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.