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Abstract: During Japan Sea back-arc opening between 21 and 15 Ma, low-K tholeiitic basalts erupted in the Abukuma Mountains on the trench side of NE Japan. The basaltic rocks in this region are divided into two units by a hiatus. The lower unit consists of the 20 Ma Tenmyosan volcanic rocks, including many monogenetic basaltic volcanoes whose vents occur throughout the district. The upper unit is the Ryozen Formation that occurred at about 16 Ma. This Formation is composed of a subaerial basaltic polygenetic volcanic edifice, with a diameter of ca. 30 km. Minor amounts of andesitic and dacitic gravels occur in the distal portion of the Ryozen edifice. Low-K tholeiitic basalts of the Tenmyosan volcanic rocks and the Ryozen Formation have similar in major and trace element compositions. Their MgO content decrease from 13.2 to 3.4 wt.% with increasing incompatible element abundances and more pronounced negative Nb and Ta anomalies. Nd, Sr and Pb isotopic compositions of the Tenmyosan basalt display a range of values for SrI (0.7040–0.7050), NdI (0.5127–0.5129) and 206Pb/204Pb (18.2–18.5). These features suggest that the Tenmyosan monogenetic volcanism at the early stage of the Japan Sea opening was generated by intermittent ascent of minor mafic melts from heterogeneous sources. Isotopic data for the Ryozen basalt define a binary mixing trend between a depleted and enriched crust-like source, varying with SiO2 content. These chemical changes show that, as the opening progressed, the asthenospheric source of the Ryozen basalts became more homogeneous and depleted (SrI; 0.7043, Nd; 0.5129, 206Pb/204Pb; 18.2), and continuous ascent of the basaltic melt was able to make a large magma chamber that was associated with significant crustal assimilation. On the other hand, hornblende dacite in the Ryozen Formation has SiO2 contents between 66 and 74 wt.%, high Mg# from 41.7 to 54.3. The dacite also shows high Sr (417–650 ppm) and Sr/Y (35–112), and high La/Yb (12–20). Compared with the basaltic rocks, the Ryozen dacite has relatively low SrI (0.70436) and high NdI (0.5128–0.5129). These features show that the dacite has no genetic relationship with the basaltic rocks and can be considered an adakite, traditionally viewed as product of eclogitic slab melting. The 14 Ma Nodegamiyama andesite has SiO2 in 54 to 55 wt.%, high Mg# (59.0–62.3), high Cr and high Ni contents. The isotopic ratios of this andesite are enriched (SrI; 0.7054–0.7055, NdI; ~0.5127, 206Pb/204Pb; ~18.5). This high-Mg andesite may be generated by melting of subducting sediments and interaction of such melts with overlying mantle peridotites. Although the downgoing Pacific plate was Cretaceous in age and thus too cold to melt under normal conditions, incursion of the hot asthenosphere above the slab during the Japan Sea opening may have caused melting to occur. [Copyright &y& Elsevier]

Abstract: Geochemical and isotopic data are reported for 20 Ma high-magnesian andesite (HMA) lavas erupted on the forearc side of Central Japan in the Choshi coastal area (Chiba). Rarely olivine-phyric (<3% vol), these rocks have ca. 56 wt.% SiO2, Na2O ranging between 3.9 and 4.2 wt.%, and relatively low K2O/Na2O ratios (ca. 0.44). Their Mg numbers are relatively high (ca. 0.64) and matched by high Cr (>345 ppm), high Sr (>520 ppm), high Sr/Y (ca. 30), and high [La/Yb]N (>8), conforming to the definition of low-silica adakites, transitional in character to HMA. However, despite slight negative Nb and Ta ‘troughs’ in a few samples, their incompatible element distribution patterns appear closer to those of intra-plate basalts, apparently confirming previous suggestions that terms such as HMA and adakite cover a diverse range of compositions. Enriched in radiogenic Sr and Nd (ca. 0.7040 to 0.7043 and 0.5128 to 0.51275, respectively), the Choshi HMA shows relatively low 206Pb/204Pb (18.25 to 18.35) and 208Pb/204Pb (38.37 to 38.48) ratios, resembling those of late Miocene intra-plate basalts in the Japan Sea and SW Japan. In this regard the Choshi HMA differs from those of the ‘classic’ Middle Miocene (14 to 12 Ma) HMA described from the Setouchi volcanic belt (e.g. Tatsumi, 1981), a short distance to the west which also shows relative high K contents. Although HMAs have been generally attributed to subduction-related processes, the lack of high field strength element (HFSE) depletions cf. large ionic lithophile elements (LILE) in most Choshi HMA suggests an absence of so-called ‘subduction signatures’. Accordingly, we suggest that the Choshi HMA probably tapped a fertile (asthenospheric) region of the convecting mantle wedge, primitive melt products having acquired HMA-like character as a result of shallow level modification by interaction with lithospheric components, aided by thermal input resulting from mantle extrusion, during the early stages of Japan Sea opening (23–15 Ma). [Copyright &y& Elsevier]