The surveyed region is one of the Southern Igneous Activity Regions (NAKAYAMA, 1962) in the Sambagawa metamorphic belt. In the western area of the region, the Chichibu Palaeozoic formation consisting of sandstone, shale and chert is distributed, but in the central and eastern area, submarine basic effusive rocks (pillow lava, pillow breccia, hyalloclastite) are in company with tholeiitic gabbro (NAKAYAMA et al., 1973), ultramafic rocks and sheet complex consisting of small gabbroic bodies. The area occupied by submarine basic effusive rocks is a depressed area in the HONSHU Eugeosyncline. The phenocrysts of basic volcanic rocks are olivine, clinopyroxene and plagioclase, and groundmass of these rocks consist of clinopyroxene and plagioclase. But these minerals are altered except for the phenocrysts of clinopyroxene by the Sambagawa regional metamorphism. These basic volcanic rocks are overlaid by acidic tuff, which originates in the acidic volcanism of the Chichibu terrain. In this region hypabyssal intrusion which took place during the movement, is associated with the upheaval of the depression area. The largest gabbroic rock in the region, which is named Kamiyama gabbro in this paper, is a tholeiitic differentiated rock based on a crystallization trend of the clinopyroxene. The differentiation of the tholeiitic magma took place in situ. The petrographic characters of gabbros which intruded as sheet into the basic effusive rocks, correspond to these of the differentiated rocks of the Kamiyama gabbro. "Peridotites" composed of olivine, Ti-augite, Ti-pargasite and Ti-biotite and gabbros have a close relationship in chemical composition of pyroxene and amphiboles. In the amphiboles of the cumulate and gabbros, (Al^<VI>+Ti) content, especially Ti content vary successively. Kamiyama gabbro and gabbros of sheet complex are crystallization products of the magma, which is the greater part of the original magma, and the cumulates were early crystallized from a very small part of the original magma in the magma chamber. The greater part of the cumulates intruded as solid intrusion along fault. Opaque minerals in the sheet rocks and Kamiyama gabbro are altered and the chemical composition of parts of one crystal is variable. Therefore, the chemical composition of the mineral can not be determined, but rarely original minerals are present as relict in some rocks. The relict minerals are ilmenite. Cr-rich clinopyroxenes are situated in the range from 0.80 to 0.90 in MgO/MgO+FeO ratio, but the clinopyroxenes of the range do not always have high Cr content.

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