Geochemistry of Garibaldi Lake andesites and dacites indicates crustal contamination involved in formation of Northern Cascade arc lavas

The Cascade Arc presents a unique setting for studying the controls on andesite genesis and the implications for growth and evolution of the continental crust. It is the type-locality for a ';hot' subduction zone, where the downgoing slab is young and subduction is relatively slow. The northern segment of the Cascade arc, the Garibaldi Volcanic Belt (GVB), hosts the youngest subducting crust in Cascadia and the termination of the subducting slab. These conditions may affect magma generation processes by reducing the amount of water reaching the area of melt generation [1,2] and imparting an adakitic signature to magmas generated there if the slab edge melts [3]. We provide insights on the origin of andesites and dacites from the Garibaldi Lake area using new high-precision Pb, Sr, Nd, Hf isotope ratios and trace element data. Andesites and dacites from the Garibaldi Lake area (The Black Tusk, Mt. Price, and The Table) are calc-alkaline and show evidence for crustal contamination such as positive correlations between Ba/Nb and SiO₂. Silica variation diagrams show no systematic trend for any of the volcanic centres, suggesting the presence of distinct magma batches. Garibaldi Lake andesites and dacites have among the least radiogenic Pb isotope ratios of all Cascade arc lavas, and define a linear array in Pb-isotope space. This most likely reflects mixing between MORB-source mantle (similar to Gorda and Explorer plate sources) and locally subducting sediments [4]. However, relative to GVB basalts and lavas from the rest of the Cascades (High Cascades), the andesites and dacites have higher ²⁰⁷Pb/²⁰⁴Pb (15.55-15.56) for a given ²⁰⁶Pb/²⁰⁴Pb (18.66-18.74). The Garibaldi Lake lavas also have higher ⁸⁷Sr/⁸⁶Sr (0.7033-0.7036) and lower ɛ_Nd (5.8-7.9) at a given ²⁰⁶Pb/²⁰⁴Pb than GVB basalts and High Cascades lavas but among the highest ɛ_Nd for a given SiO₂ for the whole of the Cascades. ɛ_Hf values (10.5-13.5) are higher at a given SiO₂ value for Garibaldi Lake evolved rocks than for GVB basalts and High Cascades lavas, except for those at Glacier Peak and Mt. Baker. We attribute these results to crustal contamination by underlying Coast Plutonic Complex country rocks [5,6], which span a wide range of compositions in isotopic space. Evolved rocks from the Garibaldi Lake area show clear geochemical differences from the High Cascades. This may reflect variations in compositions of host plutonic rocks which transfer their geochemical signatures to the forming magmas. Differences between the GVB and High Cascades andesites and dacites could also represent variations in magma generation processes such as parent magma compositions (e.g., more alkalic in the northern GVB), amount of crustal assimilation, fractionation, and mixing occurring during magma generation. [1] Green & Harry (1999) Earth Planet Sci 171, 367; [2] Harry & Green (1999) Chem Geol 160, 309; [3] Thorkelson & Breitsprecher (2004) Lithos 79, 25; [4] Carpentier et al. (2013) Chem Geol 340, 91; [5] Cui & Russell (1995) GSA 107, 127; [6] Friedman et al. (1995) Can J Earth Sci 32, 1681.