The Tomuraushi greenstone complex of the central Hidaka zone: contemporaneous occurrence of abyssal tholeiite and terrigenous sediments.

Abstract

The Tomuraushi greenstone complex in the central part of the Hidaka Belt, central Hokkaido, consists of weakly metamorphosed basalts, dolerites, hyaloclastites (greenstones) and mudstone. Modes of occurrence of the former two suggest that the basaltic magma extruded over or intruded into unconsolidated muddy sediments. The complex, approx 1 km in width, is divided into the following two parts: dolerites and massive lavas are dominant in the lower (eastern) part, while the upper part is composed largely of pillow lavas and hyaloclastites. Thirty chemical analyses of the greenstones show systematic variation in relation to the Mg-value of the rocks, suggesting that the original mafic rocks were cogenetic. Plots, particularly on the TiO2-MnO-P2O5, Cr-Ti and Ni-Ti diagrams, are mostly distributed in the fields of abyssal tholeiites. Relict clinopyroxenes have the compositions of those of non-orogenic tholeiitic suite rocks, though some of the pyroxene compositions would have been controlled by the rate of crystallization and not by the nature of host magmas. The petrological and mineralogical data suggest that the complex would have formed at the divergent plate boundary near the continent, where much terrigenous sediment was also deposited. Radiolarian fossils obtained from the associated mudstone show the formation to be late Cretaceous (Campanian).-M.Ha.