Geochemistry of ilmenite from the Asama ultramafic-mafic layered igneous complex, Mikabu greenstone belt, Sambagawa metamorphic terrane, central Japan.

Abstract

Ilmenite occurs as an intercumulus phase in most gabbroic rocks and some plagioclase wehrlites of the Asama igneous complex, an ultramafic-mafic layered intrusion in the Mikabu greenstone belt, central Japan. The Mg/(Mg + Fee+) ratio of ilmenite reaches 0.35, but generally ranges from 0.01 to 0.22; the Fe3+ content varies from 0.04 to 0.26 pfu (per formula unit calculated on the basis of 0 = 3). Ilmenite crystallized together with intercumulus clinopyroxene in plagioclase wehrlite, but most of the calculated clinopyroxene-ilmenite temperatures are much lower than the magmatic temperature. The subsolidus Mg Fe2+ exchange probably occurred between ilmenite and surrounding olivine and pyroxene during the cooling stage of the igneous body. Ilmenite, entirely surrounded by plagioclase, occurs in plagioclase wehrlite of a lower portion of the exposed layered sequence; it contains about 9 wt.% MgO, and is the most magnesian in the complex. The ilmenite shows clinopyroxene-ilmenite equilibration temperatures of 980-1100°C, and seems to preserve the magmatic composition. The Ti02 solubility of melt and the MgO distribution coefficient ilmenite/melt suggest that the Asama magma crystallizing the ilmenite contained 3-5 wt.% Ti02 and 6-7.5 wt.% MgO. The Asama magma was apparently similar to Hawaiian tholeiites, and different from mid-oceanic ridge basalts or island-arc tholeiites. The Asama complex is considered to have formed in an oceanic island.

Chromite in greenstone lavas from the Kanakasu area, Nanjo Massif of the Mesozoic Mino terrane, central Japan

“…The magma crystallizing the Kanakasu xenocrystic chromite was apparently different from island-arc or mid-oceanic ridge alkalic basalts, and similar to Hawaiian alkalic basalts. Hawaiian basalt-like rocks have been reported in greenstone bodies of some other geological terranes in Japan (Arai, 1992;Agata, 1994Agata, , 1998. The palaeomagnetism of greenstones in the Mino terrane has been investigated by Hattori and Hirooka (1979), who have shown that these greenstones exhibit geomagnetic inclinations around 08; their igneous activity probably took place in an equatorial area.…”

Identification of prospective geosites that show features of the active continental margin in eastern Kii Peninsula, Southwest Japan

“…The mountain consists of the Asama igneous body, which is intruded into Jurassic Mikabu greenstones that were metamorphosed to low grades during the high-P/T Sanbagawa metamorphism. As reported by Nakamura (1971) and Agata (1998), the igneous complex consists of a layered sequence of mafic and ultramafic rocks that are lithologically divided into three zones. The lowermost zone consists of peridotites, the middle zone of alternating gabbros and peridotites, and the upper zone of gabbroic rocks (Fig.…”

“…7 Geo logy of Mt. Asama area, illustrating the structural layering of the igneous complex and the main faults (after Agata, 1998). The photo on the right, taken from the mountaintop observatory,demonstrates how the inferred MTL trace (dotted line) and related lineaments continue to the northeast fro m the Kii Peninsula (Futami area) to the Chubu Region in Japan.…”