Download Barrovian metamorphism in the central Kootenay Arc, British

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Barrovian metamorphism in the central Kootenay Arc, British
Columbia: petrology and isograd geometry
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D.P. Moynihan and D.R.M. Pattison
Abstract: A narrow, partly fault-bounded belt of Barrovian amphibolite facies rocks transects the central Kootenay Arc in the
internal zone of the southeastern Canadian Cordillera. The following zones of increasing metamorphic grade are recognised in
metapelites: chlorite/biotite, garnet, staurolite, kyanite, sillimanite, and sillimanite + K-feldspar. The garnet and higher-grade
zones outline two joined domains: a N- to NNE-trending curved belt, >100 km long and 5–20 km wide, that is approximately
parallel to the strike of stratigraphic units; and a SSE-trending belt, >70 km long and 10–15 km wide, that transects strike.
Isograds in the N–NNE-trending belt outline an elongate bull's-eye pattern with highest-grade rocks in the centre, coincident
with the position of a structural culmination. Isogradic surfaces have an elongate domal form centred around this culmination.
Rocks in the kyanite and sillimanite zones were metamorphosed at ~25 km (⬃7 kbar (1 kbar = 100 MPa)) and >650 °C during Early
Cretaceous crustal thickening; rocks in the garnet zone experienced peak temperature conditions (⬃500 °C) at lower pressure,
suggesting the piezothermic array for the belt has a positive slope. The western flank of the N- to NNE-trending belt is cut by the
west-dipping Gallagher fault zone (GFZ), whereas the eastern boundary of the SSE-trending fork is marked by the Purcell Trench
fault (PTF). These Palaeocene–Eocene normal faults truncate Early Cretaceous isogradic surfaces and juxtapose regions with
contrasting structural and metamorphic histories. Low-grade rocks in the hanging wall of the GFZ underwent peak regional
metamorphism during the Early–Middle Jurassic, prior to intrusion of the 159–173 Ma Nelson batholith at a depth of ⬃12–14 km.
With the exception of a zone along the southern “tail” of the Nelson batholith, rocks in the hanging wall of the GFZ were not
affected by Cretaceous metamorphism or penetrative deformation. Rocks in the amphibolite-facies belt yield Palaeocene–
Eocene K–Ar and 40Ar/39Ar mica cooling ages, whereas K–Ar biotite ages in the hanging wall of the GFZ record Jurassic – Early
Cretaceous cooling, and those in the hanging wall of the PTF are mid-Cretaceous. Although the GFZ and PTF accommodated
differential exhumation during the Palaeogene, significant relief on isogradic surfaces was established prior to normal faulting,
during Early Cretaceous metamorphism and deformation.
Résumé : Une ceinture étroite de roches barroviennes au faciès des amphibolites, en partie limitée par des failles, recoupe l'arc
central Kootenay dans la zone interne de la Cordillère canadienne sud-est. Les zones suivantes, à degré croissant de métamorphisme, sont reconnues dans les métapélites : chlorite/biotite, grenat, staurotide, kyanite, sillimanite et sillimanite +
feldspath-K. Les zones à grenat et les zones à métamorphisme plus élevé définissent deux domaines conjoints : (1) une ceinture
courbée, de direction N à NNE d'une longueur de plus de 100 km et d'une largeur de 5 à 20 km, qui est presque parallèle à la
direction des unités stratigraphiques et (2) une ceinture de direction SSE, d'une longueur de plus de 70 km et d'une largeur de 10 à
15 km qui recoupe la direction. Les isogrades dans la ceinture à direction N–NNE définissent un patron de cercles concentriques
allongés dans lequel les roches à degré de métamorphisme plus élevé sont situées au centre, ce qui concorde avec une position
de culmination structurale. Les surfaces des isogrades ont une forme de dôme allongé centré autour de cette culmination
structurale. Les roches dans les zones de kyanite et de sillimanite ont été métamorphisées à une profondeur de 25 km (⬃7 kbar
(1 kbar = 100 MPa)) et à une température >650 °C durant l'épaississement de la croûte au Crétacé précoce; les roches dans la zone
à grenat ont subi les conditions de température de crête (⬃500 °C) à des pressions moindres, suggérant une pente positive pour
le groupement piézothermique de la ceinture. Le flanc ouest de la ceinture à tendance N à NNE est recoupé par la zone de faille
Gallagher (GFZ) à pendage ouest, alors que la limite est de la fourche à tendance SSE est marquée par la faille Purcell Trench (PTF).
Ces failles normales (Paléocène–Éocène) recoupent les surfaces isogrades du Crétacé précoce et juxtaposent des régions ayant des
historiques structuraux et métamorphiques différents. Les roches à faible métamorphisme dans l'éponte supérieure de la GFZ
ont subi un métamorphisme de crête au cours du Jurassique précoce à moyen, avant l'intrusion du batholite de Nelson (159–173 Ma)
à une profondeur d'environ 12–14 km. À l'exception d'une zone le long de l'extrémité sud du batholite de Nelson, les roches dans
l'éponte supérieure de la GFZ n'ont pas été touchées par le métamorphisme au Crétacé ni par une déformation de pénétration.
Les roches dans la ceinture au faciès des amphibolites ont donné des âges de refroidissement Paléocène–Éocène, déterminés par
K–Ar et 40Ar/39Ar sur du mica, alors que les âges déterminés par K–Ar sur biotite dans l'éponte supérieure de la GFZ documentent
un refroidissement au Jurassique–Crétacé précoce et que ceux dans l'éponte supérieure de la PTF datent du Crétacé moyen. Bien
que la zone de faille Gallagher et la faille Purcell Trench soient le reflet d'une exhumation différentielle durant le Paléogène, des
reliefs importants sur les surfaces des isogrades ont été établis avant les failles normales, durant le métamorphisme et la
déformation au Crétacé précoce. [Traduit par la Rédaction]
Received 10 April 2012. Accepted 17 February 2013.
Paper handled by Associate Editor Marc St-Onge.
D.P. Moynihan* and D.R.M. Pattison. Department of Geoscience, University of Calgary, Calgary, AB T2N 1N4, Canada.
Corresponding author: D.P. Moynihan (e-mail: [email protected]).
*Present address: Yukon Geological Survey, P.O. Box 2703 (K-14), Whitehorse, YT Y1A 2C6, Canada.
Can. J. Earth Sci. 50: 769–794 (2013) dx.doi.org/10.1139/cjes-2012-0083
Published at www.nrcresearchpress.com/cjes on 3 April 2013.
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Can. J. Earth Sci. Vol. 50, 2013
Introduction
Geology of the central Kootenay Arc
The southeastern Canadian Cordillera comprises a complex series of structural and metamorphic domains that record a prolonged period of orogenesis lasting from the Early Jurassic to the
Eocene (Parrish 1995; Evenchick et al. 2007). Reconstruction of the
development of the orogen is complicated by a heterogeneous
pre-Cordilleran basin architecture, multiple superposed episodes
of deformation and associated medium–high grade metamorphism, and late orogenic extension. Given this complexity, characterization of its varied structural and metamorphic domains is
a prerequisite for understanding the tectonic processes that led to
its development.
The focus of this paper is a narrow, elongate belt of Barrovian
amphibolite-facies rocks that transects the Kootenay Arc in the internal zone of the orogen (Fig. 1; Archibald et al. 1983; Read et al.
1991). We document the regional metamorphism of this metapelites
in this belt, the relationships between metamorphism and deformation, the geometry of isograds and isogradic surfaces, and interpret
the processes that gave rise to its formation.
The Kootenay Arc (Figs. 1–3) is an arcuate zone in the hinterland
of the orogen characterised by a distinctive stratigraphy and intense polyphase deformation. The structural trend is SW–NE in
the southernmost part of the Arc and changes gradually to NW–SE
in the northern part, where it is approximately parallel to the
overall trend of the orogen. The Kootenay Arc includes the zone of
overlap between Proterozoic – Early Palaeozoic rocks that formed
on the western margin of ancestral North America or precursor
rift basins, and mostly Late Palaeozoic – Mesozoic rocks that
formed in island arc or back arc settings that developed after late
Proterozoic rifting (Quesnel and Slide mountain terranes; Klepacki
1985).
Geological background
The southeastern Canadian Cordillera includes a metamorphic–
plutonic internal zone that lies to the west of a NE-vergent fold
and thrust belt and associated foreland basin (Fig. 1).
This part of the orogen developed in response to convergence
between cratonic North America and westward terranes in the
Early Jurassic, and continued convergence led to further shortening, thickening, and eastward propogation (relative to North
America) until the Eocene (Parrish 1995; Evenchick et al. 2007;
Simony and Carr 2011). Three major episodes of deformation and
metamorphism are recognised in the southeastern Canadian Cordillera: the first in the Early–Middle Jurassic, the second in the
Early Cretaceous and a third during the Late Cretaceous – Eocene
(Simony and Carr 2011 and references therein). Convergent orogenesis was followed in parts of the internal zone by a period of
extension during the Eocene.
Upper amphibolite- to granulite-facies rocks are exposed in a
series of gneissic domal culminations that are exposed in the
western part of the internal zone; individual domal culminations
include the Monashee, Valhalla, Grand Forks, and Priest River
complexes (Fig. 1; Doughty and Price 1999; Johnston et al. 2000;
Carr and Simony 2006; Hinchey et al. 2006; Cubley et al. 2012).
These culminations experienced high-grade metamorphism during the Late Cretaceous – Eocene and expose a number of thick
top-to-the-east shear zones that were active during this time (Simony and Carr 2011). The area in which the gneiss domes are
exposed experienced widespread Eocene extension (Parrish et al.
1988), and domal culminations are typically bounded by normal
faults of this age.
Lower-grade areas surrounding the gneissic domes typically record an older (Jurassic – Early Cretaceous) deformational and metamorphic history, as does much of the area that lies between the
gneiss complexes and the Late Cretaceous – Eocene foreland fold
and thrust belt (Currie 1988; Hoy and van der Heyden 1988;
Scammell 1993; Parrish 1995; Colpron et al. 1996; Digel et al. 1998;
Crowley et al. 2000; Reid 2003; Larson et al. 2007; Gibson et al.
2008). Early Cretaceous structures are dominant in a broad zone
that stretches from the central Kootenay Arc across the Purcell
Anticlinorium, as far east as the Porcupine Creek anticlinorium
(Simony and Carr 2011; Fig. 1), whereas Middle Jurassic structures are more widely preserved in low-grade domains (i.e., at
high structural levels) to the west and north of this region
(Colpron et al. 1996; Acton et al. 2002; Carr and Simony 2006;
Cubley et al. 2013). The area under consideration in this study —
the central Kootenay Arc — includes the interface between these
regions, and also lies within the area that was affected by lateorogenic extension.
Stratigraphy of the central Kootenay Arc
The Kootenay arc lies on the west flank of the Purcell
Anticlinorium, a large north-plunging structure cored by Mesoproterozoic rocks of the Belt-Purcell Supergroup (Lydon 2007) and
partially mantled by the unconformably overlying Neoproterozoic Windermere Supergroup (Fig. 1; Warren 1997 and references
therein). These supergroups comprise thick sequences of clastic
and carbonate sedimentary rocks with subordinate mafic volcanic
rocks that were deposited in partially overlapping intracontinental rift zones.
Rifting that led to formation of the western margin of ancestral
North America took place in the late Neoproterozoic (Devlin and
Bond 1988; Colpron et al. 2002) and is recorded by Neoproterozoic –
Lower Cambrian rocks of the correlative Gog and Hamill groups
(Fig. 2). The Hamill/Gog Group is dominated by coarse quartz-rich
sedimentary rocks, with minor (meta-) pelite, carbonate rocks,
and mafic volcanic rocks. There is a regional unconformity in the
Hamill Group (Devlin and Bond 1988; Likorish and Simony 1995;
Warren 1997), separating units that were deposited in faultbounded basins during rifting from an upper part distinguished
by laterally continuous units deposited in a shallow-marine setting. The unconformity within the Hamill Group is interpreted as
a “break-up” unconformity, representing continental separation
and the change from active continental rifting to thermal subsidence on the newly formed western margin of Laurentia (Devlin
and Bond 1988; Warren 1997).
In the Kootenay Arc and western Purcell anticlinorium, the
Hamill Group is overlain by the Mohican formation, a mixture
of siliciclastic and carbonate rocks, followed by the Badshot
Formation — a distinctive, Lower Cambrian pale-coloured marble. The Badshot marble is conformably overlain by rocks of the
Lardeau Group, which comprises Lower Palaeozoic pelitic and
semi-peltic schist, mafic volcanic rocks, carbonate and calcsilicate rocks, coarse siliciclastic rocks and conglomerate (Fyles
and Eastwood 1962, Fyles 1964, 1967; Hoy 1980; Colpron and Price
1995 and references therein). The lowest part of the Lardeau
Group is a fine-grained black metapelite that records deposition
under tectonically quiescent, deep-water conditions; however, a
return to active rifting is recorded by metavolcanic rocks and
coarse grits of upper parts of the Lardeau (Logan and Colpron
2006).
The Lardeau Group is unconformably overlain by the Mississippian–
Permian Milford Group, which comprises conglomerate, phyllite,
basalt, limestone, sandstone, tuff, and siliceous argillite. This is
followed in conformable succession by the Kaslo group, which is
dominated by basalt and mafic volcaniclastic rocks, but also includes serpentinite, siliceous sediments, greywacke, and minor
conglomerate. The Milford and Kalso groups are interpreted to
have been deposited in a rift setting, probably in a back-arc basin
(the Slide Mountain or Kaslo basin) to the west of cratonic North
America (Klepacki 1985; Nelson 1993; Roback et al. 1994). The
Kaslo and parts of the Milford Group represent the Slide Mountain
terrane in this part of the Cordillera.
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Moynihan and Pattison
771
Fig. 1. Geological map of the southeastern Canadian Cordillera, adapted from Wheeler and McFeely (1991). The Kootenay Arc forms an
eastward-convex arcuate belt that includes the overlap between rocks deposited on the Laurentian margin and those of pericratonic (Quesnel
and Slide Mountain) terranes to the west. A cross section from the Kootenay Arc to the foreland (Price 1981) is also shown. The subdivisions in
the cross section do not correspond exactly with those shown on the map. In the cross section, the Neoproterozoic – Lower Cambrian
Hamill–Gog groups are combined with Palaeozoic rocks, and Triassic–Palaeocene sedimentary rocks of the foreland belt are grouped together.
118 W
52 N
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FO
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Cenozoic volcanic &
sedimentary rocks
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Late Cretaceous-Palaeogene
foreland basin sedimentary rocks
Triassic - Jurassic
Mesozoic intrusions
Palaeozoic (shelf/slope)
Palaeogene intrusions
Palaeozoic (basinal)
Quesnel & Slide Mountain terranes
Undifferentiated gneiss in
metamorphic core complexes
Palaeoproterozoic basement
(Laurentian)
W
KOOTENAY ARC
PURCELL ANTICLINORIUM
Neoproterozoic - Lower Cambrian (Hamill & Gog groups)
Neoproterozoic (Windermere Supergroup)
Mesoproterozoic (Purcell Supergroup)
Porcupine Creek
Anticlinorium
E
Triangle Zone
km
0
-5
-10
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Can. J. Earth Sci. Vol. 50, 2013
Fig. 2. Geological map of the central Kootenay Arc. From Fyles (1964, 1967), Reesor (1973), Hoy (1980), Klepacki (1985), Leclair (1988), Reesor
(1996), and Warren (1997). The area covered is shown in Fig. 1.
Intrusive Rocks
A’
50 15’
Late Cretaceous/Palaeocene
A
117 00’
Middle Jurassic
Stratified Rocks
Purcell Supergroup
Sandstone, siltstone, pelite,
carbonate, mafic sills,
Lakeshore fault
lt
ard fau
West B
ern
Gallagher fault
C’
Riondel
Ainsworth
Nelson
batholith
Crawford
Bay
t Arm
Wes
Ko
ot
en
ay
La
ke
D
49 30’
E
Midge
Creek
stock
Crawford
stock
?
D’
Seeman Creek fault
Mesoproterozoic
49 45’
Proctor plut
on
Windermere Supergroup
Sandstone, conglomerate, pelite, carbonate
C
ke Fault
y Lreanch
teunrcaell T
Koo P
Neoproterozoic
Shoreline
stock
Kaslo
plu
ton
NeoproterozoicLower Cambrian
Hamill Group
Grey and brown micaceous quartzite,
mica schist, white quartzite,
amphibolite, marble, calc-silicate
B’B’
B
B
dy
Lower Cambrian
Badshot and Mohican Formations
Grey and white calcite and dolomite
marble, mica schist, amphibolite
50 00’
Bal
Lardeau Group
Calc-schist, calc-silicate gneiss, pelitic
schist, marble, quartzite, amphibolite,
greenschist, quartzofeldspathic gneiss
lt
CambrianDevonian?
Fry Creek
batholith
au
MississippianPermian
Milford Group
Siliceous argillite, schist,
grey marble, chert, quartzite
rf
Kaslo Group
Greenstone, amphibolite, chert
de
Permian
Slocan Group
Grey argillite and phyllite, light
grey to black limestone
oe
hr
Triassic
Sc
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mid-Cretaceous
Bayonne
batholith
E’
2 km
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Fig. 3. Geological cross sections of the central Kootenay Arc. All are vertical sections with equal horizontal and vertical scales. Section lines
A–E are shown in Fig. 2; section F–F= is marked on Fig. 5d. (a) A–A= is from Fyles (1967), (b) B–B= from Klepacki (1985), (c) C–C= from Fyles in
Brown et al. (1981), (d–f) D–D=, E–E=, and F–F= are from Leclair (1988).
a
A
A’
1 km
A‘
S2
S1
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Anticline
Creek
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B
B‘
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C‘
E‘
Midge Creek
fault
Seeman Creek
fault
Purcell Trench
fault
1 km
f
F
Midge Creek
fault
Wurttemberg
anticline
F‘
Purcell Trench
fault
2500 m
2000 m
1500 m
1000 m
500 m
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Fig. 4. (a) Refolded tight-isoclinal F1 fold in the Slocan Group above Fletcher Lake; hanging wall of the Gallagher fault. Limestone is folded
around recessively weathering shale. The hillside relief is ⬃500 m. (b) Quartz-filled strain shadows on pyrite grains define a (L1) stretching
lineation in phyllite of the Slocan Group; DM-07-124. (c) Polydeformed “felsite sills” in Grt–Ctd schist of the Milford Group. The “sills” contain
a margin-parallel foliation that can be traced around hinges of the tight (D2) folds. (d) Tight-isoclinal F2 fold outlined by the buff-coloured
Badshot Formation on a mountainside east of Riondel. Visible relief on the slope facing the viewer is ⬃300 m. (e) L2 stretching lineation
defined by sillimanite nodules in Hamill Group schist on the shoreline of Kootenay Lake at Riondel. (f) Shear bands in metapelite from north
of the Fry Creek batholith. S2 is deflected into a sigmoidal geometry between shear bands (C=). Length of thin section is 7.5 cm. View is to the
SW, down the plunge of the intersection of shear bands with S2. (g) Cataclasite–mylonite from the Gallagher fault. The white material in the
upper part of the photograph is a deformed quartz vein. Length of rock slab is 7.5 cm. Cut plane is approximately vertical, oriented E–W.
(h) S–C= fabric in deformed quartz monzonite from the NW margin of the Fry Creek batholith. Field of view is 7.5 cm long. Cut plane is
approximately vertical, oriented E–W. The S–C fabric indicates west-side down shearing.
Thrust duplication of the Kalso Group accompanied closure of
the Slide Mountain basin in the Late Permian – Middle Triassic
(Klepacki 1985). The Kalso Group was subaerially exposed, eroded,
and its detritus deposited in the Marten conglomerate, a Late
Permian – Middle Triassic greenstone conglomerate intercalated
with limestone (Klepacki 1985). The Marten conglomerate is unconformably overlain by the Slocan Group, which comprises
Carnian–Norian dark grey slate and phyllite, locally interbedded
with limestone, limestone arenite, quartzite, and intermediatemafic volcanic rocks. It was deposited in a regionally extensive
back-arc basin that developed east of the Late Triassic Nicola/
Quesnel magmatic arc, which formed above the Cache Creek accretionary complex to the west (Mortimer 1987). The Slocan Group
overlies a varied basement and provides part of the abundant
stratigraphic and geochemical evidence that links the Quesnel
Arc to ancestral North America in the Triassic (Dostal et al. 2001;
Unterschutz et al. 2002; Petersen et al. 2004; Simony et al. 2006;
Thompson et al. 2006). Though not exposed in the area covered by
Fig. 2, Early Jurassic volcanic and sedimentary rocks of the Rossland Group lie on top of the Slocan Group (Hoy and Dunne 1997).
The upper unit of the Rossland Group — the clastic Pliensbachian–Toarcian Hall Formation — is the youngest unit that was deposited prior to Cordilleran deformation in this part of the
Cordillera.
Middle Jurassic D1 deformation, M1 metamorphism, and
magmatism
The central Kootenay Arc is dominated by two regionallydeveloped fold generations (Fyles 1964, 1967; Hoy 1976; Leclair
1988; Figs. 2, 3) and records two episodes of regional metamorphism (M1, M2). Phase 1 folds are a series of high-amplitude isoclines with gently-plunging axes (Figs. 2–4a). The most clearly
defined F1 folds in the study area are westward-closing recumbent
anticlines cored by the Hamill Group; the largest of these, the
Riondel nappe (Hoy 1976; equivalent to the Meadow Creek anticline in the Duncan Lake area) has a 20 km long overturned lower
limb (Fig. 3). S1 schistosity is axial planar to F1 folds, and contains
a gently-plunging stretching lineation defined by aligned minerals and quartz strain shadows (L1; Fig. 4b). D1 was accompanied by
low-grade regional metamorphism that was locally overprinted
by contact metamorphism around Middle Jurassic intrusions
(Pattison and Vogl 2005). The high-amplitude, isoclinal D1 folds
are similar to Middle Jurassic structures (e.g., the west-vergent
Scrip/Carnes nappe) that are developed approximately along
strike in the Selkirk and Cariboo mountains (Raeside and Simony
1983; Murphy 1987; Brown and Lane 1988; Colpron et al. 1996).
Although there is evidence for older (Paleozoic) deformation
northwest of the study area (Read 1973; Read and Wheeler 1976;
Klepacki 1985), F1 folds in the central Kootenay Arc probably also
formed during the Middle Jurassic (see discussion in Warren 1997).
Early–Middle Jurassic deformation and regional metamorphism was followed by intrusion of Middle Jurassic plutons and
associated minor bodies. In the central Kootenay Arc, this suite is
represented by the calc-alkaline Nelson Batholith and associated
minor bodies (Fig. 2). The Nelson Batholith is an 1800 km2 composite body that was intruded during the interval 159–173 Ma
(Ghosh 1995). It ranges in composition from diorite to granite, but
is dominated by porphyritic hornblende granodiorite. The Nelson
batholith is truncated on its western side by the east-dipping
Eocene Slocan Lake (normal) fault (Carr et al. 1987), and is also
fault-bounded on part of its eastern side, as described later in the
text. Most of the Nelson Batholith is unaffected by ductile deformation; however, a subhorizontal stretching lineation and subvertical to west-dipping foliation is developed on its eastern
margin along its southern “tail” and to the north of the West Arm
of Kootenay Lake (Crosby 1968; Vogl 1992).
In addition to plutons, the region contains numerous felsic
dikes and sills. On the west side of the northern part of Kootenay
Lake, and to the north of the lake, sills of blocky, rusty-weathering
“felsite” (Fyles 1964) intrude all units. These sills are commonly
foliated and lineated and some are folded (Fig. 4c), with gently
north-plunging axes parallel to the lineation. The sills range in
size from tens of centimetres to large map-scale bodies. Smith
et al. (1991) reported a U–Pb zircon age of 173 ± 5 Ma for the largest
body of this type — the “Kaslo sill”.
Early Cretaceous D2 deformation and M2 metamorphism
A second period of deformation and regional metamorphism
(D2/M2) took place in the central Kootenay Arc, after intrusion of
the Nelson batholith suite. This is the period of metamorphism
that gave rise to the narrow Barrovian belt that is the focus of this
paper.
F2 folds are approximately coaxial with F1 (Figs. 2, 3) but are
overturned to the east (Fig. 4d). F2 axial planes and their axial–
planar schistosity (S2) dip gently to moderately-steeply to the west,
but steepen towards higher structural levels in the Purcell
Anticlinorium (Fyles 1964; Hoy 1980; Fig. 3). L2 stretching lineations (Fig. 4e), which are defined by aligned minerals (sillimanite,
micas, amphiboles, tourmaline), strain shadows on porphyroblasts, sillimanite-rich nodules, and quartzofeldspathic aggregates plunge gently, parallel to F2 fold axes. North of the West
Arm of Kootenay Lake, D2 lineations and folds plunge gently
north, exposing deeper structural levels towards the south. Approximately south of the West Arm, lineations are subhorizontal
to gently SSW-plunging (Fyles 1967; Leclair 1988; Vogl 1992).
In the region adjacent to the SSW-trending tail of the Nelson
batholith, gently-plunging stretching lineations are accompanied
by abundant shear bands and C–S fabrics, indicating dextral
shearing during D2. Foliation within the tail of the batholith displays a transition towards parallelism with this SW-trending zone
(Vogl 1992), as does the foliation in screens within the tail.
There is considerable along- and cross-strike variation in the
intensity of D2. In the eastern part of the Kootenay Arc and the
western part of the Purcell anticlinorium, D2 structures generally
form the dominant fabrics, whereas they are weakly-developed or
absent from lower-grade areas to the north and west (Figs. 3, 4a). A
southward increase in D2 strain is manifested in tightening of F2
folds (Figs. 3, 4d) and more pervasive development of D2 fabrics.
Around Duncan Lake and the northern end of Kootenay Lake, D2
folds are open to close (Fig. 3a), S1 is commonly preserved (Fyles
1964) and F2 folds have curvilinear hinges. In the Riondel area
(Hoy 1977) and further south (Leclair 1988), however, F2 folds are
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W
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g)
775
a)
b)
F1
L1
c)
d)
e)
N
UP
W
F2
S1
f)
NW
SE
C‘ (S3)
S2
L2
h)
UP
E
E
S
C
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Fig. 5. Metamorphic maps of the central Kootenay Arc. (a) A compilation showing the distribution of regional metapelitic metamorphic
zones in the central Kootenay Arc. (b–d) Mineral assemblage maps, each covering part of the area. The key for mineral assemblages is
contained in (d). Assemblages observed in rocks collected for this study are labelled in black, whereas those reported in previous studies have
red outlines (in the Web version). Samples discussed in the text or illustrated in figures are labelled. In b–d, grid lines are spaced at 5 km
intervals. Data are from this study, Fyles (1964, 1967), Reesor (1973), Crosby (1968), Hoy (1974), Archibald et al. (1983), Klepacki (1985), Leclair
(1988), and Warren (1997). The section lines of metamorphic cross sections presented in Fig. 11 are marked on (c) and (d). And, andalusite;
Bt, biotite; Chl, chlorite; Crd, cordierite; Ctd, chloritoid; Grt, garnet; Kfs, K-spar, K-feldspar; Ky, kyanite; Qtz, quartz; Sil, sillimanite;
St, staurolite.
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tight-isoclinal (Fig. 3d), rectilinear F2 fold hinges are invariably
parallel to the L2 stretching lineation and D1 fabrics are typically
not discernible (Hoy 1980; Leclair 1988).
Mid-Cretaceous magmatism
A second major plutonic suite was intruded during the interval
117–95 Ma. This suite includes hornblende and biotite granodiorite, biotite granite, and two-mica granite, which are interpreted
to have been derived from crustal anatexis (Brandon and Lambert
1993). The largest bodies of this age in the area are the Fry Creek
batholith and the Heather Creek, Drewry Point, Steeple Mountain,
and Mount Skelly phases of the Bayonne batholith. These largely
undeformed intrusions crosscut the major structures in the area
(Fig. 2) and are younger than D2/M2 penetrative deformation and
regional metamorphism.
The oldest member of this suite — the Baldy pluton — is an elongate, sheet-like granodiorite body that was deformed during D2
(Fig. 2). It has yielded a combined titanite and allanite U–Pb date of
117 +4/–1 Ma, which was interpreted to represent the age of crystallization (Leclair et al. 1993). The nearby, largely undeformed Midge
Creek stock yielded a U–Pb allanite age of 114 +1/–3 Ma. This suggests
that, contrary to earlier interpretations, D2/M2 took place during the
Early Cretaceous (Leclair et al. 1993).
White leucogranitic pegmatite, locally containing biotite or
muscovite, forms abundant sills, dikes, pods, and small plutons in
western part of the Riondel area (Hoy 1980), on much of the
Crawford peninsula, and on the west side of Kootenay Lake south
of the West Arm. Pegmatite exhibits a variety of relationships
with respect to the (D2) dominant structures. Much of the pegmatite is deformed — it is foliated, boudinaged, and locally folded.
Large bodies of pegmatite are commonly deformed on their
margins and more massive in their interior. Crosscutting, undeformed pegmatite and aplite/leucogranite dikes are also present
in the Crawford Bay – Riondel area (Crosby 1968; Livingstone 1968;
Hoy 1980).
D3, D4, D5
Over a large area adjacent to the northern, north-trending part
of Kootenay Lake, S2 in micaceous rocks is overprinted by shear
band cleavage (Platt and Vissers 1980; White et al. 1980; Williams
and Price 1990; Passchier and Trouw 2005; Fig. 4f). Shear bands dip
NW, and lines of intersection of shear bands with S2 and associated sigmoidal deflections of the foliation plunge SW. Although
most of these “extensional crenulations” (Platt and Vissers 1980)
are centimetre-scale, some metre-scale and larger folds of this
nature are also developed. These SW-plunging warps were noted
by Fyles (1967), and referred to as F3 folds by Livingstone (1968) and
Hoy (1980). The area affected by this S2 reactivation is >60 km
long, and ⬃15 km wide. On the west side of the lake, shear bands
are developed east of the Gallagher fault between the West Arm
and Kalso, and are found east of the Lakeshore fault (Fyles 1967)
north of Kaslo. On the east side of the Lake, they are restricted to
the region close to the shore of Kootenay Lake, extending north
from the highway west of Crawford Bay, past Riondel (Livingstone
1968; this work). They are also developed in the area north of
the Fry Creek batholith (Fig. 2). The age of these structures is
poorly constrained; they are crosscut by earliest Palaeocene
pegmatites (Moynihan 2012) but could be as old as Early Cretaceous (the age of D2).
Two generations of folds not observed elsewhere are developed
on and around the Crawford peninsula. Fourth-generation folds
are buckles with axes that plunge gently SW or NE and axial
planes with low to moderate dips. Unlike the F3 folds described
earlier in the text, they formed by shortening rather than extension of S2. These folds post-date intrusion of the Palaeocene
Crawford Bay stock (see later in the text) and are overprinted by
NNW-trending folds (F5) that are probably related to motion on
the Eocene Purcell Trench fault (PTF).
Faults
Hoy (1977) and Leclair (1988) identified a number of regionallysignificant, approximately strike-parallel west-dipping faults in
the central Kootenay Arc. The West Bernard fault (WBF; Fig. 2) separates the Riondel nappe from its root zone and probably had reversesense displacement (Hoy 1977). A possible along-strike continuation
to the south is the Seaman Creek fault (Leclair 1988; Figs. 2, 3), which
separates domains with contrasting Neoproterozoic – Lower
Palaeozoic stratigraphy. Leclair (1988) interpreted this fault, the
Wilson Creek fault and the Midge Creek fault (Fig. 3) as major
east-vergent contractional structures that stacked panels of rock
from the continental margin and accommodated eastward translation of rocks belonging to the Quesnel and Slide mountain terranes. Transcurrent motion on the Midge Creek fault (see Fig. 5)
was suggested by Vogl (1992), who noted that the most highly
deformed rocks in the tail zone of the Nelson batholith occur
where the fault marks the margin of the batholith.
Fyles (1967) mapped three west-dipping faults in the Ainsworth
area along the west side of Kootenay Lake. From west to east these
are the Gallagher, Josephine, and Lakeshore faults (Fyles 1967;
Figs. 2, 3). These faults are marked by zones of sheared, brecciated
rock up to ⬃30 m thick, and in places by zones of localised folding
(Fyles 1967). The Gallagher fault is particularly significant as it
marks the location of an abrupt westward decrease in the intensity of D2 deformation. S2 forms the dominant west-dipping foliation in the footwall whereas in the hanging wall penetrative D2
deformation is absent and the fault cuts S1 at a high angle (Fig. 3).
The Gallagher fault has been observed in two locations close to
the eastern margin of the Nelson batholith during this study. The
fault rock, which occupies a relatively narrow zone (less than
⬃30 m thick), comprises a mylonitised carbonate or phyllonite
matrix with clasts and lenses of deformed quartz-rich, muscoviterich, and chlorite-rich rock (Fig. 4g). The origin of these fragments
is uncertain but they are probably derived from adjacent metasedimentary rocks, quartz veins, and fragments of the Nelson
batholith. Shear bands in this zone dip to the west, more steeply
than foliation. The Nelson batholith on the west side of the fault
zone is affected by an anastomosing network of dark deformation
seams and contains abundant secondary muscovite.
Fyles (1967) mapped the Lakeshore fault north of Kaslo, and
showed it extending northwards parallel to Kootenay Lake. In
contrast, Klepacki (1985) correlated the Lakeshore fault with the
Schroeder fault, a large west-side-down structure that extends
along strike to the NW where it is plugged by a ca. 180 Ma intrusion. For reasons outlined later in the text, we consider the former
interpretation more likely. West-dipping normal faults were also
mapped by Fyles (1964) in the Duncan Lake area, one of which
projects into the NW margin of the Fry Creek batholith (Fig. 2).
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777
50 30’
a)
b)
Chlorite/Biotite
zone
Garnet zone
Staurolite zone
5590000
Kyanite zone
Dun
Sillimanite zone
can
Sil+Kfs zone
117 00’
r
ie k k
ac ee oc
Gl Cr st
50 15’
Middle Jurassic
5570000
DM-07-133
DM-07-131
50 00’
Fry Creek
batholith
Kaslo
DM-07-142
Gallagher fault
see (b)
Kootenay Lake
49 45’
DM-06-157
Nelson
batholith
Riondel
5550000
Crawford Bay
Sc
hr
oe
de
see (c)
49 30’
Mid
ge
Cre
ek
fau
lt
ault
nch f
ll Tre
Drewry
Creek intr.
15
5530000
Steeple
Mtn.
Shaw Creek
30
km
510000
Wall
stock
Bayonne
batholith
500000
117 00’
k
ree
tC
Los lt
fau
0
Fry Creek
pluton
lt
DM-07-124
see (d)
49 15’
rf
au
DM-07-166
e
Purc
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Lake
ice
Eocene
Late Cretaceous/
Palaeocene
mid-Cretaceous
Published by NRC Research Press
778
Can. J. Earth Sci. Vol. 50, 2013
Fig. 5 (continued).
Shore
c)
fault
*
DM-05-80
yC
ree
Lakeshore
5520000
k
Gallagher fault
DM-05-98
DM-05-88
DM-07-86
DM-07-58
5510000
*
Co
ee C
ree
k
DM-07-99
y Lake
G
DM-05-47
DM-05-147
Koo
tena
Nelson
batholith
DM-05-04
G’
5500000
DM-07-212
DM-05-53
DM-05-55
DM-07-178
DM-05-30
DM-05-44
DM-07-31
Crawford
Bay
stock
DM-09-CBAG
DM-09-CBA
520000
DM-07-192
510000
tor
Procpluton
Arm
West
500000
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DM-06-163
Woo
dbu
r
tock
line s
DM-06-164
Fry Creek
batholith
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Moynihan and Pattison
779
Fig. 5 (concluded).
d)
DM-05-04
5500000
DM-07-212
DM-05-55
DM-05-53
DM-07-178
st Arm
We
DM-07-192
DM-07-31
Pro
5490000
DM-07-233
plu
ldy
Ba
E
pluton
to
n
92-MC-6.1
Creek
Nelson
batholith
ult
h fa
renc
ell T
cto
Purc
r p
luton
DM-09-CBAG
DM-09-CBA
Heather
Intrusions
Late Cretaceous/Palaeocene
E’
mid-Cretaceous (deformed/undeformed)
Middle Jurassic (deformed/undeformed)
Metamorphic Zones
5480000
e C
re
ek
fa
ul
t
Midge
Creek
stock
Drewry
Point
intrusion
dg
Garnet zone
Staurolite zone
Kyanite zone
Sillimanite zone
Sil+K-spar zone
Mineral Assemblages
Chl
St+Grt+Bt
St+Bt
St+And+Grt+Bt
St+And+Bt
Kfs+Sil+Grt+Bt
Kfs+Grt+Bt
Kfs+Sil+Bt
Sil+Ky+St+Grt+Bt
Sil+St+Grt+Bt
Sil+St+Bt
Sil+Ky+Grt+Bt
Sil+Ky+Bt
Sil+Grt+Bt
Sil+Bt
Crd+And+Grt+Bt
Crd+And+Bt
Crd+Sil+And+Bt
Grt+Bt
Grt+Ctd+Chl
Grt+Bt+Chl
&
Wurtenberg
Lake
Ky+St+Grt+Bt
Ky+Grt+Bt
Ky+Bt
Ky+And+St+Grt+Bt
zones
F’
(Leclair, 1988)
510000
Ctd
500000
F
Chlorite/Biotite
zone
Observed contact aureole
Mi
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Crawford
Bay
stock
DM-05-44
And+Bt
Bt
Bt+Chl
Ctd+Chl
Chl
And+Grt+Bt
symbols with black outlines - this study
symbols with red outlines - previous work
And+Sil+Bt
* Qtz+Ky vein
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780
Can. J. Earth Sci. Vol. 50, 2013
Fig. 6. Photomicrographs and photographs of metapelitic rocks from the central Kootenay Arc. (a) F2 crenulations in chlorite-bearing phyllite
of the Slocan Group. Field of view is 6 mm; DM-06-124. (b) Chloritoid porphyroblast in Chl + Ms (muscovite) + Qtz-bearing phyllite of the
Windermere Supergroup. Field of view is 1.5 mm; DM-07-GC1. (c) Garnet–biotite–chlorite-bearing metapelite from the garnet zone near the
northern end of Kootenay Lake. Sigmoidal inclusion trails in garnet are interpreted to reflect rotation during syn-D2 growth; field of view is
6 mm; DM-07-131. (d) Garnet–chloritoid assemblage in aluminous metapelite of the Milford Group; field of view is 6 mm; DM-06-123. (e) F2
crenulations locally preserved in Grt–St schist of the Lardeau Group. Field of view is 3 mm; DM-07-86. (f) Large staurolite porphyroblasts on a
weathered surface adjacent to a quartz vein. Although they retain their idioblastic shape, the porphyroblasts are largely pseudomorphed by
chlorite and sericite. Quartz veins at this locality contain boudinaged kyanite crystals (inset); the inset photograph covers an area ⬃4 cm
wide. Milford Group; DM-05-80. (g) Partially pseudomorphed staurolite crystal in metapelite of the Milford Group. Garnet forms small crystals
that occur in the matrix and as inclusions in staurolite. Field of view is 12 mm. (h) Staurolite crystal partially replaced by kyanite; field of view
is 2.5 mm; DM-05-88. (i) Kyanite and garnet porphyroblasts visible on etched surface of Sil–Ky–St–Grt schist; shoreline of Crawford Bay; DM-0504. (j) Photomicrograph of the sample shown in (i). Staurolite has been partly replaced by kyanite; sillimanite largely grew by replacement of
biotite. Field of view is 6 mm; DM-05-04. (k) Sil–Ky–Grt schist of the Lardeau Group from the west shoreline of Kootenay Lake. Most of the rock
is a kyanite–garnet–biotite schist; however, sillimanite is present in thin Sil-rich seams and Qtz–Sil–Plag (plagioclase) veins. Rutile (Rut) forms
inclusions in some kyanite crystals. Field of view is 6 mm; DM-05-47. (l) Sillimanite crystals included in leucosome in K-feldspar + sillimanitebearing gneiss; Highway 3, west of Crawford Bay. Field of view is 6 mm; DM-05-147. (m) Garnet crystal in garnet-zone metapelite of the
Lardeau Group. The matrix fabric, which is a crenulation cleavage (S2), wraps around the crystal, indicating post-garnet tightening of S2.
Inclusion trails in the rim of the crystal also wrap around the inclusion-free core. These relationships are interpreted to result from growth of
the garnet rim during progressive tightening of S2; field of view is 6 mm; DM-07-133. (n) Another garnet crystal from the same samples as e.
F2 crenulations are included in the garnet rim, indicating growth of garnet rims after the onset of D2. Straight inclusion trails in the cores of
other crystals in this sample suggest garnet started to grow before D2 fabrics developed. Field of view is 1.5 mm; DM-07-133. (o) Kyanite crystal
that overgrew F2 crenulation; northernmost part of the kyanite zone; field of view is 3 mm; DM-07-142. (p) Contact metamorphic sillimanite
overprinted by tight (F2) crenulations; Nelson batholith contact aureole south of the West Arm. Field of view is 6 mm; 92-MC-6.1; Slocan Group.
Although most of the batholith is undeformed, this part contains
a west-dipping foliation, a down-dip lineation and well-developed
C–S fabrics indicating normal-sense motion (Reesor 1973; Warren
1997; Figs. 4h, 5b).
As discussed later, the Gallagher fault, the Midge Creek fault,
the Lakeshore fault and possibly also the Josephine fault, are interpreted to form partially overlapping strands of a west-dipping,
Palaeocene–Eocene normal fault zone that marks an important
metamorphic and geochronological discontinuity. This fault zone,
which has a strike length of >80 km is herein referred to as the
Gallagher fault zone (GFZ). The Gallagher fault is the bestcharacterised and centrally-located strand in this fault zone.
The GFZ and PTF (a NE-side-down normal fault; Figs. 2, 3) are
manifestations of a period of late orogenic extension that affected
a broad zone in the internal part of the orogen. This has been
attributed to transtension in the transfer zone between major
dextral strike-slip faults (Price and Carmichael 1986; Harms and
Price 1992).
Regional metamorphism of the central
Kootenay Arc
The isograd pattern
In map view the Barrovian garnet and higher-grade zones outline two joined, obliquely trending belts (Fig. 5). A N- to NNEtrending curved belt extends >100 km, from adjacent to the tail of
the Nelson batholith to the area east of Duncan Lake (Fig. 5a). The
axis of this belt is approximately parallel to the strike of stratigraphic units. Isograds in this part of the belt outline an elongate
bull's-eye pattern with highest-grade rocks in the centre. The
highest-grade region approximately coincides with the crest of
the D2 structural culmination (described earlier in preceding
text), which is marked by a reversal in the plunge of fold axes and
lineations. The west side of the N–NNE-trending part of the amphibolite facies belt is cut by the GFZ, which marks a metamorphic discontinuity. To the east of the axis of the metamorphic
high, grade decreases gradationally towards the centre of the
Purcell anticlinorium.
A SSE-trending fork of the amphibolite-facies belt is 10–15 km
wide and extends >70 km, continuing across the Canada–USA
border (Archibald et al. 1983). This belt is approximately parallel
to the PTF, which marks its eastern boundary. Close to the Crawford peninsula, the PTF obliquely cuts strike-parallel isograds belonging to the N–NNE-trending belt. The two amphibolite-facies
belts are separated in their region of overlap by a southward
widening region of low metamorphic grade.
Regional metamorphic zones
The regional chlorite/biotite, garnet, staurolite, kyanite, sillimanite, and sillimanite + K-feldspar zones are described in the
following text; in addition to major phases, all rocks contain accessory phases including tourmaline and (or) graphite. Ilmenite is
the dominant oxide mineral. Rutile is present in some samples
but is typically only preserved as inclusions in garnet or kyanite,
suggesting it was not stable at peak temperature (T) conditions.
Hematite is present in heavily altered rocks.
Low-grade zones (chlorite and biotite zones)
Compositional variability renders meaningful distinction between the chlorite and biotite zones difficult as many of the rocks
are high-Al pelites that do not contain biotite at low grade.
Metapelitic rocks in these zones are well-foliated phyllites
(Fig. 6a), and typically contain the assemblage Chl + Ms + Qtz ±
Bt ± Pl (Chl, chlorite; Ms, muscovite; Qtz, quartz; Bt, biotite; Pl,
plagioclase). Chloritoid (Ctd) forms small porphyroblasts as part
of the assemblage Ctd + Chl + Ms + Qtz + Pl at some localities in the
Windermere Supergroup (Purcell anticlinorium; Fig. 6b), and in
the Milford Group on the west side of Kootenay Lake. Leclair (1988)
also reported, but did not document the location of, additional
occurrences of chloritoid in the low-grade area south of the Midge
Creek stock.
Garnet zone
Metapelitic rocks in the garnet zone are generally fine-grained
schists. The diagnostic garnet (Grt)-zone assemblage Grt + Bt + Chl +
Ms + Qtz ± Pl (Fig. 6c) occurs in a few localities in the north of the
area (Fig. 5b). Hoy (1974) reported the assemblage Grt + Bt + Chl +
Ms from a number of localities east of Riondel; however, we interpret chlorite in this area as a retrograde phase. Garnet in this and
higher-grade zones is commonly partly replaced by chlorite, biotite, and other phases. The assemblage Grt + Ctd + Chl + Ms + Qtz
occurs in Milford Group schists close to where the Gallagher fault
intersects the Nelson Batholith (Fig. 5c), and east of the Gallagher
fault near where the staurolite isograd diverges from the fault
trace (SW of Kaslo; Fig. 6d). Some garnet crystals from the garnet
and higher-grade zones (up to the sillimanite zone) display welldeveloped textural sector zoning (Moynihan and Pattison 2013).
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781
a)
b)
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Ctd
F2
c)
d)
Ctd
Grt
pseudomorph
e)
f)
Grt
S2
St
g)
h)
Grt
Ky
St
St
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Can. J. Earth Sci. Vol. 50, 2013
Fig. 6 (concluded).
i)
j)
Ky
Sil
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St
Grt
k)
l)
Sil
Sil
Ky
Rut
m)
n)
S2
S2
o)
p)
F2
Sil
Ky
Grt
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Moynihan and Pattison
Staurolite zone
The staurolite (St)-zone assemblage St + Grt + Bt + Ms + Pl + Qtz
is mostly developed in rocks of the Lardeau Group and Milford
Group. Whereas the mineralogy of the rocks is similar, there
are textural contrasts between metapelites from the two units.
Staurolite-zone metapelites of the Lardeau Group (Fig. 6e) are
rusty-weathering silvery schists containing garnet crystals approximately 0.5–1 cm in diameter in a mica-rich, well-foliated
matrix. Staurolite occurs as porphyroblasts up to 1 cm long, or as
small (⬃1 mm) blocky crystals commonly aligned in trains parallel
to the foliation. In contrast, rocks of the Milford Group are what
Fyles (1967) termed “knotted schists”. They are medium–dark
grey, poorly foliated schists with large (up to 2.5 cm) staurolite
crystals and small “pinhead” garnet crystals (approx. 1 mm) in a
fine–medium-grained, commonly graphitic matrix (Figs. 6f, 6g).
Milford Group schists on the west side of Kootenay Lake are
extensively retrogressed. Most staurolite has been pseudomorphed, typically by chlorite and muscovite, but locally by chloritoid. In addition, much of the biotite has been replaced by
secondary chlorite, which commonly contains rutile inclusions.
Pseudomorphs of staurolite are idiomorphic, suggesting little deformation after their retrogression.
Kyanite zone
Textural contrasts between metapelitic schists in the Lardeau
and Milford groups described earlier in the text also pertain to the
kyanite (Ky) zone, which is defined by occurrences of the diagnostic assemblages Ky ± St + Grt + Bt + Ms + Qtz. Kyanite typically
forms blocky to elongate crystals ranging from 1 mm to 1.5 cm
long; however, as much of the kyanite has been partially replaced
by muscovite, it is commonly inconspicuous in the field. Locally,
kyanite has partially replaced staurolite crystals directly (Fig. 6h).
A feature of the kyanite zone is the abundance of non-kyanitebearing (St + Grt) assemblages within its bounds, for example, on
the west side of Kootenay Lake, and in the area north of the Fry
Creek batholith. Quartz veins containing kyanite were observed
at two locations in the Milford Group on the west side of Kootenay
Lake (Figs. 5b, 6f). As the host schists contain staurolite but not
kyanite, these were not used to constrain the position of the kyanite
isograd. The Milford Group schist is extensively retrogressed, however, and it is possible that kyanite was once distributed more
widely in these rocks.
Sillimanite zone
Metapelities in the sillimanite (Sil) zone contain the assemblage
Sil ± Grt ± Ky ± St + Bt + Ms + Qtz + Pl (Figs. 6i–6k). Sillimanite
typically forms aggregates of fine-grained crystals within biotite
crystals, and (or) as idioblastic, elongate matrix columns; in most
samples there is a gradation between these two modes of occurrence. Segregations of quartz and coarse-grained plagioclase are
conspicuous in many sillimanite-zone metapelites.
Sillimanite does not occur in contact with kyanite in rocks that
contain both phases. Locally, sillimanite is restricted to thin,
S2-parallel Sil-rich seams, Qtz–Pl–Sil veins, and their immediate
margins, whereas the body of rock contains scattered idiomorphic kyanite crystals (Fig. 6k). Several occurrences of the assemblage Sil + Ky + St + Bt + Ms + Qtz + Pl (Figs. 5d, 6i, 6j) are interpreted
to reflect metastable persistence of staurolite and kyanite into the
stability field of sillimanite rather than an unusually low-variance
equilibrium assemblage.
Sillimanite + K-feldspar zone
Hoy (1974) reported a single occurrence of coexisting K-feldspar,
sillimanite (altered to white mica), muscovite, and quartz in the
southwestern part of the Riondel peninsula. To the south, Crosby
(1968) reported two other gneiss localities with K-feldspar west of
Crawford Bay — one with sillimanite and biotite, the other with
783
garnet and biotite; muscovite was not reported from these
locations.
On the north side of Highway 3 (DM-05-147) west of Crawford
Bay, outcrops of schist/gneiss contain abundant deformed pegmatite bodies and granitic leucosomes on a variety of scales. The
host rocks contains sillimanite, biotite, and small garnet crystals
(Fig. 6l). Muscovite is present, but much or all of it is retrograde.
Granitic leucosome contains coarse-grained quartz, plagioclase,
K-feldspar, and in places, sillimanite. K-feldspar is most common
in coarse-grained leucosome but is also present in small crystals
scattered through the rock matrix. K-feldspar (Kfs) is variably replaced by myrmekite and skeletal mica–quartz symplectic intergrowths (cf. Ashworth 1972), while sillimanite is partly replaced
by muscovite. Formation and partial replacement of the peak
metamorphic assemblage (Sil + Kfs) is interpreted to have resulted
from prograde progress and subsequent reversal on cooling of the
muscovite dehydration melting reaction:
[1]
Ms + Qtz + Pl = Sil + Kfs + melt
The extent and significance of the sillimanite + K-feldspar zone is
difficult to assess. The paucity of localities with this assemblage
most likely reflects a combination of a small area that was metamorphosed at sufficiently high temperatures, and a low proportion of rocks with prograde muscovite.
Hoy (1980) interpreted the abundant granitic (Kfs + Pl + Qtz ±
Bt ± Ms ± Grt) pegmatite dikes, sills, and small plutons in the
highest-grade parts of the amphibolite-facies belt to have formed
by partial melting of surrounding metasedimentary rocks. Although these bodies were intruded during D2/M2, this is unlikely,
as only a small proportion of rocks in the sillimanite + K-feldspar
zone underwent muscovite dehydration melting. Except for unusually muscovite-rich rocks, this reaction produces small amounts
of melt (⬃5%; Clemens and Vielzeuf 1987; Patino Douce et al. 1990)
and cannot account for the voluminous granitic intrusions. It is
more likely the melt was transported from deeper levels where
other more significant melt-producing reactions (i.e., biotite dehydration) may have operated.
Microstructures
Microstructures allow assessment of the relative timing of
metamorphism and structural development. The variation in D2
strain that is evident in major structures is also manifested in
variable microstructures. In the northern part of the amphibolitefacies belt, and within the staurolite and kyanite zones on the
west side of Kootenay Lake, S2 is commonly a crenulation cleavage
(Figs. 6e, 6n, 6o), with S1 preserved in microlithons. This contrasts
with the parts of the belt in the vicinity of Riondel–Crawford Bay
and south of the West Arm, where D2 strain and metamorphic
grade is highest. Here, S2 schistosity and L2 stretching lineations
are generally the only structures preserved in the rocks.
Informative garnet microstructures are preserved in the
garnet–kyanite zones in the northern part of the area (north of
the Fry Creek batholith). Here, interpretation of microstructures
is aided by the fact that continuity is locally preserved between
the foliation in the matrix and that preserved as inclusion trails in
garnet porphyroblasts. F2 crenulations are included in some, but
not all garnet crystals. Generally, F2 crenulations in garnet are
more open than corresponding folds in the matrix. In some crystals, crenulations are included in the rim (Figs. 6m, 6n), whereas
inclusions in the core are straight. These observations indicate a
temporal overlap between garnet growth and D2 deformation.
Further examples of microstructures in rocks from this area are
illustrated in Moynihan and Pattison (2013). In the southern part
of the amphibolite-facies belt, garnet porphyroblasts are wrapped
by S2, with no continuity of included fabrics with the matrix
Published by NRC Research Press
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Can. J. Earth Sci. Vol. 50, 2013
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Fig. 7. (a) Representative X-ray maps of garnet crystals from each of the metamorphic zones showing relative concentrations of the elements
Fe, Mg, Mn, and Ca. Warmer colours (in the Web version) indicate higher concentrations. Lowest concentrations are indicated by shades of
blue, intermediate concentrations by red–pink, and highest concentrations by yellow. Scale bars are 1 mm in all cases. (b–e) Compositional
profiles for crystals shown in a; the position of the transects are shown in a. The staurolite zone example (DM-06-157) is from a rock
containing the assemblage St + Grt + Bt + Ms + Pl + Qtz + Ilm. Although the sample is located in the kyanite zone, its garnet zoning
characteristics are that of the staurolite zone, as processes related to kyanite formation did not affect this rock. Bs, backscatter image;
Ksp, K-feldspar; M/(M+F), Mg/(Mg + Fe).
fabric. Inclusion trails are typically straight or wavy, and commonly lie at a high angle to matrix S2.
In an area of low D2 strain in the north of the area, staurolite
and kyanite crystals overprint D2 crenulations (Fig. 6o), with little
post-porphyroblast tightening of the fabric. Elsewhere, however,
these minerals are bent around large garnet crystals, aligned in
the plane of S2, host strain shadows, and locally form mineral
lineations. Inclusion trails in staurolite range from straight to
curved. Regional-metamorphic sillimanite invariably defines a
shallow-plunging mineral lineation and lies in the plane of the S2
foliation. These observations indicate that peak Barrovian (M2)
amphibolite-facies metamorphism overlapped with D2 deformation and that D2 generally outlasted peak metamorphism.
Mineral compositions
Mineral compositions were determined using a JEOL JXA-8200
Superprobe at the University of Calgary Laboratory for Electron
Microbeam Analysis. Carbon-coated polished sections were analysed at 15 kV using a 20 nA beam and a range of natural and
synthetic standards. Garnet, plagioclase, and staurolite were analysed using a 5 ␮m beam diameter, with a 10 ␮m spot size used for
micas. Element-distribution maps of garnet grains were generated using WDS stage scans for the elements Fe, Mg, Ca, Mn, and
Y. The mapping was conducted with a focussed beam using step
sizes of 2–10 ␮m, beam currents of 50–200 nA and dwell times of
30–50 ms. Representative compositional data for garnet and other
minerals is included in Appendix A (see supplementary data).1 The
most complete dataset from rocks across the full range of metamorphic grades is that of garnet compositions.
Compositional zoning in garnet
Compositional zoning in garnet was assessed by carrying out
X-ray mapping on 26 crystals from 22 different rock samples,
combined with spot analyses and quantitative compositional
transects. Many garnet crystals are partially resorbed, and display
Mn-enrichment at their rims (Kohn and Spear 2000); unresorbed
examples are illustrated (Fig. 7) and discussed in the following
text.
Garnet crystals in rocks containing Grt + Bt or St + Grt + Bt
assemblages share similar, mostly smoothly varying composition
trends (e.g., Figs. 7a, 7b). The same compositional patterns are seen
in rocks from the Grt zone, the St zone and rocks from the kyanite
zone in which kyanite did not develop. Xsps (sps, spessartine) typically decreases outwards, from ⬃0.15–0.3 towards zero at the
rim. Xalm (alm, almandine) typically falls in the range 0.5–0.8; it
rises outwards from the core and flattens off towards the rim. Xprp
(prp, pyrope) increases gradually outwards from core (<0.05) to
rim (>0.1). Xgrs (grs, grossular) is relatively high in the core (typically 0.15–0.2); it is generally constant or decreases gradually
through the core, and falls more steeply close to the rim.
Mg/(Mg + Fe) increases outward from core to rim, generally with a
steeper gradient towards the rim. Partially resorbed crystals display a sharp increase in Xsps at the rim, and commonly display
further compositional modification around cracks. Moynihan and
Pattison (2013) used thermodynamic forward modeling to replicate garnet zoning and calculate a pressure–temperature (P–T)
path for a sample from the kyanite zone north of the Fry Creek
1Supplementary
batholith. The P–T path has a clockwise form with an initial steep
dP/dT, followed by an interval dominated by an increase in temperature (Fig. 10c).
Compositional zoning in garnet from two samples with the
assemblage Ky + St + Grt + Bt is broadly similar to that described
earlier in the text, but each displays additional zoning features
not seen in Ky-free samples. In DM-07-142, a metapelite containing
Ky + St + Grt + Bt + Ms + Pl + Qtz + Ilm (ilmenite) (Fig. 7c), there is
a low-Xgrs, high-Xalm band close to the rim (zone “B” in Fig. 7c). The
outer margin of this band (the boundary between zones B and C) is
marked by a discontinuity in all end-members; there is a significant rise in Xgrs, a drop in Xalm, a small rise in Xsps, and a small
drop in Xprp. This step is interpreted to mark the location of a
hiatus in garnet growth, with minor garnet dissolution. A possible
explanation for this feature is that garnet stopped growing during
formation of staurolite by a whole-rock equivalent of the KFMASH
reaction:
[2]
Grt + Chl + Ms = St + Bt + H2O
followed by renewed growth when kyanite entered the assemblage according to the reaction:
[3]
St + Ms = Ky + Grt + Bt + H2O
If so, a separate explanation is required for the origin of the
inner step (between zones A and B). This discontinuity, which is
only evident in alm and grs end-members, may reflect the consumption of a minor phase, possibly an epidote group mineral
(e.g., Menard and Spear 1993). A second sample with the assemblage Ky + St + Grt + Bt + Pl + Qtz + Ilm displays only a single
step, similar to that developed at the boundary between zones
B and C in DM-07-142.
Garnet crystals from the sillimanite zone (Fig. 7d) do not show
any discontinuities or sharp gradients. Instead, they are similar
to those seen at lower grades, with relatively flat Xgrs in their
cores, bell-shaped Xsps profiles, gradually increasing Xprp in the
core with steeper increases near the rim, and outward increasing
Xalm that flattens towards the rim. In addition, however, wellpreserved crystals feature a decrease in Xprp and Mg/(Mg + Fe) close
to the rim. This is interpreted to reflect retrograde diffusional
exchange with the rock matrix.
Crystals from the Sil + Kfs zone show little zoning, apart
from a small rimward increase in Xalm and decrease in Xprp
and Mg/(Mg + Fe) (Fig. 7e). The flat zoning and decrease in
Mg/(Mg + Fe) is similarly interpreted to result from diffusional
homogenisation and retrograde exchange with the rock matrix.
Bulk composition of metapelites and phase-diagram
topology
The bulk compositions of 46 metapelite samples, covering all
stratigraphic units and metamorphic zones, were determined by
X-ray fluorescence (XRF) using a Philips PW2440 4 kW automated
spectrometer system at the McGill University geochemical labora-
data are available with the article through the journal Web site at http://nrcresearchpress.com/doi/suppl/10.1139/cjes-2012-0083.
Published by NRC Research Press
Moynihan and Pattison
(a)
Fe
785
Grt zone
St zone
Fe
Fe
Ky zone
Sil zone
Fe
Sil+Ksp zone
Fe
A
.78 .79
A
A
B
A
B
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For personal use only.
B
DM-07-133
DM-06-157
Mn
Mn
B
.76
.76
.79
.77
DM-07-142
DM-07-192
Mn
DM-05-147
Mn
Bs, Xsps
.06
.06
.06
.06
.06
.06
Mg
Mg
Mg
Mg
Mg
.10
.09
.12
.12
.09
.11
Ca
Ca
Ca
Ca
Ca
.06
.06
.06
.06
.06
.06
(b)
0.8
A
B
(c)
A
0.75
B
DM-07-142
DM-07-133
0.75
0.7
0.7
Xalm
0.65
0.65
Xprp
Xsps
Xgrs
M/(M+F)
0.2
0.15
A
B
C
0.2
0.15
0.1
0.1
0.05
0.05
0.1
0.2
0.3
0.4
0.5
0.6
0.7
(d)
A
(e)
0.8
DM-06-157
1
2
3
4
5
6
B
DM-07-192
0.8
0.75
0.7
0.65
0.60
0.2
0.15
Xalm
0.75
Xprp
Xsps
Xgrs
M/(M+F)
0.14
0.7
0.12
0.1
0.1
0.08
0.05
0.06
0.04
0
1
2
3
4
5
mm
6
8
9 7 10
0
1
2
3
mm
4
5
Published by NRC Research Press
786
Can. J. Earth Sci. Vol. 50, 2013
Fig. 8. Modified AFM projection of metapelitic rocks analysed by
XRF. The composition was simplified to the AFM system by
projection from Ab, An, Ap, Ilm, Qtz, Ms, and where applicable, Po.
The rocks have a limited range in Mg/(Mg + Fe), but span a wide
range in A=. Metapelitic rocks of the Slocan and Milford groups
generally plot above the garnet–chlorite tie line, whereas most
samples from other units are less aluminous. Mineral composition
fields from Spear (1993). ab, albite; an, anorthite; Ctd, chloritoid;
Grt, garnet; Ky, kyanite; Sil, sillimanite; St, staurolite; Prl, pyrophyllite.
A’
Slocan Gp.
Milford Gp.
Lardeau Gp.
Hamill Gp./Mohican Fm.
Windermere Sgp.
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For personal use only.
Ky, Sil, and, Prl
+ muscovite
+ quartz
+ H2O
+ apatite
+ ab, an
+ ilmenite
+ pyrrhotite
St
Ctd
Crd
Grt
chlorite
F’
M’
A’ = Al2 O3 - 3K2 O-Na2 O-{CaO-[(10/3)P2O5]}
F’ = FeO - TiO2 - SO3
M’ = MgO
tories, and abundances of S and C established by combustion
infrared spectroscopy in the same laboratory (see supplementary
data, Appendix B1). Compositional variation is illustrated using an
aluminium–iron–magnesium (AFM) diagram (Fig. 8). The rocks
span a considerable range in A= values (range, –19.8 to 51.3; mean,
24.74; standard deviation, 17.08), but most fall within a limited
M=/(M= + F=) range (range, 0.31–0.63; mean, 0.40; standard deviation, 0.06). Metapelites of the Milford Group and most of those in
the Slocan Group are aluminous and therefore plot above the
Grt–Chl tie line, whereas the Lardeau Group and Windermere
Supergroup span a large range in A=.
To illustrate the effects of differences in bulk composition on
predicted phase relations, phase diagrams were constructed in
the chemical system MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3−SiO2−
H2O–TiO2 (Fig. 9). Diagrams were produced using compositions
representing average Milford Group (A= = 43; Mg/(Mg + Fe) = 0.37
and average Lardeau Group (A= = 29; Mg/(Mg + Fe) = 0.35 in
MnNCKFMASHT) metapelites. The phase diagrams (Fig. 9) were
calculated using the Gibbs free energy minimization software
THERIAK/DOMINO (de Capitani and Brown 1987; de Capitani and
Petrakakis 2010) in conjunction with version ds5.5 of the Holland
and Powell (1998) database. Excess H2O was assumed, and a melt
phase was not considered. Solution models are those described in
Moynihan and Pattison (2013).
Whereas there are large differences at lower temperatures,
the two diagrams are very similar above the point at which
biotite enters the assemblage (Fig. 9). The diagram representing
the average Lardeau metapelite predicts biotite above ⬃450 °C,
and a large field with the assemblage Grt–Bt–Chl. In the diagram representing the high-Al metapelite, biotite is only pres-
ent above 550–600 °C, at a higher temperature than both the
chloritoid field and the low-T limit of staurolite. The similarity in
the position of the Ky ± St + Grt + Bt + Ms + Pl + Qtz + Ilm fields in
both diagrams is of particular significance, as these assemblages
help constrain the peak P–T conditions reached in higher-grade
portions of the belt.
P–T estimates from equilibrium assemblage diagrams
Estimates of peak P–T conditions are determined by comparing
observed mineral assemblages with those predicted in phase diagrams constructed for individual samples (Figs. 10a, 10b). These
P–T estimates do not account for potential fractionation of the
effective bulk composition due to garnet growth; however, this
has a limited effect on the position of other major phase boundaries (e.g., Evans 2004; Zuluaga et al. 2005).
DM-07-133 is a metapelitic schist from the garnet zone in the
northern part of the area containing the assemblage Grt + Bt +
Chl + Ms + Pl + Qtz + Ep + Ilm + Tur. As this assemblage (with zo/cz
in place of epidote (Ep); zo, zoisite; cz, clinozoisite) is not predicted to be stable under any P–T conditions in the model
system MnNCKFMASHT, modeling was carried out in the
MnNCKFMASHTO system. An unknown variable is the Fe2+/Fe3+
ratio at the time of peak metamorphism; however, the observed
assemblage is only predicted to be stable in cases where Fe3+ is less
than ⬃6% of total Fe (magnetite appears at higher values of Fe3+), and
there is little variation in P–T position within the range Fe3+/FeTotal =
0%–6% (Fig. 10a). The observed assemblage is predicted in this model
system over a narrow P–T interval at ⬃500 °C, 5.5–6 kbar (1 kbar = 100
MPa) (Fig. 10a).
Assemblages from higher-grade zones are adequately reproduced in MnNCKFMASHT. The main P–T constraints for Ky or
Sil-bearing samples are provided by the low-T limit of Al2SiO5
minerals, the high-T limit of staurolite, the low-P limit of rutile
and the Ky–Sil phase boundary (Fig. 10b). The high-P/low-T stability
limit of plagioclase also provides an additional constraint for
some of the samples. The upper pressure limit implied by stable
ilmenite is ⬃7500–8000 bar, whereas the low-P limit for kyanite in
these samples is typically ⬃6500 bar. P–T conditions for the assemblage Ky + St + Grt + Bt + Ms + Pl + Qtz + Ilm are best constrained, falling in the narrow range of 6500–7500 bar, 640–660 °C.
The presence of Fe3+ would result in a displacement towards
higher pressure of the ilmenite–rutile boundaries, but the positions of other boundaries are predicted to be insensitive to small
variations in oxidation state.
Peak P–T conditions of samples with both kyanite and sillimanite are more difficult to assess. In Fig. 10b, Sil + Ky-bearing assemblages are plotted along the Ky–Sil boundary, unless there is
petrographic evidence that Ky is a relict phase. In these rocks,
there is no textural evidence that sillimanite formed significantly
later than other minerals as it invariably defines the gentlyplunging L2 lineation that is developed throughout the area;
equally, there is no textural evidence that kyanite was out of
equilibrium with other phases. It is likely that there was metastable persistence of kyanite in these rocks, but peak conditions may
not have been far from the equilibrium boundary.
The presence of the metamorphic K-feldspar + sillimanite in
DM-05-147 implies P–T conditions above the muscovite dehydration reaction (Ms + Qtz + Pl = Sil + Kfs + melt). For a pressure range
of 6.5–8 kbar, this suggests temperatures greater than approximately 690–710 °C (Spear et al. 1999).
These P–T estimates help constrain the piezothermic array
(Thompson and England 1984). The suggested array is approximately linear, with a positive slope (Fig. 10c), indicating greater
amounts of post-peak-metamorphic exhumation in higher-grade
parts of the belt.
Published by NRC Research Press
Moynihan and Pattison
787
Fig. 9. Simplified equilibrium assemblage diagram (EAD) for bulk compositions representing (a) average Lardeau Group (medium-Al) and
(b) average Milford Group (high-Al) metapelites. The EADs were constructed in the 10-component system MnO–Na2O–CaO–K2O–FeO–MgO–
Al2O3–SiO2–H2O–TiO2 (MnNCKFMASHT) system, using THERIAK-DOMINO software in conjunction with version db5.3 of the Holland and
Powell (1998) database. A melt phase was not considered. Minor and (or) ubiquitous phases are omitted for clarity. 1 bar = 100 kPa.
a)
9000
+Qtz+H20
MnNCKFMASHT
Grt Bt
Rut
Grt
Chl
Rut
Chlorite - in/out
8000
Biotite - in/out
Chl
Rut
6000
St Grt
Bt Ilm
Grt Bt
Chl Ilm
Garnet - in/out
Sil Grt
Bt Ilm
5000
Chl
Ilm
+M
+K s
fs
Pressure (bar)
7000
4000
Bt Chl
Ilm
3000
Staurolite - in/out
And Grt
Bt Ilm
Average Lardeau Group
A‘ = 29; Mg/(Mg+Fe) = 0.35
Crd Bt
Ilm
2000
9000
Ky Grt
Bt Rut
Grt Chl
Rut Ilm
Grt
Chl
Rut
8000
Aluminosilicate - in/out
MnNCKFMASHT
+Qtz+H20
Ky Grt
Bt Ilm
Grt
Chl
Ilm
7000
St Grt
Bt Ilm
St Grt
Chl
Ilm
Chl
Rut
6000
Cordierite - in/out
Chloritoid - in/out
Sil Grt
Bt Ilm
5000
Ctd
Chl
Ilm
Sil Grt
Bt Ilm
+
-P
l
Pl
4000
3000
Chl
Ilm
Average Milford Group
A‘ = 43; Mg/(Mg+Fe) = 0.37
+M
+K s
fs
b)
Pressure (bar)
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Ky Grt
Bt Ilm
l
-P l
+P
Grt Chl
Ilm
Ilmenite - in/out
And Bt
Ilm
Crd And
Bt Ilm
Rutile - in/out
2000
400
500
600
700
Temperature (°C)
Orientation of isogradic surfaces and thickness of
metamorphic zones
Vertical cross sections across the amphibolite-facies belt illustrate the orientation of isograds and their relationship to structures (Fig. 11). In the central and northern part of the belt (Fig. 11a),
isograds are domed over Kootenay Lake. The core of the isogradic
antiform is marked by rocks of the Sil + Kfs zone in the Riondel–
Crawford Bay areas. To the north of this core, the axis of this
isogradic antiform plunges gently to the north, whereas southwest of this culmination it plunges gently southwest. Isograds
Published by NRC Research Press
788
Can. J. Earth Sci. Vol. 50, 2013
Fig. 10. (a) Equilibrium assemblage diagram for garnet zone sample DM-07-133 in the system MnNCKFMASHTO. The field containing the
observed assemblage is shaded for different values of 100Fe3+/FeTotal. The other assemblage fields shown are for the case where Fe3+ represents
2% of total Fe. (b) P–T diagram showing the boundaries of peak metamorphic assemblages in aluminosilicate-bearing metapelitic rocks. The
boundaries are taken from EADs constructed for each sample in the system MnNCKFMASHT. The locations of samples that are plotted are
shown in Fig. 5. (c) Summary P–T diagram showing the different pressure ranges of Barrovian M2 metamorphism and contact metamorphism
discussed in the text. The P–T path from Moynihan and Pattison (2013) is also shown, as is an approximate piezothermic array. (d) Schematic
illustration of the contrasts in structural and metamorphic geology across the Gallagher fault.
b)
+Qtz+H2O
Observed assemblage Grt Chl Bt Ms Pl Ep Ilm
1% Fe 3+
DM-07-99
(Ky-St-GrtBt-Ms-Pl-Ilm)
DM-05-55
(Ky-Grt-BtMs-Pl-Ilm)
6% Fe 3+
7.5
3+
5% Fe
St Grt
Bt Ms
Pl Ilm
Grt Chl Ms
Ilm Ep
Pl
Grt Chl Bt
Ms Pl Ilm
DM-05-44
(Ky-St-GrtBt-Ms-Pl-Ilm)
6.5
Grt Bt
And Ms
Pl Ilm
DM-07-133
DM-05-53
(Sil-St-Grt-Bt-Ms-Pl-Ilm)
6
550
625
600
650
d)
9
675
Contact metamorphism St-And (3.5-4 kbar) @ 165 Ma
8
Jurassic-Early Cretaceous
Bt cooling ages
Peak M2 Barrovian regional metamorphism
7
othe
piez
rox.
app
6
rmic
700
725
750
Temperature (°C)
t
500
fa
ul
450
DM-04-17
(St-Grt-Bt
Ms-Pl-Ilm)
7
Grt
Chl
Ms
Ilm
Pl
DM-05-04
DM-05-47
DM-07-166
DM-07-178
(Sil-Ky-GrtBt-Ms-Pl-Ilm)
Gentle-Open
F2 folds
F2
er
5
3
DM-07-142
(Ky-St-GrtBt-Ms-Pl-Ilm)
8
2% Fe 3+
4
c)
8.5
Grt Chl Bt
Ms Ilm
MnNCKFMASHT
Tight F2 folds
gh
Pressure (kbar)
Grt Chl Ms
Ilm Ep
Pressure (kbar)
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For personal use only.
7
6
+Qtz+H2O
MnNCKFMASHTO
lla
8
Ga
a)
D3 shear
bands
F2
y
arra
Nelson
batholith
5
mid-K
plutons
4
Nelson batholith aureole
Crawford
Bay
(65 Ma)
Crd & And
3
Regional metamorphism
at 6.5-7 kbar (23-25 km)
Crawford Bay stock
Mineral Zones:
Grt, St, Ky, Sil, Sil + Kfs
2
400
Temperature (°C)
600
St, And
Palaeocene-Eocene mica cooling ages
700
crosscut S2 at a high angle in the centre and on the east flank of
the metamorphic antiform (Fig. 11a).
Given the constraint that isograds dip to the east on the east
flank of the isogradic dome, a minimum value for the thickness of
the St + Ky zones around the centre of the isogradic culmination
can be estimated by measuring the elevation difference between
the St and Sil isograds along the line of section G–G=; this value
(1.25 km) would be the thickness if the isograds were subhorizontal. A maximum value for the thickness of the two zones is given
by the distance between the St and Sil isograds measured parallel
to the land surface. This value is ⬃3.4 km.
Tighter constraints rely on various assumptions. One endmember possibility is that isotherms were subhorizontal during meta-
morphism, peak metamorphism was synchronous across the
area, and the domal form of isogradic surfaces is due to post-peakmetamorphic differential exhumation. If it is assumed that there
has been negligible post-metamorphic structural thickening/thinning of mineral zones, the difference in pressure between the
intersections of the St-in and Sil-in curves with the piezothermic
array (Fig. 10c) provides an estimate of the thickness of the combined St and Ky zones (~0.6 km = ~2.25 km). This value is in the
centre of the range suggested above and is used in the cross section (Fig. 11a). The other end-member possibility — that all of the
relief on isogradic surfaces was caused by domal isothermic surfaces at the time of peak metamorphism — is ruled out because
the piezothermic array has a positive slope. It is possible, howPublished by NRC Research Press
Moynihan and Pattison
789
Fig. 11. Cross sections showing the orientation of isograds and their relationships to structures. All cross sections are vertical, with equal
vertical and horizontal scales. The metamorphic zones are colour-coded according to the scheme used in Fig. 5. (a) Cross section G–G= (Fig. 5c)
across the crest of the isogradic antiform, showing truncation of the western flank of the isogradic antiform by the Gallagher fault. Gently
west-dipping isobars in the Nelson batholith are also plotted (Pattison and Vogl 2005). The Riondel Nappe (Hoy 1976) and adjacent structures
are projected onto the line of section. Isograds cut structures at moderate to high angles. (b) Metamorphic cross section along the line E–E=
(Figs. 2, 3, 5d). Isograds define an open synform. The crest of the isogradic antiform to the west is cut by the Midge Creek fault. (c) Metamorphic
section along line F–F= (Figs. 2, 3, 5d). A large low-grade region sits in the core of an isogradic synform. A sillimanite + K-feldspar zone is not
illustrated in b and c, as it has not been mapped in this area, but presumably exists at depth.
a)
Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 07/15/13
For personal use only.
G
r)
3.5 kba
r)
3.5 kba
And (~
r)
a
b
3.5-4 k
nd (<~
Crd+A
St+And
lt
fau
r
he
ag
l
l
Ga
G’
5 km
4 km
3 km
2 km
1 km
0 km
Mt. Hooker
(~
th
batholi
Nelson
Purcell Trench
b)
E
Midge Creek
fault
fault
Purcell Trench
fault
E’
2500 m
2000 m
1500 m
1000 m
500 m
c)
F
Midge Creek
fault
Purcell Trench
fault
F’
2500 m
2000 m
1500 m
1000 m
500 m
ever, that isothermic surfaces were domal to some degree at the
time of metamorphic quenching, and postmetamorphic processes enhanced the domal form of isogradic surfaces.
South of the West Arm of Kootenay Lake, a gently southplunging isogradic synform separates the southern part of the
N–NNE trending belt from SSE-trending part of the amphibolitefacies belt. (Figs. 11b, 11c). As is the case further north, isograds
transect fold axial planes and associated foliations. It is likely that
isogradic surfaces in the SSE-trending amphibolite-facies belt also
have an antiformal geometry; there is no evidence for an eastward
increase in metamorphic grade in the hanging wall of the PTF, as
would be expected if the fault cut westward-dipping isograds.
In much of the belt the garnet, staurolite, kyanite, and sillimanite zones have similar thicknesses to one another. Using the phase
diagrams (Figs. 9, 10c) and piezothermic array (Fig. 10c) as a guide,
broadly similar thicknesses for the staurolite, kyanite and possibly the sillimanite zones would be expected, with a significantly
thicker garnet zone. North of the Fry Creek batholith, on the
unfaulted eastern flank of the amphibolite-facies belt, the garnet
zone is thicker than other metamorphic zones, but elsewhere is
has a comparable thickness. The anomalously thin nature of the
garnet zone in much of the area may reflect bulk compositional
control of the position of the garnet isograd.
Metamorphic zones that outline the isogradic synform region
south of the Midge Creek stock (Fig. 5d) are poorly defined, but
appear thinner than those in the N–NNW-trending belt. It is likely
that the apparently condensed nature of metamorphic zones rePublished by NRC Research Press
790
flects, at least in part, a paucity of observations from appropriate
metapelitic lithologies.
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Contact metamorphism
Nelson batholith aureole
A contact aureole is developed around unfaulted margins of the
Nelson batholith in metapelitic rocks of the Slocan Group and
correlative units (Pattison and Vogl 2005).
The batholith was emplaced at a pressure of 3–4 kbar, equivalent to a depth of 11–14.5 km (assuming an average crustal density
of 2800 kg/m3) and overprints D1 (Early-Middle Jurassic) structures. Based on the distribution of pressure-sensitive mineral assemblages in various parts of the aureole, Pattison and Vogl (2005)
inferred post-emplacement tilting of the batholith ⬃10° to the
west.
A wide (approx. 1.75 km) contact aureole, characterised by St +
And ± Grt assemblages at medium grade is developed in the hanging wall of the Gallagher fault in the region approximately north
of Woodbury Creek (Fig. 5c). St + And-bearing assemblage, which
are typical of the eastern parts of the Nelson aureole (Pattison and
Vogl 2005) indicate emplacement at 3.5–4 kbar, equivalent to a
depth of 12.5–14.5 km (Fig. 9). Further evidence that primary intrusive relationships are preserved in this area is provided by a
localised zone of folding related to emplacement of the batholith
(Fyles 1967).
To the south, along the eastern side of the deformed “tail” of the
Nelson batholith, a contact aureole is narrow or absent. At Coffee
Creek (Fig. 5c), the eastern margin of the batholith is faultbounded, and is in direct contact with rocks of the regional (M2)
kyanite zone. South of here, there is a narrow panel of rock separating the tail of the batholith from Barrovian amphibolite-facies
rocks in the footwalls of the Gallagher and Midge Creek faults.
The regional metamorphic grade in this panel is low, and though
a narrow contact aureole is locally present, it appears to be absent
elsewhere (Vogl 1992; this work). Contact metamorphic assemblages were studied at two locations within this panel (Fig. 5d) and
in each location contact-metamorphic sillimanite is overprinted
by tight F2 crenulations (Fig. 6p). The limited evidence for contact
metamorphism along the eastern part of the Nelson batholith tail
is interpreted to reflect deformation along this margin after (D2),
and possibly also during (Vogl 1992), its emplacement.
Mid-Cretaceous contact aureoles
Mid-Cretaceous plutons crosscut Barrovian isograds, and their
andalusite-bearing contact aureoles record exhumation of rocks
in the area to upper crustal levels after amphibolite-facies (M2)
regional metamorphism. The Fry Creek batholith intruded across
the northern part of the east flank of the domal metamorphic
culmination (Fig. 5a). Reesor (1973) reported assemblages containing combinations of andalusite, cordierite, garnet, staurolite and
sillimanite from the contact aureole of the batholith. Whereas a
contact metamorphic aureole is well-developed in its eastern part,
an aureole was not identified in the western part, where it intruded rocks of higher regional metamorphic grade (Reesor 1973).
An andalusite-bearing contact aureole surrounds the 114 +1/–3
Ma Midge Creek stock and occupies a broad region to its south,
between it and an extension of the Drewry Point intrusion
(Fig. 5d). Assemblages in this region reported by Leclair (1988)
include And + Bt ± Grt, And + St + Grt + Bt and Sil + And + Bt.
And-bearing contact metamorphic assemblages are superimposed
on kyanite-bearing regional assemblages on the northern margin of
the Midge Creek stock.
And + Bt ± Grt and And + St + Grt assemblages are developed
around a small intrusion near Wurtenberg Lake (Fig. 5d), whereas
contact metamorphism around the Drewry Point intrusion produced And + St + Grt, Sil + Grt + Bt, and Crd + Bt-bearing
assemblages (Leclair 1988). The reported occurrence of St +
Can. J. Earth Sci. Vol. 50, 2013
And- and Crd-bearing assemblages from the contact aureoles of
mid-Cretaceous intrusions suggests variation in the depth of emplacement within and between intrusions (c.f. Pattison and Vogl
2005), and warrants further investigation.
Crawford Bay stock aureole
The Crawford Bay stock is a small, equigranular to porphyritic
biotite granodiorite (Crosby 1968) that is exposed on either side of
Crawford Bay, on the east side of Kootenay Lake. The stock was
intruded during the latest Cretaceous-Palaeocene (Moynihan
2012) and crosscuts D2 folds, fabrics and regional metamorphic
isograds (Fig. 2.5c); it has undergone minor ductile deformation
but is not foliated or lineated.
On the eastern side of the stock, a contact aureole is developed
in low-grade rocks of the Windermere Supergroup. Metapelitic
rocks contain the assemblages Crd + And + Bt + Ms + Qtz ± Grt and
And + Sil + Crd + Bt + Ms + Qtz (Crosby 1968; Fig. 5d), indicating
emplacement at low pressure (approximately 6–10 km depth;
Moynihan 2012). The western part of the stock intruded rocks of
the regional staurolite zone (Fig. 5d). Andalusite is present in
some metapelitic rocks near this contact, where it forms large,
generally poikiloblastic crystals that are typically elongate in
the plane of the foliation. Folding of partly sericitised contactmetamorphic andalusite crystals around F4 crenulations demonstrates a latest Cretaceous-Palaeocene or younger age for D4.
Discussion
Formation and geometry of Early Cretaceous isogradic
surfaces
Barrovian amphibolite-facies metamorphism in the central
Kootenay Arc took place in the Early Cretaceous during D2. The
metamorphism is interpreted as a consequence of burial and radiogenic heating. D2 structures are compatible with crustal thickening and the P–T path calculated by Moynihan and Pattison
(2013) suggests burial of ⬃2 kbar (~7 km) during garnet growth in
the kyanite zone.
The elongate domal geometry that characterises isogradic surfaces in the N–NNE-trending part of the Barrovian belt is interpreted to have formed during D2/M2, prior to the intrusion of
mid-Cretaceous intrusions that crosscut dipping isogradic surfaces. Isograds transect F2 axial planes at a high angle, suggesting
they were established after F2 folds formed and much of the related shortening had taken place; however, deformation of peak
metamorphic minerals indicates that, in general, D2 outlasted
peak metamorphism, and it is likely that there was some further
tightening of these folds after isograd formation. As S2 is at a
moderate to high angle to isograds, late (post peak metamorphic)
D2 shortening may have resulted in some thickening of mineral
zones and modification of the isogradic geometry.
The highest-grade rocks in the central Kootenay Arc (Sil + Kfs
zone) are located close to the crest of the structural culmination,
where there is a change in the plunge direction of F2 folds and L2
lineations (Fyles 1967). To the north of this culmination, F2/L2
plunge gently north, whereas to its south, L2/F2 plunge gently to
the south-southwest. The angle of plunge of minor folds to the
north of the culmination crest is approximately 5°–20° (Fyles
1964; Hoy 1980). Using the map and cross sections as a guide, a
comparison can be made between these plunge values and the
plunge of isogradic surfaces. The combined thickness of the staurolite and kyanite zones is ⬃2.25 km (range of 1.25–3.4 km). Combining this thickness estimate with the distance between the
staurolite and sillimanite isograds along the axis of the isogradic
antiform (Fig. 5) yields an average plunge of 4°–10° (7° using the
preferred thickness value). As the isogradic antiform has a subhorizontal axis in its crestal region (Sil + Kfs zone), this is an
overestimate of the average plunge from the crest of the isogradic
culmination to the northern tip of the staurolite zone. The estiPublished by NRC Research Press
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Moynihan and Pattison
mated plunge of isogradic surfaces is at the low end of the range of
minor fold plunges, most of which suggest greater amounts of
structural relief. This difference could be explained by a discrepancy between the orientation of minor folds and large-scale
plunge, or the possibility that the isograds were “frozen in” to the
rock after growth of the culmination had commenced.
The piezothermic array (Fig. 11b) suggests that higher-grade
rocks have been exhumed from deeper levels. Assuming the piezothermic array is representative of the entire belt, this implies
that there was Early Cretaceous differential exhumation, concentrated around the crest of the culmination. Rather than simply
recording the presence of a thermal dome, the isogradic geometry
was, at least in part, acquired as a result of progressive growth of
the structural culmination during the later stages of D2. The westward tilt of the Nelson batholith may have been acquired during
formation of this metamorphic and structural culmination. S3
shear bands, which dip NW and formed by reactivation of S2, are
developed in the NW quadrant of the culmination. Their age is
poorly constrained but it is possible that they are a manifestation
of Early Cretaceous, post-peak-metamorphic doming.
Large relief on isogradic surfaces has been reported in other
parts of the southeastern Cordillera that record Early Cretaceous
Barrovian metamorphism (Ghent et al. 1980; Digel et al. 1998;
Ghent and Simony 2005). In these areas, curviplanar isogradic
surfaces were interpreted to reflect the thermal-baric structure at
peak metamorphic conditions rather than being a consequence of
post-metamorphic differential exhumation.
Isograds that define the SSE-trending part of the amphibolitefacies belt are poorly constrained, but their geometry also suggests that relief on isogradic surfaces developed during the Early
Cretaceous. The Midge Creek stock crosscuts the hinge zone of the
south-plunging isogradic synform and west-dipping isogradic surfaces of the SSE-trending belt are cut by the Heather Creek pluton
and Drewry Creek intrusion (Fig. 5d). There is no indication from
the orientation of structures documented by Leclair (1988) that
the attitudes of Early Cretaceous structures or isogradic surfaces
were significantly modified by post-metamorphic doming or by
motion on the PTF (Fig. 11).
Following Barrovian metamorphism, kyanite zone rocks in the
vicinity of the Midge Creek stock were exhumed to <4.5 kbar
(<14.5 km depth; Fig. 10c) by ca. 114 Ma, as evidenced by the presence of andalusite in its contact aureole (Leclair 1988; Leclair et al.
1993). Similarly, andalusite developed in the contact aureoles of
the Fry Creek Stock (Reesor 1973), Drewry Point intrusion and
other mid-Cretaceous bodies that crosscut Early Cretaceous
Barrovian isograds.
Relationship to Late Cretaceous metamorphism and
deformation in the Priest River complex
The SSE-trending part of the amphibolite-facies belt extends
beyond the area covered in Fig. 4, across the USA–Canada border
into the Priest River complex (Doughty et al. 1997; Doughty and
Price 1999; Fig. 1). Peak metamorphism and deformation in the
Priest River complex took place in the Late Cretaceous – Eocene
(Doughty and Price 1999; Doughty and Chamberlain 2004). Late
Cretaceous (ca. 82 Ma) metamorphism and deformation also affected the SSE-trending amphibolite-facies belt in the area west of
Creston (Fig. 4), around the southern end of Kootenay Lake (Brown
et al. 1999). Late Cretaceous metamorphism and deformation did
not affect the northern part of this belt, however, as the 95 Ma
Steeple Mountain pluton (Brown et al. 1999; see Fig. 5) is locally
foliated adjacent to the PTF but not otherwise penetratively deformed (Leclair 1988). The Drewry Creek intrusion and Heather
Creek pluton, intrusions of presumed (but unconfirmed) midCretaceous age (Leclair 1988) are undeformed and crosscut regional isograds (Fig. 5a). While forming a continuous region with
an apparently simple isograd pattern, this belt evidently has a
polymetamorphic origin.
791
Palaeogene normal faults and their role in exhumation of
the belt
The amphibolite facies belt that transects the central Kootenay
Arc is partially bounded by two normal faults or fault zones —
the PTF and the GFZ. The PTF has long been recognised as a significant Eocene structure, but the significance of the GFZ, which
forms the western margin of the Barrovian belt, has hitherto not
been fully appreciated. Palaeocene–Eocene normal faulting augmented the isogradic structure that was established during the
Early Cretaceous, and juxtaposed regions with contrasting structural and metamorphic histories along fault-bounded margins of
the amphibolite-facies belt.
The GFZ
The western flank of the Barrovian metamorphic antiform is
truncated by the GFZ. From the West Arm of Kootenay Lake northwards to near Kaslo, the Gallagher fault separates M1 chlorite/
biotite zone hanging wall rocks from M2 amphibolite facies
(staurolite–kyanite zone) rocks in the footwall. Low-grade hanging wall rocks record a simple Middle Jurassic metamorphic and
deformation history, whereas amphibolite-facies footwall rocks
underwent further metamorphism and deformation during the
Early Cretaceous (Fig. 11d). The Gallagher fault marks the western
boundary of penetrative D2 deformation, the western boundary of
amphibolite-facies metamorphism that was concurrent with D2,
and the western boundary of young 40Ar/39Ar mica cooling ages
(Figs. 11d, 12). It also marks the western boundary of D3 structures.
Palaeocene–Eocene K–Ar cooling ages in the immediate footwall
(Mathews 1983; Archibald et al. 1984; Fig. 12) suggest that it, like
the PTF, is a Palaeogene extensional fault. To the east, K–Ar biotite
cooling ages increase gradually towards the centre of the Purcell
anticlinorium (Archibald et al. 1984).
South of the West Arm of Kootenay Lake, geological relationships are similar to those further north: a west-dipping fault (in
this case the Midge Creek fault) separates Barrovian M2 kyanite or
sillimanite-zone rocks from low-grade phyllites of the Milford
Group, which lie east of a narrow, locally developed contact aureole adjacent to the Nelson batholith. The Midge Creek fault has
previously been interpreted as a thrust or a strike-slip fault
(Leclair 1988; Vogl 1992), whereas the relationships described earlier in the text suggest normal faulting. One possibility is that a
thrust-sense/strike-slip fault was reactivated during Palaeocene–
Eocene normal faulting. Alternatively, the Gallagher fault may
extend south of the West Arm and cut the older Midge Creek fault
at a low angle. Although the Gallagher fault marks the western
limit of D2 penetrative strain north of where it forms the eastern
boundary of the Nelson batholith, deformation of contactmetamorphic sillimanite suggests some D2 strain affected rocks
in the hanging wall of the Midge Creek fault.
The Gallagher fault dies out around Kaslo, and north of here the
western boundary of amphibolite-facies rocks is marked by the
Lakeshore fault. The garnet isograd leaves the Gallagher fault
around its termination and crosses a relay zone between these
two structures, which are interpreted as individual strands of a
west-dipping normal fault zone. Interpretation of the Lakeshore
fault as a Palaeogene normal fault conflicts with Klepacki's (1985)
suggestion that the Lakeshore fault is part of a long, strike-parallel
pre-Middle-Jurassic fault – the NW-trending Schroeder fault. We
favour Fyles' (1967) interpretation (his Fig. 2) that the Lakeshore
fault continues north, approximately parallel to Kootenay Lake.
Given their relative ages, it is likely that the Lakeshore fault cuts
the Schroeder fault, rather than being part of it. Around the
northernmost part of Kootenay Lake and Duncan Lake, the western boundary of the amphibolite-facies belt does not coincide
with known faults. Normal faults were mapped in the area north
of the Fry Creek batholith by Fyles (1964) and a normal-sense shear
zone overprints the NW margin of the mid-Cretaceous Fry Creek
batholith. The zone of normal faulting may step eastwards in this
Published by NRC Research Press
792
Can. J. Earth Sci. Vol. 50, 2013
Fig. 12. Map showing K–Ar and 40Ar/39Ar dates from the area. Data
from Archibald et al. (1984) and references therein.
50 30’
120
117 00’
50 15’
45,(63)
(56)
50 00’
53
[161]
50,(51)
Kaslo
120
146
135
51
130
(59)
48,(51)
{53}
{131}
50,(55)
45
54
52,(60)
{67}
63 {75} {67}
Ainsworth
53, [126]
56,[160]
A
West
54
54
rm
60
[104]
62
48
[83]
49 30’
PTF
47,(50)
51
51,(52)
[161]
111
0
ult
k fa
ree
xt C
Ne
[157]
49
15’ 107
142
81
103
79
50
83
92
43,(51)
89
82
90
51,(54)
45,(50)
41,(49) 72
54 53,(50)
51
52
44,(50)
71
49
39
15
Thanks to Travis Murphy, Hannah Mills, Gennyne McCune, and
Chris Coueslan for assistance in the field, to Rob Marr for electron
microprobe support, and to Doug Tinkham for providing solution
models used in thermodynamic modeling. The paper benefitted
from criticisms of an early draft by Philip Simony and Sharon Carr
and from recommendations provided by associate editor Marc
St-Onge and anonymous journal reviewers. Funding was provided
by the Natural Sciences and Engineering Research Council of Canada (NSERC) discovery grants to D.R.M. Pattison and P. Simony and
by the Geological Survey of Canada through the Targeted Geoscience Initiative (TGI-3).
References
49
[131]
[99]
The PTF
The east side of the SSE-trending fork of the amphibolite-facies
belt is bounded by the Eocene Purcell Trench normal fault. This
fault forms the eastern boundary of the Priest River metamorphic
complex (Doughty and Price 1999) to the south of the central
Kootenay Arc (Fig. 1) and coincides with the location of an antecedent east-side-down normal fault that was active during the
Neoproterozoic (Leclair 1988). The fault extends northwards
across the international border and occupies the southern part of
Kootenay Lake before dying out around the latitude of the West
Arm. Along the southern, NW-trending portion of Kootenay Lake,
the fault separates sillimanite zone rocks with Palaeocene–
Eocene 40Ar/39Ar mica cooling ages in the footwall from biotite zone
rocks with mostly mid- to Late Cretaceous cooling ages (Fig. 12).
North of its termination, the sillimanite zone extends across the
Purcell Trench (occupied by Kootenay Lake), and there is little, if
any, difference in 40Ar/39Ar cooling ages (Fig. 12). Contrasts in
cooling ages demonstrate differential exhumation across the
fault during the Eocene; however, as discussed earlier in the text,
isogradic relief in the footwall of the fault existed prior to its
formation. As is the case with the GFZ, the PTF modified an isogradic geometry that was established during Early Cretaceous
metamorphism and deformation.
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GFZ
Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by UNIV CALGARY on 07/15/13
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biotite
(muscovite)
[hornblende]
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Restoration of apparent stratigraphic cut-offs in the cross section across the Kootenay Arc by Fyles in Brown et al. (1981) suggests offset of ⬃7.5 km (throw of ⬃5 km) on the Gallagher fault in
the Ainsworth–Kalso area (Fig. 3c). This estimate is similar to that
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zones against low-grade phyllites (Figs. 6a, 11a). The Josephine and
Lakeshore faults also separate lithological units for considerable
distances along strike, suggesting they are significant structures.
The faults are approximately parallel to lithological boundaries,
however, and some of the structural complexity in this zone may
arise from earlier compressional structures, as is the case along
strike (Klepacki 1985; Leclair 1988).
30
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