Constraints on slab inputs and mantle source compositions in the northern Cascade arc (Garibaldi belt) from Sr-Nd-Pb-Hf isotopes and trace elements in primitive basalts

The northernmost segment of the Cascade arc, known as the Garibaldi volcanic belt (GVB), extends from Glacier Peak in Washington to the Bridge River cones in British Columbia. GVB primitive basalts display strong arc-parallel geochemical gradients, most prominently a northerly progression from calc-alkaline to highly alkalic compositions, which present an ideal opportunity to address key questions regarding the origin of primary arc basalts including the compositions and relative inputs of sub-arc mantle sources and slab-derived fluids/melts. The gradient in GVB basalt alkalinity was proposed to result from a northerly reduction in slab-derived contributions to the sub-arc mantle wedge, a consequence of the ~4 myr decrease in slab age at the trench [1-3]. As a test of this hypothesis, we have obtained new whole-rock high-precision isotopic (Sr, Nd, Pb, Hf) and trace element data for the GVB basalt suites previously investigated by Green and others. La/Nb decreases from south (4.25 at Glacier Peak) to north (0.78 at Bridge River), confirming a progressive reduction in the "arc signature" (elevated LILE and LREE abundances relative to HFSE and HREE). ⁸⁷Sr/⁸⁶Sr ranges from 0.70310 to 0.70396, ²⁰⁶Pb/²⁰⁴Pb from 18.65 to 18.92, ɛ_Nd from 8.5 to 3.8, and ɛ_Hf from 13.3 to 8.7. Our data overlap the Sr-Nd-Pb isotopic compositions of primitive samples at Mt. Baker and Chilliwack batholith [4], but extend to more depleted compositions. GVB basalts are isotopically distinct from other Cascade arc primitive basalts, with systematically lower ²⁰⁸Pb/²⁰⁶Pb at a given ²⁰⁶Pb/²⁰⁴Pb and higher ɛ_Nd at a given ⁸⁷Sr/⁸⁶Sr. In Pb-Pb space, GVB basalts define a linear trend extending from Explorer MORB to local subducting sediments drilled at ODP Sites 888 and 1027 in the northern Cascadia basin. We interpret this array as a mixing line reflecting variable sediment input to the mantle. However, Sr, Pb and Nd isotope ratios are only weakly correlated with La/Nb and latitude, whereas ²⁰⁸Pb*/²⁰⁶Pb* and ɛ_Hf values decrease markedly to the north and are inversely correlated with TiO₂. At a given ɛ_Nd, Glacier Peak and Chilliwack samples have distinctly higher ²⁰⁸Pb*/²⁰⁶Pb* and ɛ_Hf than other GVB samples, subdividing the GVB into two parallel isotopic trends. We conclude that GVB basalt geochemistry cannot be explained solely by reductions in slab age and sediment input; rather, slab contributions are superimposed on a mantle wedge with inherent arc-parallel compositional variability that is unrelated to the subduction regime. Trace element model calculations, phase equilibria, and Ba-Nb-La/Yb-Yb systematics indicate that the mantle sources of northern GVB basalts are more enriched in trace elements and Na than those in the south, reflecting different source compositions. This study demonstrates the importance of obtaining high precision isotopic data on arc basalts, as mantle source heterogeneities in arcs may not be revealed by trace element abundances alone. [1] Green (2006) Lithos 87, 23-49 [2] Green and Harry (1999) Earth Planet. Sci. Lett. 171, 367-381 [3] Green and Sinha (2005) Jour. Volc. Geoth. Res. 140, 107-132 [4] Mullen (2011) PhD dissert.