The distributions of two mantle sources based on the Sr-Nd isotopic compositions of late Cenozoic volcanic rocks from the northern Fossa Magna, central Japan

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  • 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
  • ホクブ フォッサマグナ ニ サンスル コウキ シンセイダイ カザンガン ノ Sr-Nd ドウイタイ ソセイ カラ ミタ 2ツ ノ ソースマントル ノ ブンプ

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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.<br>   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.<br>

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