Utilizing Boron to Reveal the Influence of Subducted Slab across Central Java Island Arc, Indonesi
Esti Handini, Dr. Agung Harijoko, M.Eng.
2011 | Tesis | S2 Teknik Geologi-
The influence of subducted slab in magma genesis of Sunda Arc (Java Island, Indonesia) has been studied over years. The difference in age of subducted plate and the difference in the nature of the overriding crust are considered as important factors affecting variation in magma compositions along and across this arc. Recent works on this arc interpreted that this variation is most likely related to crustal contamination and sedimentary influx. This study utilized, the most sensitive subduction component, to test the influence of subducted plate on the magma genesis in Central Sunda Arc (CSA), focusing on the across arc variation of magma composition. The collected samples, from a north-south alignment of Quaternary volcanic centers including Merapi, Merbabu, Telomoyo, Ungaran and Muria, represent the CSA. Another two neighboring volcanoes, Dieng and Sindoro, were also analyzed. Basalt and basaltic andesite were analyzed in order to avoid the influence of crustal contamination. In this study, boron concentrations were obtained by Prompt Gamma ray Analysis with JRR-3M reactor at Japan Atomic Energy Agency. The analyzed samples from CSA are sub-alkaline basalt to basaltic andesite, except for samples from Muria, which are alkaline. Samples from Muria are shoshonite, potassic trachy-basalt and tephriphonolite. In general, boron and other incompatible elements show increasing patterns as the SiO2 increases, which can be explained by fractional crystallization. Although Muria samples are high in incompatible elements, B/Nb and B/Zr ratios are low, suggesting the small input of subduction component. Compared with other arcs, B concentrations from CSA are characterized by high values. The subalkaline suites of CSA overlap with those from Kurile, Mariana, Cascade, Northeast Japan arcs. The alkaline suite of Muria is plotted away from sub-alkaline group of CSA, and is closer to but different from OIB field. Across arc variation of subduction component shows a general decreasing trend from trench-side to back-arc side as observed in typical island arcs. In detail, however, the highest B/HFSE ratios of CSA do not appear at the volcanic front like the case of most other volcanic arcs. Instead, it appears 20-30 km behind the volcanic front. The sub-alkaline suites of CSA show the typical island arc characteristic, whilst the back arc magmatism, which is represented by Muria, distinctively shows alkaline characteristic but different from OIB source. The subduction input is observed as represented by B/HFSE all across the CSA. Even the back-arc Muria volcano shows higher B/HFSE values than MORB and OIB and is significantly indicating subduction influence. The decreasing pattern of B/HFSE across arc indicates that fluid input is high near volcanic front and gradually decreases toward back-arc. The maximum subduction input observed a little behind the volcanic front raises a question how the process of (1) dehydration of the subducting slab, (2) metasomatism of the mantle, (3) partial melting of the metasomatised mantle took place at CSA.
Kata Kunci : Prompt Gamma -ray Analysis, Subducted Slab, Instrumental Neutron Activation Analysis
