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Chemical Geology 201 (2003) 55–89

Late Paleozoic and Triassic plume-derived magmas in the Canadian

Cordillera played a key role in continental crust growth

Henriette Lapierrea,*, Delphine Boschb, Marc Tardyc, Lambertus C. Struikd

aLaboratoire Geodynamique des Chaınes Alpines, UMR-CNRS 5025, Universite J. Fourier Grenoble 1, Maison des Geosciences,

B.P. 53, 38041 Grenoble cedex, FrancebLaboratoire de Tectonophysique, UMR-CNRS 5568, Institut des Sciences de la Terre, de l’Eau et de l’Espace de Montpellier,

Universite Montpellier 2, Place E. Bataillon, 34095 Montpellier cedex 05, FrancecLaboratoire Geodynamique des Chaınes Alpines, UMR-CNRS 5025, Universite de Savoie, 73376 Le Bourget du Lac cedex, France

dGeological Survey of Canada, 101-605 Robson street, Vancouver, BC, Canada V6B 5J3

Accepted 26 June 2003

Abstract

Two major Late Paleozoic–Triassic oceanic terranes are exposed in the North America Cordillera. Slide Mountain Terrane is

made up of dolerites, pillow basalts associated with cumulate gabbros and peridotites ranging in age from Carboniferous to

Permian. Cache Creek Terrane consists of tectonic slices of Paleozoic platform carbonates, undated cumulate gabbros intruded

by dolerites, foliated ultramafic rocks and mafic volcanic rocks interbedded within Upper Triassic siliceous and volcaniclastic

sediments. Among the Permian (Slide Mountain) and Upper Triassic (Cache Creek) volcanic rocks, four types have been

distinguished. Type 1 is geochemically similar to N-MORB. Type 2 is mildly to highly enriched in LREE, Ti, Zr, Hf, Nb, Ta and

Th, and differs from Type 1 by lower eNd and higher Pb isotopic ratios. It displays alkaline affinities. Types 3 and 4 occur only

in the Cache Creek Terrane. Type 3 has flat REE patterns similar to oceanic plateau basalts and isotope compositions

intermediate between Types 1 and 2. Type 4 is distinguished from Type 3 by convex REE patterns and higher Pb isotopic ratios.

Cumulate peridotites and gabbros are LREE-depleted and their high eNd suggest that they derived from the melting of a

depleted MORB-type mantle source. The foliated ultramafic rocks are either serpentinized harzburgites (Trembleur ultramafics)

or dunites, harburgites and minor lherzolites (Murray Ridge), which are intruded by undeformed pyroxenite veins. The

ultramafic rocks have very low contents of rare earth elements (REE) and incompatible elements, and U-shaped REE patterns.

The pyroxene-bearing peridotites are distinguished from the Trembleur harzburgites by an absence of Nb negative anomalies

and lower (87Sr/86Sr)i ratios. Slide Mountain and Cache Creek Terranes formed in different tectonic settings. Slide Mountain

basin, fringing the North American margin, was probably floored by oceanic crust locally thickened by oceanic island magmas.

Indeed, in the Slide Mountain Terrane, N-MORB type basalts predominate and are associated with minor alkaline volcanic

rocks within the same thrust sheet. Correlations between incompatible elements and values of the eNd of the Slide Mountain

rocks suggest that the N-MORB-type basalts and alkaline rocks are genetically linked. N-MORB type basalts and alkaline

volcanic rocks could derive from the mixing of depleted MORB-type and enriched Oceanic island Basalt-type mantle sources.

The Cache Creek Terrane may represent remnants of a Late Triassic oceanic plateau because plume-related volcanic rocks

predominate while N-MORB type rocks are represented solely by cumulate gabbros and sheeted dykes. The Murray Ridge

peridotites could represent the roots of the oceanic plateau. We conclude that the Slide Mountain and Cache Creek Terranes

0009-2541/$ - see front matter D 2003 Elsevier B.V. All rights reserved.

doi:10.1016/S0009-2541(03)00224-9

* Corresponding author.

E-mail address: lapierre@ujf-grenoble.fr (H. Lapierre).

H. Lapierre et al. / Chemical Geology 201 (2003) 55–8956

contributed to the growth of the North American continent as when they were accreted to the craton during the Early Triassic

and Jurassic times.

D 2003 Elsevier B.V. All rights reserved.

Keywords: Canadian Cordillera; Oceanic crust; Oceanic plateaus; Geochemistry; Plate tectonics

1. Introduction

New continental crust is commonly thought to

form at island arcs, which result from the subduction

of oceanic plates into the mantle (Taylor and Mc

Fig. 1. Simplified map of the Paleozoic to Early Mesozoic terranes of th

oceanic and arc terranes. (Modified after Miller et al., 1992.)

Lennan, 1985; Ben-Abraham et al., 1981). An alter-

native or additional process is the collision and

accretion of oceanic plateaus (and possibly oceanic

islands), which form as the result of periodic large

outpouring of mafic magmas onto the ocean floor

e North American Cordillera showing the correlations between the

Fig. 2. Simplified map of the accreted terranes of the Canadian Cordillera showing the location and extension of the Slide Mountain and Cache

Creek Terranes.

H. Lapierre et al. / Chemical Geology 201 (2003) 55–89 57

H. Lapierre et al. / Chemical Geology 201 (2003) 55–8958

(Abouchami et al., 1990; Stein and Hofmann, 1994;

Saunders et al., 1996). Most of the Cordilleran oro-

gens in North America and northern South America

consist in part of oceanic terranes that accreted at

convergent plate boundaries or along transform faults.

The Cordilleran crust was probably formed by the

accretion of oceanic terranes that began to evolve into

immature continental crust before their accretion to

North or South America or just after their accretion to

the continent. Similar mechanisms have been pro-

posed for the Archean continents.

Oceanic plateaus are produced by the partial melt-

ing in the heads of mantle plumes that originate deep

in the mantle, probably at the core-mantle boundary.

This melting produces large amounts of mafic and

ultramafic magmas. Oceanic plateaus cover large

areas (around 1.5� 106 km2) and their thickness

approaches that of continental crust. According to

Schubert and Sandwell (1989), the amount of oceanic

crust produced by all the oceanic plateaus in the last

100 Ma represents 4.9% of the present continental

crust volume, and corresponds to an accretion rate of

3.7 km3 year1. At such rates, a volume equivalent to

the entire continental crust could be generated within

less than 2 Ga by plateau accretion only (Puchtel et

al., 1998). Moreover, the lithosphere of oceanic

Fig. 3. Schematic section at lithospheric

plateaus is considered to be more buoyant that normal

oceanic crust because it is hotter and composed of less

dense material. Therefore, oceanic plateaus have po-

tential to resist to subduction and to be obducted onto

or accreted to continents, thus contributing to the

growth of continental crust.

However, other processes that contribute to the

growth of continents include magma addition by crust-

al underplating involving the intrusion of sills and

plutons and overplating of volcanic rocks. Magma

additions occur in settings such as arcs, rifted conti-

nental margins and beneath flood basalt provinces. In

the case of underplating of continental crust, are the

plume heads recycled back into the asthenosphere or do

they became part of the mantle lithosphere? Stein and

Goldstein (1996) have suggested that the development

of the Arabian–Nubian shield was linked to the accre-

tion of a plume head basis some 900–700 Ma ago.

Based on the similarities of the isotopic compositions

of 900–700 Ma plume related basalts from the Arabi-

an–Nubian shield with those of the 200 Ma Israel rift

basalts, Stein and Hofmann (1992) suggested that the

accreted fossil plume head was the source of basalts in

the Arabian–Nubian shield for the last 900 Ma.

The main episodes of juvenile continental growth

took place during the Archean (2.7, 2.5, 2.1 and 1.9

scale of the Canadian Cordillera.

H. Lapierre et al. / Chemical Geology 201 (2003) 55–89 59

Ga), the Late Paleozoic (320–250 Ma) and the

Cretaceous (123–80 Ma), and were possibly caused

by superplume events. Recently, Condie (1997) pro-

posed that the growth peaks are linked to super-

plumes, which in turn are related to major events in

the mantle such as ‘‘slab avalanches’’ along the 600-

km seismic discontinuity (Brunet and Machetel,

1998). According to this hypothesis, slabs sink into

Fig. 4. Simplified map of the Cache Creek Terrane near Fort St. James (a)

Fault system (b) and near Vanderhoof (c) showing the location of the dif

work.

the lower mantle and initiate the plume production

when they arrive at the ‘‘D’’ layer.

It is commonly accepted that most Archean green-

stones belts are made up of accreted terranes with

contrasting origins. The 2.7-Ga Abitibi greenstone

belt (Ludden and Hubert, 1986) in the Superior

Province of Canada is thought to contain remnants

of oceanic plateaus and arcs that collided at 2.7 Ga

modified after Struik et al. (2001) and detailed maps of the Pinchi

ferent igneous and metamorphic rock assemblages analyzed in this

Fig. 5. Simplified geographic map showing the location of the Slide Mountain samples analyzed in this work and two simplified lithostratigraphic columns of the Slide Mountain

Terrane.

H.Lapierre

etal./Chem

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60

H. Lapierre et al. / Chemical Geology 201 (2003) 55–89 61

ago (Polat et al., 1998; Calvert and Ludden, 1999).

Because of the geochemical similarities between the

2.1-Ga Birimian tholeiites with oceanic plateaus, the

Birimian plume-related igneous episode has been

interpreted as a major crust-formation event during

the Early Proterozoic (Abouchami et al., 1990; Boher

et al., 1992).

Most of the largest oceanic plateaus (Ontong Java,

Nauru, Caribbean–Colombian) were formed during

the Cretaceous superplume event. The Permian–Tri-

assic superplume event is less well documented and

most of the Permian to Triassic plume-derived rocks

exposed nowadays are contaminated by continental

crust (Siberian trapps: Arndt et al., 1998; Emeishian

Basalts in southwestern China: Zhou and Kyte, 1988;

Yunnan, 1990). According to Condie (1997, 2001),

among the ‘‘greenstone terranes’’ accreted to the

Cordillera in western North America during the Late

Mesozoic and Early Tertiary (Coney et al., 1981),

most consist of arc-rocks. Only three represent oce-

anic plateaus, the well-known Wrangellia (Lassiter et

al., 1995) and the two terranes that form the subject of

this paper (Figs. 1 and 2; Condie and Chomiak, 1996).

Two major Late Paleozoic–Triassic oceanic ter-

ranes are exposed in the North America Cordillera.

Both extend from Alaska to northern California

(Fig. 1). The western terrane which is known as the

Cache Creek Terrane, consists of tectonic slices of

Upper Triassic mafic volcanic rocks, and Mid-Perm-

ian oceanic-island tholeiites. West of Prince George,

the eastern boundary of the Cache Creek Terrane is

marked by the Pinchi Fault (Figs. 2 and 3), which

contains slices of Paleozoic platform carbonates, un-

dated cumulate gabbros intruded by dolerites, undated

foliated ultramafic rocks and Triassic blueschists (Fig.

4). The eastern terrane which is called the Slide

Mountain Terrane is made up of dolerites, pillow

basalts associated with gabbros, peridotites and ser-

pentinites ranging in age from Carboniferous to Perm-

ian (Figs. 2 and 5). In both terranes, the mafic

volcanic rocks exhibit geochemical features of N-

MORB (Ferri, 1997; Roback et al., 1994; Smith and

Lambert, 1995; Patchett and Gehrels, 1998). On the

basis of regional tectonic syntheses, many authors

have suggested that the Slide Mountain Terrane rep-

resents a back-arc basin developed along the North

American continental margin and related to an Early

Paleozoic (Devonian to Mississippian) continental arc

(Harper Ranch Group, Finlayson Lake arc-volcanics,

Monger, 1977; Struik and Orchard, 1985; Struik,

1988; Nelson, 1993; Roback et al., 1994; Smith and

Lambert, 1995; Ferri, 1997; Piercey et al., 2001).

However, Aggarwal et al. (1984) suggested that the

Slide Mountain volcanic rocks were formed in an

ocean-island to seamount setting because Ti-rich au-

gite and Ti-kaersutite occur in the basalts, and Pb

initial ratios suggest that the volcanic rocks derive

from an enriched mantle source.

Here, we present major and trace element concen-

trations and Nd, Sr and Pb isotopic analyses of

representative rock types from the Cache Creek and

Slide Mountain Terranes from the central British

Columbia (Figs. 2, 4 and 5). On the basis of these

new data, we suggest that the geodynamic environ-

ments of Cache Creek and Slide Mountain Terranes are

different: (2) the Cache Creek Terrane probably rep-

resents the remnants of a Late Triassic plateau while

Slide Mountain basin probably was floored by oceanic

crust locally thickened by ocean island magmas.

2. Geological notes

The Late Paleozoic Slide Mountain Terrane (Fig. 2)

separates the North America craton from terranes with

island arc affinities along much of the length of the

western North American Cordillera. This terrane is

commonly considered to represent the most eastern-

most Cordilleran terrane of oceanic affinity, and con-

sists of a series of isolated allochthons, each of which

displays a mainly sedimentary lower sequence and a

predominantly volcanic upper sequence. Throughout

the allochthons, the lower sequence lies unconform-

ably or with thrust contact on Upper Devonian to

Upper Mississippian strata (Monger, 1977; Aggarwal

et al., 1984). The volcanic upper sequence consists

mainly of pillow basalts and cherts. The onset of the

volcanism as suggested by paleontological evidence,

varies throughout the allochthons. In the Slide Moun-

tain allochthon, volcanism began probably in Early

Pennsylvanian time and lasted until Early Permian.

The geodynamic setting of the Slide Mountain Ter-

rane remains a matter of debate. The presence of

associated cherts has led many workers to interpret

the Slide Mountain Terrane as the remnant of a far

traveled oceanic basin (Monger, 1977; Speed, 1979;

H. Lapierre et al. / Chemical Geology 201 (2003) 55–8962

Snyder and Brueckner, 1983; Brueckner and Snyder,

1985; Harms, 1986). Others have viewed part of the

Slide Mountain Terrane is arc- (Tomlinson, 1988;

Gabrielse, 1991; Nelson, 1993) or oceanic island-

related (Aggarwal et al., 1984; Smith and Lambert,

1995). Others have suggested that the Slide Mountain

Terrane represents part of a marginal basin, developed

along the Late Paleozoic North American continental

margin, and in close proximity to an Early Paleozoic

arc terrane (Burchfiel and Davis, 1972; Struik and

Orchard, 1985; Struik, 1988; Gabrielse, 1991; Miller

et al., 1992; Nelson, 1993; Roback et al., 1994; Ferri,

1997). However, all these authors agree that the

igneous components (basalts, gabbros and peridotites)

of the Slide Mountain Terrane display N-MORB

affinities. More recently, Patchett and Gehrels

(1998) confirmed the depleted nature of its source

(DMM, eNdi ranging between + 8.8 and + 9.2) of the

Slide Mountain magmas. Moreover, on the basis of

the negative eNdi values (� 5 to � 7) of the clastic

sediments of the Slide Mountain basin, these authors

conclude that this oceanic basin developed along the

North American continental margin.

The Cache Creek Terrane (Fig. 2) is interpreted as

an accretionary complex (Struik et al., 2001). In

central British Columbia, this terrane is bounded to

the east by the Triassic Takla Group and by Early

Jurassic volcanic formations of the Quesnel Terrane

(Quesnellia), and to the west by various Upper Paleo-

zoic to Early Jurassic terranes (Stikinia; Figs. 2 and 4).

These terranes expose rocks of oceanic and island arc

environments (Paterson, 1974; Schiarriza et al., 1998;

Struik et al., 2001), that accreted to the North Amer-

ican margin during the Mesozoic (Monger et al.,

1972; Paterson, 1974; Gabrielse and Yorath, 1991).

Near Fort St James (Fig. 4a), the Cache Creek

complex consists of Mid-Permian mafic volcanic

rocks and Upper Triassic pillowed and massive basalts

and dolerites interlayered with siliceous shales, mas-

sive and brecciated cherts, greywackes and slates

(Sowchea succession; Struik et al., 2001). The com-

plex is bounded to the east by the Pinchi Fault system

which contain slices of (i) Upper Carboniferous to

Permian platform carbonates, (ii) Upper Triassic

(210–224 Ma) blueschists (Paterson and Harakal,

1974; Ghent et al., 1996) and (iii) undated rocks of

mafic and ultramafic compositions. The mafic rocks

consist of layered cumulate gabbro intruded by fine-

grained doleritic dykes and swarms of basaltic and

doleritic dykes that cross cut fine-grained gabbros and

dolerites. The foliated ultramafic rocks exposed at

Murray Ridge (Fig. 4a and b) are composed of

harzburgite, dunite and clinopyroxene-poor lherzolite

intruded by pyroxenite veins. They differ significantly

from those exposed along Trembleur Lake shores

(Fig. 4a; Tembleur ultramafite), which consist of

intensely serpentinized harzburgite and dunite,

intruded by amphibole-bearing diorite porphyries.

In southern British Columbia (Fig. 2), at the Cache

Creek type locality and in the vicinity of Williams

Lake, the different components of the Cache Creek

Terrane occur as blocks (meter to kilometer scale)

within a Permian to Triassic sheared argiliceous matrix

(Cache Creek Melange). The blocks consist of Perm-

ian reef limestones, serpentinites, gabbros, sheared

volcaniclastic sediments, mudstones and mafic lavas.

3. Background and sampling

Sampling of the Slide Mountain Terrane was

concentrated in the Antler Formation exposed on the

Sliding Mountain, near Bakerville, in central British

Columbia (Figs. 2 and 5). The Slide Mountain terrane

consists solely of the Antler Formation of the Slide

Mountain Group. It is made up of pillow basalts,

dolerite, chert, argillite, phyllite and minor graywacke,

gabbro and ultramafic rock. It was emplaced after the

Early Permian. The section of the Antler Formation

on Sliding Mountain, consists of two units of inter-

bedded chert and argillite separated by mixed basalt

and chert; each unit is intruded by dolerite, diorite and

ultramafic dykes and sills (Struik and Orchard, 1985).

The estimated thickness of the Antler Formation is

300 m. The section is a structural stack of four thrust

sheets. Sheets III and IV consist of Lower Mississip-

pian unit of pillow basalt and minor ribbon chert

while sheets II and III comprise a Pennsylvanian chert

and Permian pillow basalts. Sills and dykes of doler-

ite, gabbro and ultramafic plutonic rocks intrude the

Pennsylvanian sediments and could represent the

feeder dykes of the Lower Permian pillow basalts

(Struik and Orchard, 1985). The basalts have been

sampled mainly in the Lower Permian strata while the

sampled ultramafic cumulate sills intrude the Penn-

sylvanian (Fig. 5).

H. Lapierre et al. / Chemical Geology 201 (2003) 55–89 63

Sampling of the Cache Creek Terrane was concen-

trated in the Sowchea succession, within the Pinchi

Fault system and in the melange exposed in southern

British Columbia (Fig. 2). The Sowchea succession

(Fig. 4a–c) consists of massive and pillowed mafic

flows (basalts and icelandites) interbedded with Upper

Triassic sediments (Cordey and Struik, 1996).

Gabbros and dykes are exposed within the Pinchi

Fault system (Fig. 4a and b). Layered gabbros (PG96-

14) are crosscut by isolated dykes of basalt (PG96-12)

and dolerite (PG96-15) while fine-grained gabbros

(01PG61) are intruded by swarms of doleritic dykes

(PG96-17, 01PG64). The foliated ultramafic rocks

were sampled at Murray Ridge (Pinchi Fault system)

and along the Trembleur Lake shore (Fig. 4a and b).

In southern British Columbia, at the Cache Creek

type locality and in the vicinity of Williams Lake (Fig.

2), we sampled different volcanic components of the

melange. Basalts 95CC1 and 95CC4 were sampled

near the Cache Creek locality. Dolerites 99CC1 and

99CC2 were collected to the north, at the junction of

highways 99 and 97N. Sample 99WL7 is a basalt

exposed 35 km north of William Lake along Highway

97. Sample 99WL2 was collected south of William

Lake. Finally, samples 99WL5 and 99WL6 were

collected 25 km north of William Lake, on Highway

97, 5 km south of McLeese Lake.

4. Analytical procedures

Fifty samples of the Slide Mountain and Cache

Creek magmatic rocks and igneous and metamorphic

rocks from the Pinchi fault were analyzed for major,

trace elements and Nd, Sr and Pb isotopic composi-

tion (Tables 1 and 2). Major and compatible trace

elements have been measured by X-ray fluorescence

(XRF) at the Laboratoire de Petrologie de l’Universite

de Claude Bernard (Lyon), or by ICP-AES at the

Universite de Bretagne occidentale (Brest), or the

Centre de Recherches Petrographique et Geochimique

(Nancy). Incompatible trace elements have been de-

termined by inductively coupled plasma spectrometry

(ICP-MS, VG-PQ2 +) at the Laboratoire Geodynami-

que des Chaınes Alpines de l’Universite J. Fourier

(Grenoble), after acid dissolution, using procedures of

Barrat et al. (1996). The accuracy on trace element

concentrations is better than 3% for all the REE based

on various standards and sample duplicates. Trace

element analyses of the foliated ultramafic rocks were

performed on a PQ2 ICP-MS at the University of

Montpellier (ISTEEM) following the procedures de-

scribed by Ionov et al. (1992).

Sr (static acquisition and Nd (dynamic acquisition)

isotopic ratios were measured at the Laboratoire de

Geochimie isotopique de l’Universite Paul Sabatier de

Toulouse on a Finnigan MAT261 multicollector mass

spectrometer using the analytical procedures de-

scribed by Lapierre et al. (1997). Results on standards

yielded 143Nd/144Nd = 0.511850F 0.000017 (2r ex-

ternal reproducibility) on 12 standards analysed.

Results on NBS 987 Sr standard yielded 87Sr/86Sr =

0.710250F0.000030 (2r external reproducibility) on

11 standard determinations. 87Sr/86Sr and 143Nd/144Nd were normalized for mass fractionation rela-

tive to 86Sr/88Sr =0.1194 and 146Nd/144Nd = 0.7219,

respectively. eNdi calculated with actual (143Nd/144Nd)CHUR = 0.512638 and (147Sm/144Nd)CHUR =

0.1967. eSri calculated with actual (87Sr/86Sr)CHUR =

0.70450 and (87Rb/86Sr)CHUR = 0.084 (Mc Culloch

and Wasserburg, 1978).

For lead separation, powdered samples were

weighed to obtain approximately 200 ng of lead.

Samples were leached with 6 N HCl during 30 min

at 65 jC before acid digestion. They were dissolved

for 48 h on a hotplate in an tridistilled HF/HNO3

mixture. After evaporation to dryness, 1 ml of HNO3

was added to the residue and kept at about 90 jC for

12–24 h. After complete evaporation, 0.5 ml of 8 N

HBr was added to the sample which was kept at 70 jCfor 2–3 h before complete evaporation. The chemical

separation of lead was done using 50 Al of anion

exchange resin (AG1X8, 200–400 mesh) and samples

were loaded and washed in 0.5 N HBr. Lead was then

eluted in 6 N HCl. Pb blanks were less than 40 pg and

are negligible for the present analyses.

Lead isotope were analysed on a VG Plasma 54

multicollector inductively coupled plasma-mass spec-

trometer (MC-ICP-MS) at the Ecole Normale Super-

ieure de Lyon. Lead isotope compositions were

measured using the Tl normalization method described

by White et al. (2000). For Pb isotope analysis,

samples were bracketed between NIST 981 standards

and calculated with respect to the value reported for

this standard by Todt et al. (1996). This technique

yields internal precision of ca. 50 ppm (2r) and an

Table 1

Major and trace element chemistry of the Paleozoic and Triassic rocks from the Canadian Cordillera

Magmatic

affinity

Type 1 Type 1 Type 1 Type 1 Type 1 Type 1 Type 1 Type 1 Type 1 Type 1

Locality N-MORB N-MORB N-MORB N-MORB N-MORB N-MORB N-MORB N-MORB N-MORB N-MORB

Rock

type

Pinchi

Fault

Pinchi

Fault

Pinchi

Fault

Pinchi

Fault

Melange Melange Slide

Mountain

Slide

Mountain

Slide

Mountain

Slide

Mountain

Basalt Dolerite Dolerite Dolerite Dolerite Dolerite Basalt Icelandite Basalt Basalt

Sample

no.

PG96-12 PG96-15 PG96-17 01PG64 99CC1 99CC2 PG96-39 PG96-40 PG96-41 85-148a

SiO2a 45.93 52.7 48.72 51.77a 51.7a 46.23a 50.58a 51.83a 49.21a

TiO2 1.9 0.46 1.75 0.86 1.24 1.46 1.68 1.7 1.39

Al2O3 14.75 14.23 14.66 16.2 15.2 15.9 15.23 14.63 15.51

Fe2O3 14.23 9.25 13.56 10.35 11.62 11.54 11.51 11.81 10.4

MnO 0.23 0.16 0.21 0.17 0.2 0.2 0.14 0.18 0.18

MgO 5.88 9.17 5.63 6.9 6.64 8.39 3.86 6.42 6.42

CaO 10.64 8.14 9.56 9.1 8.7 14.56 11.09 8.91 12.73

Na2O 2.41 3.32 3.04 3.92 4.3 0.8 0.83 3.45 3.48

K2O 0.04 0.1 0.12 0.6 0.25 0.19 0.2 0.13 0.51

P2O5 0.19 0.05 0.15 0.08 0.11 0.12 0.2 0.15 0.17

LOI 2.52 2.61 2.89 3.71 3.38 6.18 3.4 3.11 5.81

Total 98.70 100.19 100.29

Sca nd 62.31 98.37 40 41 35.5 36.2

V 500 412.53 793.5 290 340 270.6 321.5

Cr 66.1 723.22 107.64 165 155 79.3 111.5 315.9

Ni nd 148.35 31.2 63 58 41.1 40.6 65.3

Rb 0.585 1.26 0.448 35.44 12.09 4.34 2.68 26 2.4 14.46

Sr 115.506 117.90 73.437 123.22 29.21 40.54 86 175 96 182.54

Ba 53.445 61.48 10.147 24.49 181.67 220.97 120 719.4 131 190.58

U 0.010 0.04 0.082 0.08 0.05 0.04 0.06 0.18 0.15 0.28

Th 0.014 0.06 0.186 0.14 0.09 0.11 0.15 0.24 0.15 0.14

Pb 0.335 nd 0.463 0.16 0.24 0.35 0.33 0.82 0.45 0.17

Hf 0.607 0.67 1.239 1.47 0.98 1.11 2.4 3.25 3.14 2.17

Zr 13.834 21.10 25.501 49.53 28.03 25.61 90 117 114 86.36

Ta 0.152 0.03 0.165 0.05 0.05 0.08 0.14 0.16 0.15 0.13

Nb 2.796 0.88 2.678 0.66 0.56 0.99 2.99 2.59 2.02 1.55

Y 34.082 12.21 44.374 23.06 20.63 28.64 nd 36.9 34.2 33.09

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etal./Chem

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64

La 1.877 0.82 3.909 1.73 1.37 2.28 2.6 4.04 3.3 2.75

Ce 6.73 2.41 12.666 5.46 4.57 7.38 8.7 12.9 11.5 8.82

Pr 1.35 0.41 2.241 0.99 0.82 1.3 1.47 2.14 1.96 1.54

Nd 8.215 2.08 11.884 5.62 4.55 7.11 8.3 11.9 10.89 8.34

Sm 3.21 0.83 4.215 2.07 1.74 2.56 2.85 3.89 3.73 2.96

Eu 1.467 0.34 1.504 0.86 0.68 0.93 1.18 1.46 1.37 1.09

Gd 4.563 1.19 5.394 2.84 2.51 3.58 4.05 5.7 5.3 4.06

Tb 0.82 0.25 0.993 0.55 0.47 0.68 0.7 0.97 0.94 0.75

Dy 5.408 1.77 6.448 3.49 3.12 4.32 5.1 6.8 6.7 4.75

Ho 1.171 0.42 1.428 0.81 0.72 0.99 1.05 1.39 1.38 1.03

Er 3.19 1.28 4.010 2.38 2.03 2.76 2.98 3.9 3.86 3.06

Tm 2.265 0.2 1.898 nd nd nd 0.44 0.56 0.56 nd

Yb 3.024 1.33 3.875 2.29 1.95 2.67 2.85 3.54 3.53 2.76

Lu 0.462 0.22 0.579 0.35 0.3 0.41 0.451 0.55 0.56 0.45

Magmatic

affinity

Type 1 Type 1 Type 1 Type 1 Type 1 Type 1 Type 2 Type 2 Type 2 Type 2 Type 2 Type 2

N-MORB N-MORB N-MORB N-MORB N-MORB N-MORB Alkalic Alkalic Alkalic Alkalic Alkalic Alkalic

Locality Slide

Mountain

Slide

Mountain

Slide

Mountain

Slide

Mountain

Slide

Mountain

Slide

Mountain

Cache

Creek

Cache

Creek

Cache

Creek

Cache

Creek

Cache

Creek

Melange

Rock

type

Basalt Basalt Basalt Basalt Basalt Basalt Basalt Basalt Basalt Basalt Basalt Basalt

Sample

no.

85-155 84-160 84-158a 85-157 55-16 102A PG97-1 PG97-3 PG97-5 PG97-7 PG97-20b 99WL2

SiO2a 50.74a 51.76a 49.76a 48.8a 51.44a 49.98a 47.08 47.5 44.33 45.35 45.29 46.93a

TiO2 1.54 1.44 0.98 1.47 1.47 1.63 2.23 3.33 2.78 2.66 3.03 3.09

Al2O3 15.35 15.23 16.04 15.49 15.53 15.68 10.38 14.25 17.13 16.35 13.92 15.9

Fe2O3 11.76 10.17 10.14 11.93 11.63 11.31 9.7 13.58 12.09 12.34 13.28 10.93

MnO 0.16 0.15 0.16 0.18 0.19 0.18 0.26 0.17 0.17 0.15 0.15 0.18

MgO 6.07 6.38 9.23 7.04 6.09 7.02 5.59 6.43 3.82 4.28 7.43 5.17

CaO 9.47 9.76 10.91 11.97 8.96 10.52 13.14 7.11 8.67 10.01 8.74 10.76

Na2O 4.51 4.63 2.96 2.81 4.37 3.42 1.33 3.18 3.18 3.41 3.07 4.1

K2O 0.22 0.31 0.22 0.35 0.13 0.08 1.19 1.1 1.53 0.5 0.51 2.31

P2O5 0.18 0.17 0.12 0.17 0.18 0.19 0.44 0.45 0.6 0.56 0.54 0.56

LOI 3.94 2.06 3.94 3.64 2.9 3.27 7.8 3.03 4.07 2.90 3.45 8.99

Total 99.14 100.13 99.45 99.51 99.90

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Magmatic

affinity

Type 1 Type 1 Type 1 Type 1 Type 1 Type 1 Type 2 Type 2 Type 2 Type 2 Type 2 Type 2

N-MORB N-MORB N-MORB N-MORB N-MORB N-MORB Alkalic Alkalic Alkalic Alkalic Alkalic Alkalic

Locality Slide

Mountain

Slide

Mountain

Slide

Mountain

Slide

Mountain

Slide

Mountain

Slide

Mountain

Cache

Creek

Cache

Creek

Cache

Creek

Cache

Creek

Cache

Creek

Melange

Rock

type

Basalt Basalt Basalt Basalt Basalt Basalt Basalt Basalt Basalt Basalt Basalt Basalt

Sample

no.

85-155 84-160 84-158a 85-157 55-16 102A PG97-1 PG97-3 PG97-5 PG97-7 PG97-20b 99WL2

Sc 35.6 48.45 47.55 47.21 49.67 18

V 411.36 666.41 739.63 666.34 526.25 210

Cr 23.04 66.27 24.81 22.22 436.01 150

Ni 22.215 44.525 19.7 15.39 130.25 64

Rb 5.73 3.49 6.98 8.33 8.93 2.58 27.93 16.03 16.55 6.09 10.59 0.1

Sr 77.93 182.47 186.35 96.06 214.08 167.58 209.8 326.59 411.33 458.81 153.92 8.15

Ba 36.45 76.85 650.66 33.06 273.52 142.9 479.11 275.84 346.76 197.32 85.13 417.08

U 0.16 0.21 0.05 0.07 0.09 0.1 0.4 0.64 1.06 0.80 1.05 1.05

Th 0.15 0.2 0.11 0.14 0.19 0.24 1.9 2.3 2.66 2.46 3.53 2.79

Pb 0.40 0.77 0.33 0.53 0.72 0.52 5.38 0.6 0.70 0.32 0.55 2.11

Hf 2.58 2.75 1.31 2.44 2.78 2.99 4.41 5.49 5.44 5.05 7.73 6.09

Zr 104.38 111.45 45.18 98.06 110.68 117.08 188.04 239.15 235.60 214.67 335.55 262.75

Ta 0.15 0.21 0.09 0.14 0.16 0.21 1.60 2.22 2.62 2.38 2.68 2.46

Nb 1.95 3 0.91 1.82 2.32 2.97 26.53 36.41 42.09 37.98 45.25 39.19

Y 34.02 36.09 24.99 35.5 38.95 36.53 26.81 33.32 39.17 34.08 35.21 28.35

La 3.13 4.1 2 3.08 3.68 4.33 22.98 28.46 29.19 26.84 30.24 25.8

Ce 9.98 12.28 6.18 9.83 11.6 13.22 49.55 61.62 66.91 62.1 73.87 61.14

Pr 1.70 1.99 1.09 1.74 2 2.24 6.17 7.75 8.58 7.9 9.72 8

Nd 9.13 10.18 6.04 9.13 10.68 11.65 26.16 32.96 37.18 33.91 43.05 33.52

Sm 3.18 3.27 2.14 3.16 3.59 3.91 5.99 7.39 8.39 7.77 9.88 7.15

Eu 1.26 1.21 0.86 1.2 1.31 1.36 1.81 2.56 2.72 2.53 3.08 2.45

Gd 4.5 4.67 3.08 4.38 4.95 5.02 6.27 7.55 8.78 7.83 9.55 6.77

Tb 0.81 0.85 0.56 0.8 0.91 0.91 0.87 1.1 1.27 1.15 1.36 0.95

Dy 5.21 5.19 3.63 5.12 5.92 5.87 4.69 5.83 6.91 6.19 6.82 5.3

Ho 1.13 1.18 0.78 1.12 1.31 1.3 0.95 1.15 1.38 1.24 1.25 0.98

Er 3.23 3.3 2.34 3.28 3.82 3.76 2.42 2.86 3.42 3.06 2.85 2.5

Tm nd nd nd nd nd nd 0.35 0.4 0.48 0.43 0.38 nd

Yb 2.88 3.01 2.09 3.02 3.55 3.5 2.08 2.35 2.76 2.5 2.10 1.88

Lu 0.45 0.46 0.32 0.47 0.54 0.52 0.32 0.35 0.42 0.37 0.30 0.26

Table 1 (continued)

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Magmatic

affinity

Type 2 Type 2 Type 2 Type 2 Type 2 Type 2 Type 2 Type 2 Type 2

Alkalic Alkalic Alkalic Alkalic Alkalic Alkalic Alkalic Alkalic Alkalic

Locality Melange Melange Melange Slide

Mountain

Slide

Mountain

Slide

Mountain

Slide

Mountain

Slide

Mountain

Cache

Creek

Rock

type

Basalt Basalt Basalt Icelandite Basalt Basalt Dolerite Basalt Basalt

Sample

no.

99WL5 99WL6 99WL7 PG96-37 PG96-38 85-139 84-156 42-e PG97-8

SiO2a 43.58a 46.63a 51.02a 51.2a 50.77a 48.58a 47.87a 52.01a 50.4

TiO2 2.34 2.19 2.09 2.13 1.77 1.83 1.01 1.59 1.53

Al2O3 13.13 15.40 15.44 15.34 13.65 15.5 16.56 16.29 14.78

Fe2O3 11.62 12.14 9.97 11.45 11.93 13.56 9.93 10.39 10.92

MnO 0.16 0.2 0.11 0.13 0.19 0.18 0.15 0.23 0.17

MgO 4.86 5.27 6.09 4.41 6.34 7.08 9.28 7.85 7.02

CaO 17.09 9.92 9.65 10.87 10.53 9.89 12.51 5.77 8.78

Na2O 3.86 5.33 4.94 3.46 3.63 3.15 2.44 4.28 3.48

K2O 3.05 2.54 0.41 0.77 0.71 0.32 0.16 1.36 0.52

P2O5 0.26 0.34 0.23 0.17 0.2 0.22 0.11 0.22 0.18

LOI 10.11 7.45 4.25 4.63 5.53 3.12 3.81 4.38 1.75

Total 99.49

Sc 30 37 29 38 33.6 63.64

V 280 215 240 244.7 277.5 503.08

Cr 157 205 192 228.8 160.3 413.65

Ni 47 56 38 138.9 72.6 95.89

Rb 3.99 56.65 3.37 22.9 11.99 7.51 2.98 27.21 7.9

Sr 117.85 446.82 262.97 1108 159 253.12 95.69 115.43 173.64

Ba 511.89 6408.81 206.77 280 456 113.38 84.53 395.8 57.05

U 0.3 5.15 0.42 0.32 0.29 0.27 0.06 0.43 0.16

Th 0.66 9.92 0.58 0.59 1 0.67 0.22 1.74 0.27

Pb 0.85 21.8 0.43 2.41 0.86 1.3 0.2 0.94 nd

Hf 3.37 3.43 3.13 2.13 3.1 2.35 1.35 2.15 2.64

Zr 129.96 130.1 119.92 71.16 117 85.74 52.73 86.38 99.15

Ta 0.75 0.94 0.67 0.44 0.83 0.68 0.22 1.18 0.23

Nb 11.36 13.28 9.66 6.84 12.9 11.13 3.73 23.17 5.37

Y 31.53 29.77 28.34 24.29 nd 37.56 25.38 29.76 36.99

La 8.99 29.01 7.48 5.7 9.39 8.97 3.11 13.51 4.82

Ce 22.82 57 19.55 13.9 23.6 21.43 8.22 29.03 14.52

(continued on next page)

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Magmatic

affinity

Type 2 Type 2 Type 2 Type 2 Type 2 Type 2 Type 2 Type 2 Type 2

Alkalic Alkalic Alkalic Alkalic Alkalic Alkalic Alkalic Alkalic Alkalic

Locality Melange Melange Melange Slide

Mountain

Slide

Mountain

Slide

Mountain

Slide

Mountain

Slide

Mountain

Cache

Creek

Rock

type

Basalt Basalt Basalt Icelandite Basalt Basalt Dolerite Basalt Basalt

Sample

no.

99WL5 99WL6 99WL7 PG96-37 PG96-38 85-139 84-156 42-e PG97-8

Pr 3.32 6.91 2.92 2.12 3.14 3.1 1.3 3.72 2.32

Nd 15.49 25.61 13.88 10.22 15 14.46 6.63 15.77 11.98

Sm 4.35 5.1 4.01 3.19 4.01 4.15 2.21 3.83 3.86

Eu 1.6 1.61 1.48 1.04 1.37 1.54 0.71 1.22 1.37

Gd 5.36 5.6 4.9 3.92 5.08 5.34 3.17 4.64 4.59

Tb 0.89 0.79 0.81 0.67 0.81 0.91 0.56 0.75 0.88

Dy 5.41 4.67 4.95 4.27 5.55 5.53 3.54 4.74 5.7

Ho 1.16 1.01 1.01 0.9 1.11 1.21 0.79 1 1.27

Er 3.09 2.88 2.74 2.35 3.01 3.19 2.24 2.77 3.54

Tm nd nd nd 0.8 0.43 nd nd nd 0.52

Yb 2.71 2.91 2.37 2.07 2.73 2.67 2.05 2.3 3.17

Lu 3.37 0.45 0.35 0.29 0.43 0.4 0.31 0.32 0.49

Magmatic

affinity

Type 3 Type 3 Type 3 Type 3 Type 4 Type 4 Type 4 Type 1 Type 1 Type 1

OPB OPB OPB OPB OPB OPB OPB N-MORB N-MORB N-MORB

Locality Cache

Creek

Melange Melange Cache

Creek

Cache

Creek

Cache

Creek

Pinchi

Fault

Pinchi

Fault

Slide

Mountain

Slide

Mountain

Rock

type

Basalt Basalt Basalt Icelandite Basalt Basalt Gabbro Gabbro Pyroxenite Peridotite

Sample

no.

PG97-25 95CC1 95CC4 PG96-11c PG96-23 PG96-24 PG96-14 01PG61 42-1 103A

SiO2a 47.11 55.23a 48.07a 44.37 44.51 50.84 48.98 48.73a 43.85a

TiO2 1.61 1.74 1.5 2.33 2.09 0.86 0.29 1.02 0.51

Al2O3 14.13 13.01 14.35 16.94 14.09 15.05 21.3 6.38 6.98

Fe2O3 13.88 13.45 12.93 14.82 2.63 7.74 4.93 11.53 14.67

Table 1 (continued)

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MnO 0.21 0.25 0.22 0.08 0.16 0.15 0.08 0.18 0.21

MgO 6.47 7.9 10.25 3.84 5.39 6.43 5.72 18.58 29.13

CaO 9.59 5.27 8.99 8.41 10.06 11.73 12.41 12.67 4.37

Na2O 2.93 2.71 3.4 2.87 2.75 2.96 2.71 0.76 0.19

K2O 0.38 0.33 0.01 1.28 0.63 1.88 0.14 bdl

P2O5 0.13 0.18 0.11 0.09 0.14 0.31 0.09 0.15 0.09

LOI 3.42 8.46 11.58 4.91 7.2 1.94 3.27 3.35 8.56

Total 99.86 99.94 99.65 99.89 99.92

Sc 88.09 47 45 29.5 nd 162.06

V 675.71 380 330 248.9 nd 325.8 162.06

Cr 200.82 172 235 245.1 nd 226.4 434.59

Ni 70.56 65 78 186.4 nd 151 89.29

Rb 8.61 8.38 1.38 22.45 8.25 8.08 0.64 0.66 0.26 5.3

Sr 97.59 268 470 238 492 275 215.6 115.86 23.75 14.92

Ba 326.5 970 394.9 38.48 32.2 22.79 44.95 8.99 40.14 16.32

U 0.1 0.26 0.41 0.24 0.76 0.17 0.03 0.009 0.07 0.02

Th 0.33 0.37 0.34 0.23 0.16 0.15 0.04 0.013 0.18 0.09

Pb 0.29 1.66 1.4 0.27 0.26 0.08 0.32 0.05 0.54 0.51

Hf 2.45 3.09 2.31 2.61 2.2 2.28 0.65 0.72 2.04 0.75

Zr 91.24 115.58 83.32 86.91 78.44 80.56 20.89 21.02 79.54 30.63

Ta 0.29 0.4 0.26 0.4 0.33 0.34 0.03 0.03 0.15 0.06

Nb 4.73 5.94 3.92 6.94 5.36 5.58 0.58 0.41 2.19 0.852

Y 31.9 33.22 29.45 19.76 26.77 26.33 13.55 13.25 23.23 10.82

La 4.54 5.71 4.8 3.15 4.04 3.96 1.39 0.71 2.71 1.09

Ce 12.81 16.12 11.64 12.72 14.02 13.57 3.96 2.51 8.31 3.38

Pr 1.99 2.65 1.93 1.99 2.6 2.53 0.67 0.46 1.42 0.57

Nd 10.38 13.29 9.5 10.62 13.79 13.47 3.56 2.68 7.33 3.05

Sm 3.4 4.31 3.19 3.38 4.1 4.14 1.22 1.05 2.53 1.02

Eu 1.32 1.44 1.09 1.19 1.39 1.4 0.61 0.53 0.84 0.41

Gd 4.36 4.99 3.86 3.58 4.4 4.35 1.72 1.6 3.24 1.39

Tb 0.77 0.87 0.72 0.62 0.43 0.75 0.32 0.3 0.58 0.25

Dy 5.04 5.47 4.59 3.81 4.48 4.51 2.10 1.96 3.81 1.66

Ho 1.11 1.21 1.06 0.78 0.92 0.93 0.46 0.45 0.82 0.37

Er 3.01 3.22 2.85 2.06 2.4 2.42 1.31 1.34 2.38 1.09

Tm 0.46 0.51 0.45 0.33 0.38 0.39 1.67 nd nd nd

Yb 2.81 3.12 2.71 1.83 2.16 2.16 1.22 1.25 2.15 1

Lu 0.44 0.48 0.41 0.27 0.83 0.32 0.19 0.19 0.33 0.15

(continued on next page)

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Type 1 Type 1 Foliate Foliated Foliated Foliated Foliated Foliated Foliated Foliated

N-MORB N-MORB Upper

Mantle

Upper

Mantle

Upper

Mantle

Upper

Mantle

Upper

Mantle

Locality Slide

Mountain

Tremb ur Trembleur Trembleur Trembleur Trembleur Murray

Ridge

Murray

Ridge

Murray

Ridge

Rock

type

Peridotite Standard Harzb gite Harzburgite Harzburgite Harzburgite Pyroxene Pyroxenite Pyroxenite Lherzolite

Sample

no.

103B BIR SH903 SH902a SH912a TU1-2 TU1-2 PG97-17 PG01-40 PG01-77

SiO2a 43.54a nd 55.23 44.69a 44.31a 43.89a 52.73 50.1 46.23

TiO2 0.54 nd bdl bdl bdl bdl

Al2O3 6.87 nd 0.54 0.64 0.37 0.60 0.87 0.84 0.45

Fe2O3 15.07 nd 6.1 9.06 8.94 9.05 7.2 7.5 7.6

MnO 0.21 nd 0.12 0.13 0.12 0.12 0.16 0.15 0.11

MgO 29.39 nd 35.67 45.23 45.81 45.71 36.35 36.15 33.15

CaO 4.09 nd 0.75 0.24 0.37 0.56 0.95 1.15 0.75

Na2O 0.19 nd bdl bdl 0.02 0.02 0.01

K2O bdl nd bdl bdl 0.004 0.005 0.002

P2O5 0.1 nd bdl bdl 0.06 0.06 bdl bdl nd

LOI 8.98 nd 1.77 13.13 14.37 12.63 1.71 4.29 11.7

Total nd 100.18 100 100 100

Sc nd nd nd nd nd nd nd

V nd nd nd nd nd 61 3 70

Cr nd 4010 2240 2290 2530 3330 3398 1704

Ni nd 790 2160 2200 2210 892 872 2348

Table 1 (continued)

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70

d

le

ur

b

Rb 4.96 0.33 168 (ppb)b 61b 38.2b 58.2b 52.11b 0.1236 0.1782 0.0731

Sr 11.59 114.03 471 334 204 293.19 226.76 0.6433 0.6416 0.1315

Ba 14.44 8.37 1780 1210 2632 845.85 335.12 3.329 2.6925 0.4584

U 0.03 0.11 5.7 2.4 1.3 2.28 6.759 0.004 0.0033 0.0026

Th 0.07 0.03 17.2 3.1 2.4 2.36 2.264 0.0086 0.0101 0.0096

Pb 0.5 4.06 132 75 87 45.9 82.43 0.1124 0.4184 0.0942

Hf 0.81 0.62 3 2 2.8 3.64 17.29 0.0021 0.0033 0.0019

Zr 31.85 15.73 77.9 46.7 71 117.58 48.6 0.0808 0.1426 0.0752

Ta 0.06 0.05 1.3 0.6 0.5 0.642 0.45 0.0086 0.0411 0.019

Nb 0.91 0.59 24.3 23.2 21.4 12.87 8.26 0.2353 0.0232 0.0213

Y 11.23 16.73 nd nd nd 5.52 192.64 0.0625 0.095 0.0559

La 1.21 0.64 27.6 6.6 8.8 17.45 7.887 0.0199 0.0316 0.0273

Ce 3.72 2 63.1 15.5 17.2 33.33 15.06 0.0412 0.0556 0.054

Pr 0.62 0.38 7.7 1.9 2.3 4.33 1.6 0.0049 0.007 0.0064

Nd 3.29 2.45 28.9 8.5 9.8 17.19 5.78 0.0224 0.0283 0.0222

Sm 1.12 1.08 8.1 3.6 3.3 4.23 1.88 0.0053 0.0053 0.0057

Eu 0.43 0.52 1.5 0.8 0.5 1.57 0.067 0.0016 0.0015 0.001

Gd 1.49 1.75 7.2 4.4 3.3 5.29 2.32 0.0055 0.0058 0.0062

Tb 0.27 0.36 1.1 0.6 0.5 1.03 1.31 0.001 0.0013 0.0012

Dy 1.77 2.51 7.3 5.9 3.9 8.02 18.06 0.0077 0.011 0.0077

Ho 0.39 0.59 2 1.7 0.9 2.83 7.97 0.0022 0.0034 0.0022

Er 1.49 1.68 8.4 8.2 3.6 11.53 40.67 0.0099 0.0137 0.009

Tm nd nd 1.8 2.2 0.9 2.53 9.99 0.0022 0.0031 0.0021

Yb 1.08 1.59 15.4 20.2 9.7 23.74 90.28 0.0196 0.0284 0.0201

Lu 0.16 0.25 3.4 4.6 2.4 5.32 19.35 0.0046 0.0066 0.0048

a Analyses recalculated on a dry basis.

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Table 2

Sr, Nd and Pb isotopic compositions of Paleozoic and Triassic rocks from the Canadian Cordillera

Affinity Type 1 Type 1 Type 1 Type 1 Type 1 Type 1 Type 1 Type 1 Type 1 Type 1 Type 1 Type 1

Locality Pinchi

Fault

Pinchi

Fault

Pinchi

Fault

Melange Melange Slide

Mountain

Slide

Mountain

Slide

Mountain

Slide

Mountain

Slide

Mountain

Slide

Mountain

Slide

Mountain

Rock

type

Basalt Dolerite Dolerite Dolerite Dolerite Basalt Icelandite Basalt Basalt Basalt Basalt Basalt

Sample

no.

PG96-12 PG96-15 PG96-17 99CC1 99CC2 PG96-39 PG96-40 PG96-41 84-158a 84-160 55-16 102A

87Sr/86Sr 0.703294F 12 0.703097F 7 0.704508F 787Rb/86Sr 0.0158 0.0309 0.0176

(87Sr/86Sr)t 0.70325 0.70300 0.70445

eSr � 14.2 � 17.7 + 2.87143Nd/144Nd 0.513149F 6 0.513101F 7 0.513092F 6 0.513140F 4 0.513095F 8 0.513120F 6 0.513106F 6 0.513120F 5 0.513183F 8 0.513125F 5 0.513070F 7 0.513088F 6147Sm/144Nd 0.23902 0.24127 0.21427 0.23122 0.21769 0.208 0.198 0.207 0.214 0.194 0.20288 0.203

(143Nd/144Nd)t 0.51282 0.51277 0.51279 0.51282 0.512795 0.51278 0.51278 0.51278 0.51283 0.51281 0.51274 0.5127

eNd 8.8 7.8 8.4 + 8.9 + 8.4 + 9.1 + 9.1 + 9.1 + 10.1 + 9.6 + 8.6 + 8.6

(206Pb/204Pb)m 18.642 18.881 18.744 18.998 19.104 18.309 19.195 18.762

(207Pb/204Pb)m 15.534 15.504 15.503 15.572 15.582 15.565 15.578 15.527

(208Pb/204Pb)m 37.813 38.09 38.072 38.211 38.558 38.124 38.253 38.169

(206Pb/204Pb)t 18.58 18.48 18.37 18.48 18.55 17.94 18.51 18.29

(207Pb/204Pb)t 15.53 15.48 15.48 15.5 15.55 15.55 15.54 15.50

(208Pb/204Pb)t 37.78 37.67 37.8 37.91 38.32 37.85 38.04 37.79

Affinity Type 2 Type 2 Type 2 Type 2 Type 2 Type 2 Type 2 Type 2 Type 2 Type 2 Type 3 Type 3

Locality Cache

Creek

Cache

Creek

Cache

Creek

Cache

Creek

Cache

Creek

Cache

Creek

Melange Melange Melange Melange Melange Melange

Rock

type

Basalt Basalt Basalt Basalt Basalt Basalt Basalt Basalt Basalt Basalt Basalt Basalt

Sample

no.

PG96-1 PG97-1 PG97-3 PG97-5 PG97-7 PG97-20b 99WL2 99WL5 99WL6 99WL7 95CC1 95CC4

87Sr/86Sr 0.706800F 13 0.704008F 11 0.704211F11 nd 0.703825F 887Rb/86Sr 0.3851 0.1420 0.1164 0.1990

(87Sr/86Sr)t 0.70565 0.70358 0.70386 0.70323

eSr + 19.83 � 9.5 � 5.53 � 14.52143Nd/144Nd 0.512865F 6 0.51266F 3 0.512788F 5 0.512778F 4 0.512784F 4 0.512823F 6 0.5126711F 5 0.512973F 5 0.512945F 10 0.513024F 8 0.512984F 4 0.512903F 8147Sm/144Nd 0.16376 0.13843 0.13555 0.13875 0.13853 0.13875 0.12895 0.169789 0.1204 0.174676 0.19607 0.20302

(143Nd/144Nd)t 0.51264 0.512247 0.51260 0.51259 0.51259 0.51263 0.51249 0.512739 0.51278 0.512783 0.512714 0.51262

eNd + 5.3 + 2 + 4.6 + 4.3 + 4.4 + 5.2 + 2.5 + 7.4 + 8.04 + 8.12 + 6.77 + 5.00

(206Pb/204Pb)m 20.423 19.381 18.594 20.651 20.341 21.182 20.544 20.398 22.537 18.968 19.510

(207Pb/204Pb)m 15.652 15.602 15.512 15.684 15.652 15.746 15.662 15.597 15.754 15.526 15.536

(208Pb/204Pb)m 38.810 38.873 39.930 39.902 39.983 40.779 39.114 38.65 39.701 38.217 38.094

(206Pb/204Pb)t 18.70 19.22 17.79 17.57 19.66 19.86 19.63 19.79 19.93 18.57 18.78

(207Pb/204Pb)t 15.56 15.59 15.47 15.52 15.61 15.67 15.61 15.56 15.61 15.51 15.60

(208Pb/204Pb)t 37.84 38.63 38.99 37.07 39.16 39.63 38.46 38.27 38.52 38.03 37.89

H.Lapierre

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72

Affinity Type 2 Type 2 Type 2 Type 2 Type 2 Type 3 Type 3 Type 3 Type 4

Locality Slide

Mountain

Slide

Mountain

Slide

Mountain

Slide

Mountain

Slide

Mountain

Cache

Creek

Cache

Creek

Cache

Creek

Cache

Creek

Rock

type

Basalt Dolerite Basalt Icelandite Basalt Basalt Basalt Basalt Icelandite

Sample

no.

85-139 84-156 42-e PG96-37 PG96-38 PG97-8 PG97-24 PG97-25 PG96-11c

87Sr/86Sr 0.703531F13 0.715162F 10 0.705630F 14 0.705583F 1387Rb/86Sr 0.1316 2.2973 0.2552 0.2729

(87Sr/86Sr)t 0.70314 0.70830 0.70460 0.70477

eSr � 15.83 + 57.5 + 4.90 + 7.3143Nd/144Nd 0.512985F 5 0.513138F 6 0.512800F 5 0.512953F 5 0.512946F 5 0.513013F 5 0.513091F10 0.513084F 6 0.512986F 6147Sm/144Nd 0.132 0.201 0.14665 0.201 0.162 0.19481 0.18635 0.19805 0.19243

(143Nd/144Nd)t 0.51277 0.51281 0.51256 0.51281 0.51268 0.51275 0.51283 0.51281 0.51272

eNd + 8.6 + 9.6 + 4.8 + 6.4 + 7.1 + 7.4 + 9.1 + 8.7 + 6.9

(206Pb/204Pb)m 18.869 18.570 19.194 18.571 19.082 19.320 21.392

(207Pb/204Pb)m 15.569 15.597 15.669 15.597 15.541 15.554 15.680

(208Pb/204Pb)m 38.385 38.143 39.710 38.412 38.174 38.681 38.734

(206Pb/204Pb)t 18.35 17.82 18.03 18.24 18.59 18.59 19.73

(207Pb/204Pb)t 15.54 15.56 15.61 15.58 15.52 15.52 15.59

(208Pb/204Pb)t 37.97 37.52 38.17 38.21 37.85 37.89 38.12

Affinity Type 4 Type 4 N-MORB N-MORB N-MORB N-MORB Foliated Foliated Foliated Foliated

Locality Cache

Creek

Cache

Creek

Pinchi

Fault

Slide

Mountain

Slide

Mountain

Slide

Mountain

Trembleur Murray

Ridge

Murray

Ridge

Murray

Ridge

Rock

type

Basalt Basalt Gabbro Pyroxenite Peridotite Peridotite Harzburgite Pyroxenite Pyroxenite Lherzolite

Sample

no.

PG96-23 PG96-24 PG96-14 42-1 103A 103B SH903b PG97-17 PG01-40 PG01-77

87Sr/86Sr 0.704797F 11 0.702985F 11 706247F 12 0.708521F15 0.708707F 11 0.718019F 2087Rb/86Sr 0.0850 0.0107 1.0318 0.55593 0.80365 1.60996

(87Sr/86Sr)t 0.70454 0.70295 0.70317 0.70686 0.706306 0.71321

eSr + 4.1 � 18.5 � 15.44 + 37.03 + 29.1 + 127143Nd/144Nd 0.512990F 6 0.512876F 6 0.513066F 8 0.513068F 7 0.513113F 6 0.513114F 6147Sm/144Nd 0.18583 0.18582 0.20621 0.20872 0.203 0.206

(143Nd/144Nd)t 0.51273 0.51262 0.51278 0.51273 0.51278 0.51278

eNd + 7.2 + 4.9 + 8.10 + 8 + 9.1 + 9

(206Pb/204Pb)m 22.872 18.502 18.425 18.821

(207Pb/204Pb)m 15.751 15.472 15.566 15.548

(208Pb/204Pb)m 38.353 37.834 38.071 38.111

(207Pb/204Pb)t 15.652 15.46 15.55 15.54

(208Pb/204Pb)t 37.95 37.74 37.80 37.97

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H. Lapierre et al. / Chemical Geology 201 (2003) 55–8974

external reproducibility of ca. 150 ppm (2r) for206Pb/204Pb ratios determined on 20 NIST standards.

Isotopic data for the Permian rocks of Slide

Mountain have been corrected for ‘‘in-situ decay’’

with an age of 250 Ma on the basis of conodonts

(Struik and Orchard, 1985). Isotopic data for the

Upper Triassic rocks of the Cache Creek Group have

been corrected for ‘‘in-situ decay’’ with an age of

210 Ma, on the basis of radiolarian faunas (Struik et

al., 2001). We assumed an age of 210 Ma for the

rocks exposed in the Pinchi Fault system on the basis

of the 211–218-Ma ages of the blueschists (K/Ar

dating of phengitic muscovite, Paterson and Harakal,

1974).

The Pb isotopic initial ratios were corrected assum-

ing in ‘‘in-situ decay’’ using the Pb, U and Th for each

whole rock sample. The 206Pb/204Pb and 207Pb/204Pb

isotopic ratios of three samples (PG96-15, PG97-20b,

Fig. 6. Plots of Cr# (molar Cr +Al/Cr) versus Mg# (molar Mg/Mg+Fe) fo

from Dick and Bullen (1984), whereas dunite, harzburgite and plagiocla

(1980) and Leblanc et al. (1980). Spinels in the lherzolite (PG97-18) ar

pyroxenite (PG97-17, 0.74 <Cr # < 0.56) or dunite (PG97-15, 0.71 <Cr # <

PG97-8) for which the Pb and U content were not

measured have been corrected using the Pb and U

abundances calculated from the Nb/U and Ce/Pb

ratios, which are considered to be constant in the

MORB and OIB (Nb/U = 47F 10, Ce/Pb = 25F 5;

Hofmann et al., 1986). The initial 208Pb/204Pb ratio

of these three samples were calculated using the

measured Th contents.

5. Geochemistry of the Slide Mountain and Cache

Creek Terranes igneous and metamorphic

components

5.1. Main rock types

Four igneous rock types have been recognized in

the Slide Mountain and Cache Creek terranes. In order

r spinels in the Murray Ridge peridotites (abyssal peridotite field is

se and spinel lherzolite spinels fields are from Burgath and Weiser

e more Al2O3-depleted (0.75 <Cr # < 0.71) than those occurring in

0.62).

Fig. 7. Plots of Nb, TiO2, La, Ni, Ba, Sr, Th and U versus Zr. Note that the large ion lithophile elements display virtually no correlation with Zr.

H. Lapierre et al. / Chemical Geology 201 (2003) 55–89 75

Fig. 8. Chondrite-normalized (Sun and McDonough, 1989) rare earth element patterns for volcanic rocks and dolerites of the Slide Mountain

and Cache Creek Terranes: N-MORB (Type 1), alkalic (Type 2) and oceanic plateau basalts (Types 3 and 4).

H. Lapierre et al. / Chemical Geology 201 (2003) 55–8976

Fig. 9. Primitive mantle-normalized (Sun and McDonough, 1989) multi-elements plots for volcanic rocks and dolerites of the Slide Mountain

and Cache Creek Terranes: N-MORB (Type 1), alkalic (Type 2) and oceanic plateau basalts (Types 3 and 4).

H. Lapierre et al. / Chemical Geology 201 (2003) 55–89 77

H. Lapierre et al. / Chemical Geology 201 (2003) 55–8978

of decreasing abundance, they are basalts, dolerites,

cumulate ultramafic and mafic rocks (Tardy et al.,

2001, 2003).

Basalts display porphyritic, intersertal and/or

quenched textures and consist of either plagioclase

laths embedded in clinopyroxene and microcrystalline

groundmass or quenched plagioclase and clinopyrox-

ene crystals set in a vesicular groundmass. Dolerites

and fine-grained gabbros display ophitic textures.

Cumulate wehrlites and pyroxenites are restricted to

the Slide Mountain terrane (Fig. 5; Struik and Or-

chard, 1985). Layered cumulate gabbros contain cu-

mulus augite and orthopyroxene and accumulate

plagioclase.

All the igneous rocks of the Slide Mountain and

Cache Creek Terranes have undergone low-grade

metamorphism. Plagioclase is replaced by albite,

sericite or less frequently by epidote or epidote + hy-

drogarnet (in the cumulate peridotites). Olivine and

Fig. 10. eNdi versus (Sm/Yb)N (A), Th (ppm, B) and (87Sr/86Sr)i (C) di

Terranes.

orthopyroxene are replaced by smectites or serpentine.

Clinopyroxene is generally preserved but sometimes

is replaced by chlorite, actinolite or Mg-rich amphi-

bole. Oxides (Ti-rich or Ti-poor magnetite) are

replaced by titanite or Ti-rich hastingsitic magnesio-

hornblende (Tardy et al., 2003). Vesicles are filled

with calcite, chalcedony or smectiteF pumpellyiteFzeolite.

The foliated metamorphic rocks from Murray

Ridge (Fig. 4b; PG97-17, PG01-40, PG01-77; Table

1) consist of diopside + orthopyroxene + picotite (Fig.

6)F forsterite (Fo91–92). Trembleur harzburgites and

dunites (Fig. 4b; SH903b, SH902a, SH912a, TU1-2,

Table 1) are intensely serpentinized and sheared.

5.2. Effects of alteration on element mobility

The altered nature (up to lower greenschist fa-

cies) of the igneous rocks from the Slide Mountain

agrams for igneous rocks of the Slide Mountain and Cache Creek

Fig. 11. (207Pb/204Pb)i versus (206Pb/204Pb)i (A) and eNdi versus (206Pb/204Pb)i (B) diagrams for igneous rocks of the Slide Mountain and Cache

Creek Terranes.

H. Lapierre et al. / Chemical Geology 201 (2003) 55–89 79

Fig. 12. Chondrite-normalized (Sun and McDonough, 1989) rare

earth patterns and primitive mantle-normalized (Sun and McDo-

nough, 1989) multi-elements plots for cumulate rocks of the Slide

Mountain and Cache Creek Terranes.

H. Lapierre et al. / Chemical Geology 201 (2003) 55–8980

and Cache Creek Terranes means that before any

petrological inferences can be drawn from the

chemistry of the rocks, we must study the chemical

effects of element mobility. Most of the analyzed

rocks have an LOI < 4% (Table 1) with the excep-

tion of the blocks from the Cache Creek Melange

which have LOIz 7%. The high LOI values are due

in part to the presence of calcite-or smectites-filled

vesicules.

Zirconium which is considered as immobile during

low grade alteration of igneous rocks of mafic to

intermediate composition (Gibson et al., 1982) has

been plotted against minor (TiO2), compatible (Ni) and

incompatible trace elements (Sr, Ba, Nb, La, Th and U)

in Fig. 7. Nb, La and Th correlate well with Zr but

some scatter is observed in the TiO2 versus Zr plot.

Ba and Sr display no correlation with Zr, which

implies that the large ion lithophile elements (LILE)

have been mobilised. The volcanic blocks of the Cache

Creek Melange display the highest Ba and Sr abun-

dances together with the highest LOI, which may

indicate that they are the most altered rocks. Thus,

the variations of these elements will not be discussed

further.

5.3. The volcanic rocks and dolerites from the Slide

Mountain Terrane and Cache Creek Complex

From the major, trace element and isotope (Nd, Sr

and Pb) compositions of the volcanic rocks and

dolerites, four groups of rocks have been distin-

guished in both terranes.

Type 1, which comprises the majority of the Slide

Mountain basalts and dolerites, and the basaltic and

doleritic dykes of the Pinchi Fault, is characterized by:

(i) light rare earth element (LREE) depleted patterns

(Fig. 8) [0.65 < (La/Yb)N < 0.98] and (ii) a depletion

(relative to the more compatible elements) in LREE, Zr,

Hf and Th (Table 1; Fig. 9). No negative Nb and Ta

anomalies have been detected (0.9 < La/Nb < 1.7).

These features are those of N-MORB, and in good

agreement with the eNd values and Pb isotopic ratios

(Figs. 10 and 11). The Pinchi Fault basalt (PG96-12,

Table 1) differs from the other Type 1 rocks because it

has a slight positive Nb anomaly (La/Nb = 0.6; Fig. 9).

The dolerites from the Pinchi Fault and the melange

have higher La/Nb ratios (f 2.3–2.6). All these rocks

have high eNd values (+ 7.8V eNdV + 10.1, Table 2),

which correlate with low [(Sm/Yb)N < 2) ratios and Th

contents (Th < 0.24 ppm; Figs. 9 and 10C). The Slide

Mountain Type 1 is characterized by the highest eNdvalues ( + 8.6V eNdV + 10.1; Table 2). The basalt and

dolerites from the Pinchi fault and in blocks within the

melange differ from the Slide Mountain rocks by: (i)

marked negative Zr and Hf anomalies (Fig. 9) and (ii)

lower eNd values (Table 2). In the 207Pb/204Pb and eNdversus 206Pb/204Pb plots (Fig. 11A and B), Type 1

(basalt and dolerites) clusters in the MORB field.

H. Lapierre et al. / Chemical Geology 201 (2003) 55–89 81

However, two plutonic rocks of the Cache Creek

complex and one Slide Mountain basalt differ from

the other Type 1 ones by higher 207Pb/204Pb (Table 2).

Type 2 is mainly composed of alkali basalts that

are enriched in LREE [1.08V (La/Yb)NV 10.3; Fig.

8] and in Ti, Zr, Hf, Nb, Ta and Th (Fig. 9). La/Nb is

always < 0.9. The eNd values of the Cache Creek

rocks are the lowest of the analyzed rocks and in the

range of oceanic island basalts (+ 2V eNdV + 8; Fig.

10A, Table 2). eNd correlates inversely with (Sm/

Yb)N (Fig. 10A). The Cache Creek rocks (except two

samples) have also the highest (206Pb/204Pb)i that fall

in the range of OIB (Table 2; Fig. 11A). The eNdvalues of the Slide Mountain Permian volcanic rocks

are higher ( + 4.8 to + 9.6) than those of Cache Creek

and, among these rocks, some samples exhibit a trend

towards EM2 field in the eNd-(206Pb/204Pb)i plot

(Fig. 11B).

Fig. 13. Chondrite-normalized (Sun and McDonough, 1989) rare earth pat

multi-elements plots for foliated pyroxenites from Murray Ridge and Trem

Type 3 is made up primarily of Cache Creek

basalts which occur either interbedded with volcani-

clastic sediments (Sowchea succession, PG97-8,

PG97-24, PG97-25; Fig. 4) or as blocks within the

Cache Creek melange (95CC1, 95CC2; Fig. 2). This

type has flat REE patterns [(La/Yb)N = 1.1–1.4], REE

contents higher than 10 times chondritic, La/Nb ratios

close to the unity (0.8 < La/Nb < 1.22) and isotope

compositions intermediate between those of the two

other types (Tables 1 and 2; Figs. 9, 10 and 11). All

Type 3 rocks exhibit a negative correlation between

eNdi and (206Pb/204Pb)i. One sample (PG97-25) plots

in the EPR-MORB field (Fig. 11B).

Finally, the Cache Creek complex (Sowchea suc-

cession; Fig. 4) is distinguished from Slide Mountain

Terrane and Cache Creek melange by the presence of

basalts and icelandites (PG96-23, PG96-24, PG96-11c;

Table 1) with convex REE patterns [(La/Sm)N = 0.6;

terns and primitive mantle-normalized (Sun and McDonough, 1989)

bleur harzburgites.

H. Lapierre et al. / Chemical Geology 201 (2003) 55–8982

Fig. 8J]. Type 4 shares with Type 2 similar La/Nb

(0.7VLa/NbV 0.85). Types 3 and 4 have similar Nb/

Zr ratios (Table 2; Fig. 10).

5.4. Plutonic rocks of the Cache Creek complex and

Slide Mountain Terrane

The plutonic rocks are characterized by depleted

LREE patterns [(La/Yb)N =f 0.9; Fig. 12A]. The

clinopyroxenite has the highest REE contents and a

small negative Eu anomaly (Eu/Eu* = 0.9). Gabbros

(PG96-14, 01PG61) have negative Th, Nb and Ta

anomalies, which are absent in the clinopyroxenite

and wehrlite (Fig. 12B). The marked positive Eu (Eu/

Eu* = 1.33) and Sr anomalies observed in the primi-

tive mantle-normalized pattern of the gabbros can be

attributed to plagioclase accumulation (Fig. 12B). The

plutonic rocks have high eNd values ( + 8.0 to + 9.1)

and low (206Pb/204Pb)i ratios (Table 2; Figs. 10 and

11), which are similar to those of Type 1 volcanic

rocks. The Slide Mountain cumulates have higher

(207Pb/204Pb)i than the Cache Creek gabbro which

falls in the MORB field (Fig. 11A).

5.5. Foliated ultramafic rocks of the Cache Creek

complex

The REE contents of the Trembleur harzburgites

and Murray Ridge clinopyroxene-rich ultramafic rocks

are very low (0.1 times chondritic especially for the

LREE). The U-shaped chondrite-normalised REE pat-

terns of the Murray Ridge and Trembleur ultramafics

are very similar to those of very depleted abyssal

harzburgites (Frey, 1984; Sharma and Wasserburg,

1996; Fig. 13). However, harzburgites differ from the

pyroxene-rich ultramafic rocks by their marked nega-

tive Eu anomalies (0.44VEu/Eu*V 0.77), lower La/

Sm ratios (1.14V (La/Sm)NV 2.73) and negative eSr(� 15.44; Table 2). The primitive mantle-normalized

multi-element diagrams for the foliated peridotites

(Fig. 13) are characterized by (1) very low contents

in incompatible trace element (less than 0.1 primitive

mantle abundances) with the exception of LILE; (2)

positive Ba, Pb, U and HREE anomalies; and (3)

negative Th and Zr anomalies. The Murray Ridge

ultramafic rocks differ from the harzburgites by the

absence of Nb negative anomalies and high positive

eSr ratios ( + 29V eSrV + 127; Table 2).

6. Discussion

6.1. Slide Mountain Terrane

In the Slide Mountain Terrane, Types 1 and 2 are

predominantly basalts which occur within the same

thrust sheet. N-MORB type basalts predominate in the

Slide Mountain exposed near Bakerville. Indeed,

these volcanic rocks are LREE-depleted, their La/Nb

ratios range between 0.9 and 1.7, their eNd values and

Pb initial isotopic ratios fall in the range of N-MORB.

However, some Slide Mountain volcanic rocks (Type

2), sometimes less mafic than the basalts, differ from

the N-MORB type basalts by LREE-enriched pat-

terns, higher contents in TiO2, Nb, Ta and Th. Among

these volcanic rocks, some samples have lower eNdvalues (Fig. 10, Table 2) and higher Pb initial ratios

(Fig. 11, Table 2). Correlations between incompatible

elements such as Nb and Th versus (La/Sm)N shown

in Fig. 14 and the Th–Zr positive correlation (trend 1;

Fig. 7, Th < 1.7), suggest that Types 1 and 2 volcanic

rocks and dolerites are genetically linked. Both types

could derive from the mixing of two mantle sources, a

depleted mantle MORB and an enriched OIB one.

Sample 85-158A which is characterized by the lowest

Th and Nb contents (Table 1), and the higher eNdvalue ( + 10; Table 2, Fig. 10) could represent the

depleted mantle source. Sample 42-e (Type 2) char-

acterized by the highest Th and Nb contents (Table 1),

and the lowest eNd value ( + 4.8; Fig. 10) could

represent the enriched mantle source which is similar

to OIB. The differences in the incompatible trace

element contents of Type 1 basalts (Table 1) and Type

2 dolerite (sample 84-156; Table 1) can be attributed

to variations of the partial melting ratio because Type

2 dolerite share with Type 1 basalts similar eNd values(Table 2).

The cumulate ultramafic rocks of the Slide Moun-

tain share with the Type 1 volcanic rocks similar Nd

and Pb isotopic compositions suggesting that they

derived from a Depleted Mantle MORB (DMM)

source.

The Slide Mountain N-MORB type volcanic rocks

sampled near Bakerville are geochemically similar to

those of the Middle Pennsylvanian (300 Ma) Fennell

Formation and Mississippian–Upper Permian Nina

Group from British Columbia (Fig. 2; Ferri, 1997).

In all these units of the Slide Mountain Terrane, the

Fig. 14. Nb (A) and Th (B) versus (La/Sm)N plots for the Silde Mountain volcanic rocks and Nb/Zr versus Ti/Y plot (C) showing data points for

Types 2, 3 and 4 of the Slide Mountain and Cache Creek Terranes. Upper Cretaceous Colombian (Kerr et al., 1997) and Ontong Java (Neal et

al., 1997) oceanic plateau basalts are shown for comparison.

H. Lapierre et al. / Chemical Geology 201 (2003) 55–89 83

volcanic rocks exhibit features of N-MORB type

volcanics (Smith and Lambert, 1995; Ferri, 1997).

Rocks geochemically (enriched in incompatible trace

elements) similar to Type 2 are not mentioned in the

Fennell Formation and Nina Group. Nevertheless,

some volcanic rocks of the Fennell Formation differ

from the others by lower eNd values (f + 7.7; Smith

and Lambert, 1995), which are in the range of the

Slide Mountain Type 2 (Table 2). However, the

incompatible trace element chemistry of the Fennell

volcanic rocks with eNd=f + 7.7 do not depart from

those belonging to the same formation characterized

by higher eNd values, similar to those of N-MORB.

On the basis of geological and stratigraphical

evidence, the Slide Mountain Terrane is interpreted

as remnant of a Late Paleozoic back-arc basin located

west of the North American plate. Nevertheless, the

Slide Mountain N-MORB type volcanic rocks sam-

pled near Bakerville as well as those of the Fennell

Formation and Nina Group, do not exhibit geochem-

ical features typical of back-arc basin basalts (BABB).

The latter show transitional features between N-

MORB and island arc tholeiites (Saunders and Tarney,

1984). The La/Nb ratio of the Slide Mountain Terrane

is always lower than 1.7, while BABB generally have

La/Nb higher than 2.5. Their Zr contents fall in the

range of N-MORB, while BABB are characterized by

lower Zr abundances (Saunders and Tarney, 1984). In

fact, basalts developed along spreading ridges in

evolved back-arc basins (e.g. Parace Vela; Saunders

and Tarney, 1984) are undistinguishable from N-

MORB. In contrast, narrow back-arc basins are

floored with basalts characterized by (i) negative Nb

and Ta anomalies of variable magnitude, (ii) a lower

H. Lapierre et al. / Chemical Geology 201 (2003) 55–8984

TiO2 content and (iii) an enrichment in LILE, relative

to N-MORB.

Finally, the most important feature to discriminate

BABB from N-MORB is the nature of the sediments

associated with the volcanic rocks. Marginal basins

adjacent to oceanic island arcs receive volcaniclastic

debris from the bordering volcanic arc, the back-arc

spreading center and, to a lesser degree, the remnant

arc. The volcanic arc is volumetrically the most

important source because abundant volcaniclastics

are produced by explosive subaerial and/or subaque-

ous eruptions and erosion of the arc complex. Back-

arc spreading centres supply a small volume of deep

water hyaloclastites and hydrothermally derived vol-

canogenic sediments which are incorporated into the

basal parts of the sedimentary sequences (Carey and

Sigurdsson, 1984). Thus, the Pennsylvanian to Lower

Permian cherts and pillow basalts that constitute the

upper sequence of the Slide Mountain, could repre-

sent the basal part of the back-arc basin. However,

volcaniclastic sediments have never been reported in

the entire upper volcanic sequence of the Slide

Mountain. If the Slide Mountain basin was juxtaposed

to an active Devonian to Mississippian arc, then arc-

derived volcaniclastic rocks should occur in the

Mississippian–Pennsylvanian strata of the upper se-

quence of the Slide Mountain Terrane. Thus, the

absence of arc-derived volcaniclastic rocks in the

Slide Mountain Terrane can be explained in two

ways. (1) The Slide Mountain Terrane may represent

remnants of an evolved back-arc basin similar to the

present-day Parace Vela basin (Philippine sea). In this

case, a Devonian to Mississippian remnant Andean-

type arc should be present east of the Slide Mountain

Terrane. For us, no Early Paleozoic arc-rocks are

exposed east of the Slide Mountain Terrane. (2) The

Slide Mountain Terrane may represent an oceanic

basin located immediately east of the Early Paleozoic

continental (?) arc but the opening and development

of this basin is not linked to the splitting of the Early

Paleozoic continental arc. However, in both hypoth-

eses, the Slide Mountain Terrane represents an oce-

anic basin fringing the western margin of North

America (Patchett and Gehrels, 1998). The geochem-

istry of the Early Permian volcanic rocks indicate that

they could have been derived from the mixing be-

tween N-MORB and OIB Mantle sources. This sug-

gests that these rocks were likely produced by

asthenosphere-plume interaction; the latter occurring

when an aseismic ridge was located near a spreading

center. Thus, in the Early Permian, the Slide Moun-

tain basin was floored by oceanic crust locally thick-

ened by mantle plume magmas. This could have been

the case for any geodynamic setting of the Slide

Mountain Terrane.

6.2. Cache Creek Terrane

6.2.1. N-MORB type rocks

With the exception of samples 99CC1 and 99CC2

from the knockers of the Cache Creek melange, Type

1 dolerites, basalt and gabbros of the Cache Creek

Terrane were collected from two different tectonic

slices within the Pinchi Fault system (Fig. 2). The

trace element (Table 1) and isotopic (Table 2) chem-

istry of the dolerites and basalt is similar to N-MORB.

However, the basalt (PG96-14) and one dolerite

(PG96-15) are distinguished from typical N-MORB

by their slight positive Nb and Ta anomalies (0.6 < La/

Nb = 0.9; Fig. 9A), which is a feature of E-MORB.

Three dolerites (PG96-17, 01PG64 and 99CC1) La/

Nb ratios (f 2.5; Table 1) are higher than those of N-

MORB and fall in the range of BABB. Sample 99CC2

is distinguished from Type 1 rocks by its very low

trace element contents. All these rocks have uniform

Nd and Pb isotopic compositions. The cumulate

gabbros are geochemically similar to those formed

along mid-oceanic ridges.

Thus, Type 1 igneous rocks of the Cache Creek

Terrane represent fragments of oceanic crust. In the

absence of precise dating of the gabbros and doleritic

dykes, it is difficult to constrain the tectonic setting of

these rocks.

6.2.2. Plume-related volcanic rocks

Cache Creek Types 2, 3 and 4 volcanic rocks are

interbedded within the Upper Triassic sediments of

the Sowchea succession which is always in faulted

contact with tectonic slices of the Pinchi fault.

Cache Creek Type 2 exhibits trace element and

isotopic chemistry of alkali basalts, i.e., LREE

enriched patterns (Fig. 8C; La/Nb < 0.9, low eNdvalues and high Pb/Pb ratios; Table 2). Compared to

the Slide Mountain Type 2 volcanics, the Cache Creek

rocks exhibit generally lower eNd values and higher

Pb ratios. Cache Creek Type 2 volcanic rocks plot in

H. Lapierre et al. / Chemical Geology 201 (2003) 55–89 85

the OIB field (Figs. 10C and 11). eNd correlates

inversely with highest (Sm/Yb)N; Fig. 10A). Thus,

the features of the Cache Creek Type 2 rocks suggest

that they derived from an enriched OIB type mantle

source.

Type 3 exhibit flat REE patterns, and eNd values

ranging between + 7 and + 9. They display moderate

enrichment in Nb, Ta and Th, which is a feature of

oceanic plateau basalts. Because of its flat REE pat-

tern, Type 3 could be similar to T-MORB or E-MORB.

But it shares with the Ontong Java and Upper Creta-

ceous Colombian basalts similar La/Nb (0.9VLa/

NbV 1.02) and Zr/TiO2 ratios at similar Nb/Y ratios

(f 0.2; Kerr et al., 1997; Neal et al., 1997; Tardy et al.,

2001). It differs from E-MORB or T-MORB by higher

La/Nb ratios (E-MORB: La/Nb = 0.75; Sun and

McDonough, 1989; T-MORB: La/Nb of = 0.68;

Juteau and Maury, 1997) and lower LREE enrichments

[1.9 < (La/Yb)N < 3.68; Sun and McDonough, 1989;

Juteau and Maury, 1997]. Type 3 falls at or near the

boundary of the field of the Cretaceous Colombian

oceanic plateau basalts (Fig. 14C; Kerr et al., 1997).

Thus, on the basis of these characteristics, we suggest

that Type 3 displays oceanic plateau basalts affinities.

However, Th contents of Type 3 are slightly higher

than those of Type 1 and lower than those of Type 2

(Table 1, Fig. 9D).

Type 4 differs from Type 3 only by its convex REE

pattern. It exhibits trace element (Fig. 9E) and eNdvalues (Table 2), similar to those of oceanic plateau

basalts.

Thus, the mafic volcanic rocks of the Sowchea

succession display, whatever their type, features of

plume-related magmas. Because of the poor expo-

sures and tectonic imbrication, it is difficult to

estimate the thickness of the volcanic pile within

the Sowchea succession. However, it is reasonable to

consider that the Sowchea succession represents the

extrusive part of an oceanic plateau, represented by

Types 3 and 4.

Until recently, the Cache Creek Complex was

considered as a far-travelled terrane floored by oce-

anic crust developed along a mid-oceanic ridge (Mon-

ger et al., 1972). Near Fort St. James, the Cache Creek

Terrane is composed of Upper Triassic volcanic rocks

which have geochemical features of plume rocks

(OPB and OIB). These plume-related volcanic rocks

are tectonically associated with N-MORB-type doler-

ites and gabbros. Further south, N-MORB type basalts

and plume-related mafic rocks are present within the

Cache Creek melange. This suggests that the Cache

Creek Terrane represents the remnants of an oceanic

crust on which oceanic plateau and intra-oceanic

islands developed. The alkali basalts and hawaiites

(Type 2) may represent the last products of the plume

activity, after the eruption of the oceanic plateau

basalts.

6.2.3. Foliated ultramafic rocks

The tectonic setting of the foliated ultramafic rocks

remains a subject of debate. The Trembleur serpenti-

nized harzburgite which is intruded by diorite-por-

phyries of arc affinity, is geochemically similar to

extremely depleted upper mantle ultramafic rocks

(i.e., abyssal peridotites) and could represent the

remnants of the Cache Creek oceanic lithospheric

mantle. Therefore, the Trembleur ultramafites may

represent remnants of the oceanic lithosphere on

which the Cache Creek plume-related rocks were

emplaced. Later, these ultramafic rocks were intruded

by arc-related magmas.

The setting of the Murray Ridge foliated rocks is

less obvious. We had previously suggested that the

Murray Ridge ultramafites represent the roots of the

Quesnel arc (Tardy et al., 2001) because rocks of

similar chemistry are found in the Ronda and

Kohistan peridotites and could result from the

percolation of asthenospheric magmas into litho-

spheric mantle (Garrido and Bodinier, 1999; Garrido

et al., 2000). On the basis of the geochemical

features of the Murray Ridge foliated rocks, we

propose instead that these rocks represent the roots

of the Cache Creek oceanic plateau because they are

distinct from those of Ronda by an absence of

negative Nb anomalies. The Murray Ridge pyrox-

enites have significantly lower trace element con-

tents and REE patterns than lithospheric mantle

pyroxenites (Santos et al., 2002). However, the

(87Sr/86Sr)i of the Murray Ridge foliated ultramafic

rocks are higher than those of lithospheric mantle

pyroxenites (Santos et al., 2002). The presence of

undeformed pyroxenite veinlets cross cutting the

foliated ultramafic rocks and the absence of the

negative Nb anomaly, are consistent with this hy-

pothesis, because plume magmas are characterized

enrichment of Nb relative to La.

H. Lapierre et al. / Chemical Geology 201 (2003) 55–8986

The Cache Creek Terrane represents an accretion-

ary prism that includes also arc-lavas and sedimentary

rocks (Quesnel arc; Struik et al., 2001), Paleozoic

platform carbonates and high-pressure metamorphic

rocks. This accretionary prism formed when the

thickened Cache Creek oceanic crust collided with

the Quesnel arc during the Early Jurassic. Because

thick oceanic plateaus resist subduction (Nur and Ben

Avraham, 1982; Burke, 1988), they may be accreted

to continents (Saunders et al., 1996; Kerr et al., 1997).

Cache Creek Triassic oceanic crust could not be

subducted under the Quesnellia arc, and a significant

part of this crust was accreted to the North American

margin during the collision of the Quesnellia arc with

North America.

The tectonic setting of the Side Mountain Terrane

is that of an Upper Paleozoic oceanic domain fringing

the western margin of North America. Between the

Late Permian and the Jurassic (Struik and Orchard,

1985), the Slide Mountain oceanic basin was thrust

upon the sediments deposited on the North American

margin and was accreted to the craton. The subduction

of this oceanic crust led to the development of the

Quesnellia arc. The N-MORB type oceanic rocks

found in the Cache Creek Terrane are assumed to be

Late Paleozoic to Triassic in age (Struik et al., 2001).

The Cache Creek Complex itself appears to be com-

prise the remnants of a Triassic–Jurassic subduction

complex caught between the colliding blocks of

Quesnel and Stikine arc terranes. The minimum age

of the collision between the Quesnel and Stikine arcs

is Middle Jurassic (Struik et al., 2001). Rocks of

oceanic affinity of the Cache Creek Terrane are

presently exposed, because they represent remnants

of oceanic crust thickened by plume rocks. If the

Cache Creek Terrane had been floored only by oce-

anic crust developed along a mid-oceanic ridge, no

remnants of this crust would be found now, because

this crust would have been completely destroyed by

subduction leading to growth of the Quesnellia and

Stikinia arcs.

7. Conclusion

Incompatible trace element chemistry and Nd and

Pb isotopic compositions of the Slide Mountain and

Cache Creek volcanic and cumulate rocks of mafic

composition show that these rocks were generated

from the mixing of depleted N-MORB and OIB

mantle sources. N-MORB type basalts predominate

in the Slide Mountain basin while volcanic rocks with

oceanic plateau and alkalic affinities are the main

components of the Cache Creek Complex. These data

constrain the geodynamic environment of the Slide

Mountain and Cache Creek oceanic terranes. The

Slide Mountain basin developed offshore the North

American craton from a ridge-centered or near-ridge

hotspot during the Late Paleozoic while the Cache

Creek Complex is related to the activity of a large

plume during the Late Triassic. The presence in the

Cache Creek Complex of cumulate gabbros intruded

by dolerites of N-MORB affinities and depleted

harzburgites and dunites intruded by subduction-re-

lated diorite porphyries suggest that remnants of deep

parts of an oceanic crust are exposed within this

terrane. Assuming that the Murray Ridge depleted

pyroxene-rich foliated ultramafic rocks could repre-

sent the depleted mantle rocks of an oceanic plateau,

the Cache Creek Terrane probably represents the

remnants of an oceanic crust thickened by plume-

related magmas, i.e., oceanic plateau basalts and their

mantle residues. These data emphasize the importance

of oceanic plateaus in the formation of juvenile mafic

crust in North America during the Early Mesozoic.

Moreover, the Late Triassic plume-related rocks ex-

posed in the accreted terranes of the Canadian Cor-

dillera are related to the major mantle plume activity

that occurred during the Permian–Triassic.

Acknowledgements

This work was funded by the Laboratoire Geo-

dynamique des Chaınes Alpines, (LGCA), Universite

de Savoie and Ministere de l’Education Nationale et

de la Recherche. Field expenses were supported by

the Nechako NATMAP project. Many thanks to

Pierre Brunet (LMTG, UMR-CNRS 5563, Toulouse),

Beatrice Gallaud (ISTEEM, Montpellier), Francine

Keller (LGCA, UMR-CNRS 5025, Grenoble), Phil-

ippe Telouk (ENS, Lyon), Francis Coeur for their

technical assistance. Thanks to Nick T. Arndt, Kent

Condie, Stephen J. Piercey and Steve L. Goldstein

for their helpful and constructive comments on the

manuscript. [SG]

H. Lapierre et al. / Chemical Geology 201 (2003) 55–89 87

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