Enriched mid–ocean ridge basalt–type geochemistry of basalts and gabbros from the Nikoro Group, Tokoro Belt, Hokkaido, Japan

摘要

The Tokoro Belt is a subduction complex located in eastern Hokkaido, Japan. The Nikoro Group, a constituent of the Tokoro Belt, is composed mainly of Late Jurassic to Early Cretaceous igneous rocks intercalated with bedded chert and limestone. These rocks have been regarded as fragments of seamounts. Here, we report new whole–rock geochemistry, clinopyroxene major and trace element compositions of the basalts and gabbros, and re–evaluate the origin and geodynamic setting of the Nikoro Group. The gabbros showed ophitic texture and contain fresh, large oikocrystic clinopyroxenes. Fe–Mg partitioning between the clinopyroxenes and whole–rock (i.e., melt) can be regarded as in equilibrium. The trace element composition of the clinopyroxenes within the gabbros and the whole–rock geochemistry of the gabbroic rocks are almost identical to the basalts. This evidence suggests that the whole–rock compositions were not extensively modified and the gabbroic rocks represent the melt composition. Whole–rock trace elements indicated Enriched Mid–Ocean Ridge Basalt (E–MORB)–type patterns as well as ‘garnet signatures’ [e.g., (Sm/Yb)_(N) > 1]. After correction of the gabbros and basalts for the fractionation effect to Mg# = 0.72, the whole–rock chemistry suggests a significantly shallow lithosphere–asthenosphere boundary depth of ~ 0–km. Whole–rock (Sm/Yb)_(N) ratios also confirm the same results. These geochemical results constrain the geodynamic setting of the Nikoro Group; the greenstone most likely originated from a plume–influenced ridge in the Pacific Ocean basin during the Middle Jurassic to Early Cretaceous.