STRUCTURAL RELATIONS OF THE SOUTHERN QUESNEL LAKE GNEISS,
ISOSCELES MOUNTAIN AREA, SOUTHWEST CARIBOO MOUNTAINS, BRITISH
COLUMBIA
by
JOHN R. MONTGOMERY
A.B. OCCIDENTAL COLLEGE, 1982
A THESIS SUBMITTED IN PARTIAL FULFILMENT 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
June, 1985
e John R. Montgomery, 1985 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the 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 or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission.
Department of Geological Sciences
The University of British Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5
Date: ft^y /9S>S^ ABSTRACT
The southern extension of the Quesnel Lake Gneiss lies approximately 10 km northeast of the Intermontane-Omineca Belt tectonic contact in the southwestern
Cariboo Mountains, British Columbia. The aim of this thesis is the investigation of the structural development and style at a deep structural level relative to the 1MB-OB contact, and to determine the nature origin of the southern extension of the Quesnel
Lake Gneiss. Omineca Belt rocks in the Quesnel Lake region are the Late Proterozoic to Late Paleozoic Snowshoe Group metasediments. The Snowshoe Group rocks in this study area comprise a package of variably micaceous schist, quartz-biotite gneissose schist, calcareous metasandstone, marble and amphibolite which represent deformed and metamorphosed continental margin deposits. The Quesnel Lake Gneiss is a predominately subalkaline granodioritic intrusive into these sediments that has been modified by subsequent deformation and metamorphism. High Sr content, low initial
S7Sr/uSr ratios and an alkalic component imply a mantle source although possible Pb inheritance in zircons and regional Sr data suggest a certain amount of assimilated continental crust A U-Pb zircon age on the Quesnel Lake Gneiss indicates intrusion in Mid-Paleozoic, probably Devonc—Mississippian time. A regional metamorphic event affecting the entire sedimentary and intrusive package is interpreted to have occurred in the Middle-Jurassic as suggested by sphene U-Pb geochronometry and regional stratigraphic relations.
The structural sequence observed in this area is composed of five phases of folding followed by a brittle fracturing and faulting phase. The entire sequence of deformation is seen in both the Snowshoe Group and the Quesnel Lake Gneiss. A pervasive metamorphic foliation defines the compositional layering (S0/1) and is axial planar to isoclinal first phase folds in both rock packages. Syn-metamorphic second phase deformation is evidenced as tight similar-style folds with an axial surface penetratively developed at a low angle (10-15°) to the compositional layering. Syn- to
ii post-metamorphic third phase deformation produced southwest verging folds with only locally penetrative axial surfaces developed at approximately 40° to SO/1 compositional layering and northwest plunging fold axes nearly coaxial with F2 folds. The Quesnel
Lake Gneiss shows a lack of F3 macroscopic folds. Fourth and fifth phase folds are brittle, broad warps that are only locally developed in the more micaceous units. A series of t' vs. a plots on second and third phase folds in both rock types indicates a ductile regime associated with high shear strain during F2 deformation with decreasing shear strain and less ductile behavior during the third phase of deformation.
This change in behavior corresponds with the waning of metamorphism.
At least one regional metamorphic episode has affected this area in association with the deformational sequence outlined above. The metamorphic peak occurs post-F2 and pre- to syn-F3 deformation producing Barrovian-type assemblages of the amphibolite facies. Metamorphic temperatures of approximately 590° C at 5.5 kb were determined by garnet-biotite geothermometry in sillimanite-bearing schists northeast of the Quesnel Lake Gneiss.
A tectonic history for the rocks in this map area began with the deposition of the Snowshoe Group sediments in a continent margin basin from the Late Proterozoic to the Early Mississippian. Intrusion into this package by the Quesnel Lake granitic body occurred between 317 and 400 Ma ago. The first phase of deformation recognized in the Snowshoe Group and Quesnel Lake Gneiss is absent in the
Quesnellia and Slide Mountain rocks and may also be of Paleozoic age. The accretion of Quesnellia onto the continental margin in Early Jurassic time is inferred to have initiated the subsequent deformation and regional metamorphism.
iii Tahle of Contents
I. INTRODUCTION 1 Location 1 Regional Geology 1 Thesis Area 3
II. STRATIGRAPHY 6 Introduction 6 Snowshoe Group 8 Caput Mountain Area 8 Isosceles Mt. Area 12 Southwest Gneiss Contact 15 Quesnel Lake Gneiss 15 Major and Minor Element Chemistry 17 Geochronometry 25 Summary and Discussion 28
III. STRUCTURE 30 Introduction 30 Snowshoe Deformation F1-F5 30 Quesnel Lake Gneiss FI-F5' 40 Correlation of Deformation 48 Summary and Discussion 50
IV. METAMORPHISM 55 Introduction 55 Mineral Relations 55 Garnet- Biotite Geothermometry 66 Summary 68
V. SUMMARY 72
REFERENCES CITED 78
APPENDIX A '. 82
APPENDIX B 85
APPENDIX C 87
iv List of Figures and Plates FIGURE PAGE
1. Location map 2
2. Regional map 4
3. Map of study area 7
4. Structural succession of Caput Mtn. area 11
5. Structural succession of Isosceles Mtn. area 13
6. Harker diagram 19
7. ACF diagram 20
8. Alkaline/subalkaline discriminant diagram 22
9. Rb vs. Y + Nb discrimant diagram 24
10. Rb-Sr diagram 26
11. Ll and Ll' lineations 32
12. Isoclinal Fl fold ....33
13. S2 axial surfaces 35
14. L2 lineations 36
15. F3 fold : 37
16. F3 axial surfaces 38
17. L3 lineations 39
18. S4 and S4' axial surfaces and L4 and L4'lineations 41
19. F4 open fold 42
20. F5 open fold 43
21. S5 and S5' axial surfaces and L5 and L5'lineations 44
22. Fl' folded vein 46
23. F2' fold 47
24. t' vs o diagram 52
25. a3 slip direction 54
26. S0/1 fabric 56
27. F3 crenulation of micas 57
v 28. Composite garnet 58
29. Garnet with kyanite inclusion 60
30. Staurolite over- F2 crenulation 61
31. Kyanite over F2 crenulation 62
32. Sillimanite (fibrolite) 64
33. Metamorphic mineral zones 65
34. ASK phase diagram 67
35. Garnet-biotite geothermometry 69
36. Mineral growth vs. deformation 70
Plate 1: Geologic map of the Isosceles Mountain area
Plate 2: Geologic cross sections A-A' and B-B' (
vi Acknowledgements
J.V. Ross, thesis supervisor, suggested the project and provided support, supervision and advice throughout the research.
Valuable assistance in laboratory analyses was provided by J. Knight and K.
Scott. K. Scott also provided Rb-Sr analyses. B. Cousens provided elemental analyses.
J. Mortensen enthusiastically contributed to this project with his zircon and sphene analyses. L. Erdman helped with data reduction and other computer problems.
Discussions with J. Fillipone, D. Parkinson, J. Mortensen, R.L. Armstrong, H.J.
Greenwood and S. Garwin helped clarify numerous issues and problems.
E. Bomer provided able assistance in the Field. J. O'Brien was a source of patience and moral support
National Research Council of Canada grant number A-2134 to J.V. Ross provided funding for this project
vii I. INTRODUCTION
Location
The study area is located at the southwestern edge of the Cariboo Mtns., central B.C., approximately 140 km east of Williams Lake, B.C. and 5-10 km south of the east arm of Quesnel Lake (Fig. 1). Headwaters of the Horsefly River divide the study area with Caput Mountain to the south and Isosceles Mountain to the northeast
A total of 40 km2 was studied during the summer of 1983.
Regional Geology
Five major tectonic provinces are defined in the Canadian Cordillera of which the Omineca Belt (OB) and the Intermontane Belt (1MB) are the most important to this study (see Fig. 1). Rocks within the Omineca Belt in the Quesnel Lake region are Upper Proterozoic and Paleozoic psammitic and pelitic metasedimentary rocks and bodies of granitic gneiss (Campbell and Campbell, 1970; Campbell, 1978). This package represents a miogeoclinal wedge of sediments that was deposited along the western edge of the North American craton from Proterozoic through Late Paleozoic time
(Wheeler and Gabrielse, 1972). Intermontane Belt rocks are composed of Upper
Paleozoic ophiolitic rocks (Montgomery, 1978), overlain by Triassic and Lower Jurassic sedimentary and volcanic rocks (Campbell and Campbell, 1970). These two packages are a part of the Slide Mountain and Quesnel terranes, respectively (see Monger, 1977;
Monger and Price, 1979; Monger et al., 1982). The contact between the 1MB and OB is tectonic and is thought to record the accretion of an allochthonous oceanic/arc terrane (1MB) with the cratonic margin (OB) (Monger et al., 1972; Ross et al., 1985).
In the Quesnel Lake region the Omineca Belt rocks are composed of the recently redefined Snowshoe Group of Late Proterozoic to Late Paleozoic age (Struik,
1983). These miogeoclinal sediments also contain elongate bodies of granitic gneiss, the most prominent being a northwest-trending body occurring adjacent to Quesnel Lake
1 Figure 1: Location map of the Quesnel Lake and Crooked Lake map areas in central British Columbia. 3
(Campbell and Campbell, 1970). Northwest of Quesnel Lake and between the north and east arms of the lake, the gneiss has been interpreted to be a metamorphosed intrusive (Rees, 1983; Struik, 1983).
Structures in the region are dominated by northwest-plunging anticlinoria and synclinoria. Metamorphic grade increases down-structure from the northwest to southeast culminating at the edge of the Shuswap Complex as defined by the sillimanite isograd.
The metamorphic isograd surfaces plunge to the northwest away from this culmination and cut across stratigraphy (Campbell, 1978). A transition in structural style accompanies
the metamorphic grade change with rocks becoming more highly strained towards the southeast (Campbell, 1970).
Thesis Area
This project is part of a program initiated to understand the tectonics of the
1MB and OB in the Quesnel Lake region (Ross et al., 1985). The aim of this
particular thesis is the investigation of the structural development and style at a deep
structural level relative to the 1MB-OB contact, and to determine the nature and
origin of the southern extension of the Quesnel Lake Gneiss.
Rocks comprising the 1MB-OB tectonic boundary have undergone a polyphase
deformational history that is frequently coincident within an accretionary tectonic setting.
By studying the structural styles and deformational phases at a deep structural level, an
attempt can be made to understand infrastructure behavior and variation in strain
within an accretionary zone.
This study area lies on the northeast limb of the northwest plunging ML
Perseus antiform (Fig. 2). Campbell (1970, 1978) shows the compositional layering on
this limb dipping predominately northeast and indicates, on his map (1:125,000), an
apparent lack of major anticlinal culminations inboard of those seen at the 1MB-OB
contact This implies a variation in the development of major folds away from the 4
L. Triassic Black phyllite M. Paleozoic QUESNEL LAKE GNEISS
SNOWSHOE GROUP Hadrynian- L. Paleozoic ANTLER FM. pelitic schists, quartzite, M. Paleozoic marble, amphibolite
/ Third phase antiform / Decollement S Convergence boundary X /
Figure 2: Quesnel Lake-Crooked Lake map area along the Intermontane- Omineca Belt boundary. Map pattern exhibits major northwest-plunging antiform-synform series at this contact 5 contact There is also a marked increase in metamorphic grade in an easterly direction.
Previous work on the Quesnel Lake Gneiss attempted to define its origin, specifically whether it represents a basement slice or a metamorphosed granitic intrusive
(Campbell and Campbell, 1970; K.V. Campbell, 1971; Fletcher, 1972; Rees, 1983).
Although the gneiss has been interpreted to be an intrusive north of Quesnel Lake
(Struik, 1983), the lack of continuity of the gneissic bodies northwest of the lake, coupled with the progressively higher grade of metamorphism and more intense deformation to the southeast, indicated the need for further investigation of the southern extension of the gneiss. The presence of this large gneiss body within the metasedimentary sequence also allows insight into the interaction between two packages
of contrasting rheology during a polyphase deformational history. II. STRATIGRAPHY
Introduction
Rocks in this study area are of two main types, miogeoclinal metasedimentary rocks of the Upper Proterozoic to Middle Paleozoic Snowshoe Formation and a granitic gneiss body named the Quesnel Lake Gneiss (Campbell, 1963; Campbell and Campbell,
1970). The Snowshoe Formation, in the Yanks Peak-Roundtop Mountain area north of
Quesnel Lake, was originally thought to be the uppermost part of the Upper
Proterozoic to Lower Cambrian Cariboo Group (Holland, 1954 and Sutherland-Brown,
1957). Regional mapping by Campbell in the mid-1960's to early-1970's showed that
the Snowshoe Formation was not part of the Cariboo Group, but was likely a western
equivalent of the Late Proterozoic Kaza Group. Recent mapping by Struik (1981, 1982,
1983) has redefined the relationship of the Snowshoe Formation, elevated it to Group
status, and separated it from the Cariboo Group by the northeast-dipping Pleasant
Valley thrust
In the study area mappable units are defined in both the metasedimentary
sequence and the Quesnel Lake Gneiss. Polyphase deformation and metamorphism of
the rocks has resulted in the complete transposition of bedding and has eliminated the
possibility of determining original sedimentary thickness. Primary sedimentary features
have also been obliterated. Mappable formations described herein therefore comprise a
structural succession rather than a chronostratigraphic sequence.
Units within the Snowshoe Group are described in three areas: the Caput
Mountain area, the Isosceles Mountain area and the southwest gneiss contact area (Fig.
3). A description of lithologies within the Quesnel Lake Gneiss is then presented along
with major- and minor-element chemistry and geochronometry. A discussion of the age
and regional significance of the sequence completes this section.
6 Figure 3: Generalized map of the study area (see also Plate 1). 8
Snowshoe Group: Caput Mountain area
Two major stratigraphic divisions are made in the Caput Mountain area on the basis of predominance of quartzose (Domain A) or pelitic and calcareous rocks
(Domain B) within each division.
Domain A, in the southwestern portion of the Caput Mountain area, contains a total structural thickness of approximately 210 m. This domain is characterized by quartz-rich semi-pelitic schists, a capping of crudely foliated quartz-feldspar gneiss and a deficiency of metacarbonate and amphibolite units. The domain has been further broken down into five informal units. These units will be described in detail beginning with the structurally lowest unit
Unit 1 is a gneissose quartz biotite schist and is restricted to the southwest portion of the Caput Mountain area. This gneissose unit may be correlative with the
ML Perseus Gneiss (D.C. Elsby, pers. comm., 1984) and has a thickness of approximately 35 m in the area. In outcrop Unit 1 is rather blocky and resistant to weathering and exhibits a dark brown to grey appearance. Gneissic banding is defined by alternating bands of quartz-plagioclase and biotite which vary in thickness from 0.5 to 2.0 cm. Minor amounts of slightiy chloritized garnets are occasionally seen within the more biotite-rich layers. The unit shows a consistent composition laterally and is richer in plagioclase and garnet near the top. The upper contact is gradational on the scale of 0.5 m and is defined by an increasing mica and quartz content.
Unit 2 is semi-pelitic, 10 m thick, with a minor lenticular calcitic marble occurring near the middle. Tan to brown weathering outcrops are thinly foliated and are defined by alternating quartz-rich and mica-rich layers. Mica (± garnet) layers preserve a pervasive crenulation fabric. The mica/quartz ratio varies throughout the
unit This quartz-rich schist is in sharp contact with the overlying blocky- weathering
Unit 3. 9
Unit 3 is green to grey, waxy-textured mica schist This relatively resistant unit is characterized by thin quartzofeldspathic layers alternating with layers of chloritized mica that contain occasional small garnets. Thickness is approximately 15-20 m. Contact with Unit 4 is sharp with a sudden decrease in mica content
Unit 4 is more psammitic than the underlying units with a basal calcareous metasandstone grading upwards into quartz-rich semi-pelitic schist The metasandstone occurs in the bottom 5 m of this 100 m thick unit and is blocky in outcrop.
Preferentially weathering calcareous layers approximately 1.0-3.0 cm thick alternate with quartzose layers 1.0-2.0 cm thick in rhythmic cycles. Rusty tan to grey weathering quartz-rich semi-pelitic schists dominate the remainder of the unit and contain quartz-rich layers 2.0-8.0 cm thick separated by thin partings of mica. The entire unit becomes more quartz and feldspar rich at the top and grades into the structurally higher Unit 5.
Unit 5 is a quartz-feldspar gneiss with a total thickness of 55 m. The buff-weathering, off-white unit is characterized by its resistant nature and small amount of mica. Approximately 10% white mica defines a crude schistosity and
separates 0.1-1.0 cm layers of quartz and feldspar. Rare garnet is also seen. The
upper contact with the lowest unit of Domain B is gradational at the 1.0 m scale
with increasing mica content
The division between Domains A and B is made at the upper contact of Unit
5. Domain B features a greater pelitic content as well as a number of marble layers
and amphibolites. Two subdivisions are made within the domain: a lower
quartz-feldspar-mica schist and a large upper unit containing pelitic schist, marble,
amphibolite and semi-pelitic schist A thickness of approximately 950-1000 m is
contained in Domain B and caps Caput Mountain and the neighboring Peak 7398.
Unit 6 is a grey to brown weathering quartz-feldspar -mica schist comprising
a total thickness of 400 m. The unit is fairly resistant in outcrop and is well-foliated 10 and crenulated as defined by planar parallel alignment of micas. The rock is divided into layers of quartz and feldspar 1.0-5.0 mm thick with interspersed mica in thin partings. Minor amounts of garnet and occasional staurolite grains are also seen in this unit. The upper contact of this unit is sharp and is defined by the first marble layer on the southwest ridge of Caput Mountain.
Unit 7 begins with a grey micaceous marble approximately 1.5 m thick. Above it lies a varying sequence of garnet-kyanite-mica schist, marble, and garnet-quartz-mica schist with local amphibolite seen on Peak 7398. The lenticular nature of the marble beds, either as an original depositional feature or the result of layer parallel, isoclinal folding (Fl), has hindered their use as markers in correlating lithologic sequences across the map area. An isoclinal Fl fold hinge of marble on the southern face of
Caput Mountain indicates the tectonic thickening and complex stratigraphic inversion that exists in these rocks.
Four major metacarbonate layers occur in this uppermost unit (see Fig. 4). The marbles are variably grey to tan weathering and contain minor 0.5-1.0 cm thick layers of mica and resistant lenses of quartz and feldspar. The structurally lowest marble lens is, however, massive and up to 1.5 m thick with a lateral extent of 20-25 m. The
marble member that defines an Fl fold hinge on Caput Mountain is 2.5 m thick.
Tan to reddish-brown weathering pelitic schist is interlayered between the
marble members. The well-crenulated schists contain high-grade mineral assemblages of
quartz-plagioclase-biotite-muscovite-garnet ± staurolite ± kyanite. Individual layers of schist
vary in thickness from 10-80 m.
Minor amphibolite members occur on Peak 7398 with no apparent spatial
relationship to carbonate units. They range in thickness from 0.3-1.0 m and contain
dominantly hornblende-plagioclase-quartz± garnet
A semi-pelitic garnet-quartz-mica schist is seen above the highest marble layer
capping both Caput Mountain and Peak 7398, although any further evidence for 11
Domain B
qtz-bi schist marble
amphibolite
gar-mica schist
marble
Unit 7 gar-ky-mica schist
Domain A marble/amphibolite
qtz-f spr-ms gar-mica schist 5 z. ----7 gneiss
qtz-mica schist amphibolite calc. metaSst 3 grn/gray schist qtz-mica schist 2 semi-pelitic schist 7 qtz-bi schist marble
variably micaceous schist Unit 6
Figure 4: Structural succession in the Caput Mountain area. 12
correlation is lacking.
Isosceles Mountain area
The Isosceles Mountain area lies northeast of Caput Mountain across the
Horsefly River and just northeast of the northwest trending Quesnel Lake Gneiss
(QLG). The section will be described from the gneiss contact structurally upwards toward the northeast (see Fig. 3).
Contact between the metasediments and the more massive QLG is everywhere concordant with compositional layering (SO/1) approximately parallel. Interlayered QLG is commonly seen within a zone up to 35 m into the metasediments with lenses and layers ranging in thickness from 1-20 m (see Fig. 5). Gneiss interlayers are more
leucocratic than the main body of the gneiss.
Unit 8 is a layer of garnetiferous quartz-mica schist which ranges in thickness
from 12-15 m, and lies directly above the gneiss contact The gneiss-schist contact is
commonly obscured by layer parallel intrusion of late-stage (post-F3) unfoliated pegmatite and where seen is roughly gradational on a scale of 0.5-1.5 m. This unit is
variably pelitic with alternating quartz-rich and mica-rich layers ranging from 0.5-2.5
cm thick. Layers appear randomly cyclic with no evidence for relic graded bedding. An
interlayer of QLG forms the upper contact, gradational on the 0.5 m scale with an
increase in feldspar and hornblende content and a corresponding decrease in quartz and
mica. This interlayer can be traced parallel to the northeast QLG -Snowshoe contact
the length of this map area and ranges in thickness from 8.0-12.0 m thick.
Unit 9, which lies in sharp contact above this gneiss layer, is a medium-coarse
grained garnetiferous quartz-mica schist This unit is 12-15 m thick and contains
numerous 0.5-1.0 cm diameter garnets with local kyanite and staurolite. Minor layers
and lenses of QLG 1.0-4.0 m thick are seen locally within this unit (see Fig. 5). South of Isosceles Mtn. 2 km NW 2 km SE
Figure 5: Structural succession along the northeastern Quesnel Lake Gneiss 14
Unit 10 is a tan to silvery-brown weathering micaceous marble ranging from
1.5-2.0 m thick. Mica, feldspar and quartz layers 0.5-1.5 cm thick outline complex fold geometries within the unit Sharp contact is made with the overlying 25-30 m of variably micaceous schist of Unit 11. Unit 11 is grey to tan garnet-quartz-mica schist which locally contains an amphibolite layer 0.5-1.0 m thick. Unit 12 is another marble
2.0-3.0 m thick. This marble is relatively clean and massive, tan to grey weathering, and is commonly adjacent to thin (0.25-0.75 m) amphibolites.
Unit 13 lies in sharp contact above this marble and is a greenish-grey to tan weathering garnet-quartz-mica schist ranging from 100-110 m thick. The unit varies from a more chloritized greenish-grey color at the base, to more tan to reddish-brown upwards. Cycles of quartz-rich and mica-rich layers 0.5-3.0 cm thick alternate throughout this unit Large kyanite blades up to 5.0 cm long are observed within this schist just below Isosceles Mountain lake.
Unit 14 is a quartz-feldspar augen gneiss. This gneiss is approximately 2.0 m thick and contains 5.0-10.0 cm thick layers of quartz and feldspar with minor amounts of mica alternating with 1.0-3.0 cm thick mica-rich layers that locally contain garnet porphyroblasts. Quartz and feldspar asymmetric augen are approximately 0.5 cm in diameter. The entire unit is divided into 2-5 mm laminae by fine mica partings.
Unit 15 comprises the remaining 900 m of section observed. It is dominated by variably micaceous schist but also contains layers of graphitic schist, a local lens of actinolite schist, minor marble and amphibolite. This sequence was observed only along the northeast trending Isosceles Mountain ridge and not followed along strike.
Unit 15 begins with 45-50 m of grey to tan weathering garnet-quartz-mica ± kyanite schist This rock is very well-foliated and crenulated and contains a lens of almost pure actinolite schist approximately 1.0 m thick and 6.0 m long. Contact is gradational with an overlying very fine-grained graphitic schist 10 m thick. This rock is layered on the scale of 1.0-2.0 cm with micaceous layers separated 15 by thin quartz laminae. Above this is a fairly quartz-rich semi-pelitic schist, 175 m thick, containing several micaceous layers. This schist becomes progressively more micaceous. Next is a very quartz-rich schist sequence, 50 m thick, followed by a metacarbonate-amphibolite pair, each approximately 1.5 m thick. Silver-grey to tan weathering, variably micaceous schist completes the remainder of the unit Large volumes of unfoliated quartz-kspar-muscovite pegmatite intrude this seqence predominately along compositional layering but also into a late-stage, east-west trending fracture system.
Southwest Gneiss Contact
The southwest Quesnel Lake Gneiss-Snowshoe Group contact has approximately the same structural relationships as described for the northeast, Isosceles Mountain area contact Concordant, layer-parallel contact between the metasediments and the Quesnel
Lake Gneiss is seen at two locales (see Fig. 3). The zone of gneiss-metasediment
interlayering is approximately 25 m wide on the southwestern ridge of Dutchman
Mountain. This zone contains 3 to 4 alternating layers of gneiss and quartz-mica schist
ranging in thickness from 2-6 m. Numerous pegmatites also intrude within this zone
along compositional layering. Structurally below this contact zone the metasediments are
medium-grained garnet-kyanite ±staurolite schists. This sequence shows some rhythmic
alternation of quartz-rich layers with very garnetiferous mica-rich layers ranging from
2.0 m to 2.5 m. Undifferentiated pelitic schists continue down-section to the southwest,
out of this project's map area.
Quesnel Lake Gneiss
The Quesnel Lake Gneiss examined in this area is a southern extension of a
body of gneiss that trends northwesterly for approximately 50 km on the western edge
of the Omineca Belt The gneiss is bounded on both northeast and southwest contacts 16 by metasediments of the Snowshoe Group. A total structural thickness of 1700 m is exposed across Dutchman Mountain ridge. The southwestern contact has been seen in two locales while the northeastern, upper contact has been mapped along strike for approximately 8 km. The gneiss lenses out to the southeast, beyond the area mapped
(K..V. Campbell, 1971; R.B. Campbell, 1968). The contacts are concordant and are commonly obscured by quartz-feldspar pegmatite sills and dikes. The southwest contact is well-exposed and displays concordantly interlayered QLG and quartz-mica schists.
The northeast contact is more obscured by pegmatites but where observed also shows concordant interlayers of gneiss.
A variety of lithologies occur within the gneiss body proper: a micaceous quartz-feldspar ± hornblende gneiss, pinkish pyroxene-hornblende-feldspar gneiss, dark green amphibolitic gneiss, and a garnetiferous syenitic gneiss. Many generations of veins of feldspar and quartz ± mica appear in all units both within and across compositional layering. At least one major interlayer of Snowshoe Group schistose rock occurs well within the gneiss body and is interpreted to represent either an infold of the enveloping rock or a xenolith. Lateral correlation of these lithologic units is problematic due to their gradational character both across and along strike. A generalized correlation is attempted based on lithology and structural association (see Plate 1).
An epidote- biotite- quartz- feldspar± hornblende gneiss is the most pervasive unit in the body. It is seen along the entire length of the northeast contact and in places across the Dutchman Mountain ridge. The well-layered biotitic end-member of the unit
is especially distinct at the Snowshoe Group metasediment contact and on Dutchman
Mountain. This unit is grey to dark brown weathering schistose gneiss with variable banding of leucocratic and melanocratic layers from 0.0 to 2.0 cm thick. Biotite and
hornblende content vary along and across the unit and contacts with other gneiss units are gradational on the 1.0 m scale. 17
A medium-coarse grained pinkish pyroxene-hornblende-feldspar gneiss is the other dominant unit within the exposed body. This unit is medium to thickly banded and occurs far from the Snowshoe Group contact. Dark green amphibolitic gneiss and a garnetiferous syenitic gneiss commonly occur as lenses or layers up to 2.0 m thick within leucocratic gneisses. The dark green gneiss consists of anastamosing layers of feldspar and minor quartz around pods and layers of hornblende, pyroxene and minor biotite. The garnetiferous syenitic gneiss appears almost skarn-like with very coarse grains of potassium feldspar, epidote and garnet This lithology is seen both along the southwest ridge of Dutchman Mountain and near Dutchman Mountain lake.
Major and minor element chemistry
Major and minor element chemical analyses were carried out on eight representative samples of Quesnel Lake Gneiss, two samples of Snowshoe Group schist and four samples of Mt Perseus Gneiss which occurs bordering the 1MB-OB contact immediately south of this study area. These analyses were undertaken in an attempt to characterize the lithologic variation within the gneiss and to a) define whether this variation represents an originally differentiated intrusive modified by subsequent deformation and metamorphism or is in part due to infolds/interlayers of the enveloping country rock, and b) make a comparison with the ML Perseus Gneiss which has undergone the same metamorphic and deformational history as the Quesnel
Lake Gneiss. Amphibolite-grade metamorphism, solution transfer during polyphase deformation, and abundant fluid phase migration as evidenced by pervasive pegmatization has led to both an obscuring of contacts between lithologies and a segregation of felsic and mafic phases within the units. Because of the varied lithologies present within the gneiss, it is often difficult to recognize a metaigneous or metasedimentary protolith at the mesoscopic level. Tabulated results of both major and minor element analyses are given in Appendix A. Major element abundances are shown 18 on a Harker diagram (Fig. 6). Although there are few analyses some apparent trends are present While a simple differentiated intrusive body would show smooth variation curves (Hyndman, 1972) this relationship is not obvious here. Apparent compositional
trends observed are a decrease in A1203, CaO, MgO, Fe203 with increasing Si02
content and approximately constant K20 and Na20 content with increasing Si02.
Irregularities in the trend are especially evident at lower (<65%) Si02 levels. QLG sample 402, a syenitic gneiss, is most anomalous with respect to a linear differentiation trend. This alkali-rich, silica-poor rock is enriched also in various minor elements (Ba,
Ce, Nd, Sr). QLG sample 306, a very micaceous quartz-feldspar schistose gneiss, is taken from a unit suspected of being an infold of Snowshoe Group metasediment due
to its extremely micaceous nature, and shows a depletion of Na20 and CaO characteristic of sediments (Chappie and White, 1974). The Snowshoe Group schists and
Mt Perseus Gneiss plot with approximately the same compositional trend as the
Quesnel Lake Gneiss samples. The lack of linearity indicates that a significant amount of migration and mobilization of major elements has occurred in these rocks.
Figure 7 is an ACF diagram with associated compositional fields from Winkler
(1979). The Snowshoe Group schists plot within and adjacent to the greywacke field.
Most of the QLG samples plot within the greywacke and basalt and andesite fields.
The greywacke field also approximates a granitic composition field. QLG sample 306 lies adjacent to the shale region which contains Mt Perseus Gneiss sample E000. The
CaO-rich QLG sample 402 is a highly alkaline syenitic gneiss (see Fig. 8).
Chappie and White (1974) have defined a set of geochemical parameters useful in distinguishing between two contrasting types of granites derived from either igneous or sedimentary source material (I-type or S-type). 19
• Quesnel Lake Gneiss • Snowshoe a Mt- Perseus Gneiss • CM 0.8 • • A 2 0.5 • t- • 0.2 • •
001 8.0 • • Ol 6.0 • • f_14.0 A < 1 2.0 • • °a
CO • • O 6.0 • • • CM A O • • °o Li. 2.0 Q0.15 • •• So. 1 o • A • • 0.05 • a •I n 4.0 • o A • A CO • • 2 2.0 • •• • • ? «-° o • 2.0 A f an • • A • o • A • a° CM 3.0 • • CO • • • • 2 1.0 • 0.9 • • O CM 0.5 • • • a _: • • • A • A
~ 0.4 O 0.2 • • CM • • a. • A • 1 I I • I I • •» - u . I 50 55 60 65 70 75 80 Si02
Figure 6: Harker variation diagram plotting the weight percent of the major element oxides vs. Si02 of the Snowshoe Group, Quesnel Lake Gneiss and ML Perseus Gneiss. 20
Figure 7: ACF diagram for the Snowshoe Group, Quesnel Lake Gneiss and ML
Perseus Gneiss. A=Al203-(K.2O + Na2O), C=CaO, F=FeO + MgO+MnO, with compositional fields from Winkler (1979). 21
Their chemical parameters are summarized below:
I-TYPE S-TYPE
Relatively high sodium, Na20 normally Relatively low sodium, Na20 normally
>3.2% in felsic varieties, decreasing to <3.2% in rocks with approx. 5% K20, >2.2% in more mafic types decreasing to <2.2% in rocks with
approx. 2% K20
Mol Al203/(Na20 + K20 + CaO) < 1.1 Mol Al203/(Na20+K20 + CaO)>l.l C.I.P.W. normative diopside or < 1% >1% C.I.P.W. normative corundum normative corundum Broad sprectum of compositions from Relatively restricted in composition to felsic to mafic high Si02 types Regular inter-element variations within Variation diagrams are irregular the plutons; linear or non-linear variation diagrams
Application of these cutoffs and ratios to the analysed samples allows separation of the samples into one of the two types. It is recognised that the use of these parameters (which are based on the more mobile major elements) is not completely valid for these highly metamorphosed and deformed rocks, but they may be useful in recognizing general trends. Although most QLG samples are categorized as I-type, 262
QLG and 306 QLG are S-type, as is E000 of the ML Perseus Gneiss.
I-TYPE S-TYPE
E362 E000 E200 QLG 262 E371 QLG 306 QLG 232 QLG 377 QLG 402 QLG 293 QLG 249 QLG 27
The Quesnel Lake Gneiss and Mt. Perseus Gneiss were plotted on an alkaline/subalkaline discriminant diagram (Fig. 8; Irvine and Baragar, 1971). All samples plot in the subalkaline field except Quesnel Lake Gneiss sample 402 which is highly alkaline. Quesnel Lake Gneiss samples 232 and 377 plot just above and below the
boundary line, respectively. 22
• Quesnel Lake Gneiss
A Mt. Perseus Gneiss 16.Or
S 12.0|
ALKALINE + S 8.01 CO
4.0 SUB-ALKALINE
o.oi 50 60 70 80 Si02
Figure 8: Alkaline/ subalkaline discriminant diagram with weight percent NajO+K20
vs. weight percent Si02 (Irvine and Baragar, 1971). 23
The samples were also analysed for the minor elements: Ba, Ce, Cr, Nb, Nd,
Rb, Sr, V, Y, Zn and Zr (see Appendix A). The samples contain highly variable quantities of Ba with values ranging from 30 ppm to 3030 ppm. Snowshoe Group schists are characterized by low concentrations of most of the minor elements relative to the Quesnel Lake Gneiss. QLG sample 306 however, is depleted in most minor elements relative to the other QLG specimens and is more akin to Snowshoe Group schists. The unit represented by this sample is interpreted to be Snowshoe Group metasediment and can be shown to core a minor antiform. Strontium values are anomalously high in the QLG ranging from approximately 600 ppm to nearly 6000 ppm in a syenitic gneiss, QLG 402. This gneiss is also the most enriched in most
other minor elements. The ML Perseus Gneiss samples contain minor element
abundances intermediate between the Snowshoe Group metasediments and the Quesnel
Lake Gneiss.
In a recent study by Pearce et al. (1984) systematic trace element differences
have been shown to occur between plutonic rocks produced in different tectonic
settings. The four intrusive settings that were distinguished in their study are:
syn-collision (syn-COLG), volcanic arc (VAG), within plate (WPG) and ocean-ridge
(ORG). Granites analysed from these settings show almost complete separation in
Rb-Y-Nb space. The 14 analyses carried out for this study have been plotted on a
Rb vs. Y + Nb discriminant diagram (Fig. 9). As can be seen from the diagram the
analyses all plot along the WPG/VAG boundary, and near the syn-COLG 'triple
point'. The WPG field is interpreted to be representative of granites that have
intruded into (a) continental crust of near normal thickness, (b) strongly attenuated
continental crust or (c) oceanic crusL Pearce et al.'s dividing line between (a) and (b)
is arbitrarily defined as the appearance of associated dike swarms; the dividing line
between (b) and (c) is taken as the shelf edge. The majority of the granites from the
data bank of Pearce et al., are quartz syenites, granites and alkali granites, although 24
Y + Nb (ppm) P«arce et al, (1084)
Figure 9: Discrimination plot of Rb vs. Y + Nb for plutonic rocks from different tectonic/intrusive settings (Pearce, et al., 1984). 25 most granites from sub-group (b) belong to calc-alkaline suites. The VAG field represents calc-alkaline and tholeiitic granites resulting from subduction of oceanic crust
The syn-COLG field is interpreted to be representative of both syn- and post-tectonic granites resulting from either continent-continent or arc-continent collision (Pearce et al., 1984). Assuming immobility of Y + Nb, the QLG protolith may have intruded into any of the three magmatic/tectonic settings, WPG, VAG or syn-COLG. Possible variation of Rb content due to the relative mobility of K during metamorphism and pegmatization inhibits any further elucidation of the original magmatic/tectonic setting.
Crustal contaminants will plot in either the VAG or syn-COLG fields (Pearce et al.,
1984) as do the Snowshoe Group samples.
Geochronometry
Rb-Sr dating was attempted. Samples from four localities within the Quesnel
Lake Gneiss were analysed for Rb/Sr and 87Sr/86Sr ratio. Analytical data from this study are contained in Appendix B. A line fit to the four points has a slope corresponding to an age of 950 ± 15 Ma and an 87Sr/86Sr lower intercept of 0.7030
±0.00005. Figure 10 shows the results of these analyses.
A line fit to these points should not be interpreted as representing the age of the gneiss because the wide range of Rb/Sr-87Sr/86Sr results from analyses of Quesnel
Lake Gneiss over a large region suggests initial Sr isotope heterogeneity. The sample points are part of a large field of Sr data from Quesnel Lake Gneiss with maximum bounding isochrons of 1100 Ma and 163 Ma (R. L. Armstrong, pers. comm., 1985).
Ambiguity of Rb-Sr dates from the Quesnel Lake Gneiss as a whole, is possibly the result of :
variable 87Sr/86Sr initial ratios
magma enriched in Sr from original source with varying degrees of mixing with
crustal rocks
mobilization or exchange of Sr during metamorphism 26
0.708
0.707H
0.706H 87 Sr 86 Sr 0.705H
0.704H
0.703H
0.702
87Rb/8eSr
Figure 10: Rb-Sr diagram with four whole rock analyses from the Quesnel Lake Gneiss. 27
U-Pb zircon and sphene geochronometry has been carried out on sample 377
QLG, a granodioritic gneiss, as well as a sample of orthogneiss from the Boss
Mountain area. The Boss Mountain Gneiss occurs in a structural culmination at the
1MB-OB boundary approximately 25 km south of this study (see Fig. 2). The results of this study are described in detail elsewhere (Mortensen, et al., in prep.) and are
summarized briefly below.
Preliminary zircon results from sample 377 QLG suggest a crystallization age of
approximately 340 Ma, however the zircons appear to have undergone a complicated
history of Pb inheritance, Pb loss and possible metamorphic overgrowth (Mortensen,
pers. comm., 1985). These factors preclude a precise interpretation of the emplacement
age of this body of Quesnel Lake Gneiss.
The Boss Mountain Gneiss is also intrusive into the Snowshoe Group
metasediments and appears to have experienced the same metamorphic and
deformational history as the Quesnel Lake Gneiss (Fillipone, 1985). A minimum age of
317 ±6 Ma has been inferred for the Boss Mountain Gneiss (Mortensen, pers. comm.,
1985). Results from analysis of these two gneiss bodies, as well as other analyses on
Quesnel Lake Gneiss further north, suggest an intrusive event of Mid-Paleozoic,
probably Devono-Mississippian age.
Analysis of the U-Pb systematics in coarse, euhedral, unstrained sphenes that
appear to be metamorphic in origin from sample 377 QLG results in a discordant
point between 174 and 183 Ma. The reason for the discordance is uncertain. This date
is interpreted to represent the approximate age of metamorphism. With a U-Pb
blocking temperature in sphene of 550-600° C (Parrish and Roddick, 1985), and a
metamorphic temperature estimate of approximately 590° C from garnet-biotite
geothermometry in the enclosing schists (this study) this interpretation seems valid and
is consistent with previous interpretations for the timing of major regional
metamorphism (Pigage, 1978).. The possibility exists, however, that the analysed sphene 28 consisted mainly of euhedral metamorphic overgrowths on primary igneous sphene cores, and the discordance therefore represents a more complex history of inheritance, Pb loss and partial reequilibration during metamorphism. Further work is in progress to evaluate this possibility.
Summary and discussion
The Snowshoe Group contains variably micaceous schists, quartz-biotite gneissose schists, calcareous metasandstones, marble and amphibolite. This probably is a metamorphosed and deformed sequence of shales, shaly sandstone, quartz wackes, limestone, chert and possible tuffs deposited on the the continental margin. The predominately clastic character suggests a fairly deep water environment A submarine
fan complex with turbidites, grain flows, and interbedded limestones has been proposed
for the time correlative Kaza Group northwest of this area (Murphy and Journeay,
1982). The quartzo-feldspathic nature of many of the coarser units, and the various
amphibolite layers and lenses occurring within the succession is also suggestive of a plutonic or volcanic provenance, respectively.
The rocks defined in this study area are probably in the lower part of the
Snowshoe Group (Struik, 1983a,b). Lithologies in the area do not match published
descriptions of the Snowshoe Group and may well belong to a lower part, as yet
undescribed in the literature.
Major and minor element chemistry and geochronometry has helped clarify the
relationship between the Quesnel Lake Gneiss and the Snowshoe Group
metasedimentary package. Field and geochemical relations (see also Structure chapter)
and geochronology show that the Quesnel Lake Gneiss is probably an originally
differentiated intrusive modified by subsequent deformation and metamorphism. The
Quesnel Lake Gneiss and Mt. Perseus Gneiss plot dominantly in the subalkaline field
with two notable exceptions, QLG samples 402 and 232 (see Fig. 8). This suggests a
dominantly calc-alkaline body with an alkaline component High Sr content, low initial 29
87Sr/86Sr ratios and an alkaline component imply a mantle source although possible Pb inheritance in zircons and regional Sr data suggest a certain amount of assimilated crust The Rb vs. Y + Nb discriminant plot of Pearce et al. (1984) fails to give unequivocal results with QLG samples plotting in both the within plate (WPG) and volcanic arc (VAG) fields. A tectonic setting in which the Quesnel Lake Gneiss represents a calc-alkaline intrusive with variable assimilation of crust is consistent with geochemical, geochronologic and stratigraphic information. Intrusion took place during the
Mid-Paleozoic, probably Devono-Mississippian time. The ML Perseus and Boss
Mountain intrusions also occurred at about the same time.
A regional metamorphic event affecting the entire sedimentary and intrusive package is interpreted to have occurred during the Middle Jurassic as suggested by sphene U/Pb geochronometry and regional stratigraphic relations. III. STRUCTURE
Introduction
Rocks in this region have experienced a polyphase deformational history in which up to six phases of deformation are recognized. This deformational sequence consists of five phases of superposed folding followed by a brittle fracture episode.
Axial surfaces (both penetrative and non-penetrative) and linear elements (mineral lineations, fold axes) of the various fold sets were measured and recorded with regard to orientation and superposition. Different phases of deformation were then distinguished on the basis of orientation and superposition of these structures. The earliest recognizable structures in both the Snowshoe Group metasediments and the Quesnel
Lake Gneiss are overprinted by tight to isoclinal second phase folds whose axial surfaces make a small angle (10-15°) with the compositional layering. SW-verging third-phase folds are more upright and serve to tighten the earlier structures while producing the dominant map pattern in the area. Late, more brittle phases of folding produce a broad warping of the layering with locally developed crenulations.
Deformational phases in the Snowshoe metasediments will be referred to
sequentially as F1-F5 with corresponding planar and linear elements referred to as SI
and Ll, respectively. Phases developed in the Quesnel Lake Gneiss will be designated
with a prime symbol : Fl', SI', etc.. The following sections will first describe the
sequence of deformational phases in the Snowshoe Group and then the Quesnel Lake
Gneiss, followed by a correlation of phases between the two rock types. A summary
of the sequence of structural events will complete this chapter.
Snowshoe Deformation F1-F5
The earliest phase of deformation (Fl) recognizable in the Snowshoe Group
metasediments is represented by the pervasive compositional layering that exists
throughout the area. This compositional layering is a metamorphic foliation parallel to
the axial surface of Fl folds and is designated S0/1. The parallelism of SO
30 31 compositional layering and SI axial surface of Fl rootless isoclines coupled with the lack of minor fold pairs in the very micaceous schists implies that SO/1 is transposed bedding. Evidence for an early version of SO/1 can be seen in thin section as planar to helicitic inclusion trails in garnets (Fig. 26). It is this pervasive foliation that is folded by all subsequent deformation.
Linear features (LI) related to this phase are a few curvilinear minor fold axes and are seen trending roughly SE to NW with moderate to shallow plunges (Fig. 11).
No preserved mineral lineations are recognized.
Fl folds are not ubiquitous in the Snowshoe Group metasediments and are usually seen as small (1.5 to 4.0 cm) rootless isoclines of quartz ± feldspar. These Fl isoclines lie with their limbs and axial planes parallel with the SO/1 compositonal layering and are most well preserved in the more quartz-rich schists and some marble units. Fl rootless isoclines within the SO/1 layering are seen folded about S2. One outcrop-size Fl closure is seen in a marble unit on the southern flank of Caput
Mountain and is illustrative of the tectonic thickening of these rocks (Fig. 12). The limbs of this isocline are overprinted by both S2 and S3 axial surfaces.
Second phase deformation (F2) appears to be the most intense producing shear folds with a pervasive axial planar schistosity. F2 folds are shallow to moderately plunging, moderately inclined, tight to isoclinal shear style folds (Class II). These folds have highly attenuated limbs with thickened hinges and commonly show a complete transposition of SO/1 in their cores. F2 folds are mostly seen as minor folds ranging from crenulation to outcrop-size. One major F2 synform occurs on the southern flank of Peak 7398 as defined by F2 cleavage/SO/1 relationships, sense of rotation of F2 minor folds, and the general variation in orientation of SO/1 surfaces. F2 folds fold the SO/1 surface which contains Fl isoclines.
S2 surfaces are a penetrative schistosity that is developed at low angles
(10-15°) to SO/1 and is defined by parallel orientation of micas. This surface is L1 Snowshoe
Figure 11: LI and LI' minor fold axes in the Snowshoe Group and the Quesnel Lake Gneiss. 33
Figure 12: Isoclinal Fl closure in a marble unit on Caput Mountain F2 and F3 overprints can also be seen on the limbs of this structure. 34 oriented approximately 115/40NE and is pervasive throughout the area (Fig. 13).
L2 linear features developed are a penetrative mineral elongation lineation parallel to F2 minor fold axes. Aligned minerals are elongate quartz "rods" and mica edges plus some staurolite grains. Near coaxiality with F3 structures can lead to some confusion of phases unless an overprinting relationship is observed. L2 linear structures are curvilinear and plunge shallowly to moderately to the SE/NW describing a great circle locus oriented approximately 120/50NE (Fig. 14).
Third phase structures are the prominent SW-verging, NW-plunging fold sets seen throughout the region. This study's area is located on the northeast limb of the
Mt Perseus antiform, a major third phase culmination. F3 folds described herein are all SW-verging, with consistently oriented axial surfaces (w.r.t SO/1) and are therefore minor folds to this major culmination. These folds are outcrop-size, gently plunging, moderately inclined, close to tight folds and show less thickening of the hinge and limb attenuation than F2 folds (Fig. 15). An F3 crenulaton is also developed, most pervasively in the more micaceous schists. This crenulation is also asymmetric toward the southwest
Development of a locally penetrative axial planar schistosity to the F3 minor
folds and crenulations is mostly confined to the hinge regions of these folds. This surface (S3) is defined by the interspersed alignment of micas and is commonly
developed at angles of approximately 35-40° to SO/1. S3 has a relatively consistent
orientation of 115/55NE (Fig. 16).
Linear elements produced during F3 deformation are L3 mineral elongations that
are parallel to F3 minor fold and crenulation axes. Muscovite and biotite edges and
local hornblende in amphibolite units are the minerals defining L3. L3 is slightly
curvilinear about later structures but have an average plunge of 15/340 (Fig. 17).
Fourth phase deformation in the Snowshoe rocks has produced broad open
warps of the layering. F4 folds are shallowly plunging, gently inclined open folds with 35 S2 Axial Surfaces Snowshoe
Quesnel Lake Gneiss
Figure 13: Contoured equal-area stereonets of S2 axial surfaces in the Snowshoe Group and the Quesnel Lake Gneiss. Contoured at intervals of 1. 3, 5. and >10%. 36 L2 Lineations
Snowshoe-Caput Snowshoe-Northeast
Quesnel Lake Gneiss
Figure 14: L2 mineral lineations and minor fold axes. Contoured at intervals of 1 3 5, and >10% . 37
Figure 15: Southwest-verging F3 fold in a Snowshoe Group quartz-mica schist on Caput Mountain. This fold plots as a Class lc fold (see Fig. 25). S3:SNOWSHOE GROUP
4
Figure 16: Contoured F3 axial surfaces in the Snowshoe Group. Contoured at intervals of 1. 3, 5 and >10%. 39
L3 Lineations Snowshoe- Caput Snowshoe-Northeast
75 lines 32 lines
11 lines
Figure 17: L3 mineral lineations and minor fold axes. Contoured at intervals of 1, 3, 5 and >10%. 40 an axial surface oriented approximately 110/20SW (Fig. 18). These folds are locally seen to be slightly asymmetric toward the northeast and range in size from 1 m wavelength/0.1 m amplitude to crenulation-size (Fig. 19). S4 surfaces are non-penetrative axial plane cleavage. L4 lineations are minor fold and crenulation axes and cleavage/SO/l intersections that plunge gently to the SE.
Locally developed fifth phase structures 'sometimes show ambiguous liming relationships with F4 structures and are taken to be closely related in time of development F5 folds are also broad warps and are described as steeply plunging, upright folds that developed at a high angle to F4 structures (Fig. 20). F5 axial surfaces trend approximately 065/90 (Fig. 21). These folds are not seen in the Caput
Mountain area and are best developed NE of the QLG in the Isosceles Mountain area. F5 deformation failed to produce any penetrative planar or linear features with only a fracture cleavage and cleavage/SO/l intersection lineation observed.
The final deformation recorded in the Snowshoe rocks is a brittle fracturing and faulting phase. Fractures that appear to be systematically related to the later episodes of folding (fractures perpendicular to fold axes, radial fractures and a conjugate set intersecting perpendicular to the fold axis) are well developed as well as near-vertical NE-trending minor faults. These NE-trending faults show a consistent last movement of a left-lateral sense of not more than 15 m interpreted by subhorizontal slickensides and offset of stratigraphy. In the Caput Mountain area nearly vertical pegmatites are seen trending approximately 040 and 080. These pegmatites are not appreciably folded and may be filling a conjugate fracture system related to the late stages of folding.
Quesnel Lake Gneiss Fl'-F5'
The earliest recognizable surface developed in the Quesnel Lake Gneiss is a prominent penetrative gneissic layering parallel to Fl* axial surfaces and designated 41
L4 Lineations Quesnel Lake Gneiss Snowshoe
13 lines 42 lines S4 Axial Surfaces Quesnel Lake Gneiss Snowshoe
planes 24 planes
Figure 18: S4 and S4' axial surfaces and L4 and L4' minor fold axes in Snowshoe Group rocks and Quesnel Lake Gneiss. Contoured at intervals of 1, 3. 5 and >10%. Figure 19: F4 open fold (in foreground) in Snowshoe Group rocks northeast of the Quesnel Lake Gneiss. 43
Figure 20: Upright F5 brittle warp in Snowshoe Group rocks near Isosceles Mountain. 44
Figure 21: S5 and S5' axial surfaces and L5 and L5' minor fold axes in Snowshoe Group metasediments and Quesnel Lake Gneiss. Contoured at intervals of 1, 3, 5 and >10%. 45
SO/1'. This gneissic layering in most units is a well defined banding of felsic and mafic phases and is parallel to the compositional layering in the Snowshoe metasediments.
Fl' folds in the Quesnel Lake Gneiss occur as extremely flattened and elongated isoclines of feldspar± quartz veins ranging from 2.0 cm to 10 cm in amplitude with a very high amplitude to wavelength ratio. The limbs of the folded veins have been thinned with a concurrent thickening of the hinge, sometimes resulting in 'knot' folds (Fig. 22). Fl' isoclines show a consistent NE sense of rotation, and plunge at shallow angles to the southeast
LI' lineations in the gneiss are the few Fl' minor fold axes that could be
measured which plunge gently to the east and southeast Too few LI' features were
found to determine a locus of distribution, but plot in the same region as LI in the
Snowshoe rocks (Fig. 11).
Second phase deformation (F2') in the gneiss is delineated by shallowly
plunging, moderately inclined, tight shear folds. These F2' folds have greatly attenuated
limbs and thickened hinges and are seen from the microscopic to outcrop scale (Fig.
23). A variably penetrative S2' schistosity is well developed in the cores of folds in
all units and is pervasive throughout the more micaceous units. S2' is developed at a
low angle (10-15°) to the SO/1' compositional layering and has an average orientation
of 125/45NE (Fig. 13). Transpositon of SO/1' into the S2' surface is seen locally in
the cores of F2' folds.
L2' lineations are penetrative mineral elongations parallel to F2' minor fold axes
and are composed of hornblende and/or biotite. These lineations are moderately
curvilinear and show a great circle distribution oriented 115/35NE (Fig. 14).
The remaining phases defined in the QLG are usually only recorded locally in
the more micaceous units of the gneiss. A locally developed third phase of
deformation (F3') is seen which tightens and coaxially refolds the F2' folds. These F3' 46
Figure 22: Isoclinal Fl folded vein with its axial surface parallel to the SO/1' compositional layering. 47
Figure 23: Tight F2' fold in the Quesnel Lake Gneiss with attenuated limbs and thickening in the hinges that plots as a nearly Class II fold (see Fig. 24). 48 minor folds lack a penetrative schistosity although an axial planar cleavage with some planar alignment of micas is seen in the cores of F3' folds. This S3' cleavage is
developed at an angle of approximately 40° to SO/1' and has an average orientation
of 115/40NE. Linear features (L3') are a mineral elongation lineation parallel to minor
fold axes and oriented approximately 10/340.
Fourth phase structures developed in the micaceous gneisses are shallowly
plunging, gendy inclined open folds. These F4' folds fail to produce a penetrative axial
surface and show only local development of an axial planar cleavage oriented
approximately 110/20SW. L4' features are simply minor fold and crenulation axes with
no mineral elongation produced (Fig. 18).
F5' structures are seen in only one unit of the gneiss. Structures produced
during this phase of deformation are steeply plunging, upright open folds which locally
produce a basin and dome type interference pattern with F4' structures (Fig. 21).
A late brittle fracturing and minor faulting episode is also observed in the
Quesnel Lake Gneiss. Characteristic fractures perpendicular to fold axes, conjugate
fractures and radial fractures that are systematically related to the later stages of
folding are well defined in this relatively massive body. A series of NE-trending
vertical minor faults are seen along the northeast Gneiss/Snowshoe contact as well as
two larger lineaments trending 072 and 043 that disect the Dutchman Mountain area.
All of these lineaments and minor faults show minor offsets of not more than 20 m
with a left-lateral last movement
Correlation of Deformation
Deformational phases observed in the Snowshoe metasedimentary package and
the Quesnel Lake Gneiss are correlated on the basis of orientation and superposition
of the structures produced. The earliest surface recognized in both rock types is a
compositional layering that is parallel to the axial surface of highly attenuated first 49 phase isoclines. This layering (SO/1 and SO/1') is sub-parallel across the
Snowshoe/QLG contacts on a fine scale (cm to mm), indicating that this contact is folded by SI and tightly transposed. Interlayers of Gneiss and Snowshoe rocks at these contacts contain this parallel SO/1 layering.
Both the Snowshoe and Quesnel Lake Gneiss have a penetrative second phase fabric (S2 and S2') developed at a low angle (10-15°) to this compositonal layering.
S2 and S2' are roughly parallel and are axial planar to tight shear-style folds.
Phases F3-F5 all have similar trends and styles in both rock types although they are differentially developed in the two packages. These later phases are well developed and preserved throughout the Snowshoe Group metasedimentary rocks but are only locally developed in the more micaceous units of the QLG. This difference is a reflection of the contrasting rheologies of the two rock types with the Snowshoe metasediments behaving more anisotropically, preserving the complete sequence of deformation. The Quesnel Lake Gneiss tended to act as a more rigid body that failed to completely record the later phases of folding. Based on the similar trends and superposition of the structures developed in both rock types, the deformational histories of both the Snowshoe Group metasediments and the Quesnel Lake Gneiss appear to coincide. A correlation of phases is summarized in the table below.
SNOWSHOE QLG STRUCTURES DEVELOPED
F6 F6' Systematic fractures and minor faults F5/4 F574' Open warps producing basin and dome structure locally developed in the QLG F3 F3' Nearly coaxial refolding of F2 folds; S3 developed approximately 35° to SO/1 F2 F2' Tight similar-style folds; S2 developed at low angle (10-15°) to SO/1 Fl Fl' Pervasive metamorphic foliation axial planar to isoclinal folds 50
Summary and Discussion
The structural geometry of the study area is summarized in the above table and can be seen both in map view and cross-section (Plates 1 and 2). The pervasive metamorphic foliation representing SO/1 is seen to be deformed, dominantly by four successive phases of folding. First phase folds are commonly seen only as small rootless isoclines within the SO/1 layering, but a larger isolated Fl closure is seen on
Caput Mountain (see Fig. 12). Tectonic thickening of these rocks is evidenced by this isoclinal closure which is in turn overprinted by second and third phase folds. Second phase tight shear folds refold SO/1 about an axial surface which makes a low angle to SO/1. This tight more ductile second phase of folding is seen mostly as minor, outcrop-size folds which from their consistent sense of rotation show evidence for a major synformal closure below Peak 7398 in the Caput Mountain area. Throughout the whole of the study area third phase folds, having axial planes oriented approximately
115/45NE, refold the earlier structures and show a consistent sense of rotation to the southwest In a regional context, this consistent southwest sense of rotation is due to the location of the study area on the northeast limb of a major third phase antiform,
the Mt Perseus antiform, whose hinge is located at the southwest margin of the
Caput Mountain area. These F3 minor folds are seen throughout the Snowshoe rocks
but are much less developed in the Quesnel Lake Gneiss. Fourth and fifth phase structures comprise open warps, are non-penetratively developed and are localized in
the more micaceous units of both rock types.
Fl folds are developed with axial surfaces parallel to the SO/1 compositional
layering in both the Snowshoe metasediments and the QLG. Rootless isoclines of
quartz ± feldspar in the Snowshoe rocks are interpreted to represent extension gashes
that developed at low(?) angles to original bedding parallel to the maximum
compressive stress. These rootless isoclines are therefore likely fractures that can be
progressively developed during the initial compaction of a sedimentary pile and 51
throughout the whole of its deformational history (Etheridge, et al., 1984). The isoclinal
"hooks" merely show evidence for movement along and into parallelism with the now transposed layering with progressive deformation. The larger isoclinal Fl closure of carbonate on Caput Mountain, however, shows evidence of a certain amount of shortening associated with Fl deformation.
A series of t' vs a plots (Ramsay, 1967) have been made in similar lithologies in the Snowshoe and one in the QLG to illustrate the variation in strain during deformational phases F2-F4 (Fig. 24). For a package of rocks of a similar composition and viscosity the change from a Class IB buckle fold to a Class II shear fold is accompanied by a decrease in viscosity contrast with a corresponding increase in ductile behavior and an increase in shear strain acting across the layers. Comparison of t' vs. a plots in similar lithologies for F2, F3 and F4 folds in the Snowshoe rocks shows the reverse sequence with a transition from Class II folds developed during F2 deformation to Class lb buckle folds developed during F4 deformation.
F2 folds in both the Snowshoe and Quesnel Lake Gneiss are shear folds that plot as nearly Class II folds (Fig. 24). As can be seen by their highly attenuated limbs and thickened hinges these folds were formed in a ductile regime associated with high shear strain and low viscosity contrast F3 folds have coaxially refolded F2 folds and are not well-developed in the QLG. These folds are Class IC folds tending toward the lb end-member and are associated with declining shear strain, less ductile behavior and increasing viscosity contrast than that associated with F2 folding (Fig. 15).
The lack of development of F3 folds in the gneiss is a reflection of these parameters and highlights the effects of viscosity contrast between the gneiss and the Snowshoe metasediments. Behavior of the gneiss as a more rigid body renders it less susceptible to the coaxial refolding and tightening effects of F3 on F2 structures.
F4 and F5 folds are Class IB buckle folds associated with brittle behavior and plane strain with a high degree of viscosity contrast (Figs. 19 and 20). This is 52
Figure 24: A t' vs. a plot (Ramsay, 1967) of the variation in orthogonal thickness in relation to the angle (a) from the axial surface. Second phase folds in both the Snowshoe Group rocks and the Quesnel Lake Gneiss and a third phase fold in the Snowshoe Group are plotted. 53 evidenced by the localized development of these structures in the most micaceous units of the Snowshoe and QLG which have the highest viscosity contrast relative to their enclosing units. This comparison of fold geometries developed during sequential phases of deformation is illustrative ' of a change from more ductile behavior and high shear strain during F2 through a more ductile/brittle behavior with declining shear strain during F3 to brittle behavior and plane strain during F4 and F5.
Determination of the a-slip direction associated with F3 deformation has been made by finding the intersection of the locus of deformed L2 mineral lineations with the S3 axial surface (Fig. 25). The slip direction is seen to make a low angle with
the F3 fold axis in the Caput Mountain area while the a3 slip direction determined in the Snowshoe rocks NE of the gneiss is at a high angle to the F3 fold axis.
These differently oriented a3 slip directions can be interpreted in light of their location with respect to major structural culminations. The Caput Mountain area is located near
the hinge of the ML Perseus antiform and the low angle a3 slip direction in the
Caput area is reflecting a high amount of extensional strain concentrated in the core
of this major culmination. To the northeast, on the limb of this culmination, the a3
slip direction is at a high angle to the F3 fold axis indicating movement perpendicular
to the F3 axial trend and a high amount of shear strain concentrated on the limbs
of this major culmination. The northwest- southeast nearly coaxial trends of the
dominant phases of deformation (F2 and F3) coupled with the a3 slip direction
information indicates a movement directon nearly perpendicular to the existing 1MB-OB
tectonic boundary to which these deformational phases are associated with. Across this
boundary there is a major viscosity contrast which allows extensional strain to be
developed closely parallel with the fold axes in the antiformal closures and very high
shear strain to be developed at a high angle to the fold axes on the limbs which is
associated with movement of material out of the flattened synformal regions (Ross, et
al., 1985). 54
aa CAPUT MTN. (hinge)
N
Figure 25: a3 slip direction determined by finding the intersection of the locus of L2 lineations and the S3 axial surface in the Caput Mountain area, near the hinge of the F3 ML Perseus anuform, and the Isosceles Mountain area, on the limb of this major cuhriination. Average S3 surfaces and L2 distributions are taken from Figures 16 and 14, respectively. IV. METAMORPHISM
Introduction
Rocks in this study area have undergone at least one regional metamorphic
event producing mineral assemblages characteristic of the Barrovian-type of
metamorphism in the pelitic schists of the Snowshoe Group. These mineral assemblages
are described, and their compositional variation and relationships to the fabric are
attributed to the various deformational phases outlined above (see Structure section).
Textural relationships are used to constrain the timing of metamorphism with respect to
deformation.
Mineral Relations
Biotite and muscovite define the compositional layering SO/1 that is seen
throughout the area. These micas occur as idiomorphic platy grains commonly
•containing zircons(?) with pleochroic haloes and show retrograde alteration to sericite
and/or chlorite. Biotite and muscovite have grown along a well-developed F2
crenulation cleavage and show only minor growth parallel to the more weakly
developed F3 axial surfaces. Figure 26 shows the crenulated SO/1 fabric transposed
parallel to the S2 crenulation cleavage defined by biotite and muscovite. F3
crenulations can be seen in Figure 27 with a weak development of mica alignment
parallel to the S3 surface. Evidence of F4 deformation is seen as kinks in biotite and
muscovite laths. Sericitic alteration of micas, garnet, staurolite and kyanite probably
occurred sometime after F3 deformation after the last period of mica growth.
Garnet occurs in most of the schistose rocks of the Snowshoe Group and is
seen as xenomorphic to idiomorphic porphyroblasts having a wide range of inclusion
densities. In the Caput Mountain area there is evidence for two stages of garnet
growth. Figure 28 shows a composite garnet with an early sub-idiomorphic garnet
outlined by aligned quartz inclusions (SO/1) with second stage growth surrounding
55 56
I
SO/1
Figure 26: Photomicrograph and line drawing illustrating SO/1 fabric transposed by S2 crenulation cleavage and early SO/1 fabric in garnet 7.2 mm diameter field of view. 57
Figure 27: Photomicrograph and line drawing of F3 crenulations of muscovite and biotite illustrating a lack of penetrative S3 mica growth. 1 cm diameter field of view. 58
Figure 28: Photomicrograph and line drawing of a composite garnet from the Caput Mountain area with early poikiloblastic garnet and second stage growth with inclusions of kyanite. 7.2 mm diameter field of view. 59 grains of kyanite. Planar to slightly crenulated inclusion trails of aligned quartz, magnetite/ilmenite and biotite can be seen in Figure 29 and are interpreted to represent growth during the early stages of F2 deformation.
Garnets in schists northeast of the Quesnel Lake Gneiss also show evidence for two stages of growth with the first stage overgrowing the early F2 crenulation of the
SO/1 fabric. Figure 29 shows a two stage garnet with a more inclusion free core and a rim containing inclusions of kyanite, staurolite and quartz. Electron microprobe analyses carried out on four samples northeast of the Quesnel Lake Gneiss indicate zoning with an increasing Mg/Fe ratio and Ca and Mn depletion from core to rim.
Staurolite porphyroblasts are seen only in samples northeast of the Quesnel
Lake Gneiss in high-grade Barrovian assemblages containing garnet-kyanite-staurolite-biotite-muscovite-quartz-plagioclase. Staurolite grains observed in the Caput Mountain area are seen only as inclusions in garnet suggesting their complete consumption during continued prograde metamorphic reactions. Figure 30 shows staurolite enclosing an F2 crenulation with the inclusion trails curved at the edge of the grain. This is interpreted to illustrate post-F2 staurolite growth continuing into F3 deformation.
Kyanite porphyroblasts are seen in only one locality in the Caput Mountain area although inclusions of kyanite in garnet occur in a few locations. Kyanite is more common just southwest and northeast of the Quesnel Lake Gneiss. Figure 31 shows well-developed poikiloblastic kyanite overgrowing the F2 crenulation cleavage in sample
0626-36 from the southwest flank of Caput Mountain. These grains are not completely optically continous around the crenulation hinges indicating some further deformation, probably F3 tightening and overprinting of the earlier F2 structures. These kyanites are part of the mineral assemblage garnet-kyanite-biotite-quartz with minor amounts of plagioclase and muscovite and no staurolite recognized. Figure 29: Photomicrograph and line drawing of garnet with inclusions of kyanite, sericite and quartz. .5 mm diameter field of view. 61
Figure 30: Photomicrograph and line drawing of staurolite overgrowing F2 crenulation of opaque inclusions. .5 mm diameter field of view. 62
Figure 31: Photomicrograph and line drawing of kyanite overgrowing F2 crenulation cleavage from Snowshoe Group rocks on Caput Mountain. 7.2 mm diameter field of view. 63
Kyanite porphyroblasts found in schists both northeast of the Quesnel Lake
Gneiss and at the southwest gneiss contact all show a retrograde sericitic rim that obscures contact and equilibrium relationships with other mineral phases. Figure 29 shows this textural relationship in the mineral assemblage: garnet- kyanite- biotite- muscovite- quartz ± plagioclase ± clinozoisite.
Sillimanite (fibrolite) is observed in units along the northeast contact with the
Quesnel Lake Gneiss. Figure 32 shows knots of fibrolite associated with muscovite, quartz, plagioclase and biotite. Fibrolite is also seen growing at the expense of garnet
No fibrolite is seen in garnet-kyanite schists further to the northeast at the edge of this field area, approximately 1 km from the Quesnel Lake Gneiss contact
Three distinct metamorphic mineral zones can be recognized in this area: the garnet, kyanite, and sillimanite (fibrolite) zones (see Fig. 33). The garnet zone is confined to the southwest edge of the Caput Mountain area with the kyanite zone encompassing most of the remaining map area. The sillimanite (fibrolite) zone is a narrow strip approximately 1 km wide along the northeast margin of the Quesnel Lake
Gneiss, with the reappearance of the kyanite zone approximately 1 km further to the northeast These zones have been qualitatively defined by observing mineral appearances
in units of similar bulk composition.
The presence of this narrow fibrolite zone may be either a reflection of the
regional isograd trends, thus implying a structural culmination along this zone, or it
may be a local thermal anomaly associated with higher temperature around the gneiss
body. There is no structural evidence of a major culmination around the gneiss,
although the original configuration of the post-F2 (metamorphic peak) isotherms is not
known and may be at high angles to the F2 enveloping surface. Therefore, a
structural culmination in the rock package may not necessarily correspond to a
metamorphic (or thermal) culmination. Figure 32-. Photomicrograph of sillimanite (fibrolite) growing at feldspar grain boundary associated with quaru, muscovite and biotite from the northeast Quesnel Lake Gneiss contact area. 1.5 mm diameter field of view. Figure 33: Metamorphic mineral zones and garnet-biotite geothermometry sample locations (northeast corner). 66
Garnet-Biotite Geothermometry
Temperatures of metamorphism have been calculated from the compositions of coexisting garnet and biotite, using the experimental calibration of Ferry and Spear
(1978), in four samples of sillimanite-bearing pelitic schists from northeast of the
Quesnel Lake Gneiss (see Fig. 33). The activities of garnet components in garnet solid
solution have been calculated using the solution model proposed by Newton and
Haselton (1981) and used by Lang and Rice, (1985), Engi, (1984), and Getsinger,
(1985). Newton and Haselton (1981) consider all interactions between Fe, Mg, Mn and
Ca ideal except for Ca-Mg.
Pressure estimates used in calculating the temperature were made using Al2Si05
phase relations (from Holdaway, 1971), application of the garnet+rutile=ilmenite +
kyanite+quartz (GRAIL) geobarometer of Bohlen, et al. (1983), and the presence of
mineral assemblages characteristic of Carmichael's (1969) bathozone 5 (see Fig. 34).
Data used in GRAIL calculation is contained in Appendix C. The presence of
sillimanite in all four samples allows use of the kyanite-sillimanite transition as a
pressure indicator with the assemblage garnet(alm77) + ilmenite +kyanite + quartz without
rutile further constraining the maximum pressure to below 6.5 kb. A lack of potassium
feldspar and no evidence for melting provides an additional boundary. The small
temperature dependence of the garnet-biotite geothermometer on pressure (approximately
4° per kb) provides further definition of the pressure regime. Combination of the
above parameters indicates a pressure of 5.5 ±0.5 kb existed at the peak of
metamorphic conditions.
Calculation of peak metamorphic temperature has been made using the Newton
and Haselton (1981) garnet solution model with the Ferry and Spear (1978)
geothermometer. Using a pressure estimate of 5.5 kb (discussed above) the samples
from the sillimanite zone indicate an average temperature of. 586±23°C. This average
was obtained by calculating temperatures for each garnet-biotite pair in the four 67
Figure 34: ASK phase diagram showing limiting pressure boundary lines, (after Holdaway, 1971) 68 samples and then averaging the combined temperatures. Figure 35 shows the geothermometry results plotted versus the mole fraction of Mn in garnet
(Mn/Fe + Mg+Ca+Mn). No apparent dependence of temperature on Mn content can be seen in this plot, supporting Newton and Haselton's ideal mixing model for Mn in garnets and in accord with previous worker's conclusions (Lang and Rice, 1985; Engi,
1984). Mg/Fe ratios used in geothermometric calculations are included in Appendix C.
Extensive retrograde effects are seen in schists at both the southwest margin and northeast of the Quesnel Lake Gneiss although relatively minor effects are seen in the Caput Mountain area. These effects are mostly a wide sericitic rim around garnet, staurolite, and kyanite sometimes with complete replacement, and local alteration of biotite to chlorite. Abundant retrogression is generally restricted to zones of deformation and/or obvious hydrothermal activity (Etheridge, et al., 1983). The abundant pegmatization at the gneiss margins and in the Isosceles Mountain area (see
Stratigraphy) is evidence for the presence of a large amount of fluid. This late-stage pegmatization is probably an expression of a post-deformational intrusive event documented in this region (Pigage, 1978; Getsinger, 1985). The contrasting rheologies of the gneiss and the metasediments, especially in the late, more brittle stages of deformation, would allow movement along the contact surface and possible infiltration of a fluid phase.
Summary
Mineral growth in relation to the deformational phases is summarized in Figure
36. A metamorphic peak of approximately 590° C at 5.5 kb. in sillimanite-bearing pelitic schists just northeast of the Quesnel Lake Gneiss existed shortly after F2 and into the early F3 deformation. Metamorphic mineral growth waned during F3 time with only minor mica growth parallel to the S3 axial surfaces. Post-F3 deformation was brittle in nature with no associated metamorphic mineral growth. Extensive 69
GARNET/BIOTITE GEOTHERMOMETRY
700n
650 i O •
600H CD B _. 3 2 550 CD a E ,® 500 * 0716-76 • 29b • 100 450 J P= 5.5 kb • 40
400 i i .00 .005 .010 .015 .020 .025 .030 .035 gar Xmn
Figure 35: Garnet-biotite geothermometry results plotted as T°C vs. the mole fraction of Mn in garnet Samples have an average temperature of 586±23°C at approximately 5.5 kb . 70
F1 F2 F3 F4&5 Biotite
Muscovite Garnet
Staurolite Kyanite Sillimanite
Figure 36: Summary of metamorphic mineral growth in relation to deformational phases. 71 retrograde effects in the northeast portion of the map area suggests an influx of a fluid phase which is also evidenced by a large volume of post-F3 (post-metamorphic) pegmatization and regional intrusive activity (Pigage, 1978; Getsinger, 1985). V. SUMMARY
Rocks in this study area are of two main types: the metasediments of the
Snowshoe Group and the granodioritic Quesnel Lake Gneiss. Metasediments of the
Snowshoe Group comprise a package of variably micaceous schists, calcareous metasandstone, relatively thin micaceous marbles sometimes with associated amphibolites, minor amphibolites not associated with marble layers, and very fine grained quartzose graphitic schist Sedimentary protoliths are suggested to be a sequence of shales and shaly sandstones, deep water limestones, chert and possible tuffs. This sequence is
interpreted to represent deposition in relatively deep water possibly onx the continental rise. This interpretation is in accord with previous suggestions that the Snowshoe Group is a western facies equivalent of the Kaza Group, although this suggestion has since been questioned due to structural dislocations between the Snowshoe Group and the
Kaza Group (Campbell, 1970; Struik, 1982).
The Quesnel Lake Gneiss is believed to be intrusive into the metasedimentary package and is dominantly of granodioritic composition with a minor amount of amphibolitic gneiss and syenitic gneiss. Field and geochemical relations and geochronologic information show that the Quesnel Lake Gneiss represents an originally differentiated, dominantly calc-alkaline intrusive modified by subsequent deformation and amphibolite-grade metamorphism. Minor infolds/interlayers/xenoliths of Snowshoe rocks
do occur well within the gneiss body. Major element trends suggest an approximate compositional equivalence to the Snowshoe Group rocks and plot on an ACF diagram
dominantly in the granite/greywacke and basalt/andesite fields. The Quesnel Lake
Gneiss is relatively enriched in many minor elements (Ce, Nd, Sr). High Sr content,
low initial 87Sr/86Sr and an alkalic component imply a mantle source. Use of a Rb vs.
Y+Nb discriminant plot (Pearce, et al., 1984) fails to give unequivocal results with
samples concentrating in both the within-plate granite (WPG) and the volcanic arc
granite (VAG) fields (Fig. 9). A magmatic setting in which the Quesnel Lake Gneiss
72 73 represents a calc-alkaline intrusive is consistent with geochemical, geochronologic and stratigraphic information. Intrusion of the body into the sediments took place between
315 and 400 Ma ago; at the same time the ML Perseus and Boss Mountain intrusions were emplaced.
A regional metamorphic event affecting the entire sedimentary and intrusive package occurred during Early to Mid-Jurassic time as suggested by sphene U-Pb geochronometry which is in accord with previous interpretations (Pigage, 1978).
The structural sequence observed in this area is composed of five phases of folding followed by a brittle fracturing and faulting phase. The entire sequence of deformation is seen in both the Snowshoe Group metasediments and the Quesnel Lake
Gneiss. The pervasive metamorphic foliation which defines the compositional layering
(SO/1) is the earliest recognizable surface and is the axial plane to isoclinal first phase folds in both rock packages. First phase folds in the Snowshoe Group metasediments are usually seen only as rootless isoclines of quartz-feldspar although a large isoclinal
Fl closure is seen in a marble unit on Caput Mountain and shows evidence for a certain amount of shortening associated with Fl deformation (Fig. 12).
Syn-metamorphic F2 deformaton is evidenced as tight similar-style (Class IC) folds with the S2 axial surface penetratively developed at a low angle (10-15°) to the compositional layering (SO/1). F2 folds are well-developed in both the Snowshoe
Group and the Quesnel Lake Gneiss and have highly attenuated limbs and thickened hinges. These folds have been classified as nearly Class II shear folds (Fig. 24).
Syn- to post-metamorphic F3 deformation produced southwest verging folds with only locally penetrative axial surfaces developed at approximately 40° to the SO/1 compositional layering and northwest plunging fold axes that are nearly coaxial with F2 folds. Effects of F3 deformation was to tighten and refold F2 folds. The Quesnel
Lake Gneiss shows a lack of F3 macroscopic folds indicating its more rigid behavior.
F3 folds are classified as Class IC folds and show less limb attenuation and hinge 74 thickening than second phase folds (Figs. 15 and 23).
F4 folds are shallowly southeast plunging, gendy southwest dipping, broad, open warps that are only locally developed in the most micaceous units. Interference with the steeply northeast plunging, upright, open F5 folds which develop at nearly 90° to
F4 folds, locally results in formation of broad basin and dome structures.
F2 folds in both the Snowshoe Group and the Quesnel Lake Gneiss were formed in a ductile regime associated with high shear strain and low viscosity contrast
F3 folds, preferentially developed in the Snowshoe Group metasediments, have nearly coaxially refolded F2 folds and are Class IC folds associated with a decreasing amount of shear strain, less ductile behavior and increasing viscosity contrast The lack of development of F3 folds in the Quesnel Lake Gneiss is a reflection of these parameters and highlights the effects of viscosity contrast between the gneiss and the metasediments. Behavior of the gneiss as a more rigid body renders it less susceptible to the coaxial refolding and tightening effects of F3 deformation on F2 structures.
Fourth and fifth phase structures are Class IB buckle folds associated with brittle behavior and a high degree of viscosity contrast as evidenced by their localized
development in the most micaceous units of the rock package.
This comparison of fold geometries developed during sequential phases of
deformation illustrates the change from more ductile behavior and high shear strain
during F2 through a less ductile behavior with declining shear strain during F3 to
brittle behavior and plane strain during F4 and F5. This observation is in accord with
the waning of the metamorphic thermal event
Determination of the a3 slip direction (Ramsay, 1967) using the intersection of
the distribution of deformed L2 lineations with the S3 axial surface has been made in
both the Caput Mountain area and northeast of the Quesnel Lake Gneiss (Fig. 25).
The Caput Mountain area is located near the hinge of the Mt Perseus F3 antiform
and has a slip direction at a low angle to the F3 fold axis reflecting a high amount 75 of extensional strain concentrated in the core of this antiform. Northeast of the
Quesnel Lake Gneiss which is on the limb of the ML Perseus antiform, the slip direction is approximately 90° to the F3 fold axis indicating movement perpendicular to the fold axis and a high amount of shear strain concentrated on the limbs. The northwest-southeast trends of the dominant phases of deformation (F2 and F3) coupled
with the a3 slip direction information indicate a movement direction at a very high angle to the present 1MB-OB tectonic contact with which these deformational phases are associated.
At least one regional metamorphic event has affected this area in association with the deformational sequence outlined above. A summary of mineral growth related to deformation is presented in Figure 36 and shows the metamorphic peak occurs post-F2 and pre- to syn-F3 deformation. Metamorphic temperatures of approximately
590° C at 5.5 kb was determined in sillimanite-bearing schists northeast of the
Quesnel Lake Gneiss. This pressure estimate indicates a depth of approximately 16-18 km during deformation. Extensive retrograde effects in the northeast portion of the map area suggests influx of a fluid phase which is also evidenced by a large volume of post-F3 (post-metamorphic) pegmatization and regional intrusive activity (Pigage,
1978; Getsinger, 1985). Most of the map is area within the kyanite zone (Fig. 33).
The southwest corner of the map area is in the garnet zone and a narrow sillimanite zone borders the northeast edge of the Quesnel Lake Gneiss.
The same metamorphic and deformational history outlined for this map area can be observed in the Boss Mountain area (J. Fillipone, pers. comm., 1983; see Fig.
2) and northeast of Quesnel Lake in the Three Ladies Mountain area (Getsinger,
1985). Recent work along the 1MB-OB boundary and in the adjacent Eureka Peak synform has defined a metamorphic and deformational history for the rocks of
Quesnellia that is also similar, but lacking evidence for Fl deformation seen in the
Snowshoe Group and the Quesnel Lake Gneiss (Bloodgood, 1985; Fillipone, 1985; Ross, 76 et al., 1985). The earliest common phase of deformation (F2 in OB rocks) is thought to be related to the accretion and obduction of Quesnellia onto the Snowshoe Group.
Fl deformation is not seen in Quesnellia and therefore occurred prior to the
Mid-Jurassic accretion. The pervasive SO/1 compositional layering that is axial planar to
Fl folds is a metamorphic foliation that was likely produced during Fl deformation.
A tectonic history for the rocks in this map area began with the deposition of the Snowshoe Group in a continent margin basin from the Late Proterozoic to the
Early Mississippian (Struik, 1982). Intrusion into this package by the Quesnel Lake granitic body occurred between 317 and 400 Ma ago. The Boss Mountain and ML
Perseus intrusions also took place in this time period although their more S-type character (Fillipone, pers. comm., 1985) suggests either a different source or more assimilation of crustal material. The Paleozoic history of the entire package of rocks remains somewhat enigmatic.
The presence of Fl folds and SO/1 compositional layering within these gneissic bodies indicates their intrusion prior to, but possibly coeval with, Fl deformation. This places a maximum age for the Fl event at approximately 317 Ma (minimum intrusive age for the Boss Mountain Gneiss). The SO/1 metamorphic layering is composed of muscovite and biotite indicating metamorphic conditions up to greenschist facies. Garnet may also have grown during this first regional event as evidenced by the early sub-planar fabric seen as inclusions. Fl rootless isoclines of quartz and feldspar are interpreted to represent extension fractures that can be progressively developed during the compaction of a sedimentary pile as fluid pressures exceed the tensile strength of the rock material (Etheridge, et al., 1984). The larger Fl closure on Caput Mountain
(Fig. 12) as well as macroscopic folds in the Quesnel Lake Gneiss are more substantial evidence for tectonic shortening during the Fl evenL
Accretion of the allochthonous Quesnellia and Slide Mountain terranes initiated subsequent deformation and regional metamorphism. Timing of this accretion has been 77 shown to be Early-Middle Jurassic based on stratigraphic evidence to the south
(Monger, 1984). A limit on the time of deformation is given by a 163 Ma Rb-Sr whole rock date on a post-tectonic pluton in the Wells Gray Provincial Park area approximately 35 km northeast of this study area (Pigage, 1978). This timing places a minimum age of Early Jurassic for Fl deformation. A sphene date from a Quesnel
Lake Gneiss sample (this study) suggests a 180 Ma age for the metamorphic peak which has been determined to postdate F2 deformation from petrofabric relations.
F2 folds are shown elsewhere to be east-verging (Getsinger, 1985; Fillipone,
1985). A lack of major culminations and stratigraphic facing indicators hinders vergence determinaton in this study area. F3 folds are southwest verging throughout the region and create the existing large-scale map pattern. This vergence reversal may be explained as the result of a flip from obduction of the allochthonous terranes to easterly subduction (Ross, et al., 1985). Other interpretations call for a tectonic
"wedging" of the allochthonous terranes between basement and sedimentary cover of the continent (Price, 1984). A vergence reversal may also be shown to be a dynamically necessary consequence of accretion by modelling crustal strength vs. depth profiles in an orogenic belt (D. Murphy, pers. comm., 1985). F3 deformation was also a time of waning temperature leading to less ductile behavior and increasing viscosity contrasts, and less strain.
An influx of post- F3 pegmatite and regional intrusive activity in the Late
Jurassic also corresponds to the timing of F4 and the closely related F5 deformation.
High angle block and strike slip faults may be occurring in response to regional dextral translation thought to have affected the area in the Late Cretaceous (Gabrielse,
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Winkler, H.G.F. 1979. Petrogenesis of metamorphic rocks. Springer-Verlag, 348 p. APPENDIX A
Sample Description and Location
Sample Description Latitude Longitude
E000 qtz-dioritic gneiss 52 21' 00" 120 34' 45" E200 qtz-dioritic gneiss 52 19' 00" 120 28' 00'
E362 qtz-dioritic gneiss 52 20' 00" 120 32' 00'
E371 qtz-dioritic gneiss 52 20' 15" 120 32' 30"
-17 Sn qtz-fspr-mica schist 52 24' 55" 120 36' 25" 32a Sn qtz-mica schist 52 26' 50" 120 30' 15" 27 QLG hbld-bi-epi gneiss 52 26' 50" 120 30' 35" 232 QLG qtz-fspr-bi gneiss 52 28' 10" 120 36' 20" 249 QLG hbld-bi-fspr gneiss 52 27" 55" 120 36' 10" 262 QLG qtzrfspr-bi gneiss 52 28' 10" 120 34' 45" 293 QLG hbld-epi-fspr gneiss 52 28' 15" 120 34' 30"
306 QLG qtz-bi-fspr schist 52 28' 00" 120 34' 30" 377 QLG hbld-fspr gneiss 52 27' 45" 120 35' 00' 402 QLG syenitic gneiss 52 27' 40" 120 35' 15"
82 SAMPLE sio2 T102 A1203 Fe203 MnO MgO CaO 2 Na 0 K20 P LOI 2°5
EOOO 73.06 0.22 14.01 2.51 0.04 1.03 0.54 2.41 5.07 0.20 0.91 E200 74.14 0.21 12.74 2.71 0.05 0.67 1.77 3.14 3.96 0.05 0.56 E362 74.73 0.18 12.49 2.93 0.04 0.60 1.43 3.25 3.66 0.06 0.63 E371 60.46 0.63 14.24 7.24 0.10 5.46 6.41 1.92 2.63 0.14 0.77
-17 Sn 72.24 0.27 12.80 2.74 0.05 3.05 0.71 4.68 2.27 0.07 1.12 32a Sn 68.70 0.57 13.90 4.80 0.05 3.24 1.75 3.30 2.11 0.10 1.76
27 QLG 72.37 0.14 13.07 2.18 0.05 0.89 1.78 4.73 4.15 0.06 0.58 232 QLG 50.32 1.05 16.64 9.58 0.15 6.81 7.86 2.49 3.30 0.62 1.18 249 QLG 69.67 0.28 12.95 3.97 0.07 2.64 3.41 3.70 2.51 0.09 0.71 262 QLG 71,54 0.40 11.80 6.08 0.07 2.71 1.94 1.83 3.30 0.04 0.29 293 QLG 72.35 0.25 13.52 2.34 0.04 0.92 2.14 3.17 4.51 0.10 0.66 306 QLG 63.06 0.73 16.90 6.02 0.06 3.66 0.33 0.85 5.70 0.07 2.62
377 QLG 58.68 0.57 16.54 6.31 0.15 3.10 5.45 3.39 4.23 0.46 1.12 402 QLG 58.04 0.25 18.90 3.53 0.13 0.33 5.38 2.20 9.82 0.15 1.27
NOTE:All Fe as
Analyses normalized to 100%
Analyses were carried out on an automate XRF in the Department of Oceanography, U.B.C SAMPLE Ba Ce Cr Nb Nd Rb Sr V Y Zn Zr ST. ERR. 60 13 5 1 10 2 6 10 3 2 7
EOOO 335 (42) (32) 12 (23) 201 100 - (12) 42 80 E200 761 (31) (19) 24 - 136 233 (12) (17) 27 84
E362 893 (26) (26) 24 - 124 260 (14) (18) 22 101 E371 826 (53) (33) 14 (24) 82 561 174 (22) 54 90
-17 Sn 2689 - - 18 95 66 321 54 (11) 47 95 32a Sn 301 (51) 98 14 (24) 92 356 59 27 47 191
27 QLG 855 (45) (17) 38 - 160 597 (28) - 29 306
232 QLG 772 104 48 24 52 138 1316 250 35 81 89 249 QLG 942 (18) 42 16 - 86 852 80 (13) 40 98
262 QLG 3030 133 140 11 (39) 86 619 80 (10) 47 184
293 QLG 2598 68 - 15 - 118 1131 (42) - 24 174
306 QLG 1393 (45) 110 21 (26) 185 188 99 (23) 74 143 37 7 QLG 1473 353 (17) 41 117 143 2378 159 32 117 330 402 QLG 2658 620 - 24 140 149 5907 124 59 25 335
NOTE: Analyses in parentheses have a standard error of >20% of their measured value. Bars indicate analyses with a standard error of >50% of their measured value. Results are in ppm. Analyses were carried on an automated XRF in the Department of Oceanography, U.B.C.
oo APPENDIX B
Rb/Sr Analytical Techniques
Rb and Sr concentrations were determined by replicate analysis of pressed powder pellets using X-ray flourescence. U.S. Geological Survey rock standards were used for calibration; mass absorption coefficients were obtained from Mo Ka Compton scattering measurements. Rb/Sr ratios have a precision of 2% (1 a) and a precision of
5% (1 a). Sr isotopic composition was measured on unspiked samples prepared using standard ion exchange techniques. The mass spectrometer, a V.G. ISOMASS 54R, has data acquisition digitized and automated using a HEWLETT PACKARD HP 85 computer. Experimental data have been normalized to a !6Sr/8SSr ratio of 0.1194 and
87 86 adjusted so that the NBS standard SrC03 (SRM987) gives a Sr/ Sr ratio of
0.71020+2 and the Eimer and Amend Sr a ratio of 0.70800±2. The precision of a single 87Sr/86Sr ratio is 0.00010 (1 a).
85 Rb/Sr Analytical Data
QJ Q~J QC
SAMPLE DESCRIPTION Sr ppm Rb ppm Rb/ Sr Sr/ Sr (+0.00015)
249 hbld-bi-fspr gneiss 877 86.3 0.285 0.7065
293 hbld-epi-fspr gneiss 1172 120 0.295 0.7074
377 hbld-fspr gneiss 2515 148 0.171 0.7051
402 syenitic gneiss 6609 169 0.074 0.7041
oo APPENDIX C
ELECTRON MICROPROBE ANALYSES
Samples were collected from a zone of sillimanite-bearing schists in the
Isosceles Mountain area for microprobe analyses of garnet and biotite (Figure 33). The results from these analyses were used in geothermometry calculations in order to characterise the peak metamorphic temperatures that existed in the area.
Analyses were collected on a three channel, automated ARL SEMQ
Experimental Mineralogy microprobe at the University of British Columbia. Instrument conditions for these analyses were: an acceleration potential of 15 kV, a specimen current (on aluminum) of 40 nanoamps, and an electron beam diameter of approximately 10-12 microns. Counting time of 20 seconds on peak and 10 seconds on background was used for all elements, with total counts normalized to a reference beam current determined on aluminum.
Natural and synthetic minerals from the University of British Columbia collection were used as standards (Table 1). All analyses were corrected for background and dead time with count readings corrected for matrix effects using a Bence-Albee correction procedure (Bence and Albee, 1968) with alpha factors taken from Albee and
Ray (1970). Grains of garnet were analysed at both core and rim with adjacent rim biotite grains selected to represent equilibrium compositions used in geothermometric calculations.
87 88
TABLE 1 : Standards for microprobe analysis of garnet(*) and biotite
ELEMENT STANDARD LOCALITY UBC ID
F flourophlogopite synthetic 24
Na albite Langlois, OR 20
Mg forsterite 7 22
Al andalusite ? 26
Si, Ca* wollastonite Willsboro, NY 21
K orthoclase 7 96
Ba benitoite 7 35
Ti* rutile synthetic 13
Mn* pyroxmangite Taguchi Mine, Japan 245
Fe* fayalite synthetic 250
Mg*, Al*, Si* pyrope Kakanui, NZ 235 BIOTITE ANALYSES
5AMPLE 29-•1B1 29--1B3 29-•1B4 29-1B5 29-1B6 29-2B1 29-2B4 Weight percent F 0. 54 0.,6 9 0. 53 0.61 0.37 0.55 0.49 NaO 0. 35 0. 24 0. 24 0.30 0.23 0.28 0.30 MgO 10. 81 10.,7 0 11. 08 10.64 11.57 10.70 11.15 A1203 18. 39 18.,8 8 18. 86 18.94 18.68 18.97 17.59 Si02 37. 13 36.,3 6 36. 85 37.03 35.52 36.95 36.67 K20 9. 14 8.,8 0 9. 24 9.25 7.98 9.25 9.14 CaO 0. 04 0.,0 0 0. 00 0.03 0.02 0.00 0.00 BaO 0. 21 0.,0 8 0. 15 0.06 0.07 0.15 0.10 Ti02 1.9 5 1..3 6 1.3 6 1.53 1.34 1.57 1.74 MnO 0. 03 0.,0 1 0. 02 0.01 0.01 0.06 0.06 FeO 18. 83 19.,7 5 19. 57 18.89 20.47 19.25 19.54 TOTAL 97. 40 96.,8 7 97. 91 97.29 96.31 97.71 96.77 Formula F 0. 252 0.,32 5 0. 249 0.287 0.176 0.258 0.233 NaO 0. 099 0.,07 0 0. 070 0.088 0.067 0.082 0.087 MgO 2. 396 2..39 5 2. 390 2.361 2.602 2.369 2.500 A1203 3. 225 3.,34 2 3. 300 3.324 3.321 3.322 . 3.119 Si02 5. 524 5.,46 0 5. 471 5.514 5.358 5.489 5.518 K20 1. 734 1..68 5 1.75 0 1.758 1.536 1.754 1.754 CaO 0. 000 0.,00 0 0. 000 0.000 0.000 0.000 0.000 BaO 0. 012 0.,00 0 0. 152 0.000 0.000 0.000 0.000 Ti02 0. 218 0..15 4 0. 000 0.171 0.152 0.175 0.196 MnO 0. 000 0.,00 0 0. 000 0.000 0.000 0.000 0.000 FeO 2. 342 2..48 0 2.43 0 2.352 2.583 2.391 . 2.458 TOTAL 15. 814 15..91 8 15. 886 15.864 15.808 15.851 15.880 BIOTITE ANALYSES
SAMPLE 40-2B1 40-.2B2 40-3B1 40-3B2 76-1B1 76-1B2 76-3B1 76-3B2 100-2B2 100-2B2-2 Weight percent
F 0.21 0.30 0.20 0.39 0.38 0.46 0.43 0. 51 0.53 0.64 NaO 0.24 0.21 0.22 0.21 0.21 0.16 0.10 0. 17 0.23 0.29 MgO 11.04 10.74 10.42 10.68 10.33 10.45 11.13 10. 68 11.22 10.64 A1203 18.63 18.44 18.57 18.50 18.47 18.92 19.74 19. 16 18.52 18.86 Si02 36.64 36.50 35.15 36.13 36.52 36.91 35.08 36. 02 36.67 38.31 K20 9. 18 9.24 9.10 9.24 9.03 9.22 5.54 7. 06 ,9.15 9.14 CaO 0.01 0.01 0.00 0.03 0.00 0.00 0.02 0. 06 0.00 0.02 BaO 0.09 0.12 0.05 0.09 0.28 0.29 0.22 0. 28 0.13 0.13 Ti02 1.39 1.59 1.75 1.34 1.77 1.60 1.40 1. 57 1.67 1.48 MnO 0.01 0.03 0.00 0.03 0.03 0.06 0.05 0. 03 0.01 0.00 FeO 20.16 20.28 20.73 19.86 20.07 19.55 20.83 20. 30 18.98 17.50
TOTAL 97.62 97.46 96.20 96.49 97.09 97.63 94.54 95. 83 97.11 97.01
Formula
F 0.097 0. 142 0. 944 0.187 0. 179 0.215 0.205 0. 243 0.253 0.298 NaO 0.070 0.059 0. 066 0.061 0. 060 0.046 0.029 0. 049 0.660 0.082 MgO 2.449 2.393 2. 362 2.404 2. 309 2.318 2.518 2. 399 2.497 2.344 A1203 3.269 3.250 3. 327 3.293 3. 266 3.319 3.532 3. 402 3.260 3.285 Si02 5.455 5.457 5. 345 5.456 5. 479 5.494 5.323 5.42 6 5.477 5.661 K20 1.743 1.763 1. 765 1.779 1. 729 1.751 1.072 1. 356 1.744 1.722 CaO 0.000 0. 000 0. 000 0.000 0. 000 0.000 0.000 0. 010 0.000 0.000 BaO 0.000 0.000 0. 000 0.000 0. 017 0.017 0.013 0. 016 0.000 0.000 Ti02 0.156 0.178 0. 200 0.152 0. 199 0.179 0.159 0. 178 0.187 • 0.164 MnO 0.000 0.000 0. 000 0.000 0. 000 0.000 0.000 0. 000 0.000 0.000 FeO 2.510 2.536 2. 636 2.508 2. 518 2.434 2.644 2. 558 2.371 2.163
TOTAL 15.759 15.792 15. 801 15.854 15. 761 15.781 15.507 15. 641 15.864 15.732
oV© GARNET ANALYSES
SAMPLE 29-2G1-KR) 29-2Gl-2(C) 29-2G2-KR) 29-2G2-2(C) Weight percent
MgO 3.42 1.84 3.53 2.42 A1203 21.02 20.86 21.51 21.71 Si02 36.67 36.27 36.77 36.85 CaO 1.93 4.20 1.78 3.22 Ti02 0.00 0.02 0.03 0.02 MnO 1.47 0.83 1.49 2.02 FeO 34.72 35.65 35.37 34.73
Total 99.23 99.67 100.49 100.97
Formula MgO 0.413 0.223 0.422 0.288 A1203 2.009 2.004 2.033 2.048 Si02 2.975 2.957 2.94 9 2.949
CaO 0.168 0.367 0.153 0.267 Ti02 0.000 0.000 0.000 0.000 MnO 0.101 0.058 0.101 0.137
FeO 2.355 2.431 2.373 2.325
Total 8.021 8.040 8.033 8.025 GARNET ANALYSES
SAMPLE 29-lG2-l(R) 29-lG2-2(C) 29-1G3-KR) 29-lG3-2(C) Weight percent
MgO 3. 52 1.65 3.33 1.76 A1203 22. 02 21.52 22.01 20.90 Si02 36. 82 36.60 36.75 36.62
CaO 1. 67 4.47 2.00 4.05 Ti02 0. 00 0.03 0.02 0.03 MnO 1.4 3 1.42 1.23 ' 2.83 FeO 35. 07 34.83 35.72 34.10
Total 100. 53 100.53 101.06 100.29
Formula
MgO 0. 419 0. 198 0.396 0.213 A1203 2. 075 2.044 2.069 1.996 S102 2. 943 2. 950 2.931 2.967 CaO 0. 143 0.386 0.171 0.351 Ti02 0. 000 0.000 0.000 0.000 MnO 0. 097 0.097 0.083 0.194 FeO 2. 344 2.348 2.383 2.310
Total 8. 020 8.026 8.034 8.033 GARNET ANALYSES SAMPLE 76-2G1-KR) 76-2Gl-2(C) 100-2G2-1(R) 100-2G2-2(C) 29-1G1-KR) 29-lGl-2(C) Weight percent MgO 3. 49 1. 73 3. 22 1.78 3.23 • 2.01
A1203 21. 39 20. 34 22. 14 21.45 22.09 21.69 Si02 36. 75 36. 62 36. 15 36.69 36.69 36.61
CaO 1.8 5 4. 09 3. 33 3.76 2.49 4.27 Ti02 0. 00 0. 08 0. 06 0.02 0.01 0.00 MnO 0. 96 3. 92 0. 72 0.57 0.90 0.75 FeO 35. 06 32. 59 33. 39 35.02 33.63 34.35
TOTAL 99. 50 99. 37 98. 99 99.29 99.04 99.69
Formula MgO 0. 419 0. 211 0. 388 0.215 0.389 0.242 A1203 2. 035 1. 959 2. 109 2.053 2.099 2.066 Si02 2. 967 2. 992 2. 923 2.979 2.958 2.958 CaO 0. 160 0. 358 0. 288 0.327 0.215 0.369 Ti02 0. 000 0. 000 0. 000 0.000 0.000 0.000 MnO 0. 066 0. 271 0. 049 0.039 0.061 0.051 FeO 2. 367 2. 227 2. 258 2.378 2.268 2.321
TOTAL 8. 015 8. 023 8. 019 7.993 7.991 8.009 GARNET ANALYSES SAMPLE 40-2Gl-l(R) 40-2Gl-2(C) 40-3Gl-l(R) 40-3Gl-2(C) 76-1G1-KR) 76-lGl-l(C) Weight percent MgO 2. 36 0. 92 2..3 7 1.91 3.28 1.16
Al-203 21. 77 21. 73 21..7 5 21.57 21.98 20.80 S102 36. 13 36. 86 36..5 1 37.09 36.09 36.73 CaO 5. 04 8.4 2 4,.7 7 5.78 2.36 4.28 Ti02 0.0 2 0. 16 0..0 8 0.02 0.09 0.01 MnO 0.0 5 2. 09 0..0 2 0.12 0.81 5.37 FeO 33. 58 30. 23 34..7 0 33.94 34.08 31.48
TOTAL 98. 95 100. 41 100..2 0 100.44 98.70 99.83
Formula MgO 0.28 5 0. 109 0..28 4 0.228 0.397 0.141 A1203 2. 083 2. 051 2,.06 1 2.035 2.103 1.990 S102 2. 993 2. 953 2..93 5 2.970 2.931 2.990 CaO 0.43 8 0. 723 0.,41 1 0.496 0.206 0.373 T102 0.00 0 0. 000 0..00 0 0.000 0.000 0.000 MnO 0.00 0 0. 142 • 0,.00 0 0.000 0.056 0.370 FeO 2. 279 2. 025 2..33 3 2.272 2.314 2.143
TOTAL 8.02 5 8. 012 8..03 0 8.011 8.012 8.013 X X X 1 SAMPLE MgGt FeGt CaGt ^Gt SUM 1g/Fe Gt Mg/Fe Bi K T°C (at 5.5 kb)
40- 2G11B1 0.2853 2 .2798 0.4386 0.00 3 .0037 0.1251 0.9758 0. 1282 538.2 40- 2G11B2 0.2853 2 .2798 0..438 6 0.00 3 .0037 0.1251 0.9433 0. 1326 547 .7 40- 3G11B1 0.2839 2 .3331 0.,411 2 0.00 3 .0282 0.1217 0.8960 0. 1358 550.4 40- 3G11B2 0.2839 2 .3331 0.,411 2 0.00 3 .0282 0.1217 0.9584 0. 1269 531.5 76- 1G11B1 0.3966 2 .3144 0.,205 7 0.0556 2 .9723 0.1714 0.9171 0. 1869 625.9 7 6-1G11B2 0.3966 2 .3144 0.,205 7 0.0556 2 .9723 0.1714 0.9525 0. 1799 612.5
7 6-2G11B1 0.4197 2 .3668 0.,160 1 0.0657 3 .0123 0.1773 0.9526 0. 1861 618.0
7 6-2G11B2 0.4197 2 .3668 0.,160 1 0.0657 3 .0123 0.1773 0.9376 0. 1891 623.7 100 -2G21B2 0.3879 2 .2576 0.,288 4 0.0490 2 .9829 0.1718 1.0532 0. 1631 590.7 100 -2G21B3 0.3879 2 .2576 0.,288 4 0.0490 2 .9829 0.1718 1.0835 0. 1586 581.7 29- 1G11B1 0.3888 2 .2679 0.,215 0 0.0614 2 .9331 0.1714 1.0230 0. 1676 590.2 29- 1G21B3 0.4192 2 .3440 0., 1423 0.0968 3 .0023 0.1788 0.9655 0. 1852 613.9
2 9-1G21B4 0.4192 2 .3440 0., 1423 0.0968 3 .0023 0.1788 1.0092 0. 1772 598.5 29- 1G31B5 0.3958 2 .3826 0. 1708 0.0832 3 .0324 0.1661 1.0042 0. 1654 579.1 29- 1G31B6 0.3958 2 .3826 0. 1708 0.0832 3 .0324 0.1661 1.0074 0. 1649 578.1
29- 2G11B1 0.4130 2 .3553 0.,167 6 0.1007 3 .0366 0.1753 0.9907 0. 1769 601.2 29- 2G21B4 0.4216 2 .3730 0., 1530 0.1009 3 .0485 0.1777 1.0169 0. 1747 594.8
ave. T°C 586.8 + 23.7
,cc/2089+.00956 P(bars)NN YGt
(1661 755( )} X 2089 + .00956 P(bars) "' 0.7820-ln K» Ca T°(K) = 0.7820 - In K1 (after Lang & Rice, 1985) Kl= (Mg/Fe Gt / Mg/Fe Bi) 96
aalm= tyalm XFe)3
Gt
Xpe= Fe/Fe+Mg+Ca+Mn
C1 / RT (~W X. X )) yalm=exp 03 (Newton & Haselton, (1981))
-13807-6.3T (joules, K)
ave. ^=0.1246 ave. X„ =0.0814 Ca T (K)=(586.8 C + 273.15)=859.95 K
a . =0.4565 aim
Using GRAIL geobarometer after Bohlen, et al., (1983) :
3 2 a., a. a = il 1 K = —~ (assuming a^, a , a , a^^ = 1 for pure end members) TEMPERATURE (°C) (after Bohlen, et al., 1983)