Across-arc variation of Magma Composition in Central Sunda Arc, Indonesia: A test of slab influence to mantle source

Sunda arc, a part of Pacific ring of fire, extends from West Java to Flores. The arc developed since Tertiary period at a convergent tectonic plate margin, where India-Australian plate is subducted northward beneath Eurasian plate. Central Sunda Arc (CSA) is represented by a series of Quartenary volcanoes from the fore arc toward the back arc including Merapi, Merbabu, Telomoyo, Ungaran and Muria. Estimated depth of Wadati-Benioff zone beneath CSA ranges from 190 km for Merapi to 350 km for Muria. Field works have been conducted for brief geologic observation and rock sample collection from Merbabu, Telomoyo, Muria, including Genuk on the north and Patiayam on the south of Muria. Data from Merapi is compiled from previous studies. X-Ray Fluorescence, Prompt Gamma Ray and Instrumental Neutron Activation Analyses were used to obtain whole rock compositions. Previously reported trace element of Altered Oceanic Crust (AOC) and Indian Ocean sediment are employed to estimate the derived fluid composition, by considering mobility of the elements and assuming 1.5% weight fraction of hydrous fluid extracted from them. By applying subduction component elements, we tried to estimate the slab influence to mantle source in magma genesis of CSA. High Al₂O₃ (~18 wt%), low Cr (~29 ppm) and Ni (~27 ppm) of the volcanic rocks characterize CSA. K₂O content increases gradually with the depth of Benioff zone from each volcano. Most samples from Merapi, Merbabu, Telomoyo and Ungaran are classified as subalkaline, whereas Muria samples fall on both alkaline and subalkaline fields. In detail, Merapi samples could be divided into medium-K and high-K, Merbabu medium-K, Telomoyo and Ungaran high-K, and Muria samples range from high-K to shosonitic. We only selected unfractionated lavas to avoid assimilation, including basalt, basaltic andesite, basanite, and trachy basaltic andesite. We also exclude samples with hornblende, micas, and K-feldspar to avoid boron fractionation and assimilation. Magma generation beneath CSA has a strong indication of subducted slab influence. Examination of mobile/immobile element ratios (B/Nb-K₂O/Nb, B/Nb-Rb/Nb, B/La-K₂O/La, and B/La-Rb/La) diagrams, indicates that total fraction of slab derived fluid among CSA are approximately 4% for frontal arc volcanoes and <1% for back arc volcano assuming that Merapi represents 2% of fluid contribution. Among those total fractions of slab derived fluid, contribution from sediment derived fluid is higher in frontal arc volcano compared to the back-arc one. Highest sediment contribution is observed at Telomoyo instead of Merapi the very front arc volcano.