HR: 0830h
AN: V31E-0973    [PDF]
TI: Geochemical Characteristics of Volcanic Rocks from the Southern Okinawa Trough and its Implications for Tectono-magmatic Evolution
AU: * Shinjo, R
EM: rshinjo@sci.u-ryukyu.ac.jp
AF: Dept. Physics and Earth Sciences, University of the Ryukyus, Senbaru-1, Nishihara, Okinawa, 903-0213 Japan
AU: Hokakubo, S
AF: Dept. Physics and Earth Sciences, University of the Ryukyus, Senbaru-1, Nishihara, Okinawa, 903-0213 Japan
AU: Haraguchi, S
EM: haraguti@ori.u-tokyo.ac.jp
AF: Ocean Research Institute, University of Tokyo, 1-16-1 Minamidai, Tokyo, 164-8639 Japan
AU: Matsumoto, T
EM: takshi@nme.co.jp
AF: Marine Science Dept., Nippon Marine Enterprises, ltd., 14-1 Ogawa-cho, Yokosuka, Kanagawa, 238-0004 Japan
AU: Woodhead, J
AF: School of Earth Sciences, University of Melbourne, Parkville, Victoria, 3010 Australia
AB: The Okinawa Trough is a site of ongoing backarc rifting behind the Ryukyu arc-trench system. Recent intensive surveys, including submersible dives, at the southern Okinawa Trough (SOT) have revealed details of bathymetric, geological, and geophysical features. Here, we present the petrological and geochemical characteristics of volcanic rocks collected during these cruises, and discuss its relation to the evolutionary stage of rifting. Based on bathymetirc and magmatic features, SOT can be divided into two (i.e., eastern and western) segments with non-transform offset at $\sim$ 123.5$\deg$ E. The eastern segment represents a well-developed rift system with E-W-trending central graben and separated NE-SW-trending volcanic front; these two features merge at $\sim$ 125$\deg$ E. In contrast, the western segment is in the incipient rifting stage; rift axis exists close to 100 km contour of the Wadati-Benioff zone. The most notable feature is the presence of 'abnormal' volcanic chain (Cross Backarc Volcanic Trail, CBVT), which trends NE-SW and is obviously oblique to the axial trend. All rocks are subalkaline, but range from basalt to rhyolite; dacite-rhyolite are dominant in the eastern volcanic front and CBVT. Basalts from both segments are low-K tholeiites; they have high abundance of LILEs relative to HFSEs, negative Nb anomalies on MORB-normalized diagrams, and range of $^{143}$Nd/$^{144}$Nd (0.5128-0.5129) and $^{87}$Sr/$^{86}$Sr (0.7034-0.7048). Pb isotope systematics indicate $^{206}$Pb/$^{204}$Pb=18.398-18.582, $^{207}$Pb/$^{204}$Pb=15.594-15.652 and $^{208}$Pb/$^{204}$Pb=38.570-38.912, clearly above the Northern Hemisphere Reference Line. These elemental and isotopic variations are compatible with derivation from Indian Ocean MORB-like mantle with strong overprint of subduction components from the slab. There is clear difference among more felsic rocks between two segments. At similar silica contents, most of felsic rocks from the western segment, including CBVT rhyolites, have higher LILE contents, $^{87}$Sr/$^{86}$Sr and $^{208}$Pb/$^{204}$Pb and lower $^{143}$Nd/$^{144}$Nd; these feature can be explained by significant crustal contamination of mantle-derived magmas. On the other hand, the eastern segment felsic rocks have a similar range of Sr-Nd-Pb isotopes for basalts. We suggest that the marked spatial differences in geochemical characteristics of felsic rocks reflects different stage of the backarc rifting. Geophysical surveys provided evidence that crustal thickness is relatively thin ($\sim$ 20km) under the eastern segment, whereas thick continental crust ($\sim$ 25-30km) without significant thinning have been proposed in the western segment. Sibuet et al. (1998) proposed the linkage of subducted Gagua Ridge for CBVT rhyolites, because of a sharp change in subduction angle (slab tear?) near 123$\deg$ E. The CBVT rhyolites are, however, not adakitic. Instead, we suggest mantle flow originated from the magmatism at the Northern Taiwan Volcanic Zone, which commenced since $\sim$ 3 Ma and was resulted from post-collisional extensional collapse, have important implications for required heat source and comparatively depleted mantle source inferred from mafic inclusion in CBVT rhyolites.
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
DE: 3640 Igneous petrology
DE: 9320 Asia
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting