1987 Volume 82 Issue 12 Pages 419-432
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.