The Asama igneous complex, central Japan: An ultramafic-mafic layered intrusion in the Mikabu greenstone belt, Sambagawa metamorphic terrain

Abstract

The Asama igneous complex comprises layered mafic and ultramafic plutonic rocks exposed over about 500×6000 m in the Mikabu greenstone belt, Sambagawa metamorphic terrain of Mie Prefecture; its margins terminate by faults, and there is no trace of chilled rocks. The exposed layered sequence is about 460 m thick, and includes dunite, plagioclase wehrlite, olivine gabbro and two-pyroxene gabbro. The crystallization sequence of essential cumulus minerals is olivine, followed by plagioclase and clinopyroxene together, and finally the appearance of orthopyroxene. Olivine systematically varies in composition from Fo 89 to Fo 78 with stratigraphic height in the lower to middle portion of the layered sequence. The composition of clinopyroxene changes from Ca 49Mg 46Fe 5 to Ca 40Mg 47Fe 13 upward in the layered sequence; cumulus orthopyroxene, which occurs at the top of the exposed layered sequence, has a composition of Ca 2Mg 74Fe 24. Cumulus chromite occurs as disseminated grains in peridotitic rocks, and tends to increase its Fe 3+ (Cr+Al+Fe 3+) ratio with stratigraphic height. The most aluminous chromite [ Cr (Cr+Al) = 0.48 ] occurs in dunite that crystallized shortly before plagioclase began to separate as an essential phase. The Cr (Cr+Al) ratio of the most aluminous chromite, coupled with the crystallization order of essential minerals, suggests that the Asama parental magma was moderately enriched in plagioclase and clinopyroxene components in the normative mineral diagram plagioclase-clinopyroxene-orthopyroxene. It was similar to a Hawaiian tholeiite and different from the Bushveld and Great “Dyke” parental magmas that were more enriched in orthopyroxene component; it also differed from mid-oceanic ridge basalts that are more depleted in the orthopyroxene component. The Asama clinopyroxene and chromite show characteristically high TiO 2 contents and are also similar to those in Hawaiian tholeiites. The Asama igneous complex probably resulted from the crystallization of a magma of a Hawaiian (oceanic-island) tholeiite composition and formed in an oceanic island regime.