Petrology of an epizonal gabbroic suite: the Batow pluton, Yamizo Mountains, Central Japan

Abstract

The Batow pluton, an epizonal gabbroic body, Central Japan, was studied to delineate its geologic, petrographic and geochemical characteristics. Though the pluton is small, it exhibits a wide compositional range from melagabbro to granodiorite and varies from 42.8 to 62.7 wt.% SiO₂. The Batow rocks have similar features to calc-alkaline, magnetite-series and I-type plutonic rocks and are characterized by high abundances of lithophile elements, comparable to those in a shoshonite rock association.
Crystallization path calculations and petrographic observations suggest that the wide compositional variation was brought mainly by amphibole- and plagioclase-dominated fractionation of a gabbroic magma with minor fractionation of clinopyroxene, biotite and potash feldspar. This fractionation occurred at a comparatively shallow crustal level, deeper than 2kb, before upward migration.
The granitic rocks characterized by an abundance of amphibole (up to 39%) and a scarcity of biotite (less than 4%) were formed at a higher P _H2O and lower temperature than early-crystallized gabbros. The close association of clinopyroxene- and biotite-rich gabbro, whose clinopyroxene and plagioclase often show reverse zoning in the Mg/(Mg+Fe^*) ratio and An content, with amphibole-rich gabbro suggests that P _H2O had increased rapidly during crystallization of the gabbroic melt. The intrusion of H₂O-saturated granodioritic melt to the gabbroic melt may have caused the rapid increase of P _H2O.