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 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.

Geochemistry of adakitic quartz diorite in the Yamizo Mountains, central Japan: Implications for Early Cretaceous adakitic magmatism in the inner zone of southwest Japan

“…The Bato pluton is composed mainly of hornblende gabbro, with subordinate clinopyroxene–biotite gabbro and melagabbro accompanied by quartz diorite (Tanaka et al . 1987).…”

Zircon U–Pb dating of gabbro and diorite from the Bato pluton, northeast Japan

“…Crystallization path calculations and petrographic observations led Tanaka et al () to infer that the wide compositional variation of the diorite–gabbro complex in the Bato pluton resulted mainly from amphibole‐ and plagioclase‐dominated fractionation of a gabbroic magma with minor fractionation of clinopyroxene, biotite, and K‐feldspar. However, zircons in the diorite were about 35 % inherited zircons with older 238 U– 206 Pb ages than the solidification age of the diorite.…”

“…The major‐element composition of the Bato pluton covers the range from gabbro to diorite (Tanaka et al ). Our dated samples show representative SiO 2 contents for the gabbro and the diorite (Figure ).…”

“…Their K–Ar ages (108–65 Ma; Shibata, Hachisy, & Uchiumi, ) are similar to those of the Abukuma plutons (104–86 Ma; Kawano & Ueda, ; Takagi & Kamei, ). The Bato pluton, one of the Yamizo plutons, is composed mainly of hornblende gabbro with subordinate quartz diorite (Takahashi, Kagashima, & Mikoshima, ; Tanaka, Huang, Nakamura, Kurokawa, & Nobusaka, ). Tanaka et al (), who conducted field observations and used petrographic and geologic data to study the formation process of the Bato plutonic rocks, concluded that chemical variations in the pluton were caused by fractional crystallization of amphibole and plagioclase from a primitive basaltic melt.…”

“…Tanaka et al () estimated the cooling age of the Bato pluton to be 105 ±7 Ma based on the K–Ar age of hornblende from quartz monzodiorite, but the solidification age of the Bato pluton based on the U–Pb age of zircon has not been estimated. Moreover, on the basis of a preliminary investigation we conclude that the relationship between the gabbro and diorite in the pluton cannot be explained only by fractionation of a gabbroic magma.…”