Petrology of dunite/harzburgite with decimeter-scale stratification in a drill core from the Tari-Misaka ultramafic complex, southwestern Japan

Abstract

A drill core from Tari-Misaka ultramafic complex, southwestern Japan, shows a one meter interval in which a decimeter scale stratification of dunite/harzburgite is well preserved. This part was petrologicaly examined in detail. Special attention was focused on the morphology and chemistry of chromian spinel. Chromian spinel is more abundant, more euhedral and higher in Ti and Fe³⁺ contents, and slightly lower in Cr/(Cr+Al) ratio in dunite than in harzburgite. Harzburgite becomes orthopyroxene-poor, and its chromian spinel tends to be similar to dunite spinel near (usually within 5 cm) the lithological between dunite. It was presumed that the dunite was formed as veins as a result of a reaction between harzburgite and an exotic melt: olivine crystals was formed by a reaction between hartzbergite and the melt.
    The petrological and chemical features of this part of the drill is basically similar to those in dunite with large chromitite pods, and harzburgite widely distributed in the Wakamatsu mine area. It is suggested that the dunites within harzburgite have the same origin irrespective of their dimension and the presence of chromite pods or not. The lower Cr# of spinel in the drill-core rocks than those in widely distributed dunite is possibly due to the difference in amount of orthopyroxene reacted with the melt. A large amount of orthopyroxene reacted and turbulency of melt current may be indispensable for the formation of podiform chromitite. The large melt conduit represented by dunite of semi-regional distribution in the Wakamatsu mine area possibly fulfilled the all of the conditions for the formation through this process of chromitite pods.