北部フォッサマグナに産する後期新生代火山岩のSr-Nd 同位体組成からみた 2 つのソースマントルの分布

Abstract

We analyzed the Sr-Nd isotopic and geochemical compositions of 112 late Cenozoic volcanic rocks from the northern Fossa Magna, central Japan. The volcanic rocks are divided, on the basis of Nd isotopic compositions, into a high-Nd group (HND) and a low-Nd group (LND). The HND has higher Zr/Nb and Hf/Nb ratios than the LND volcanic rocks. The isotopic ratios and high field strength element (HFSE) ratios indicate that the HND volcanic rocks were derived from a relatively depleted mantle compared with the LND rocks. The relationship between the HND and LND rocks, in terms of their isotopic and Zr/Nb, Hf/Nb ratios, is similar to that observed between Quaternary rear-arc and frontal-arc volcanic rocks of the NE Japan arc. The rear-arc rocks from NE Japan have higher Nd isotopic and HFSE ratios than the frontal-arc rocks, indicating the injection of depleted asthenospheric mantle beneath the rear-arc, associated with Miocene opening of the Sea of Japan. The HND rocks occur in the central part of the Fossa Magna region in the form of a NW-trending wedge. This wedge shape of the HND area is consistent with the wedge-shaped rifting that occurred perpendicular to the strike of the arc and that opened the Fossa Magna contemporaneously with the opening of the Sea of Japan. The Sr and Nd isotopic compositions of the HND and LND rocks fall within the mantle array and extend into the high-Sr isotopic field. Some of the LND rocks contain lower-crustal mafic xenoliths with Sr-Nd isotopic compositions that overlap those of the LND rocks. In the Sr-Nd isotope diagram, the trend of the HND rocks can be extrapolated to the most radiogenic Sr isotopic compositions obtained for the xenoliths. This result suggests that the lower-crustal materials, which have the most radiogenic Sr isotopic compositions, contributed to the Sr isotopic enrichment of the LND and HND magmas.