Abstract
This thesis presents the results of a geochemical study of the Banda Arc (East Indonesia) where magma genesis is influenced by subducted source components that are controlled by an active arc-continent collision. The main objective of this study is to investigate the role of subducted continental material on the magma
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genesis using isotopic and chemical compositions of samples from volcanoes and sediments distributed along the whole stretch of the arc. Sr-Nd-Pb isotope ratios and major and trace-element contents were determined for a total of 152 rocks from seven volcanic islands, 127 surface sediments from seven locations and five sediment samples from the DSDP Site 262 hole in the Timor Trough. The composition of these sediments is considered to represent that of a subducted component with continental affinity which is currently involved in magma genesis. This data set is used to (1) assess the importance of subducted continental material as opposed to material assimilated from the arc crust; (2) quantify the contribution of subducted continental material to magma sources; (3) discuss the mode of transfer from the slab to magma sources in the overlying mantle wedge, and (4) determine the role of sediment provenance in generating isotopic heterogeneities within the arc. Large variations in the isotopic composition were found between the volcanoes. These variations are matched by similar variations in the sediments along the arc. This parallelism is most evident for Pb isotopes and is considered as strong evidence for the involvement of subducted continental material in magma genesis. Conventional bulk mixing models indicate that the contribution of subducted continental material increases along the arc from < 1% in the NE to 5-10% in the SW. Important within-suite Sr-Nd isotopic ranges were also found for individual volcanoes. Assimilation is thought to be significant in one volcano and probably occurs to some extent on most islands. However, it cannot explain the conspicuous 'continental' signatures of the more mafic Banda Arc volcanics. Trace-element compositions of the volcanics show upper continental crust signatures. Many ratios of incompatible elements (e.g. LILE/HFSE and LILE/REE) and rare-earth patterns display an increasing correspondence with those of the sediments in the same direction. REE mixing-melting models were applied using a typical MORB source mantle and a representative sediment as end members. The results are consistent with the isotopic mixing models in terms of quantities and with the NE-SW along-arc changes. These models indicate that the bulk addition of subducted continental material to mantle sources is an important characteristic of the Banda Arc which can explain many of its trace-element signatures. Addition in the form of large-degree melts rather than mechanical mixing is considered to be the most plausible bulk-addition mechanism, particularly in the SW Banda Arc. Nevertheless, some trace-element ratios (e.g. BaJNb, Th/Nb, Th/Zr) cannot be expl~ned by bulk mixing and must be due to a process of selective mobilization, probably by fluids escaping from the slab. It is suggested that bulk transfer of subducted continental material dominates and is most conspicuous in the SW whereas fluid transfer is more evident in the NE. Despite the evidence for the involvement of subducted continental material, the Banda Arc volcanics are characterized by higher 208Pbp04Pb at a given 206Pbp04Pb compared to the sediments. This difference becomes more pronounced from NE to SW along the arc. High 208Pbp04Pb is a typical characteristic of the Indian ocean MORB and alB (so-called Dupal anomaly), and the high 208Pbp04Pb component in the volcanics could thus be derived from a mantle contribution. Because of the high Pb concentration in continental material compared to the Pb concentration in any solid mantle type, it is suggested that small-degree partial melts from a mantle source are involved. The terrigenous fraction in the sediments is thought to be responsible for the overall 'continental' character of the arc. From the combined isotopic and trace-element ratios in this fraction two provenance areas can be distinguished: in the NE part of the arc the sediments originate from Phanerozoic New Guinea and in the SW they originate mainly from Proterozoic Australia. A further subdivision can be made on the basis of Th/Sc, 147Sm/l44Nd, Pb and Nd isotopes into: North New Guinea+Seram, South New Guinea, Timor and Australia. In trace-element ratios and REE patterns the sediments are generally similar to published estimates for average Upper Continental Crust and Post Archean Australian Shale. No systematic difference was found between the compositions of sediments from shelf, wedge and back arc. The results for the Banda Arc highlight the importance of the subducted component in controlling inter- and intra-arc variations in chemical and isotopic signatures. Therefore, data from local sediments are indispensable for models of arc magma genesis in individUal cases. The potential variability of subducted sediments also has important implications for the origin and scale of mantle heterogeneities.
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