1984 Volume 34 Issue 187 Pages 307-321
The Hidaka Series of the Shimokawa mining area consists mainly of an apparent ophiolite suite that comprises ultramafic rocks, gabbro, diabase, massive and pillow basalts, with numerous intercalated pelitic, psammitic and rarely calcareous layers. These rocks are widely altered and the assemblages of replacing min-erals range from prehnite-pumpellyite facies to a transition state from greenschist to amphibolite facies. The alteration mineral zoning and the compositional variation of amphibole from actinolite to magnesio-hornblende downward indicate a thermal gradient estimated to be about 140°C/km. The chemical compositions of amphibole and chlorite formed by the alteration are analogous to those from mid-ocean ridges. It may be considered that the mafic rocks and sediments in Shimokawa area were formed on a sea-floor growing new oceanic crust where an adequate amount of clastics was supplied from the adjacent land and altered by circulating hydrothermal water.
