Origin of Jove Peak Orbiculite in Wenatchee Ridge area, northern Cascades, Washington

Abstract

The sill-like, metagabbroic Jove Peak Orbiculite intruded the Chiwaukum Schist during the Skagit metamorphism, and was later intruded by quartz diorite. Orbicules of hornblende nucleated on pyroxene or hornblende phenocrysts, or on xenoliths of gabbro, hornblendite(?), pyroxenite(?), mica schist, and amphibolite. The igneous origin of the orbicules is shown, and other characteristics clearly indicate post-orbicular recrystallization. The orbiculite matrix contains abundant relict igneous features. Subsequent medium-grade metamorphism is shown. This dual igneous-metamorphic origin (as opposed to migmatitic) is also shown by bimodal plagioclase composition and by remarkable compositional differences in plagioclase across interfingering mica-schist-orbiculite contact.