Generation of high-Mg andesites in the Kueishantao volcano, the southernmost part of the Okinawa Trough

Kueishantao is an emerged volcanic islet located at the western end of the Southernmost Part of Okinawa Trough (SPOT). The Okinawa Trough, extending from SW Kyushu, Japan to NE Taiwan, is widely regarded as a backarc basin that is built behind the Ryukyu arc-trench system owing to subduction of the Philippine Sea plate underneath the Eurasian plate. The SPOT, however, is not a simple backarc basin but an embryonic rift zone in which early arc volcanism occurs as a result of the Ryukyu subduction. The Kueishantao is one of such volcanoes thus formed in the SPOT and consists mainly of andesitic lava flows dated to be ~7000 yr old. In this study, we report whole rock major and trace element, and Sr-Nd-Pb isotope compositions of the Kueishantao andesites. The results indicate that some of the samples have unexpectedly high magnesium, with MgO ≥ 5 wt.% and Mg# > 0.5, relative to their silica contents (SiO2 ≈ 60 wt.%), which allow them to be coined as high-Mg andesites (HMAs). In the incompatible element variation diagram, these Kueishantao HMAs exhibit enrichments in the large ion lithophile elements and Th, U and Pb, and depletions in the high field strength elements, features typical of arc lavas from the Ryukyu subduction zone as well as convergent margins worldwide. More interestingly, their overall geochemical compositions are very similar to those of the mean continental crust proposed by Rudnick and Fountain (1995). The Kueishantao HMAs have uniform isotope compositions, with low εNd (–4.3 to –5.0), high Sr (87Sr/86Sr ≈ 0.706) and Pb (18.76, 15.69 and 39.06 of 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb, respectively) ratios. Such “continental” isotopic signatures have led previous workers to argue significant crustal contamination during magma ascent as a major petrogenetic process, but our evaluation shows that this simple binary mixing model fails to explain their geochemical and Pb isotope systematics. We propose, instead, that the Kueishantao HMAs result from partial melting of the altered Philippime sea crust and overlying subducting sediments, followed by a melt-mantle wedge interaction. This interpretation is in consistency with seismic tomographic data beneath the SPOT area characterized by a complex collision/extension/subduction tectonic context off NE Taiwan.