On the Gneisses and Migmatites of the Central Hidaka Metamorphic Zone

Abstract

Various kinds of Metamorphic rocks which are regarded as the product of the Hidaka Orogeny, constitute a prominent extended zone along the Hidaka mountain range., Around the upper course of the River Satsunai which is in the central part of the zone, argillaceous metamorphic rocks are widely developed., Their many rock facies are distinguishable and each facies is zonally arranged with N-S trend., In the argillaceous metamorphic rocks of this area, the writer has descriminated from east to west across the metamorphic zone, five different zones, viz., 1., hornfels zone, 2., banded gneiss zone, 3., coarse granitic migmatite zone, 4., breccia like quartz dioritic migmatite zone and 5., Plagioclase porphyroblast biotite gneiss zone., In this paper their chief petrographical characteristics are described and some considerations about their chemical compositions are also offered., As to the chemical compositions of these gneisses, the following features are conspicuous : 1) In the banded gneisses, K₂O, Al₂O₃ contents are greater than in the rocks of any other facies., 2) Biotite-gneiss containing plagioclase porphyroblast has the highest Na₂O/K₂O, and contrasts with the banded gneiss which has high K₂O and low Na₂O contents., 3) K₂O is comparatively more abundant also in the coarse granitic migmatite, this abundance may be due to the presence of much orthoclase in the coarse granitic migmatite., 4) In the breccia like quartz dioritic migmatite (agmatitic migmatite), the containing hornfels is more basic than is the hornfels of the hornfels zone., From the petrographical view-point, the rock forming biotites were studied., Their chemical compositions are varied corresponding to the differences in the bulk composition of the host rocks., Their relation is given in the Al₂O -FeO -MgO Diagram., (Fig., 13)