Quartenary Gede Salak volcanic complex, Banten area, at the junction between Sumatra arc and Java arc, Indonesia

Pleistocene Gede Salak volcanic complex is located at Banten, northwestern edge of Java island (NWJ), forming a part of Sunda arc. The volcanism is associated with the subduction of the India-Australia plate beneath Eurasian plate at the rate of 7 cm/y. This volcanic complex consists of Gede, Salak, Batur and Wadas volcanoes. To southeast is located Pinang volcano, and to south is volcanic complex of Rawa Dano. These volcanoes are located near Sunda Strait, a transitional zone between Java arc and Sumatera arc where oblique subduction is observed. The distance of all these volcanoes from Java trench varies from 250 km to 300 km. This study is the first geochemical study of volcanic rocks characterizing across-arc variation of Java-Sumatra junction. Gede Salak volcanic complex consists of pyroclastic flow deposits in the western part and lava flows in the eastern part. The later development of dome Wadas formation is probably associated with fault structures trending northwest to southeast. Pinang volcano mainly consists of basaltic lavas. Rawa Dano volcanic complex consists of two caldera, Anyer caldera and Dano caldera, which produced large amount of volcanic tuff called Tufa Banten. Samples from this volcanic complex include basaltic to trachytic rocks, in the range of medium-K to high-K. MgO content is less than 3 %. Elements of Rb, Zr, Ce, and La increase with increasing SiO₂. Chondrite-normalized REE patterns are similar to those of island arc basalts. When compared to volcanic samples from western Java volcanoes (WJ), REE pattern is similar to those from back-arc volcanoes. Gede Salak volcano is slightly enriched in the subduction component, as illustrated by the low Nb/Zr and elevated Ba/Zr ratios. B/Nb and B/Zr ratios are in the range of 1.5 - 5.4 and 0.03 - 0.10 respectively, which are higher than those of the back arc volcano in central Java (CJ), but lower than those of the frontal volcanoes there. Across arc variation of NWJ including GSVC, Pinang and Rawa Dano volcanic complex shows a decreasing trend of subduction component from the volcanic front to back arc. Although the trend is similar to other arcs, the decreasing rate is small. Comparison with WJ and CJ suggests that the mantle underneath NWJ has a small but definite influence of subduction component, i.e. altered oceanic crust (AOC) and sediment (SED) to MORB source. Fluid (SED and AOC) contribution at NWJ is higher than that of WJ but less than that of CJ.