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.