Petrogenesis of Oligocene to Miocene volcanic rocks from the Toyama basin of the SW Japan arc: Temporal change of arc volcanism during the back-arc spreading in the Japan Sea

Oligocene to Miocene volcanic rocks from the Toyama basin of the SW Japan arc, that were formed during back- arc spreading in the Japan Sea, were examined to reveal their petrogenesis and temporal change of arc volcanism during the Japan Sea opening. The arc volcanism in the Toyama basin initiated with rhyolitic pyroclastic flows (Tori Formation) containing hecatolite (moonstone) in 23-22 Ma. Enriched Sr-Nd isotope (SrI = 0.70769- 0.70944; NdI = 0.51203-0.51224) suggests that contemporaneous andesitic magma (Kamiwazumi and Matsu-nagi Formations) mixed or assimilated basement granitoids and gneisses of the Hida belt to generate rhyolitic magma. Subsequently, andesitic volcanism (Iwaine Formation) occurred in 18-17 Ma after magmatic hiatus. Andesitic lavas of the Iwaine Formation are composed of high magnesian andesite (HMA), high-Sr andesite and tholeiitic andesite. HMA has Mg# > 64, high Cr and Ni concentrations, not so high Th/Yb and (La/Sm)N ratios, and slightly enriched Sr-Nd isotope (SrI = 0.70482; NdI = 0.51279). High-Sr andesite has relatively low SiO2 content (<60 wt%), high Sr (>2000 ppm) and K2O contents (3.98 wt% in the maximum), indicating that it is low- SiO2 adakite. These geochemical characteristics suggest that HMA and high-Sr andesite were produced by partial melting of the mantle wedge saturated by H2O derived from slab fluid and metasomatized by slab melt, respectively. Although chemical variation diagrams suggest tholeiitic andesite seems to have been generated from basaltic magma, it has enriched Sr-Nd isotope (SrI = 0.70713-0.70756; NdI = 0.51237-0.51241). Thus, tholeiitic andesite is considered to have been produced by AFC (assimilation and fractional crystallization) after generation of basaltic parental magma. Andesitic magmatism of the Iwaine Formation was followed by rhyolitic magmatism of the Iozen Formation in 17-16 Ma. The petrogenesis of the rhyolite from the Iozen Formation can be explained by low-rate mixing between andesitic magma (Iwaine Formation) and the Hida belt. The petro-geneses of the andesites, especially HMA and high-Sr andesite, are related to slab melting. Because the old and cold Pacific plate was subducting beneath the Toyama basin during the Japan Sea opening, additional heat source such as upwelling of the asthenospheric mantle into the mantle wedge is required. Moreover, back-arc spreading in the Japan Sea was driven by upwelling of the asthenospheric mantle into the mantle wedge.