Abstract
A small separate body of ultramafic rock was found in the Paleozoic volcanic and sedimentary sequence in Nakatsubara of the Nanjo Mountains. It consists mainly of euhedral to peri-euhedral olivine and chromite, and poikilitic clinopyroxene and plagioclase, with a small amount of interstitial bronzite. Some large olivine crystals contain globular inclusions, 0.1 to 0.5mm in diameter, comprising minute crystal aggregates of kaersutite, dendritic titanaugite and chromite in the saponite mesostasis. The bulk chemical composition of the ultramafic rock is similar to that of a picrite, relatively rich in alumina and soda. It also exhibits comparatively high Fe/Fe+Mg and presence of normative hypersthene. The chemistry of the essential minerals is investigated. From the Mg-Fe partitioning between olivine and chromite, the temperature of their crystallization is estimated about 1, 250°C. The texture and the mineralogy suggest that the Nakatsubara ultramafic rock was essentially formed in a magma chamber by crystals accumulating on the floor. The presence of orthopyroxene discloses this ultramafic rock coming from the magma of a tholeiitic composition.
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