Petrogenesis and ^(230)Th-^(238)U disequilibrium at Mt. Shasta, California, and in the Cascades

Abstract

Petrogenesis at Mt. Shasta is dominated by mixing of magmas and/or assimilation of wall rock, as is shown by petrographic, major and trace element chemistry, and ^(238)U-^(230)Th disequilibrium data. At least three end- members are involved in these mixing processes. Lavas of very young Cascades lavas, from Mt. Garibaldi in the north to Lassen Peak in the south, are characterized by a large range of thorium isotopic ratios, although series of samples from single volcanoes are characterized by approximately constant (^(230)Th/^(232)Th). There is a monotonic decrease in this ratio from Crater Lake south through Lassen Peak, perhaps reflecting increasing thickness of the underlying crust. Th/U fractionation in Cascades lavas, as evidenced by (^(230)Th/^(238)U) ≥ 1, is in the opposite sense to that in most island arc lavas. This trend suggests that fluid transport, which is thought to produce uranium enrichment in island arc, is lacking or somehow modified in the petrogenesis of the Cascades lavas.