Geology and petrography of the Tanohata zoned pluton in northern Kitakami Mountains

Abstract

Geological and petrographical studies were carried out on the rocks from Early Cretaceous granitoids (112-122Ma) in the Tanohata area, northeastern Japan. It was revealed that the granitic complex called the Tanohata pluton consists of six granitic intrusives, which are different in lithology and intruded at the different stages. The largest body is a concentric texturally and compositionally zoned pluton. This body is definede as the Tanohata aoned pluton. Other five smaller bodies are distrbuted aroud the zoned pluton. The granitoids that make up the Tanohata zoned pluton are progressively more felsic inward; from biotite hornblede quartz diorite_??_tonalite (A-2 type) at the margin to biotite leucogranodiorite (A-7 type) in the center through biotite hornblende tonalite_??_granodiorite (A-3, 4 type), hornblende biotite granodiorite (A-5 type) and biotite granodiorite (A-6 type). The mineral assemblages of these rock types are as follows: pl+opx+cpx+ho+bio+qz+K-f (A-2 type), pl+cpx+ho+bio+qz+K-f (A-3 type), pl+ho+bio+qz+K-f (A-4, 5 type), pl±ho+bio+qz+K-f (A-6 type), and pl+bio+qz+K-f (A-7 type, pl+opx+cpx+ho+bio) is foud with the A-2 and A-3 types. The zoned pluton is generally fine-grained and well-foliated at the margin and becomes progressively coarse-grained and poorly-foliated towards center. Mafic minerals decrease and quartz and K-feldspar slightly increase from the margin to the center (from 27-21 to 7-4 in the color index). The anorthite content of plagioclase core decrease from 60-52 at the margin to 35-28 in the center. Continuous geological and petrographical changes of these rocks (at least the A-2_??_A-7 types) suggest that this zoned pluton formed by differentiation of a single magma, and that the crystallization sequences of principal minerals is plagioclase, orthopyroxene, clinopyroxene, hornblende, biotite, quartz, and K-feldspar.