Geochemistry and tectonic interpretation of some metavolcanic rock units of the western North Cascades, Washington

Abstract

Metavolcanic rocks of the western North Cascades of Washington, including the Chilliwack Group (CG), the Elbow Lake Formation (ELFm), and the Yellow Aster Complex (YAC), have been analyzed for clinopyroxene composition and whole-rock major, trace, and rare-earth elements (REE9s) to determine their original chemistry and tectonic setting. Covariant trends, elemental abundances, and ratios of high field-strength elements (HFSE9s) and REE9s indicate that the previously correlated CG and ELFm, and possibly equivalent basalts of the YAC, are separate rock units which evolved in distinct tectonic settings. Volcanic flows within the upper part of the upper Paleozoic CG are composed of tholeiitic basalt with lesser basaltic andesite and minor dacite. The CG originated as an intraoceanic island arc. Volcanic rocks of the ELFm are largely alkalic basalt, distinguished from the CG by HFSE enrichment and low Y/Nb and Zr/Nb. The ELFm is interpreted to represent structurally dismembered oceanic islands of Pennsylvanian to Jurassic age. An affinity with the Cache Creek terrane is suggested by correlation with the Orcas Chert of the San Juan Islands, which bears a Tethyan fauna. Basalt dikes of the YAC are of continental affinity, perhaps produced during an episode of continental rifting. High Cr and Ni contents, large-ion lithophile element enrichment, and transitional REE patterns distinguish YAC dikes from the CG and ELFm.