Diverse Origins of Xenoliths from Seamounts at the Continental Margin, Offshore Central California

Abstract

A diverse assemblage of small mafic and ultramafic xenoliths occurs in alkalic lava from Davidson and Pioneer seamounts located at the continental margin of central California. Based on mineral compositions and textures, they form three groups: (1) mantle xenoliths of lherzolite, pyroxenite, and dunite with olivine of4Fo90; (2) ocean crust xenoliths of dunite with olivine 5Fo90, troctolite, pyroxenegabbro, and anorthosite with low-K2O plagioclase; (3) cumulates of seamount magmas of alkalic gabbro with primary amphibole and biotite and anorthosites with high-K2O plagioclase. The alkalic cumulates are genetically related to, but more evolved than, their host lavas and probably crystallized at the margins of magma reservoirs. Modeling and comparison with experimentally derived phases suggest an origin at moderate pressures ( 0 5^0 9 GPa).The high volatile contents of the alkalic host lavas may have pressurized the magma chambers and helped to propel the xenolith-bearing lavas directly from deep storage at the base of the lithosphere to the eruption site on the ocean floor, entraining fragments of the upper mantle and ocean crust cumulates from the underlyingabandoned spreadingcenter.