Geochemical secular variation of magma source during Early to Middle Miocene time in the Niigata area, NE Japan: Asthenospheric mantle upwelling during back-arc basin opening

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In the Niigata region of the NE Japan arc, basaltic and intermediate to felsic volcanic rocks (silicic andesite, dacite and rhyolite) have been produced during the Early Miocene, Middle Miocene and Pliocene ages, spanning the pre-Japan Sea opening to post-opening stages. Early Miocene basaltic rocks are characterized by enriched Sr and Nd isotopic signatures (initial 87Sr / 86Sr (SrI) = 0.70557 to 0.70592 and initial 143Nd / 144Nd (NdI) = 0.51255 to 0.51262), whereas Middle Miocene and Pliocene basaltic rocks are slightly enriched than MORB in terms of SrI (0.70314–0.70416) and NdI (0.51286–0.51310). Early Miocene basaltic rocks are also characterized by higher abundances of HFSE and LREE, and higher Zr / Y compared to Middle Miocene and Pliocene basaltic rocks. The geochemical features of Early Miocene basaltic rocks are almost identical to those of basaltic rocks found in continental rift zones, such as the Rio Grande rift. The different geochemical signatures of Early and Middle Miocene basaltic rocks can be ascribed to the geochemical differences in the mantle source. Apart from andesitic rocks of the Kakuda area, most of Early and Middle Miocene intermediate to felsic volcanic rocks from the Niigata region show significantly higher SrI values (0.70673–0.70773) and lower NdI values (0.51234–0.51254) than Early Miocene basaltic rocks, indicating a lower crustal origin for these more felsic volcanic rocks. A possible tectono-magmatic model for the production of post-Early Miocene volcanic rocks from the Niigata region includes: (1) at the Early Miocene (22–20 Ma); commencement of asthenospheric mantle upwelling followed by both partial melting of the overlying lithospheric mantle and crustal rifting, resulting in the production of continental rift zone-type basaltic magmas which have either erupted on the surface or initiated partial melting of the lower crust to produce more felsic magmas, and (2) at the Middle Miocene (after 15 Ma); thinning of the overlying lithosphere due to the opening of the Japan Sea. This was associated with asthenospheric upwelling followed by partial melting of the asthenosphere to generate extensive basaltic magmas. These were either erupted or melted the lower crust to generate more felsic magmas. Such tectono-magmatic processes may also be applicable to the generation of Early to Middle Miocene basaltic to felsic volcanic rocks in many other parts of the back-arc margin of the NE Japan arc.