Late Paleozoic and Triassic plume-derived magmas in the Canadian Cordillera played a key role in continental crust growth

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 εNd 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 εNd 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 εNd 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 contributed to the growth of the North American continent as when they were accreted to the craton during the Early Triassic and Jurassic times.

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 Lennan, 1985, Ben-Abraham et al., 1981. An alternative 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 Abouchami et al., 1990, Stein and Hofmann, 1994, Saunders et al., 1996. Most of the Cordilleran orogens 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 proposed for the Archean continents.

Oceanic plateaus are produced by the partial melting 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 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 potential 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 crustal underplating involving the intrusion of sills and plutons and overplating of volcanic rocks. Magma additions occur in settings such as arcs, rifted continental 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 accretion 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 Arabian–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 Ga), the Late Paleozoic (320–250 Ma) and the Cretaceous (123–80 Ma), and were possibly caused by superplume events. Recently, Condie (1997) proposed that the growth peaks are linked to superplumes, 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 the lower mantle and initiate the plume production when they arrive at the “D” layer.

It is commonly accepted that most Archean greenstones 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 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–Triassic 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, Condie, 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 oceanic plateaus, the well-known Wrangellia (Lassiter et al., 1995) and the two terranes that form the subject of this paper (Fig. 1, Fig. 2; Condie and Chomiak, 1996).

Two major Late Paleozoic–Triassic oceanic terranes 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-Permian oceanic-island tholeiites. West of Prince George, the eastern boundary of the Cache Creek Terrane is marked by the Pinchi Fault Fig. 2, Fig. 3, which contains slices of Paleozoic platform carbonates, undated 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 serpentinites ranging in age from Carboniferous to Permian Fig. 2, Fig. 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 represents 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 augite 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 concentrations and Nd, Sr and Pb isotopic analyses of representative rock types from the Cache Creek and Slide Mountain Terranes from the central British Columbia Fig. 2, Fig. 4, Fig. 5. On the basis of these new data, we suggest that the geodynamic environments of Cache Creek and Slide Mountain Terranes are different: (2) the Cache Creek Terrane probably represents the remnants of a Late Triassic plateau while Slide Mountain basin probably was floored by oceanic crust locally thickened by ocean island magmas.