheared crreenstone. at Sheared, altered granogabbro.-|^?| Mainly purplish brown t u f f s and grey andesitic flows, \/\ Fault, marked by carbonated breccia zone. |\ | Contact between volcanics, intrusives, etc. L A J Mainly fault-contacts. Horizontal Scale 1 inch= 500 feet. V e r t i c a l Scale 1 inch = 250 feet. Figure A C-3 |c.^| Sheared, granogabbro. \cz| Xenolith of quart7 gabbro. Reddish brown siliceous tuff. C-s\ Vertical basalt dyke. Quartz-carbonate gneiss. |c-zj Sheared calcareous tuff. \c-i\ Foliated lens of quartz-carbonate. \c-3 [ Serpentinized alteration zone. |c-x | Schistose, altered tuff. |o-y/| Pror>ylitl zed tuff. |c-/o| Sheared gabbroic intrusive. f\ | Oberved contacts, mainly fault contacts. | \ J Anproximate contacts. |# | Fault observed, assumed. ^\ Strike, '.i-. of faultR, 'contacts. 

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