Ultramafic and mafic rocks of Shimo-Ina district, Nagano Prefecture

Abstract

The Sanbagawa, Mikabu metamorphics and the Chichibu Paleozoics are diatributed in this district. Green rocks occuring in the crystalline schist, are composed of massive green rock and the green intrusive rock (the Mikabu intrusive), which include meta-diabase, meta-gabbro and amphibolite. Most of the ultramafic rocks concordantly intrude along the boundary between the massive green rock and the schist. They are mainly composed of dunite, banded diallage peridotite, diallage peridotite, fine-grained and coarse-grained gabbro, and are companied by gabbro pegmatite and albitite. Most of the ultramafic rocks are affected more or less by serpentinization and other metasomatic actions. Although these alterations are varied in different rocks, they are always characterized by the transformation of olivine to antigorite and of plagioclase to grossularite. Some gabbroic rocks are also altered into rodingite. Compared with the ordinary grossularite, the grossularite in these rocks has lower refraction indices (1.720-1.733) and smaller density (3.50) and lattice constant (11.88 A). From the properties, it is considered that the grossularite probably inculdes the hydrogrossular molecules.