日高変成帯，札内川七ノ沢におけるIタイプ・Sタイプトーナル岩マグマの起源：Sr・Nd同位体比からの検討 (original) (raw)
2014, The Journal of the Geological Society of Japan
The Hidaka Metamorphic Belt (HMB) is a tilted crustal section of a magmatic arc formed by magmatic activities at and Ma. We investigated the Sr and Nd isotopic compositions of the-Ma I-and S-type tonalites distributed in the central part of the HMB to clarify their origins. Emplacement of I-type hornblende-biotite tonalite magma triggered partial melting of metamorphic pelitic-psammitic country rocks, resulting in the in situ formation of migmatitic S-type muscovite-biotite tonalite. The Sr and Nd isotopic ratios of the hornblende-biotite tonalites are increasingly enriched with increasing whole-rock SiO concentrations, suggesting that the magma experienced fractional crystallization and assimilated gneissic country rocks. The discordance of the Nd isotopic compositions of the hornblende-biotite tonalites compared with the mafic metamorphic rocks in the lower sequence of the HMB indicates that the hornblende-biotite tonalite magma was not derived exclusively from mafic metamorphic sources. Isotopic signatures show that the magma source was likely similar to that of synchronous norites in the Oshirabetsu region.
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