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 genesis using isotopic
and
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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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