Early alkalic plutonism in the calc-alkalic batholithic belt of California

Abstract

Four alkali-rich plutons exposed in southern and eastern California are chemically distinct from the quartz-rich, calcalkalic batholithic rocks that typify the region. These predominantly monzonitic plutons are older (175 to 230 m.y.) and located farther east than the majority of typical batholithic rocks, but there is spatial and temporal overlap. Most of the monzonites contain minor quartz, but one unit is feldspathoidal. Na 2 O, K 2 O, and Sr are abundant, and SiO 2 is low relative to the calcalkalic rocks. Total REE is high, and patterns have steep negative slopes. The chemical data suggest an upper mantle source—perhaps eclogitic—for the monzonitic magmas. These plutons, together with other early Mesozoic alkali-rich intrusions in California, Washington, and British Columbia, may represent a previously unrecognized phase of alkali-rich magmatism associated with the early stages of continental margin orogeny.