Geochemical characteristics of the volcanic rocks from the Tokara Islands, Ryukyu volcanic arc, Japan.

The tectonic setting of the Ryukyu volcanic arc extended from the southern Kyushu to Ioutori-shima is changing from continental margin to island arc. Most of the oceanic islands in the Tokara Islands are Quaternary volcanoes and some of them have eruption recodes in historical time. The volume of each volcanic island above sea level is less than 10% of the total volume estimated from the precise bathymetric map. The volcanic front of the Ryukyu arc is getting closer to Okinawa Trough at the southern part of the arc indicated in a precise bathymetric map. In order to understand the geochemical variation of the volcanic rocks located in the toransitional setting, we have analyzed volcanic rocks collected from entire volcanic arc and some Okinawa Trough. The samples are included not only onland but also dredge haul samples obtaied during the cruises KT00-15, KT07-2 and KT07-21. Analyzed volcanic rocks are ranging from basalt to rhyolite. The most abundant rock samples are andesite (SiO2= 57 wt.% and MgO = less than 6wt.%). Some of the dredged rhyolites are poorly vesiculated. The volcanic rocks, except for Kuchino-shima, in which volcanic rocks have abundant hornblende phenocryst, are mainly composed of plagioclase, augite and hypersthene. Basaltic rocks are only found dredged sample from Yokoate-jima and are rich in Al2O3 (~ 18 wt. %) due to the large amount of plagioclase phenocrysts. Most of volcanic rocks are plotted in the medium-K rock series, whereas samples from Ioutori-shima and dredged samples from Daini-amami bank and Kuchino-shima are plotted in the low-K rock series. Chondrite normalized REE patterns in the volcanic rocks are slightly enriched in light REE (La/Yb = ~3.6). They does not indicate remarkable changes in the REE patterns with increasing silica. La/K2O ratios, which are indicated LILE / HFSE, define two distinct trends. Some dredged samples including Okinawa Trough are higher ratio (La/ K2O = ~9), whereas volcanic rocks of the Tokara Islands are relatively low (La/K2O = 5.7). This ratio expected to indicate relative abundance of volatile components in the source magma. Therefore, volcanic rocks from the Tokara Islands are derived from water rich magma and those of the submarine samples including Okinawa Trough are derived relatively dry magma. Chemical variations of the volcanic rock dose not indicate systematic spatial distribution along the arc. The wet magma, which represents the Tokara volcanic islands, may be coexisting with dry magma, which represents volcanic rocks of Okinawa Trough, simultaneously even in the same volcano.