Petrochemistry of volcanic rocks of the Nishiyatsushiro Group in the South Fossa Magna

Abstract

The Kurosawa, Takahagi basalt members and Kamimura volcanic member of the Nishiyatsushiro Group mainly consist of basaltic pillow lavas and hyaloclastites erupted in the lower bathyal and abyssal zone, and were intruded by a large number of basaltic dikes of the same age. Plotting on the TiO_2-FeO/MgO, TiO_2-MnO-P_2O_5, and 2Nb Zr/4-Y diagrams, and patterns in the N-MORB-normalized diagram indicate that tholeiites of the Kurosawa and Takahagi basalt members have petrochemical character similar to back-arc basin basalt rather than island-arc tholeiite. Alkali basalt dykes have oceanic island alkali basalt-like character similar to alkali basalt of the Takakusayama Formation. These tholeiites and alkali rocks have relatively low Sr isotopic ratios and high Nd isotopic ratios which are similar to those of alkali rocks from the Takakusayama Formation and basalts from the Izu-Ogasawara arc. Therefore, these basalts seem to have been derived from slightly depleted mantle beneath the back-arc region. On the Y/Nb-Zr/Nb diagram, tholeiites are plotted both in the N-MORB and OIB fields near T-MORB field, and alkali basalt dykes and alkali rocks of the Takakusayama Formation are plotted more OIB side than the tholeiites. Accordingly, these rocks may have been formed in relation to mixing of N-MORB and OIB. Bimodal volcanism of basalt and dacite in the Nishiyatsushiro Group of middle Miocene occurred in relation to deep fracture formed in the back-arc side of the proto-Izu-Bonin arc. After the volcanism, collision of the Misaka and Tanzawa blocks onto the Honshu arc took place.