Isotopic ages of the Pankenushi olivine gabbro complex, Hidaka magmatic belt, central Hokkaido, Japan: implications for the tectonic setting of Hidaka magmatism

Abstract

Isotopic ages (K-Ar hornblende: 17.5±0.6 Ma; K-Ar biotite: 17.3±0.4 Ma; Rb-Sr biotite: 17.3±0.1 Ma) were determined for a ferrogabbro of the Pankenushi gabbroic complex, Hidaka magmatic belt, central Hokkaido. These ages are consistent with a previously reported U-Pb zircon age (18.5 Ma), indicating the Pankenushi complex was emplaced in the Early Miocene. Closure temperatures were estimated for the K-Ar hornblende and biotite ages. A very high cooling rate (10²–10³ °C/myr) suggests that the intrusion of mafic magma with an N-MORB signature was shortly followed by rapid uplift and denudation associated with the eastward tilting of the crustal sequence of the Hidaka magmatic belt. The tectonic setting of the magmatism may have been the opening of the Kuril backarc basin or collision between the Kuril arc and the NE Japan arc.