Anomalous intra-plate high-Mg andesites in the Choshi area (Chiba, Central Japan) produced during early stages of Japan Sea opening?

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.% SiO 2, Na 2O ranging between 3.9 and 4.2 wt.%, and relatively low K 2O/Na 2O 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).