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.
Sm-Nd and Rb-Sr isotopic compositions and REE abundances were measured on the Ishikawa composite mass, which is one of the older granitic rocks in the Abukuma mountains. Three mineral and two whole-rock samples define a Sm-Nd isochron yielding an age of 111±42 Ma with an initial 143Nd/144Nd ratio of 0.51251±3. Eight whole-rock samples give a Rb-Sr isochron age of 106±16Ma with an initial 87Sr/86Sr ratio of 0.70518±15. These ages are interpreted to represent the time of intrusion, supporting the Jurassic age of the Gosaisho metamorphic rocks confirmed by the fossil evidence. The Nd model ages of the Jshikawa mass relative to CHUR are as young as intrusion ages, whereas those relative to DM are 600-800Ma. On the basis of the REE patterns and εNd-εSr relationship, the Ishikawa mass is thought to be derived from the source material similar to CHUR. It is therefore suggested that the intrusion of the Ishikawa mass occurred subsequently to the differentiation of the source magma from the mantle.