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

Abstract

Geol. and geochem. studies were carried out on the small (3.5 × 2 km) Batow pluton (Japan). Petrog. and chem. analyses of rocks and minerals (clinopyroxene and amphiboles) are given. The rocks have a wide compositional range from gabbro to granodiorite, with the SiO2 content 42.8-62.7 wt.%. Petrog., these are similar to rocks of calc-alk. I-type magmatic series. The rocks are chem. characterized by enrichment of lithophile elements, comparable to the shoshonite rock assocn. Calcd. crystn. path and petrog. observation suggest that the wide compn. range results mainly from amphibole- and plagioclase-dominated fractionation of gabbroic magma with minor fractionation of clinopyroxene, biotite, and K-feldspar. This fractionation takes place in the shallow crust (at >2 km), before upward migration of the magma. The granitic rocks with amphibole (<39%) and biotite (<4%) formed at a higher hydrostatic pressure and a lower temp. than early crystd. gabbros. The close assocn. of clinopyroxene- and biotite-rich gabbro (clinopyroxene and plagioclase show reverse zoning in the Mg/(Mg + Fe) ratio and CaAl2Si2O8 content in places) with amphibole-rich gabbro suggests that water vapor pressure increased rapidly during crystn. of the gabbroic melt. The intrusion of H2O-satd. granodioritic melt to the gabbroic melt might have caused the rapid increase of water-vapor pressure. [on SciFinder(R)]