Metamorphosed Dolerite Intrusives from the Western Zone of the Hidaka Metamorphic Belt, Hokkaido

Metamorphosed dolerite intrusives in ultramafic tectonites, cumulates, and metagabbros are first described from the Western Zone of the Hidaka Metamorphic Belt. The dolerite intrusives ranging from a few cm to 30m in thickness occur as a dyke or sheet. Most of the dolerite intrusives are metamorphosed into amphibolite or metadolerite in various degrees. The metamorphic grade attains generally up to the amphibolite facies, and partly to the hornblende granulite facies in the ultramafic tectonites. Primary textures and minerals are well preserved in some metamorphosed intrusives. After mylonitization of the ultramafic tectonite the dolerite magma is considered to have intruded into the tectonites and into the already consolidated cumulate and gabbro complexes. Petrochemical and mineralogical data indicate that all of the dolerite intrusives are co-magmatic and are formed by crystallization of an olivine tholeiite magma which is highly undersaturated in silica and less differentiated

The Basic Plutonic Rocks of the Hidaka Metamorphic Belt, Hokkaido. Part II : The Ameyama Layered Gabbros in the Tokachi Province

The occurrence of cumulus gabbro in the Hidaka Metamorphic Belt has been noted in the Western Zone of the Metamorphic belt as cumulus series of dunite-wehrlite-troctolite-olivine gabbro-anorthosite and various amphibolites which were obducted westward (Miyashita and Hashimoto, 1975). Besides the Western Zone gabbro series, many small bodies of cumulus gabbros have been known as fault bounded blocks throughout the whole metamorphic belt in the Axial and Eastern zones (Hashimoto, 1975). The Ameyama gabbro which occurs as an outlier of the metamorphic belt, is described and is compared with the Western Zone gabbros. Diabase which is accompanied by the Ameyama gabbro is also described. The chemical characteristic of the diabase show that it is nearly the same as the Hidaka Western Marginal Zone diabases but a slight difference in the trend of differentiation is revealed