HR: 1340h
AN: V13A-1444    [Abstracts]
TI: Generation of high-Mg andesites in the Kueishantao volcano, the southernmost part of the Okinawa Trough
AU: * Chu, C
EM: julie0104@yam.com
AF: National Taiwan University, No.1, Sec 4, Roosevelt Road, Taipei, 106 Taiwan
AU: Chung, S
AF: National Taiwan University, No.1, Sec 4, Roosevelt Road, Taipei, 106 Taiwan
AU: Shinjo, R
AF: University of The Ryukyus, Senbaru 1, Nishihara, Okinawa, 903-0213 Japan
AU: Wang, S
AF: National Taiwan University, No.1, Sec 4, Roosevelt Road, Taipei, 106 Taiwan
AU: Chen, C
AF: National Taiwan University, No.1, Sec 4, Roosevelt Road, Taipei, 106 Taiwan
AB: 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 (Chung et al., 2000). The Kueishantao is one of such volcanoes thus formed in the SPOT and consists mainly of andesitic lava flows dated to be $\sim$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 $\geq$ 5 wt.$%$ and Mg\# $>$ 0.5, relative to their silica contents (SiO$_{2}$$\approx$ 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 $\epsilon$Nd (-4.3 to -5.0), high Sr ($^{87}$Sr/$^{86}$Sr­™$\approx$ 0.706) and Pb (18.75, 15.68 and 39.02 of $^{206}$Pb/$^{204}$Pb, $^{207}$Pb/$^{204}$Pb and $^{208}$Pb/$^{204}$Pb, respectively) ratios. Such ­øcontinental­ñ isotopic signatures have led previous workers (Chen et al., 1995) to argue significant crustal contamination 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 subducting sediments and subsequent melt-mantle interaction, an interpretation in consistency with seismic tomographic data beneath the SPOT characterized by a complex collision/extension/subduction tectonic context off NE Taiwan.
DE: 9320 Asia
DE: 3640 Igneous petrology
DE: 3655 Major element composition
DE: 3670 Minor and trace element composition
DE: 1749 Volcanology, geochemistry, and petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting