Geochemical and Nd-Sr-Pb isotopic evidence on the origin and geodynamic evolution of mid-Cretaceous continental arc volcanic rocks of the Spences Bridge Group, south-central British Columbia
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| Authors: | Smith, AD; Thorkelson, D |
| Year: | 2002 |
| Journal: | Geol. J. 37: 167-186 Article Link (DOI) |
| Title: | Geochemical and Nd-Sr-Pb isotopic evidence on the origin and geodynamic evolution of mid-Cretaceous continental arc volcanic rocks of the Spences Bridge Group, south-central British Columbia |
| Abstract: | The mid-Cretaceous Spences Bridge Group (SBG) comprises a series of basaltic to rhyolitic lavas and related volcaniclastic rocks (Pimainus Formation) overlain by a succession of mainly amygdaloidal andesites (Spins Formation) related to the closure of the Methow-Tyaughton basin and accretion of the Insular terrane in the North American Cordillera. Geochemical variation in the SBG is related primarily to metasomatic processes in the mantle wedge. Pimainus lavas include low- to high-K, tholeiitic and calc-alkaline types, and have isotopic compositions (epsilon(Nd)(100Ma) = + 5.2 to + 7.0, epsilon(Sr)(100Ma) = - 10 to - 20, Pb-206/Ph-204 = 18.82 to 18.91, Pb-207/Ph-204 = 15.55 to 15.60, Pb-208/Ph-204 38.24 to 38.43) between the ranges for primitive arcs and accreted terrane crust. Crustal sources are identified only for some low-medium K dacite and rhyolite compositions. The occurrence of intermediate compositions with high MgO contents (up to 6 wt%) and the presence of adakitic trace element features in medium-high K felsic lavas attests to metasomatism of the mantle wedge by slab melts during Pimainus volcanism. Spins lavas have comparable K2O and Ph isotopic compositions to the Pimainus, even higher MgO (up to 9.2 wt%), and display a mild intraplate character in having up to 0.6 wt% P2O5, 15 ppm Nb, and 240 ppm Zr. Spius Nd - Sr isotopic compositions (epsilon(Nd)(100Ma) = + 5.3 to + 6.9, epsilon(Sr)(100Ma) = - 14 to - 25) define an array extending from Pimainus to alkaline seamount compositions. The low epsilon(Sr) values, elevated high field strength element contents, and moderate silica contents suggest Spins volcanism was related to the introduction of small melt fractions from the asthenosphere into the mantle wedge which had previously generated Pimainus melts. The range of compositional types in the Pimainus Formation constrains tectonic scenarios to include an elevated slab thermal regime, likely from approach of an ocean ridge system toward the continental margin. Spins volcanism may have been generated by asthenospheric upwelling triggered by slab window development or slab-hinge rollback on closure of the Methow-Tyaughton basin. Copyright (C) 2002 John Wiley Sons, Ltd. |
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