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 (Spius 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 (" Nd .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. Spius lavas have comparable K 2 O and Pb 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% P 2 O 5 , 15 ppm Nb, and 240 ppm Zr. Spius Nd À Sr isotopic compositions (" Nd (100Ma) ¼ þ 5.3 to þ 6.9, " Sr (100Ma) ¼ À 14 to À 25) define an array extending from Pimainus to alkaline seamount compositions. The low " Sr values, elevated high field strength element contents, and moderate silica contents suggest Spius 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. Spius volcanism may have been generated by asthenospheric upwelling triggered by slab window development or slab-hinge rollback on closure of the Methow-Tyaughton basin.

The Eocene–Oligocene magmatic hiatus in the south-central Canadian Cordillera: a capture of the Kula Plate by the Pacific Plate?

“…They also differ significantly from Eocene alkaline rocks of the Penticton Group of southern British Columbia, which have ε Nd values ranging from -4 to -6 and high initial Sr isotopic ratios (0.706 450 -0.706 494; Dostal et al 2003). However, the BDM values overlap the data reported by Ghosh (1995), Smith and Thorkelson (2002), and Dostal et al (2005) for Mesozoic basement rocks in the Eocene volcanic belt in central and southern British Columbia ( Fig. 10; Hazelton Group, Dostal et al 2005).…”

“…The source, therefore, probably had an arc-like trace element composition and probably resembled island-arc basalts. Such rocks are present in the Mesozoic basement of Stikinia (Smith and Thorkelson 2002;Dostal et al 2005). The large volume of andesitic and dacitic rocks with high ε Nd and low 87 Sr/ 86 Sr values suggests melting of a juvenile mafic source at the base of the crust induced by the rise of fluids from the Benioff zone at a depth of ca.…”