Chemical characteristics of basic metamorphic rocks from the Main Zone of the Hidaka metamorphic belt, Hokkaido: an affinity to mid-ocean ridge basalt.

摘要

High-grade basic, pelitic and psammiticmetamorphic rocks are exposed in the western part of the MainZone of the Hidaka metamorphic belt. Major and trace elementconcentrations of basic metamorphic rocks were determined. Thesamples analyzed were classified into two rock types, i.e.,amphibolite that is widely-exposed throughout the study area, andbasic granulite which occurs exclusively along the margin of agabbroic body in the northern part. Both show similarities inchemistry to normal mid-ocean ridge basalts (N-type MORB).Most of the amphibolites have variable and higher concentrationsof Rb, Ba, K_2O and Cs than typical N-type MORB, probably dueto enrichment in those elements through metamorphic or alterationprocesses. In the other hand, the basic granulites are poorer in Rband Th, that may have been lost during high-grade metamorphismassociated with deformation. The basic metamorphic rocksoriginated from basalts or gabbros, of which parental magmaswere derived form a mantle source depleted in light rare earthelements. Possibly, the basic magmas may have solidified at seafloor where terrigenous sediments deposited.