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

摘要

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 ~(206)Pb/~(204)Pb (18.25 to 18.35) and ~(208)Pb/~(204)Pb (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 describ