High-Al podiform chromitites in dunite-harzburgite complexes of the Sangun zone, central Chugoku district, Southwest Japan

Abstract

There are many ultramafic complexes in the Sangun zone of the central Chugoku district, Southwest Japan, some of which have chromitite bodies. Lithological variations of these complexes were determined for primary mineral assemblages in order to understand the origin of podiform chromitites. They mainly consist of harzburgite, dunite and chromitite, and are intruded by some gabbroic dikes. Harzburgite is always dominant over dunite. Chromitite is closely associaied with dunite: a chromitite body is enclosed by a dunite envelope and large chromitite bodies are exclusively found in relatively dunite-dominant complexes. Cr # (=Cr/(Cr + Al) atomic ratio) of chromian spinet varies only slightly, from 0.4 to 0.6 for all ultramafic rocks from all ultramafic complexes, with only one exception, Ochiai-Hokubo complex, (Arai et al. 1988) of the Sangun zone, central Chugoku district, Southwest Japan. Spinels in peridotites from Wakamatsu mine of the Tari-Misaka complex, the largest chromitite body of this district, are slightly high in Cr # (around 0.55) and in Fe 3+ (Cr + Al + Fe 3+) atomic ratio. The podiform chromitite of this district is of interaction origin between melt and harzburgite. Themagma mixing between a secondary Si-rich melt formed by the interaction and a primitive magma in the upper mantle is essential to the chromitite precipitation. Mechanism of spinet precipitation is the same as that given by Irvine (1975, 1977). The magma interacted with the harzburgite in the Sangun zone could be MORB or back-arc basin basalt. The podiform chromitites of this district have relatively Al-rich chromian spinet, indicating involvement of both an exotic Al-rich melt and of relatively fertile harzburgite.