THE ASAMA IGNEOUS COMPLEX, CENTRAL JAPAN - AN ULTRAMAFIC-MAFIC LAYERED INTRUSION IN THE MIKABU GREENSTONE-BELT, SAMBAGAWA METAMORPHIC TERRAIN

The Asama igneous complex comprises layered mafic and ultramafic pluto nic rocks exposed over about 500 X 6000 m in the Mikabu greenstone bel t, Sambagawa metamorphic terrain of Mie Prefecture; its margins termin ate by faults, and there is no trace of chilled rocks. The exposed lay ered sequence is about 460 m thick, and includes dunite, plagioclase w ehrlite, olivine gabbro, anorthositic gabbro and two-pyroxene gabbro. The crystallization sequence of essential cumulus minerals is olivine, followed by plagioclase and clinopyroxene together, and finally the a ppearance of orthopyroxene. Olivine systematically varies in compositi on from Fo(89) to Fo(78) With stratigraphic height in the lower to mid dle portion of the layered sequence. The composition of clinopyroxene changes from Ca49Mg46Fe5 to Ca40Mg47Fe13 upward in the layered sequenc e; cumulus orthopyroxene, which occurs at the top of the exposed layer ed sequence, has a composition of Ca2Mg74Fe24. Cumulus chromite occurs as disseminated grains in peridotitic rocks, and tends to increase it s Fe3+ / (Cr + Al + Fe3+) ratio with stratigraphic height. The most al uminous chromite [Cr/ (Cr + Al)= 0.48] occurs in dunite that crystalli zed 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 As ama parental magma was moderately enriched in plagioclase and clinopyr oxene components in the normative mineral diagram plagio clase-clinopy roxene-orthopyroxene, It was similar to a Hawaiian tholeiite and diffe rent from the Bushveld and Great ''Dyke'' parental magmas that were mo re enriched in orthopyroxene component; it also differed from mid-ocea nic ridge basalts that are more depleted in the orthopyroxene componen t. The Asama clinopyroxene and chromite show characteristically high T iO2 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.