THE GEOLOGY OF PART OF THE SHULAPS ULTRAMAFITE,
NEAR JIM CREEK, SOUTHWESTERN BRITISH COLUMBIA
by
JOE JOCHEN NAGEL
Sc., University of California at Los Angeles, 1972
A THESIS SUBMITTED IN PARTIAL FULFILLtffiNT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE
in
THE FACULTY OF GRADUATE STUDIES DEPARTMENT OF GEOLOGICAL SCIENCES
We accept this thesis as conforming to the required standard
THE UNIVERSITY OF BRITISH COLUMBIA August 1979
© Joe Jochen Nagel, 1979 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study.
I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission.
Department Of G^nlogi na.] .Sn.i fitipps
The University of British Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5
August 20, 1979 ABSTRACT
The Shulaps ultramafite lies at the eastern edge of the
Coast Plutonic Complex approximately 150 miles from Vancouver,
B.C. It is one of the largest "alpine-type" peridotites in
British Columbia, consisting primarily of harzburgite and subordinate dunite lying in an elongate zone trending northwest. Irregular bodies of gabbro and clinopyroxenite occur on the western side of the ultramafite associated with pillowed volcanic rocks. The ultramafite is bounded on the northeast by the Yalakom fault, a major regional structure, and on the southwest by rocks on the Fergusson Group, an assemblage of chert, clastic and volcanic rocks, probably of
Triassic age.
This study outlines the distribution of peridotite, gabbro and other rocks in the area just west of Shulaps Peak, a major topographic feature in the area. The peridotite is almost completely serpentinized and pervasively sheared. It is in tectonic contact with all other rock types, and contains isolated inliers, ranging from a few feet to over 500 feet, of gabbro, greenstone, chert and clastic rocks. Within the area mapped, the serpentinite could be termed a tectonic melange. An interesting feature of the serpentinite is the irregular occurrence of olivine porphyroblasts, formed by the reaction serpentine + brucite =olivine. The gabbro is foliated and in places layered, although no cumulate textures were observed. It is in gradational contact with pillowed volcanic rocks and in tectonic contact with serpentinite.
The evidence gathered by previous workers and during this study argues strongly for the hypothesis that the
Shulaps ultramafite, and perhaps some of the associated rocks, are an allocthonous piece of oceanic crust (ophiolite) which has been emplaced into its present position by plate-tectonic processes. This emplacement probably took place between
Middle Triassic and Lower Jurassic time. iv.
TABLE OF CONTENTS
INTRODUCTION 1
STRATIGRAPHY 10
DESCRIPTION OF UNITS 14
SEDIMENTARY ROCKS 14
EAST LIZA VOLCANICS 19
SHULAPS PEAK VOLCANICS 21
EAST LIZA GABBRO 21
MAIN GABBRO 26
SMALL BODIES, PODS AND DIKES OF GABBRO 28
DIORITE AND QUARTZ DIORITE 32
REACTION ZONES 34
MYLONITE 34
ULTRAMAFIC ROCKS 35
STRUCTURE 56
SPECULATION ON ORIGIN AND EMPLACEMENT 64 V.
LIST OF FIGURES
1. Index map of the Jim Creek area 2
2. View of the central part of the Jim Creek area 3
3. Geological map of the Jim Creek area >M poukis.1; m MapC«.tinet
4. Geological map of the Shulaps Range (after Leech, 1953) 7
5. Outcrop of clastic rocks with interbedded chert, near 15 middle fork of Jim Creek
6. Ribbon chert near East Liza Basin 15
7. Photomicrograph of feldspathic wacke from East Liza 17 Basin (#N706)~
8. Pillowed volcanic rock near East Liza Creek 17
9. Pillow breccia near East Liza Creek 18
10. Photomicrograph of pillowed volcanic rock from near 18 East Liza Creek (;W598)
11. Photomicrograph of volcanic rock from near East Liza 22 Basin (#NbIl)
12. layering in gabbro from southwestern part of map area 22
13. Layering in gabbro from southwestern part of map area 23
14. Stereonet summary of poles to layering and foliation 23 in gabbro
15. Contact of East Liza Gabbro and sheared serpentinite, with 25 central zone of pumpellyite-rich rock
16. Photomicrograph of pumpellyite from contact zone shown : 25 in Figure 15 (#N551)
17. Typical outcrop of Main Gabbro near the head of Jim Creek 29
18. Dike completely altered to rodingite, near the southern 29 part of the map area
19. Summary of optically determined pyroxene compositions 30
20. Exotic inclusion of granitic rock in serpentinite nea.r 33 Shulaps Peak VI.
LIST OF FIGURES (CONT)
21. Quartz diorite intrusion and hornfels, near triple fork 33 in Jim Creek
22. Photomicrograph of mylonite from contact of East Liza 36 Gabbro and serpentinite (#N586)
23. Mylonite stringers at contact of sedimentary rocks and 36 serpentinite on centre fork of Jim Creek
24. Sheared serpentinite near centre fork of Jim Creek 38
25. "Roundstone Breccia" near triple fork of Jim Creek 38
26. Layered peridotite near western tributary of Jim Creek 40
27. Photomicrograph of relict olivine in peridotite, showing 40 deformation lamellae (#N35)
28. Relict grains of clinopyroxene and clinopyroxene exsolution 41 lamellae in serpentinized peridotite (#N45)
29. Inclusion trains of magnetite in sheared serpentinite (#N245) 41
30. Serpentinite with olivine porphyroblasts, from east of 45 the map area
31. Serpentinite with olivine porphyroblasts, found as float 45
32. Elongate olivine porphyroblast with pyramidal termi- 46 nation (#N215)
33. Regenerated olivine layers in outcrop near Shulaps Peak 46
34. Rim of magnesite enclosing olivine porphyroblast 47
35. Olivine porphyroblasts showing 120 degree grain 47 boundaries (#N312)
36. Mesh texture preserved in regenerated olivine (#N711) 48
37. Mesh texture preserved in regenerated olivine (#N711) 48
38. Regenerated olivine (#N710) 49
39. Regenerated olivine replacing serpentine vein (#N215) 49
40. Olivine porphyroblast almost completely serpentinized (#N440) 51 vii.
LIST OF FIGURES (OONT.)
41. Schematic T-X^ diagram for the system MgO-Si02-R20-C02 at 52 elevated pressures and temperatures.
42. Isobaric equilibrium curves at low C0? content for the system 53 MgO-SiOg-RgO-COg.
43. Summary of structural measurements 58
44. Sections A-A' and B-B' 60
45. Section C-C 61
46. Section D-D' 62
47. Hypothetical, schematic cross-section through the Shulaps 68 Range in the vicinity of Shulaps Peak. v o tl 48. Sample locations -i-n-pocket h0tu*+ ACKNOWLEDGEMENTS
Thanks are due to Drs. H. J. Greenwood, K. C. McTaggart and P. B.
Read for help in the field and in many subsequent discussions.
H. J. Greenwood, K. C. McTaggart and J. V. Ross were also instrumental
in helping the author overcome his considerable writers' inertia
by way of their encouragement and patience. I would also like to
thank my wife Sharon for her support at all times. 1.
INTRODUCTION
Purpose
The aim of the present study is to examine a part of the
Shulaps alpine-type ultramafite, especially with respect to its contact relationships with other rock types. Alpine-type ultra- mafites are relatively common in the Canadian Cordillera, and the
Shulaps body represents one of the large examples. Although descriptions of many of these may be found in the literature, comparatively little work has been published since the advent of the theory of plate tectonics. In this study, the author will describe the geology of part of the western flank of the ultra• mafite, and interpret the features observed in terms of the plate tectonic model.
Location and Accessibility
The map area covers a part of the Shulaps Range, which is located approximately 100 miles north of Vancouver, B.C., near the town of Lillooet (Fig.l). Marshall Lake, just outside the map area, is accessible by sixty miles of good gravel road from
Lillooet. On the eastern side of Marshall Lake, a mining road branches into the drainage basin beneath Shulaps Peak. The road is passable to a four-wheel drive vehicle. The eastern branch extends to 6000 feet and the western branch to 7000 feet.
Although it may snow at any time of the year at the higher elev• ations, the area is relatively clear from the end of June to the beginning of September. Figure 1: Index map of the Jim Creek area Figure 2. View of the central part of the Jim Creek area. Shulaps Peak is to the right, just off the picture. The prominent outcrops on the right hand side of the photograph are gabbro. Most of the foreground is underlain by serpenti- nite. 4.
Physiography
The map area lies on the western flank of the Shulaps Range
immediately northeast of Marshall Lake. It is bounded on the northeast by a prominent ridge which incorporates Shulaps Peak,
and which forms the crest of the range. Glacial action has
formed a number of small valleys and basins which are drained by
small creeks. The range-crest is at approximately 9500 feet and
local relief exceeds 5000 feet. Tree line is at 6500 feet and, because exposure is poor below tree line, most mapping was done
above this elevation. Topography reflects the geology; the steep
ridges are primarily resistant gabbro, and the basins and lower
elevation are underlain mainly by more easily eroded serpentinite
or sedimentary rocks (Fig.2).
Previous Work
The earliest work in the vicinity of the Shulaps Range was
done by Drysdale (1916,1917). He made a general reconnaissance
of an area which included a part of the Shulaps ultramafic rocks.
McCann (1922) continued this work with an emphasis on the
economic aspects of the region. Both authors considered the
Shulaps ultramafites to be volcanic. Cairnes (1937,1943) mapped
the area around the Bridge River mining camp and northward, and
established a regional stratigraphy. He determined the ultra-
mafic rocks in the area to be intrusive rather than volcanic,
but his mapping did not extend into the Shulaps Range.
Leech (1953) mapped the Shulaps Range in detail at a scale
of one inch to one mile. His major interest was the ultramafic
body, but his map includes a small marginal area of the surround- ing rocks. Although Leech's mapping did not overlap with that of
Cairnes, the lithologies in the two areas were very similar and he correlated the stratified rocks found in the Shulaps Range with the units Cairnes had defined in the Bralorne area. Some of the units, however, he was unable to correlate. In agreement with
Cairnes, he found the ultramafite to be intrusive rather than volcanic, but found insufficient evidence for correlation with the ultramafic rocks Cairnes had mapped to the west.
Present Work
This thesis area is centered on the area around Jim Creek and includes ultramafic, gabbroic, volcanic, and sedimentary rocks (Fig.3). During the summer of 1973, approximately six square miles were mapped at a scale of 1000 feet to one inch. Of the 709 specimens:; collected during mapping, 125 were chosen for more detailed study. Ninety-seven thin sections were examined in conjunction with forty-one X-ray diffractograms. X-ray fluores• cence data were collected for six representative gabbros to determine their suitability for Rb/Sr dating. Because the total rubidium content was low (approx.30 ppm) and the spread in Rb/Sr ratios quite narrow, no analyses were performed.
Regional Geology
The Bridge River region lies on the eastern edge of the Coast
Plutonic Complex, just west of the Intermontane Belt (Monger,et al.
1972). An "eugeosynclinal" assemblage of basic volcanics, argillit greywacke, ribbon chert, and minor limestone and conglomerate underlies most of the area. Igneous rocks include ultramafites, 6. gabbros, and granitic rocks. The granitic rocks are mainly of
Cretaceous age.
A large proportion of the rocks in the Bridge River area belong to the Fergusson Group. These are characterized by sequences of ribbon chert and greenstone, with minor quartzite and limestone. Fossils are scarce, but conodonts collected in a limestone approximately four miles southwest of the thesis area indicate a Middle Triassic age (Cameron and Monger, 1971). Similar rocks make up the Hurley and Noel Groups, defined by Cairnes in the Bralorne area. The problems involved in correlating these to rocks found in the Shulaps Range are dealt within a later section.
The main structural trend of the region is northwesterly, with numerous faults lying along this trend. The most important of these is the Yalakom Fault, which lies on the eastern flank of the Shulaps Range. This fault is a northerly extension of the
Fraser River fault system (Duffell and McTaggart, 1952) and marks a major regional structural discontinuity. The pre-Cretaceous rocks of the region have been complexly folded and faulted and subjected to a low grade of regional metamorphism.
Ultramafic rocks are scattered throughout the Bridge River region. They consist of podiform to lensoidal masses ranging from tens of feet to miles across, usually elongate along the regional structural trend. Many of them are fault-bounded on at least one side (Cairnes, 1943). The Shulaps ultramafite, underlying approx• imately 150 square miles, is by far the largest of these.
LEGEND (after Leech, 1953) QUATERNARY Q Quaternary Cover
CRETACEOUS (?) AND TERTIARY TAYLOR GROUP: Micaceous arkosic sandstone and grit, 8 chert-pebble conglomerate, shale UPPER TRIASSIC OR (?) LATER
5 I Gabbro and diopside-pyroxenite
Shulaps Ultrabasic Rocks: peridotite, minor enstatite pyroxenite
UPPER TRIASSIC HURLEY GROUP: Argillaceous and tuffaceous siltstone and sandstone, conglomerate, limestone, chert TRIASSIC AND/OR EARLIER Greenstone, argillite, chert, chloritic phyllites; minor limestone
Greywacke (tuffaceous in part), argillite, siltstone, grit, conglom• A erate; limestone, chert; probably Jurassic and Lower Cretaceous
P3 Undifferentiated sedimentary and volcanic rocks; probably Mesozoic
Greenstone-gabbro complex; andesite and/or basalt, diorite, gabbro. The extrusive part is probably Triassic in age. 9.
Local Geology
The thesis area lies on the southwestern flank of the Shulaps ultramafite, and includes many of the rock types found on a regional scale. Leech (1953) recognized sedimentary rocks of the
Hurley Group, a greenstone-gabbro complex, gabbro and diopside- pyroxenite, and some undifferentiated Mesozoic sedimentary rocks
(Fig.4). Rocks mapped as Fergusson Group by Leech outcrop just south and east of the thesis area. The remainder of rock outcrop• ping within the thesis area consist of ultramafic and gabbroic rocks.
The stratigraphic nomenclature adopted by Leech for the area will not be adhered to in this report for reasons outlined in a subseq• uent section.
Sedimentary, gabbroic, and ultramafic rocks display a complex outcrop pattern, with "islands" of gabbroic and sedimentary rocks totally surrounded by sheared serpentinite. These islands range
in size from only a few feet across to masses over 500 feet across,
and appear to be randomly distributed. In detail, the rock dist•
ribution appears to be chaotic. 10.
STRATIGRAPHY
Ambiguity of Lithologic Units
Correlation of rock units mapped in this work with those described by Leech and by Cairnes presents difficulties. Similar difficulties were experienced by both of those authors in correl• ating rock units within their own respective areas. These problems center around the four factors discussed below.
An obvious problem is the difficulty of obtaining detailed structural information. Cairnes (1937,p.9) described the Fergusson series as, " ... highly deformed both by folding and faulting and lacking recognizable horizons such as might facilitate interpreta• tions and Leech (1953,p.12) said, "... (they) are some thousands of feet thick, but deformation, the lack of marker horizons, and the difficulty of determining the tops of beds prevent accurate measurement.". Cairnes (1937,p.15) made a similar statement about the Noel Formation. These characteristics have hampered detailed structural mapping, leaving the relative structural positions of the rock units some doubt.
A serious factor making correlation difficult is the overall similarity of rock types in the various units. The Fergusson
Group, Noel Formation, Pioneer Formation, and Hurley Group are said by Cairnes to contain similar lithologies, differing only in relative proportions of each. Cairnes (1937, p.14) said of the Noel Form• ation, "At a few places within the outcrop areas of this formation narrow belts or lenses of cherty sediments, much resembling those of the Fergusson series, were noted.", while of the Hurley Group
(1937, p.18), "many of the Hurley beds are banded, light and dark grey, argillaceous and tuffaceous rocks indistinguishable in appearance from the banded Noel strata.". The same problem is encountered in working with volcanic and intrusive rocks. Cairnes
(1937, p.16) noted of the Pioneer Formation, "The formation is essentially volcanic, but includes intrusive phases and ... in places it was not possible to differentiate between Pioneer extr- usives and related rocks and the later Bralorne intrusives.".
Also of the Pioneer Formation (1943,p.3), "Difficulties were experienced in making everywhere an effective separation between the Pioneer volcanic rocks and those of the Fergusson Group
Although the units as defined by Cairnes are mappable in the
Bralorne and Tyaughton Lake area, the overall lithologic simil• arity of the various units make correlation tenuous where sedimen• tary outcrops occur as inliers within serpentinite.
Stratigraphic relationships among the rock units are not well defined. Cairnes (1937,p.15) said of the Noel Formation,
"Contact relations with the underlying Fergusson series were noted at a couple of widely separated points and yielded contrad• ictory interpretations.". Also (1937,p.17),"The contact relations of the Pioneer greenstone with the underlying Noel Formation are difficult to interpret.". Of the Bralorne intrusions (1937,p.21),
"The rock may vary within the limits of a hand specimen or over narrow or broad outcrop areas from coarse to fine-grained, and the latter types may be indistinguishable from associated green•
stone formations.". Cairnes (1937,p.20) also experienced some
difficulties in interpreting the relationships among the Bralorne
intrusions, Hurley Group and the Pioneer Formation, the latter which conformably underlies the Hurley Group. In places, Pioneer greenstone grades into the coarser-grained Bralorne augite- diorite. This and other relationships noted by Cairnes indicate that the Bralorne intrusions and the Pioneer greenstone are closely related in origin. However, the Bralorne intrusions cut the overlying Hurley Group, which suggests that the Pioneer green• stones are considerably older; than the Bralorne intrusions
(Cairnes 1937,p.24). These relationsups were interpreted by
Cairnes as evidence that Bralorne intrusions and Pioneer green• stone were derived from one magma chamber, with periods of intru• sive and extrusive activity interrupted by a period of relative quiescence, during which the Hurley Group was deposited. Whatever the interpretation, the stratigraphic relations between rock units are poorly defined.
The limited age data on the ages of the rock units further hampers successful correlation. Fossils within the sedimentary rocks are scarce, and radiometric dates for the igneous rocks are not available. Paleontologic dates have been established only for the Fergusson and Hurley Groups, and indicate Middle and Late
Triassic- ages, respectively (Cairnes, 1937; Cameron and Monger, 1971).
Because fossil localities are few, and their stratigraphic relations in doubt, these ages may not be representative of the rock units in question. For these reasons, age relationships are of limited practical use in correlation.
Correlation
Leech, mapping the Shulaps Range, correlated his units on lithologic grounds with those of Cairnes. For reasons given above, these correlations are not unequivocal. Within the thesis area the problem is aggravated in that most of the sedimentary rocks 13.
are very similar, and in many places occur as isolated inliers within serpentinite, or in areas of poor exposure. Therefore, no attempt is made here to correlate the sedimentary and volcanic rocks with previously established units. Detailed descriptions are made on the basis of location rather than under formation names. DESCRIPTION OF UNITS
The most common rocks of the area include sedimentary rocks, volcanic rocks, gabbroic rocks with associated clinopyroxenite, and ultramafic rocks. In addition, quartz diorite, rodingite, and mylonite underlie small areas. 1
Sedimentary Rocks
Sedimentary rocks crop out in East Liza Basin, on the prominent ridge at the southwest corner of the map area, and as small bodies scattered throughout the rest of the map area. They consist of argillaceous and arenaceous rocks, ribbon chert, and minor amounts of limestone. Ribbon chert and argillaceous rocks are interbedded but in most exposures one of the other rock type is dominant.
The ribbon chert and clastic rock sequences commonly form relatively resistant; dark brown to black outcrops (Fig.5). Where ribbon chert is absent bedding is commonly difficult to find.
Ribbon chert consists of one to two inch beds of chert separated by thin argillaceous partings, and forms distinctive outcrops
(Fig.6). The chert layers commonly show irregular folding and are dark grey, white, or more commonly a rusty brown. Arenaceous rocks are light grey and are commonly interbedded with black argillaceous rocks. This combination forms rather distinctive banded outcrops in East Liza Basin, the bands measuring one to two
inches. Outcrops of light grey lithic wacke with angular black
fragments were found near the southwest corner of the map area.
The majority of the sedimentary outcrop, however, consists of fine- Figure 6. Ribbon chert near East Liza Basin. grained, black, argillaceous rock. Most of the fine-grained rocks
have a steeply dipping foliation which trends northwesterly. In many places where this foliation is well developed, the rock has a
slaty or phyllitic appearance.
Microscopically, the chert consists of micro-crystalline
aggregate of quartz which is cut by coarser veins of quartz. In
one specimen, irregular patches and stringers of argillaceous mater
ial with pale green actinolite needles was noted.
Most of the fine-grained clastic rocks are feldspathic wackes,
consisting of angular grains of quartz and plagioclase feldspar
averaging .1 to .2 millimeters in a fine-grained matrix which const
itutes approximately 25% to 60% of the total rock (Fig.7). Some of
these rocks also contain abundant chert fragments. Minor amounts
of actinolite, epidote, carbonate, chlorite, and white mica are
usually present. Bedding is marked by thin laminae of argillaceous
or carbonaceous material and most of the samples examined show evidence of deformation, often with extreme disruption of bedding.
Although some of the rocks may be tuffaceous, no definitive tex-
tural evidence of pyroclastic origin was found. One of the coarser
lithic wackes consists of grains averaging .5 to 1.0 millimeters.
Lithic fragments include chert, fine-grained volcanic rocks, sand•
stone, and argillite, in addition to quartz, plagioclase, and potassium feldspar grains.
Limestone is blue-grey to white, and occurs both as sporadic,
discontinuous beds(?), and as a few isolated blocks in East Liza
Basin.
Most of the outcrops in the head of the cirque facing East
Liza Basin consists of a black volcanic rock with calcite amygdules 17.
Figure 8. Pillowed volcanic rock near East Liza Creek. Figure 10. Photomicrograph of pillowed volcanic rock from near East Liza Creek. Amygdules are partially filled with pumpelly- ite (#N598). 19.
associated with abundant ribbon chert and limestone rubble. Very similar outcrops near a small inlier of argillite and ribbon chert approximately 1000 feet to the west. Abundant rubble of the same volcanic rock occurs near the base of Shulaps Peak, the only out• crop being a three foot inclusion in serpentinite. The relation• ship between the sedimentary rocks and this volcanic rock is unclear, but small amounts of limestone associated with the volcanic rock suggest they are related. Serpentinite is in tectonic contact with these rocks and where the contact is exposed, the volcanic rocks are strongly deformed.
East Liza Volcanics
Volcanic rocks underlie part of the eastern side of the area, in the vicinity of East Liza Basin. These rocks grade into gabbro to the southeast. Both fine-grained, pillowed extrusives, and medium- to fine-grained volcanic rocks were recognized in the field.
The distribution of the two types is irregular, however. A sub• ordinate amount of tuffaceous rock is also associated with these volcanic rocks.
Volcanic pillows are recognizable in some weathered outcrops as rather vague, ellipsoidal shapes, but very little detail is visible. Most pillows are one to two feet across. The best exp• osures are seen on certain planar joint surfaces (Fig.8); here, the chilled margins of the pillows are clearly visible. Most of the pillowed rocks are grey-green with rusty weathering surfaces. Loc• ally, they are brecciated with angular fragments of pillows held in dark green matrix (Fig.9). The stratigraphic orientation of the pillow lavas could not be determined. A thin section from one of the pillows shows an amygdaloidal, slightly porphyritic texture (Fig.10). Phenocrysts make up less than 5% of the rock and consists of plagioclase and minor augite.
Most are smaller than 2 millimeters. Amygdules are generally less than 1 millimeter in size and consist of mixtures of chlorite, carbonate, and iron-rich pumpellyite, usually in radial aggregates.
The groundmass consists of a fine-grained aggregate of plagioclase laths, up to approximately .3 millimeters, and smaller grains of actinolite, probably derived from augite. Scattered veins of chlorite, epidote, zoisite, and carbonate are also present. The plagioclase is albitic, with N 1.54. Alteration of the primary mineralogy precludes an exact name for these rocks, but the texture and composition suggests basalt or andesite.
The coarser-grained rocks associated with the pillowed vol- canics are characterized in the field by a more regular weathering pattern, and darker green color than the pillowed extrusives. Mafi grains averaging 1 millimeter and constituting 30% to 40% of the rock are plainly visible in hand specimen.. These rocks occur near both gabbro and pillowed volcanics in scattered locations and may represent small flows or dikes.
Microscopically, the rocks consist of 60% to 70% albitized and saussuritized plagioclase laths, and 30% to 40% titaniferous augite with ophitic texture (Fig.11). The average grain size is approximately 1 millimeter. Sphene, partially altered to leucox- ene, and patches of chlorite are abundant. The composition of one
of the pyroxenes, determined optically, is approximately Ca41Mg„1 21.
Shulaps Peak Volcanics
Volcanic rocks associated with small bodies of gabbro outcrop near the base of Shulaps Peak. These rocks are very similar to the
East Liza Volcanics, although in general they are more deformed and lack pillowed varieties. A.thin section consists of approximately
50% albitized plagioclase, with the remainder composed of actino• lite, zoisite, and chlorite, along with abundant sphene and patches of carbonate. The grain size ranges up to 1 millimeter, but the original texture has been completely disrupted by deformation and alteration. Although the petrographic evidence is inconclusive, the general similarity of these rocks with those near East Liza
Basin, and the fact that both volcanic units are associated with gabbro, suggests they may be part of the same sequence.
East Liza Gabbro
The East Liza Gabbro forms the eastern part of the Greenstone-
Gabbro Complex mapped by Leech (1953). The unit is heterogeneous and appears to grade into extrusive rocks to the west. Gabbro ranges from fine-grained to pegmatitic varieties, along with sub• ordinate amounts of pyroxenite.
Much of the medium- to coarse-grained gabbro has a well devel• oped layering and/or foliation, defined by segregation and alignment of mafic grains (Fig.12,13,14). The layering is commonly of a wispy nature; individual layers are discontinuous and could not be traced more than 5 to 10 feet. There is commonly a difference in grain size between individual layers, so that medium-grained gabbro in many cases has pegmatitic layers within it. The foliation and lay- 22.
Figure 12. Layering in gabbro from southwestern part of map area. Figure 14. Stereonet summary of poles to layering and foliation in gabbro ( 24 points ). ering are parallel, generally dipping steeply and striking westerly,
although subject to local variations. The origin of the layering
is unknown, but rhythmic units and textures indicative of cumulate
origin were not found.
The relationship between rocks of different grain size is un•
clear. In some well exposed areas, the gabbro is a confusing
assemblage of pegmatitic pods and stringers, medium-grained foliated gabbro, and fine-grained dikes. In places, the fine-grained rocks
truncate the foliation in the coarser-grained varieties, but the relationship between the pegmatitic and medium-grained rocks is less apparent. Some of the unfoliated pegmatitic pods appear to be the result of replacement, but interpretation is hampered by the fract• ured and sheared condition of the rocks.
In general, the gabbro averages 50% mafic constituents, but locally this limit is exceeded. In hand specimen, the mafic miner• als appear relatively unaltered, but feldspar consists of a dull, greyish-white material in which cleavage faces and twin lamellae are usually absent. Microscopic examination shows the feldspar to be almost totally altered to a fine-grained mixture rich in albite, epidote, clinozoisite, actinolite, and chlorite. The common mafic mineral is augite, in part altered to actinolitic amphibole.
Most grains show undulatory extinction, and in some specimens granu• lation around grain edges. The pyroxene from one of the finer- grained rocks has an approximate composition Ca^^Mg^^FegQ, determined optically. In one specimen, the main mafic mineral was found to be a brown hornblende, perhaps a uralitic alteration of primary pyroxene.
Small veins in the gabbro usually contain albite, clinozoisite, and chlorite, with occasional white mica. Quartz was not detected in any of the rocks. Although the primary textures was partially obsc- 25.
Figure 15. Contact between East Liza Gabbro and sheared serpentinite, with central zone of pumpellyite-rich rock.
Figure 16. Photomicrograph of pumpellyite from contact zone shown in Figure 15 (#N551). 26.
ured in many specimens by alteration and small scale shearing, the fine-grained rocks are diabasic, while the coarse-grained varieties appear hypidiomorphic-granular.
In the contact area between the ultramafite and the East Liza
Gabbro, which is well exposed on the northern perimeter of the gabbro.., a zone of altered rock approximately fifty feet thick separates intensely sheared serpentinite from gabbro (Fig.15).
The rock in this zone is partly mylonitized and consists almost entirely of pale-green pumpellyite and green hornblende, with some chlorite. Small veins in this rock are composed entirely of pump• ellyite (Fig.16). The zone of pumpellyite rock is not laterally continuous; approximately 1000 feet to the west the serpentinite is in direct contact with the gabbro. Also near this locality are two blocks of medium-grained, foliated amphibolite, composed of altered feldspar and green hornblende in equal proportions. One of these is approximately 25 feet across, the other only a few feet. Both are within the serpentinite and appear to be identical, although they are separated by 200 feet. Their source is unknown, as no other rock resembling them was found within the map area.
Main Gabbro
What is referred to in this account as the Main Gabbro under•
lies the northeast side of the map area, forming very prominent
cliffs (Fig.17). This unit is at least in part tabular in cross-
section, dipping to the northeast at a moderate angle. In plan, the
gabbro consists of northwest and southeast sections separated by
a narrow segment that is obscured by rubble. Sheared serpentinite structurally underlies and overlies the southeast section and at least part of the northwest section. Although gabbroic rockgrpred- ominate, a sizeable part of the unit consists of clinopyroxenite.
Layering and foliation were observed in only three localities. All contacts are tectonic within the map area but Leech states (1953, p.40) that the gabbro intrudes the Hurley Group to the north.
Although less strongly layered and containing larger amounts of pyroxenite, the Main Gabbro is lithologically similar to the
East Liza Gabbro, and consists of a complex assemblage of altered gabbroic rock, ranging from fine-grained to pegmatitic. Two pyroxene compositions, determined optically, one in each of the
major extensions are CsL^^S^Q^^gi^) and Ca^Mg2gFe2g(NW) . Layered and foliated rubble is abundant, suggesting that these features may be more common than exposed outcrop would indicate. Of the three layering attitudes measured, two are at a high angle to the general trend of those in the East Liza Gabbro.
Clinopyroxenite occurs as lense-shaped masses at least several hundred feet in length, although exact boundaries are difficult to trace in the field. It occurs on the perimeter and within the Main
Gabbro, and also associated with some gabbro pods within the serpen tinite. The contact between pyroxenite and gabbroic rock is very irregular on a fine scale, suggestive of intrusion or possibly repl acement.
Several specimens were collected across a well exposed contact near the head of the western tributary of Jim Creek, where the contact is well exposed within a pod of gabbro and pyroxenite. Mic roscopic examination shows the pyroxenite immediately adjacent to the contact to be coarse-grained, the largest grains greater than 28.
5 millimeters with numerous stringers of granulated pyroxene. The
pyroxenite two feet from the contact is also coarse-grained, but
lacks the granulated zones. In addition, scattered veins and
patches of serpentine are present. The pyroxene in both specimens
has strongly undulatory extinction, and numerous triple-grain boundaries in the latter specimen indicate that some recrystalliz-
ation may have taken place. Optical properties show the pyroxene
composition to be approximately Ca^Mg^QFe^. The contact consists of a two-inch-wide zone of fine-grained, dark grey rock composed of equal proportions of actinolitic amphibole in a very fine-grained groundmass rich in pumpellyite and chlorite. This rock grades into a diabasic rock with an average grain size of 1 millimeter, consist• ing of albite, actinolite, and chlorite, with accessory sphene.
The pumpellyite-rich contact zone is clearly the result of metasomatic reactions between the pyroxenite and gabbroic rock similar in nature to the narrow zone between part of the East Liza
Gabbro and serpentinite, but the origin of the pyroxenite is: unknown.
Contact relationships and distribution of the pyroxenite suggest that it is genetically related to the gabbro.
Small Bodies, Pods, and Dikes of Gabbro
In addition to the East Liza and Main Gabbrox, numerous smaller bodies of gabbro are found within the map area. The largest of these is near the head of Jim Creek, but others are scattered throughout the area. With the exception of the small amount of gabbro associ• ated with volcanic rocks near Shulaps Peak, and some small outcrops Figure 18. Dike completely altered to rodingite, near the southern part of the map area. N 34: Pyroxenite N408: Pyroxenite N118: Gabbro N575: Gabbro N154: Gabbro N667: Gabbro N195: Gabbro N677: Volcanic N236: Pyroxenite N703: Gabbro N298: Gabbro
Figure 19. Summary of optically determined pyroxene compositions. For locations, see Figure 48. near the triple; fork in Jim Creek, all of these gabbro bodies are completely surrounded by sheared serpentinite. Lithologically these bodies are indistinguishable from the gabbroic rocks found in the Main and East Liza Gabbros, although most are medium- to fine-grained and unfoliated.
In addition to the relatively large inliers mentioned above, smaller lenticular pods of gabbroic rock are found throughout the sheared serpentinite. The sizes of these pods range from approx• imately 50 feet to only a few feet. Gabbroic dikes are also present, although subordinate in number to the pods. All show some meta- somatic alteration and deformation along their borders, with the smaller pods and dikes, in most cases, completely altered to white rodingite (Fig.18). In general, however, this zone is only a few inches thick and the pods appear remarkably undeformed. Optically determined pyroxene compositions from all of the gabbroic and volc• anic rocks are summarized in Figure 19.
The origin of these tectonic inclusions is uncertain. None of the dikes is traceable for more than a few tens of feet before being truncated by serpentinite. Crude alignment of some of the smaller pods suggest that they may be dismembered dikes. Leech (1953) rep• orts gabbroic dikes of various sizes in Shulaps ultramafic rocks outside the map area, so some of the pods may be derived from such dikes. Lithologic similarity to the Main and East Liza Gabbros suggest them as a possible source. However, in the vicinity of
Shulaps Peak, at 8,400 feet, a tectonic inclusion approximately 15 feet across consists of 60% quartz, 40% albite, with minor tremolite
(Fig.20). The grain size averages 2 millimeters. This rock is a brilliant white and none other resembling it was found within the map area. This exotic inclusion suggests that the source of the gabbroic (tectonic) inclusions is not necessarily close at hand.
Diorite and Quartz Diorite
A few small outcrops of grey, smoothly weathering diorite and quartz diorite occur just north of the triple fork in Jim Creek.
Acicular hornblende prisms are prominent in hand specimen. In thin section this rock averages 50% brown hornblende prisms up to 3 mill• imeters in length, along with subordinate reddish-brown biotite; strained quartz and plagioclase make up the remainder. Quartz content ranges from 5% to 25%. Hornblende is altered to chlorite and clinozoisite, in some specimens almost completely, and small sheaves of muscoivite. • and clinozoisite are scattered as inclusions throughout the albitic feldspar. Accessories include titanite, apatite, and zircon. Quartz and feldspar are commonly graphically intergrown and most of the original texture has been obscured by recrystallization.
Although the contact of the quartz diorite with the serpent• inite is not exposed in this locality, it is intrusive into an associated inlier of sedimentary rocks and has formed a narrow zone of hornfels (Fig.21). In addition, the outcrop pattern suggests that the quartz diorite truncates a narrow sliver of foliated gabbro lithologically identical to the Main Gabbro, which also intrudes the sedimentary rocks in this locality. Therefore, the quartz dior• ite is probably younger than the East Liza and Main Gabbros. A ten foot thick dike of altered quartz diorite very similar to the above occurs within a narrow sedimentary inlier approximately 3000 feet to the northwest. Here, the dike is clearly truncated by sheared 33.
Figure 21. Quartz diorite intrusion and hornfels, near triple fork in Jim Creek. 34. serpentinite, suggesting an earlier age for the quartz diorite.
Reaction Zones
Reaction zones occur at all serpentinite contacts. In general, these zones are less than a few inches wide, but many small tecton• ic inclusions are completely replaced; such small pods are present throughout the serpentinite in the map area.
These rocks are usually white or pale green, fine-grained, and have been given the name "rodingite" after type localities near Dun
Mountain, New Zealand (Bell, et al, 1911; Grange, 1927). The rocks examined consist primarily of mixtures of diopside, tremolite, garnet of the grossular-hydrogrossular series, chlorite, and occas• ional vesuvianite. In a few instances, compositional zoning parall• el to the serpentinite contact was noted. The serpentinite near major contacts is in many places veined by chrysolite. In a few places, talcose rocks are present near these major contacts. Some small scale mining activity is centered on small deposits of neph• rite that develops in these reaction zones. Detailed studies of such rocks have been presented elsewhere (Coleman, 1967; Larrabee,
1969).
Mylonite
The term mylonite was originally defined by Lapworth (1885) to refer to intensely deformed rocks characterized by brittle fail• ure. The use and misuse of the mylonite terminology was summariz• ed by Christie (1960,1963), and more recent work has shown that mylonites may actually be the result of ductile processes (Ross,
1973; Bell and Etheridge, 1973). Because of the disagreement as to 35. the exact mode of origin, Bell and Etheridge (1973) have proposed the following more generalized definition of the term mylonite:
"a mylonite is a foliated rock, commonly lineated and containing megacrysts, which occurs in narrow, planar zones of intense deform• ation. It is often finer-grained than the surrounding rocks, into which it grades.". Mylonite" as used here, adheres to the above definition.
Mylonitic rocks occur sporadically along serpentinite contacts.
Most of the mylonite is confined to those contacts with larger
gabbroic or sedimentary bodies, but occasional small mylonitic
stringers can be found within the serpentinite. These may be devel•
oped from the remnants of tectonic inclusions. Mylonites are most
commonly pale green, with irregular banding and streaking. They
consists of mixtures of tremolite, diopside, and chlorite, with
occasional pumpellyite and brown spinel (Fig.22). In some places,
small stringers are intercalated with serpentinite at the margin
of the mylonite zone (Fig.23). These zones usually grade within ten
feet into relatively undeformed rocks with the typical sedimentary
or igneous mineral assemblages described previously. On the basis
of the mylonite occurrences,it appears that the serpentinite margin
in general has been a zone of intense deformation.
Ultramafic Rocks
The ultramafic rocks within the map area have been almost
completely serpentinized; primary minerals were only found in one
locality. In addition, many of the rocks have been deformed to
such an extent that the original textures are completely obliterated. 36.
Figure 23. Mylonite stringers at contact of sedimentary rocks and ser• pentinite on center fork of Jim Creek. Serpentinite is on the right. 37.
For these reasons, characterization of the protolith is difficult.
In other parts of the Shulaps Range, however, the rocks are less altered and have been described in detail by Leech (1953). He found that approximately 85% of the Shulaps ultramafic rocks were harz- burgite (10% - 20% pyroxene), the remainder consisting of dunite.
In addition, approximately 10% of the ultramafic rock was found to be layered, the layers consisting of narrow, discontinuous layers of orthopyroxene. The ultramafite within the map area is assumed here to have once had a similar composition. Textural evidence supports this conclusion.
The ultramafite consists of a complex mixture of two types of serpentinite. "Sheared serpentinite" is light green and contains closely spaced, slickensided shear surfaces (Fig.24). Generally, this type of rock is fissile, but in places is quite coherent, with a mylonitic texture. No relict textures remain in this type of rock. "Blocky serpentinite" forms the bulk of the ultramafite, weathering in shades of green and rusty brown. In most specimens serpentine pseudomorphs after pyroxene can be clearly seen on fresh surfaces. All gradations between these two serpentinite varieties are common. In occasional exposures the blocky serpentinite occurs as rounded masses up to ten feet in diameter in a matrix of sheared serpentinite (Fig.25). These "roundstone breccias" probably repres• ent localized zones of differential movement, but none was mappable for any great distance. Although outcrops of the serpentinite mixtures are complex in detail, with shear zones of many sizes and orientations bounding blocky serpentinite, most have a dominant, measurable foliation. The origin of this foliation is discussed in a subsequent section. 38.
Figure 25. "Roundstone breccia" near triple fork of Jim Creek. 39.
Layered peridotite was observed in only three outcrops; two
are near the fork in the western tributary of Jim Creek, and the
third within the narrow band of serpentinite separating the East
Liza Gabbro from sedimentary rocks to the south. The layering con•
sists of thin stringers, only a few crystals wide, of pyroxene pseudomorphs which weather in slight relief (Fig.26). The inconsis•
tency in the orientation of layering and the very restricted extent
of the layered outcrop within sheared serpentinite suggest that
these examples are isolated blocks within the serpentinite complex.
The three serpentine "polymorphs" clinochrysotile, lizardite,
and antigorite have been described in detail by Whittaker and Zuss- man (1956) and Aumento (1970). Additional comments on identification were offered by Hostetler, et al.(1966). Aumento pointed out the
difficulties in making positive identification of mixtures of these minerals using powder diffraction techniques. Special sample prep•
aration is necessary in most cases. Optical techniques are of
limited usefullness because of typically fine grain sizes and simil•
arity of optical properties. Although special sample preparation was not attempted, comparison of standard X-ray diffractograms of
bulk samples with those of Aumento (1970), coupled with textures
observed in thin section, indicate that the Shulaps serpentinites
probably consist of mixtures of clinochrysotile and lizardite. Anti•
gorite may be present in some specimens of dense, mylonitic serpen•
tinite. In any case, little is known of the genetic significance
of the three serpentine varieties. The general term "serpentine"
is used here to refer to all serpentinites found in the area. Figure 27. Photomicrograph of relict olivine in peridotite, showing deformation lamellae (#N35). 41.
Figure 28. Relict grains of clinopyroxene and clinopyroxene ex- solution lamellae in serpentinized peridotite (#N45).
Figure 29. Inclusion trains of magnetite in sheared serpentinite (#N215). 42.
Primary periodotite minerals were observed in only one set of outcrops in the south-central part of the map area. The rock is grey-green in hand specimen with approximately 15% milky-green pyroxene pseudomorphs. Two specimens were collected from the out• crop, and thin section examination shows one to be 60% serpentin- ized and the other approximately 80%. The original rock consisted of approximately 15% orthopyroxene, 10% clinopyroxene, 72% olivine, and 3% chromian spinel. The orthopyroxene occurs as large grains
(2 mm) and is completely altered to a brownish serpentine mixture, while the clinopyroxene is in clusters of unaltered, smaller cry• stals (1 mm). Olivine grains range in size from 0.5 millimeters to over 5.0 millimeters, and are all partly serpentinized. Most of the grains show distinct deformation lamellae (Fig.27). Semi• quantitative microprobe analysis yields a composition of Fo (91).
Chromian spinel occurs as anhedral grains averaging 2.0 millimeters and sheathed in a zone of colorless chlorite. The cores of these grains are translucent brown, while the remainder is altered to an opaque material, probably magnetite. Veins of serpentine-brucite- magnetite are scattered throughout.
In thin section, blocky serpentinites are quite variable in appearance. Most specimens contain some orthopyroxene pseudomorphs, which are recognizable because they are commonly replaced by serp• entine (clinochrysotile?) with a slightly higher birefringence than the groundmass. These pseudomorphs commonly show kink bands. In some specimens small exsolution lamellae and/or grains of clinopy• roxene have resisted serpentinization, although the total clinopy• roxene is generally less than 1.0% (Fig.28). Most of these rocks were originally harzburgites. Primary olivine is completely absent from these rocks, and serpentinization and deformation have almost 43.
completely obscured original textures. Serpentine grains are
generally smaller than 0.2 mm, although the sheared varieties
are usually finer-grained. "Mesh texture", islands of serpentine
bounded by intersecting serpentine veinlets, is commonly developed,
but not the rule. Brucite was not detected.
Both the sheared and the blocky serpentinites contain opaque minerals ranging to 10%. These occur in two forms; large (1.0 mm)
anhedral to subhedral grains, and smaller (1.0 mm) anhedral grains
and stringers. The larger grains are probably magnetite derived
from original chromian spinel; they have occasionally retained a
translucent brown core. In the more deformed serpentinites the
smaller grains commonly occur as trains within the foliation plane
(Fig.29). These are probably secondary magnetite, but may also be
in part remnants of earlier chromian spinel.
Regenerated Olivine
Olivine regenerated from serpentine is present in the area around Shulaps Peak, although the distribution is sporadic and un• predictable in detail (Fig.3). Buff-weathering olivine porphy• roblasts, ranging in size from 0.1 mm to 10.0 mm, occur within both sheared and blocky serpentinite and give the rock a very distinctive spotted appearance (Fig.30,31). Most porphyroblasts are ovoid, while some are elongate along (001) and occasionally show a pyramid• al termination (Fig.32). The direction of elongation is different than that reported from elongate olivines in talcose metamorphic rocks (Evans & Tommsdorff, 1974). Although in most instances the grains are randomly distributed, some specimens of sheared serpent• inite have small grains concentrated along foliation planes, and in one outcrop near Shulaps Peak, the olivine is concentrated in
discrete layers up to 2 cm thick (Fig. 33). The porphyroblasts
generally constitute from 20% to 50% of the rock, and
magnesite is a common accessory, usually as small grains in the
serpentine matrix. In one specimen, however, a narrow rim of
magnesite encloses the olivine grains (Fig. 31, 34). Grain
boundaries between ovoid grains commonly form 120 degree angles
(Fig. 35). In only two specimens do the olivine porphyroblasts
show deformation lamellae, although most are fractured to
various degrees. Small opaque inclusions are present in all
grains, but in widely ranging concentrations.
The most striking feature of the regenerated olivines is
the relict mesh textures.which are preserved within them. These
textures are outlined by small opaque inclusions (magnetite?)
in the olivine. In some thin-sections the textures can be
traced directly into the surrounding serpentine matrix (Fig. 36,
37), while in others the relationship is less clear (Fig. 38).
Olivine clearly replaces a serpentine vein on one specimen
(Fig. 39).
Semi-quantitative analyses of four regenerated olivines performed with electron microprobe yielded three compositions
of Fo 97 ± 1% and one of Fo 95.5 ± 1%. The latter sample is
one with a large concentration of inclusions, which may have
affected the result. These compositions are more magnesium-
rich than the compositions reported by Leech (1953) for primary olivines, and are consistent with a metamorphic mode of origin
(Trommsdorff and Evans, 1972). Figure 30. Serpentinite with olivine porphyroblasts, from east of the map area.
Figure 31. Serpentinite with olivine porphyroblasts, found as float. In this specimen, each olivine grain is enveloped in a thin layer of magnesite. 46.
Figure 32. Elongate olivine porphyroblast with pyramidal termi• nation (#N215).
Figure 33. Regenerated olivine layers in an outcrop near Shulaps Peak. 1 mm
Figure 34. Rim of magnesite enclosing olivine porphyroblast. Same specimen shown in Figure 31.
1 mm
Figure 35. Olivine porphyroblasts showing 120 degree grain boundaries (#N312). Figure 37. Mesh texture preserved in regenerated olivine (#N711). Figure 39. Regenerated olivine replacing serpentine vein (#N215). All of the regenerated olivines are affected by a second
generation of serpentinization, which usually progresses along
grain boundaries and fractures. Small amounts of brucite are
commonly present in these rocks, usually associated with the
second-generation serpentinization or in small veins close by.
One sample which in hand specimen appears to have rather large
porphyroblasts, is seen'in thin section to have been almost
completely serpentinized, and only small islands of relict
olivine remain (Fig. 40).
The assemblages present in rocks containing regenerated
olivine are the following:
Forsterite + Serpentine ± Brucite
Forsterite + Serpentine ± Magnesite
The reaction producing these assemblages can be described
within the system Mg0-Si02-R"20-C02, which has been investigated
in detail by Johannes (1969), and Greenwood (1967). A
schematic diagram of the stability ranges in this system is
given in Fig. 41 and a detailed plot of the portion relevant
to the Shulaps rocks in Fig. 42.
Almost all of the regenerated olivine examined appears to
be the result of the reaction:
1 serpentine + 1 brucite = 2 forsterite + 3H20
x At which takes place only under very low values of QQ2- 1000 o bars the reaction proceeds at approximately 360 C., rising to o
410 C at 4000 bars. With increase of Xori forsterite breaks
down by the reaction: 2 forsterite + 2 H2O + 1 CO2= 1 serpentine +
1 magnesite 51.
Figure 40. Olivine porphyroblasts almost completely serpenti- nized (#N440). Uncrossed polars. adopttd from Johannts(l969)
Figure 41. Schematic T-X^ diagram for the system MgO-Si02-H20-CD2 at elevated pressures and temperatures. Abbreviations: A=anthophyllite; B=brucite; E=enstatite F=forsterite; M=magnesite; P=periclase; Q=quartz S=serpentine; T=talc Figure 42. Isobaric equilibrium curves at low C0o content in the T-
field for the system MgO-43i02-B^O-C02 See Figure 41 for abbreviations. Generally, the CO2 component appears to have been minimal, as the above reaction was illustrated by only one specimen, found
as float (Fig. 34). The highest values of X„n reached in the are exemplified by talc-magnesite rock which is locally abundant.
The origin of the regenerated olivine is problematical.
The very irregular distribution of olivine-bearing outcrops, and absence of a local heat source suggest that the porphyroblasts were not formed in situ. (See Fig. 37) Rather, they appear to be within small exotic blocks which are now incorporated in the serpentinite melange. The olivines must have been formed between two events: (1) initial serpen• tinization of a peridotite protolith, and (2) a second episode of serpentinization and some deformation which has affected the secondary olivine. Given the lack of information on the early history of the Shulaps ultramafite, these are rather broad constraints, but a brief consideration of the possible mode of origin of these interesting rock is appro• priate .
The metamorphic olivine could have been formed in one of the three ways: (1) regional metamorphism (2) frictional heating associated with emplacement of the ultramafite (3) contact metamorphism by a nearby intrusion. Regional metamorphism is precluded in that the rocks containing the olivine are found in only a small area. If a substantial area of serpentinite had been affected, one would expect to find more in evidence now, even after dismemberment. Frictional heating has been investigated by a number of workers (Minear &
Toksoz, 1970; Mercier & Carter, 1975), and is insufficient to produce the amount of heat required for significant metamor• phism. The restricted occurrence of the secondary olivine also argues against this hypothesis. Contact metamorphism by a neighbouring intrusive appears to offer the best solution.
Such an intrusion would produce porphyroblasts within a restricted area which, after dismemberment, could assume the distribution now observed. In addition, other cases of such contact metamorphism in the cordillera have been well documented
(Frost, 1975; Pinsent & Hirst, 1977). This contact metamorphism must have happened relatively early, as there are no suitable intrusions in the immediate area.
It should be mentioned that in addition to the area above, similar olivine-bearing rocks were reported by Leech (1949, p. 150) near the head of Burkholder Creek. The total area underlain by such rocks was estimated to be 400 acres, consid• erably more than in the present area of study. Leech attributed the origin of the olivine to dynamothermal metamorphsim, the heat being furnished mechanically. STRUCTURE
Introduction
Obtaining detailed structural measurements in the map area is hampered by a number of factors. Bedding in the sedimentary rocks is difficult to find, and in most cases stratigraphic top is not apparent. Most of the ultramafic rock consists of sheared serpentinite with a chaotic array of shear planes. Also, individual measurements are subject to magnetic compass error. In most outcrops, however, a dominant foliation was measurable. Structural data obtained are summarized in Figure 43. Attitudes measured in the area of
East Liza Basin are distinguished for reasons given below.
Structural Relations
Almost all ultramafite in the Jim Creek area is pervasively sheared and serpentinized, making interpretation of contacts with other rock types difficult. All ultramafite contacts appear sheared, and are characterized by reaction zones. In addition, no metamorphic aureole was observed at any contact. For these reasons, the contact of serpentinite with all other rock types was interpreted to be tectonic, and defines a major structural boundary within the mapped area. The contact between the East Liza Gabbro and the East
Liza Volcanic appears to be gradational. To the west, sedimentary rocks in East Liza Basin are in contact with the
East Liza Volcanics. This contact dips southeast at a moderate angle, conformable with the bedding attitudes in the argillaceous rocks. Although the contact is not abrupt, the rock near it is severely distrupted and sheared, and was mapped as a fault. This fault must be an early feature, as it has been folded parallel to the regional trend.
The Main Gabbro was not seen in contact with sedimentary rocks. Leech (1953) states that the gabbro is the younger on the basis of sedimentary fragments within the gabbro to the east of Liza Lake. This contact, however, is a sheared one
(Leech, 1949, p.116), and this writer does not feel that the evidence is conclusive. In the inlier near the head of Jim
Creek, however, a small body of gabbro lithologically identical to the Main Gabbro is intrusive into argillaceous sedimentary rocks. Although this contact is within an exotic block, it supports the view that the gabbro is younger than the sedimen• tary rocks.
Structural measurements on sedimentary rocks were obtained from two general areas; in and around East Liza Basin, and around the west tributary of Jim Creek. The latter may give a misleading picture as the rocks may be exotic blocks. For this reason, these two areas are distinguished in Figure 43. In general, the rocks, in East Liza Basin strike northwesterly, and have a pronounced sub-vertical foliation. In one outcrop, Poles to serpentine foliations Poles to foliations in country rocks
A= western half of area •= East Liza Creek area • = eastern half of area * = Jim Creek area
Lineations in country rocks Poles to bedding in country rocks.
• = East Liza Creek area • •= East Liza Creek area A = Jim Creek area A = Jim Creek area
• Figure 43. Summary of structural measurements 59.
mesoscopic symmetrical folds of approximately five inch amplitude were observed. The fold axis plunges nine degrees to the northwest, although other outcrops suggest gentle plunges to the southeast.
Overall Structure
The irregularity of many of the contacts in the Jim Creek area makes extrapolation to depth very speculative. Although some of the outcrop pattern shown on Leech's map just to the southeast suggests rather large-scale folds, such structures cannot reasonably be tied into the present mapping. For this reason, the following remarks are restricted to the area mapped for the present study. Cross-section A-A' to D-D' are shown in
Figures 44 and 45. Although the cross-sections are of necessity rather schematic, a striking feature is the serpentine melange zone seen in sections B-B' to D-D'. This zone consists of sheared serpentinite with inclusions of almost all other rock types encountered in the area. Serpentine foliation wraps around these inclusions, and the larger ones are surrounded by a rodingite reaction zone. Most of the inclusions are gabbroic, and are most abundant in thearea around and west of
Shulaps Peak. The cross-sections suggest thateven the Main
Gabbro may represent a large inclusion.
Briefly, the structural history of the area is as follows.
The sedimentary rocks, the oldest in the area, were intruded by gabbro at some time before the Upper Triassic. These rocks then underwent some metamorphism and deformation before Figure 44. Sections A-A' and B-B' For legend, see Figure 3. Figure 45. Section C-C' For legend, see Figure 3. emplacement of the serpentinite. This is clearly seen on the northeast side of the range, where isoclinal folding in the country rocks is not observed in the ultramafite (P. Mejstrick,
Pers. Comm.). Emplacement of the ultramafite followed, probably during the Upper Triassic or the Lower Jurassic, although it may not have been a single episode. Further work along the western flank of the ultramafite is necessary to give a more complete picture. Speculation on Origin and Emplacement
One of the aims of this study was to determine what place the Shulaps ultramafite has in the geological history of British Columbia, as examined in the light of plate tectonic theory. Knowledge of the processes of seafloor spreading and continental drift has served to put alpine-type ultramafic rocks in an important position with regard to understanding the evolution of continental margins. As yet, the processes taking place in the mantle which lead to seafloor spreading are poorly understood, and bodies such as the Shulaps ultramafite, thought to be mantle derivatives, may give some answers. A related problem is that of the mechanics of emplacement of such large bodies of ultramafic rock.
A number of writers have recently attempted to interpret the plate tectonic history of the Canadian Cordillera
(Monger, et al, 1972; Ross, 1973; Godwin, 1975). The scope of the present study is not broad enought for such a general discussion, but the place of the Shulaps ultra• mafite in the tectinic regime of B. C. will be outlined in view of present evidence.
The term "ophiolite" is currently used to refer to the suite of rocks commonly associated with and including alpine-type ultramafites such as the Shulaps body. The term has undergone some change since it was first proposed, and recently was defined (G.S.A. Penrose Conference on Ophiolites,
1972) as follows:
A completely developed ophiolite consists of mafic to
ultramafic rocks in the following sequence from the
bottom and working up:
(1) Ultramafic complex consisting of harzburgite,
lherzolite and dunite, usually with a metamorphic
tectonite fabric.
(2) Gabbroic complex ordinarily with cumulus textures
and usually less deformed than the ultramafic
complex.
(3) Mafic sheeted dike complex
(4) Mafic volcanic complex, commonly pillowed.
(5) Associated rock types:
- ribbon chert, shale, minor limestone
- sodic felsic intrusive and extrusive rocks
Ophiolites are currently thought by most workers to represent
pieces of oceanic crust and upper mantle, for reasons
summarized by Williams and Smyth (1973).
The Shulaps ultramafic complex possesses all of the
features of an ophiolite, with the exception of the sheeted dike complex. It does not, however, represent the ophiolite succession; the elements are dismembered and no-longer in the proper sequence. In outline:
(1) Most of the Shulaps ultamafite consists of harz-
burgite, with subordinate dunite; tectonite fabric
has not been documented, but may be present.
(2) The East Liza and Main Gabbros represent part(?)
of a gabbroic complex. Layering is present,
although definite cumulus textures were not ob•
served. Although sheared in places, the gabbro is
not extensively deformed.
(3) A mafic sheeted-dike complex was not observed.
It can be noted that this element of the ophiolite
succession is the one most commonly missing in
other examples.
(4) Pillowed mafic volcanic rocks are associated with
the East Liza Gabbro.
(5) Associated with the Shulaps ultramafite are ribbon
chert, greywacke and minor limestone.
The regional setting is much the same. A large part of the Bridge River area is underlain by the Fergusson Group, through which are scatterd numerous ultramafites, although non nearly as large as the Shulaps body. One of these, the
Pioneer ultramafite, was studied in detail by Wright (1974).
He concluded that the Pioneer ultramafite was a part of a dismembered ophiolite.
The Shulaps ultramafite is bounded on the east by the Yalakom Fault, which is a northern extension of the Fraser
River fault system (McTaggart, 1967). This fault is sub- vertical and sharply truncates the ultramafite. The western boundary of the ultramafite appears much more complex, however. Although the Jim Creek area may not be represen•
tative, it suggests, with its melange zone and pervasive
shearing, that it may represent a part of the base of the
ultramafite, along which emplacement by thrust-faulting took place.
The blocks of varied lithologies scattered throughout
the serpentinite melange could be pieces of country rock which were torn up during emplacement, with the predominant
foliation in the serpentine being produced during the same
event. This would mean that the ultrmafite in the Jim Creek
area must have been largely serpentinized before final emplace• ment. The melange zone would thus represent the base of the
ultramafite, along which it was transported to its present position. A schematic cross-section of this arrangement is
shown in Figure 46. A later fault, shown just to the east of
the crest of the range, may account for the fact that
relatively unaltered peridotite is found in a topographic•
ally low part of the body, and also for the sharp topographic break east of the range crest. It should be mentioned,
however, that a reconnaisance to the northern tip of the
ultramafite, an area which appears to be in the same struc•
tural position as the Jim Creek area, did not show an extensive
serpentine melange zone. This problem can only be resolved by more extensive detailed mapping. SHULAPS PEAK
MELANGE ZTj YALAKOM FAULT /DEXTRAL STRIKE-SLIP \ ISINCE MIDDLE JURASSIC 1
JURASSIC 6 CRETACEOUS
Figure 46. Hypothetical, schematic cross-section through the Shulaps Range in the vicinity of Shulaps Peak. ( 3x vertical exaggeration) There may have been some deformation after emplacement of the ultramafite, as suggested by some outcops on the central tributary of Jim Creek. Here, a small antiformal inlier (?) of sedimentary rocks has serpentinite wrapped conformably around it; they may have been folded contemp• oraneously. If there was post-emplacement folding, some of the serpentinite may have been re-mobilized, accounting for some of the irregular serpentinite distribution around
East Liza Basin. It appears there as if the serpentinite may have been "squeezed" in and around some of the more competent rock units, possibly in response to folding.
The mode of emplacement of the Shulaps ultramafite is thus presumably by thrust faulting, associated with an active plate margin. This may be either by the process of obduction, as suggested by Coleman (1971), or by complex faulting which appears to take place near a subducting lithospheric plate, as exemplified by the Franciscan complex in California
(Ernst, 1965 ; Suppe, 1972).
The age of emplacement is bracketed on the one hand by the age of the Fergusson Group, Ladinian to Carnian
(Cameron and Monger, 1971), and on the other by detrital chromite in clastic rocks of late Lower Jurassic age
(Leech, 1953, p. 39). This is in agreement with paleo- magnetic evidence from Triassic rocks in the insular belt and from the interior, which support the idea of an active suture zone during that period of time (Nelson, 1976). Literature Cited
Aumento, F. (1970): Serpentine mineralogy of ultrabasic intrusions in Canada and on the Mid-Atlantic ridge; Geol. Surv. Can. Paper 69-53, pp. 1-51.
Bell, et al (1911): The geology of the Dun Mountain subdivi• sion, Nelson; New Zealand Geol. Surv. Bull. 12.
Bell, T.H. & Etheridge, M.A. (1973): Microstructure of mylonites and their descriptive terminology; Lithos 6(4), pp. 337-348.
Cairnes, C.E. (1937): Geology and mineral deposits of the Bridge River mining camp, British Columbia; Geol. Surv. Can. Memoir 213.
(1943): Geology and mineral deposits of Tyaughton Lake map-area, British Columbia; Geol. Surv. Can. Paper 43-15.
Cameron, B.E.B. & Monger, J.W.H. (1971): Middle Triassic. conodonts from the Fergusson Group, northeastern Pemberton map-area (92J), British Columbia; Geol. Surv. Can. Paper 71-lb, pp. 94-96.
Christie, J.M. (1960): Mylonitic rocks of the Moine thrust- zone in the Assynt region, northwestern Scotland; Edinburgh Geo. Soc. Trans. 18, pp. 79-93.
(1963): The Moine thrust-zone in the Assynt region, northwest Scotland; Univ. Calif. Publ. Geol. Sci. 40, pp. 345-440. Coleman, R.G. (1967): Low-temperature reaction zones and alpine ultramafic rocks of California, Oregon and Washington; U.S.G.S. Bull. 1247.
(1971): Petrologic and geophysical nature of serpentinites; Geol. Soc. Amer. Bull. 82(897-918).
Drysdale, C.W. (1916): Bridge River map-area, Lillooet mining division; Geol. Surv. Can. Summ.Rpt. 1915, pp. 75-85.
(1917): Bridge River map-area; Geol. Surv. Can. Summ Rept. 1916, pp. 45-53.
Duffell, S. & McTaggart, K.C. (1952): Ashcroft map-area, British Columbia; Geol. Surv. Can. Memoir 262.
Ernst, W.G. (1965): Mineral paragenesis in Franciscan metamorphic rocks, Panoche Pass, California; Geol. Soc. Amer. Bull. 76(879-914).
Frost, B.R. (1975): Contact metamorphism of serpentinite, chlorite blackwall and rodingite at Paddy-Go-Easy Pass, Central Cascades, Washington; Jour:. Pet. 16(2), pp. 272- 313.
Godwin, C.I. (1975): Imbricate subduction zones and their relationship with Upper Cretaceous to Tertiary porphyry deposits in the Canadian Cordillera; Can. Jour. E. Sci. 12(8), pp. 1362-1378.
Grange (1927): On the rodingites of Nelson; New Zealand Inst. Trans. 58, pp. 160-166. Greenwood, H.J. (1967): Mineral equilibria in the system MgO-SiC^-^O-CC^ ,in Researches in Geochemistry, Abelson, P.H., ed., John Wiley & Sons, pp. 542-567.
Hostetler, P.B. et al (1966): Brucite in alpine serpentinites Amer. Min. 51, pp. 75-98.
Johannes, W. (1969): Ah experimental investigation oi the
system Mg0-Si02-H20-C02; Amer. Jour. Sci. 267, pp. 1083-. 1104.
Lapworth (1885): On the stratigraphy and metamorphism of the rocks of the Durness-Eriboll District; Proc. Geol. Assoc. Vol. VIII, pp. 438.
Leech, G.B. (1953): Geology and mineral deposits of the Shulaps Range; B.C. Dept. Mines Bull. 32.
McCann, W.S. (1922): Geology and mineral deposits of the Bridge River map-area, British Columbia; Geol. Surv. Can. Memoir 130.
McTaggart, K.C. & Thompson, R.M. (1967): Geology of part of the northern Cascades in southern British Columbia; Can.' Jour. E. Sci. 4, pp. 1199-1228.
Mercier, J. & Carter, N.L. (1975): Pyroxene geotherms; Jour. Geophy. Rsrch. 80 (23), pp. 3349-3362.
Minear, J.W. & Toksoz, M.N. (1970): Thermal regime of a downgoing slab and new global tectonics; Jour. Geophys. Rsch. 8, pp. 1397. Monger, J.W.H. et al (1972): Evolution of the Canadian Cordillera: A plate-tectontic model; Amer. Jour. Sci. 272, pp. 577-602.
Nelson, J.L. (1976): Origin of Georgia Depression; The Coast Plutonic Complex/Insular Belt Province Boundary on Hardwicke and West Thurlow Island, B.C.; Univ. B.C. M.Sc. thesis.
Pinsent, R.H. & Hirst, D.M. (1977): The metamorphism of the Blue-River ultramafic body, Cassiar, British Columbia, Canada; Jour. Pet. 18(4), pp. 567-594.
Ross, J.V. (1973): Mylonitic rocks and flattened garnets in the southern Okanagan of British Columbia; Can. Jour. E. Sci. 10, pp. 1-17.
Suppe, J. (1973): Geology of the Leech Lake Mountain-Bell Mountain region, California; Univ. of Calif. Pub. Geol. Sci. V. 107.
Trommsdorff, V. & Evans, B.W. (1972): Progressive metamorphism of antigorite schist in the Bergell tonalite aureole (Italy); Amer. Jour. Sci. 272, pp. 423-437.
Whittaker, E.J.W. & Zussman, J. (1956): The characterization of serpentine minerals by X-ray diffraction; Min. Mag. 31, pp. 107-126.
Williams, H. & Smyth, W.R. (1973): Metamorphic aureoles beneath ophiolitic suites and alpine peridotites: tectonic implications with West Newfoundland examples; Amer. Jour. Sci. 273, pp. 594-621. 74.
Wright, R.L. (1974): The geology of the Pioneer ultramafite, Bralorne, B.C.; U.B.C. M.Sc. thesis.