Dunite--Harzburgite--Chromitite Complexes as Refractory Residue in the Sangun--Yamaguchi Zone, Western Japan

Abstract

Dunite, harzburgite and chromitite of alpine-type ultramafic complexes emplaced in the Paleozoic sediments in the Sangun—Yamaguchi zone, western Japan, are massive and almost lacking in layered structure. Constituent minerals are more or less deformed and equilibrated at a relatively low temperature, about 700 °C or lower. Chromian spinels in ultramafic rocks from dunite—harzburgite—chromitite complexes in the Sangun—Yamaguchi zone are characterized by the uniformity of the Cr/Cr + Al ratio, around 0.5, regardless of locality and rock type, which is in contrast to the wide variation of the ratio of chromian spinel from the ordinary alpine-type dunite—harzburgite complex. Mg/Mg + Fe” ratio of chromian spinel, on the other hand, is variable in parallel to the volume per cent of chromian spinel in ultramafic rocks. Olivine in ultramafic rocks is uniform in chemical composition, from Fo88 to Fo92, except for that in chromitite, of which it is Fo95 to Fo97. Primary chemical compositions (especially the Mg/Mg + Fe” ratio) of the constituent minerals have been modified to some extent by element redistribution at low temperature (700 °C or lower), the degree of modification depending on the volume ratio of the minerals. For example, the Mg/Mg + Fe” ratio of chromian spinel in peridotitic rocks has been lowered substantially, and inversely, that of olivine in chromitite has been raised. Primary Mg/Mg + Fe” ratios of olivine and chromian spinel can be estimated, assuming a certain value of high temperature, e.g. 1200 °C, and the partition coefficient between olivine and chromian spinel at that temperature. As a result, the Mg/Mg + Fe” ratio of olivine (0.88 to 0.92), and that of chromian spinel (0.78 to 0.80) were uniform, irrespective of rock type and locality, at the high-temperature stage. Dunite—harzburgite—chromitite complexes in the Sangun—Yamaguchi zone were accumulated as refractory residue after the relatively large-scaled partial fusion of some primordial peridotites, which resulted in the chemical uniformity of the residual minerals. They may have initially constituted the lowest part of an ophiolitic suite as ultramafic tectonite and been emplaced as dismembered portions after the disruption of the parent body.