HIGH PRECISION CHEMICAL AND ISOTOPIC CONSTRAINTS ON THE ORIGIN AND CRUSTAL PROCESSING OF NORTHERN CASCADE ARC LAVAS

The northern segment of the Cascade Arc, the Garibaldi Volcanic Belt (GVB), hosts some of the youngest and hottest subducting crust globally [1], along with the termination of the subducting slab at the Nootka Fault. The unique tectonic and thermal setting of the GVB may give rise to unusual melt production and lava compositions. The contributions of parental mafic magmas and crustal processing to evolved lavas at Mt. Garibaldi Volcanic Complex (GVC) and Mt. Meager in the GVB are assessed using new high-precision Pb-Sr-Nd-Hf isotope and trace element data.

Compared to the rest of Cascadia, evolved lavas of GVC have relatively high ε_Hf, ε_Nd and unradiogenic Pb. Lack of evidence for garnet fractionation, including no correlation between Pb isotopes and Sr/Y, suggests oceanic crust melts do not constitute a major component of GVC evolved lavas. Mt. Meager is located north of GVC, above hotter subducting lithosphere, yet evolved lavas show lower ε_Hf, ε_Nd and more radiogenic Pb than those of the GVC, inconsistent with a slab-melt origin for GVB lavas.

In Pb isotope diagrams, evolved lavas from both the GVC and Mt. Meager lie along a mixing line between Northern Cascadia subducting sediment [2] and new data for Juan de Fuca oceanic crust. Local country rocks also lie along the Pb-Pb mixing line, and have similar ε_Hf, ε_Nd and Sr isotopic compositions to the evolved lavas. However, evolved GVC and Mt. Meager lavas trend towards high Ba/La and Pb/Ce with increasing SiO₂, indicating interaction with country rocks.

Cascadia magmas define three separate mixing arrays in Pb-Pb plots, one for the High Cascades and Glacier Peak, the southernmost GVB volcano; one for mafic alkalic GVB lavas; and the other for Mt. Adams-Simcoe [3]. Isotopic data for evolved GVC and Mt. Meager lavas lie on the High Cascades array and project towards a common parental basalt composition represented by Glacier Peak high alumina olivine tholeiite.

The alkalinity of evolved GVB lavas does not increase northwards, contrary to GVB mafic lavas [4], further implying the lack of a genetic relationship between these GVB lavas.

[1] Syracuse et al. (2010) Phys. Earth Planet. Int. 183, 73-90. [2] Carpentier et al. (2014) Chem. Geol. 382, 67-82. [3] Mullen et al. (2014) this meeting. [4] Mullen & Weis (2013) G3 14, 3126–3155.