Geochemistry of Miocene basaltic rocks temporally straddling the rifting of lithosphere at the Akita-Yamagata area, northeast Japan

Middle Miocene bimodal volcanism by asthenospheric upwelling: Sr and Nd isotopic evidence from the back-arc region of the Northeast Japan arc.

“…This observation is based on Sri ratios for Early Miocene to Quater nary basaltic rocks (22-0 Ma) from the trench side and the transitional zone, and those of Oligocene to Middle Miocene age (30-15 Ma) from the back arc side. Similarly, Ujike and Tsuchiya (1993) showed that basaltic rocks older than 16 Ma from the Akita-Yamagata area in the back-arc side of the NE Japan arc are characterized by high Sr isotopic ratios, whereas those younger than -16 Ma have low Sr isotopic ratios. They considered that the older basaltic rocks were derived from either a more radiogenic source, or a source con taminated by such material, while younger magmas were derived from isotopically more depleted sources.…”

“…The temporal variation of Sr and Nd isotopes for Tertiary and Quaternary basaltic rocks and their source materials of the Northeast Japan arc have been discussed in relation to the tectonic evolu tion by Kurasawa and Konda (1986), Nohda et al (1988), Shuto et al (1993) and Ujike and Tsuchiya (1993). These investigations have shown that Sr isotopic compositions of basalts from the back arc side of the NE Japan arc changed drastically from higher ratios to lower ones at around 15 Ma.…”

Geochemistry of igneous rocks of Shimane Peninsula, formed within a Miocene back-arc rifting zone at the Japan Sea margin.

“…The . N-MORB normalized incompatible element abundance patterns of basaltic rocks of Akita-Yamagata area , NE Japan (Ujike and Tsuchiya, 1993), Matsue basalt (solid lines: this work, broken lines: Iwamori, 1992) , and alkalic basalts of Sekiryo area, SW Japan (Iwamori, 1992).…”

“…8)., most middle Miocene rocks are characterized by enrichments in LIL elements compared to HFS elements. Depletion in HFS elements (such as Ti, Nb, Hf, Zr) may be caused by their retention in a Ti-bear ing phase in the hydrous mantle during partial melting (Hellman and Green, 1979), but this con Iwamori, 1992), and basalts from Akita-Yamagata area, NE Japan (Ujike and Tsuchiya , 1993). Compositional fields (after Meschede, 1986): WPA = within plate alkali basalts; WPT = within plate tholeiites; VAB = volcanic arc basalts.…”

“…1), voluminous basaltic magma formed in a failed rift zone during the middle Miocene (Tsuchiya, 1988(Tsuchiya, , 1990. Ujike and Tsuchiya (1993) showed that, before rifting, basalts were generated by subduction-related magmatism, whereas, after rifting began, magma was generated from the subarc lithospheric mantle with an arc signature, and mixed with sub ducti on related magma. Their IE patterns are similar to those of middle Miocene rocks of Shimane Peninsula in terms of a weak Nb depletion, and higher La/Sm ratios than N-MORB (Figs.…”