The origin of geochemical variations in a late permian volcanic arc, eastern klamath mountains, California

Abstract

Volcanic are remnants present in the eastern Klamath terranes record an intermittent volcanism from Ordovician to Early Jurassic times. The Upper Permian volcanics (Nosoni, Dekkas and Bully Hill formations) consist mainly of andesites with minor basalts and rhyolites, metamorphosed in the prehnite-pumpellyite facies. Their petrographic and geochemical characters indicate a mature island-arc setting, but their tholeiitic affinity suggests an intra-oceanic environment. Nd isotopic compositions of the Nosoni-Dekkas lavas ( εNd T = +4 to +8), as well as a preferential enrichment of light rare-earth over high-field-strength elements and systematic negative Ce anomalies, point to MORB-type mantle sources contaminated by a few percent of slab-derived sediments. Large variations in the chondrite-normalized La/Yb ratio (0.95 to 3.7) are interpreted in terms of variable depth and degree of partial melting. Two distinctive basalt-types are found at the top of the Nosoni Formation (Dekkas Saddle) and the Bully Hill Formation. The former are high-Mg basalts showing a strong enrichment in incompatible elements (La/Yb ∼ 50, 8.7 ppm Th, ∼ 50 ppm Nb) but low TiO 2, Al 2O 3, Yb, Zr and Sr contents, with a pronounced positive Eu anomaly. They may result from small degrees of partial melting of a refractory peridotitic source enriched by felsic melts derived from subducted MORB, with garnet in the residue. These lavas probably record a transient anomalous thermal regime in the deepest part of the subduction zone, and can be compared to some extent to the arc alkali series. On the other hand, the Bully Hill basalt shows boninitic affinities and probably originated from a shallower lithospheric mantle source. The large range of compositions recorded by the Permian arc lavas gives evidence of efficient extraction of melts segregated at different depths.