Petrochemistry and tectonic significance of the emerged late Cenozoic basalts behind the Okinawa Troughs Ryukyu arc system

Abstract

Mineral chemistry, major and trace elements, and Sr-Nd isotopic compositions are presented for late Cenozoic basalts from the Senkaku Islands: Kobi-syo (KBS) (< 1 Ma) and Sekibi-Syo (SBS) (~ 2.6 Ma) in the back-arc region of the southern Ryukyu island arc, comprising the most distant volcanic zone associated with the Ryukyu Trench. The Senkaku Islands arc also landward of the Okinawa Trough, a young back-arc basin and at the easternmost extent of the Northern Taiwan Volcanic Zone. The SBS and Okinawa Trough basalts have similar large ion lithophile (LIL)/high field strength (HFS) element ratios and Sr-Nd isotopic compositions, indicative of a common mantle source. Two groups of SBS basalts (both transitional between high-Al and tholeiitic series) are recognized on the basis of the stage of volcanic activity, trace element abundances, and Sr isotopic composition, and are probably derived from two types of parental magmas. The KBS basalts (transitional between high-Al and alkali series) have similar 87Sr 86Sr (~ 0.7038–0.7040) but higher alkali contents and LIL HFS element ratios compared with the SBS basalts ( 87Sr 86Sr ~ 0.7037–0.7041). In addition, the KBS basalts have lower 143Nd 144Nd (~ 0.51271) than the SBS basalts (~ 0.5128–0.5129) projecting below the mantle array on a Sr-Nd isotope plot towards an EMI-like component. Enriched subcontinental mantle rather than crustal contamination and/or the effect of subduction components is important in the generation of the geochemical characteristics of the KBS basalts.