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Origin and evolution of mafic volcanics of Sumatra (Indonesia) : their mantle sources, and the roles of subducted oceanic sediments and crustal contamination

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Gasparon, Massimo (1993) Origin and evolution of mafic volcanics of Sumatra (Indonesia) : their mantle sources, and the roles of subducted oceanic sediments and crustal contamination. PhD thesis, University of Tasmania.

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Abstract

The Quaternary volcanoes along the Sunda arc of Indonesia are a well known
example of subduction-related volcanism. Relatively primitive rocks in the arc are
rare, so that in the last two decades much attention has been dedicated to the study of
the isotopic systematics of the mafic volcanics. These suggest that sediments - or
fluids derived from the sediments - subducted along the Sunda Trench have some
effect on the chemistry of the arc volcanics only in some sectors of the arc, whereas
the isotopic signature of other sectors mainly reflects the composition of their mantle
source. However, very little is known about the composition of the mantle beneath
the arc, and good quality, representative analyses of oceanic sediments from the
northeastern Indian Ocean are scarce. Moreover, the importance of crustal
contamination has not been sufficiently tested, at least partly due to the lack of
exposure of arc basement in the best studied areas.
Good-quality, modern analyses of volcanic rocks from the Sunda arc mainly come
from the relatively easily-accessible areas, but little is known about the least
accessible volcanoes in the largest island in the arc, Sumatra, despite the fact that the
occurrence of both granitic rocks and very primitive basalts was reported some 70
years ago.
In the first part of this thesis, after a brief review of the studies on the volcanism in
the Sunda-Banda arc, an attempt is made to define the chemical and isotopic
composition of "typical" oceanic sediments, and continental crust, for use in the
modelling of contamination processes along the Sunda Trench.
"Basement" granitoids and arc-related granitoids from several localities in Sumatra
were investigated and described, and compared with the better known granitoids of
peninsular southeast Asia.
Two groups of ganitoids have been identified. Arc-related granitoids, generally older
than the Quaternary arc volcanics, are found along the arc, follow the calcalkaline
trend of arc rocks, and have Sr, Nd, and Pb isotope systematics similar to those of
Indian Ocean basalts (including basalts from south Sumatra). East of the Semangko
fault, granitoids and pyroclastic rocks are mainly of "S-type", and have considerably
high initial 87Sr/86Sr values, similar to those found in the Central Granitoid Province
of Southeast Asia. Therefore, the Semangko fault and the Quaternary arc may
represent an important tectonic boundary, defining the southeastern margin of the SIBUMASU terrane. No systematic variations in geochemical and isotopic
composition from north to south Sumatra have been observed in the two groups of
granitoids. This suggests that there are no major variations in the isotopic
composition of the source of arc volcanism and of the continental crust from north to
south Sumatra.
Oceanic basalts from the Northeastern Indian Ocean were studied to test the
possibility of the existence, beneath the Sunda arc, of mantle sources with OIB
composition and "enriched" (EM, HIMU) isotopic signatures.
Basalts with an isotopically "enriched" component are described for the first time in
three new locations in the Northeastern Indian Ocean: the Cocos Plateau, the
Investigator Ridge, and site DSDP 22211. Only the occurrence of this type of basalts
along the Investigator Ridge is consistent with a "fixed hot spot" model of evolution
of the Northeastern Indian Ocean. Pre-"Gondwana rifting" basalts in the Argo
Abyssal Plain seem to have isotope systematics similar to Atlantic and Pacific
MORB.
For the sediments, the large (but qualitatively poor) existing database was integrated
with new data to provide a more precise picture of variations of chemical and isotopic
composition of the sedimentary cover in time and distance from and along the arc.
The variability in major and trace element composition observed in oceanic sediments
can be modelled as the result of variable degrees of dilution of a detrital component
by organogenic calcite and silica. Pb isotopes can distinguish between pre- and postMiocene
sediments, and the Sr, Nd, and Pb isotope systematics in old sediments are
consistent with their derivation from volcanogenic material with isotope systematics
similar to those of Indian Ocean basalts, and material from pre-Mesozoic continental
crust similar to that found in Sumatra. On the other hand, post-Miocene sediments
seem to be enriched in a U-rich component.
Using the new data, the distribution of volcanism along Sumatra and the west Sunda
arc and the effects of crustal and sediment contamination are briefly reviewed and
discussed. These data suggest that neither contamination of the mantle source by bulk sediment
nor bulk assimilation of crustal material by an uprising melt can satisfactorily account
for the Sr, Nd, and Pb isotope systematics observed in most arc rocks in the west Sunda arc.
Fluids released from the subducted sediments do not seem to be able to reproduce the
range and spatial distribution of 87Sr/86Sr - 143Nd/144Nd values in the west Sunda
volcanic rocks. Pb isotopic values and their variations along the arc also suggest that
subducted sediments did not have important effects on the Pb systematics of the west
Sunda volcanic rocks. On the contrary, assimilation of partial melts of crustal material by uprising magmas
can account for the Sr, Nd, and Pb isotope systematics in the Sumatran and Jawanese
arc rocks, and is consistent with the spatial distribution of Sr, Nd, and Pb isotope
values observed from north Sumatra to Lombok. This model requires that Indian
Ocean OIB-like mantle, as well as MORB mantle, be present in the mantle wedge,
and also requires the existence of pre-Mesozoic continental crust in Sumatra and west
Jawa, but its absence in the Bali sector from east Jawa to Lombok.
Relatively low 3He/4He, and high B/Be and a 18 0 values, usually interpreted as
evidence for metasomatism of the mantle wedge by slab-derived material, are rare in
the west Sunda volcanics, and are best explained by processes occurring at shallow
levels (degassing of the magma, percolation of meteoric fluids, assimilation of crustal
material), in the magma chamber or at near-surface levels. 230Th/232Th values in
Holocene rocks are indicative of an OIB source, and there is no evidence for 230Th -
238U disequilibrium and metasomatism of the mantle source by slab-derived fluids.
In the second part of this thesis, all three localities in Sumatra where olivine-phyric
basalts had been previously reported (Sukadana, Bukit Telor, and Bukit Mapas) have
been studied, together with other alleged occurrences of olivine-phyric basalts
elsewhere in Indonesia. Whole rock major, trace and Sr, Nd and Pb isotope
analyses, together with detailed mineralogical studies, were carried out to determine
the composition of the source and the evolution of these isotopically and
compositionally rather unusual (in an arc environment) rocks, and their similarity to
OIB and MORB basalts from the Indian Ocean and Australia is evaluated
The basaltic andesites of Bukit Mapas are olivine-phyric, but closely resemble the
other Sumatran arc volcanics in their mineral and whole-rock composition, and Sr,
Nd, and Pb isotope systematics. These rocks are characterised by the coexistence of
Al-spinel and Cr-spinel inclusions in olivine phenocrysts, probably as the result of
mixing at relatively shallow depth between a "normal" basaltic melt and a high-Al
melt derived from the melting of crustal material.
Two types of olivine-bearing basalts (Sukadana basalts) were identified in south
Sumatra. The high-Ti basalts have TiO 2 > 2.3%, are slightly alkaline, and have
HFSE, and MREE and HREE concentrations similar to those of OIB, but are
relatively depleted in LILE and LREE. Isotopically they resemble basalts from the
Ninetyeast Ridge and some Indian Ocean MORB, and there is no evidence for their
derivation from isotopically strongly enriched hot spot magmas. Despite their
proximity to the arc, there is no evidence for metasomatism of their source by
subducted sediments, and the depletion in LREE and L1LE suggests that their source
might have suffered a previous episode of depletion: maybe the extraction of arc
melts, as suggested by the fact that these rocks are spatially and temporally associated
with arc rocks, and that arc-related granitoids in Sumatra derived from an isotopically
similar source.
Low-Ti basalts have TiO2 < 1.6%, are tholeiitic, and have variable major and trace
element composition, with the most "depleted" samples showing trace element
composition similar to E-MORB, but with negative HFSE peaks and depleted HREE.
The low-Ti basalts may derive from a source similar to that of the high-Ti basalts
through a combination of two processes: higher degrees of partial melting,
responsible for the lower content in incompatible elements, and crustal
contamination, which accounts for the Sr, Nd, and Pb isotopic variations, and for the
correlation between isotope ratios and LILE/HFSE values. The composition of some
spinel inclusions in the low-Ti basalts suggests that relatively large degrees of crustal
contamination may produce melts which may have major and trace elements and Sr,
Nd, and Pb isotopic ratios very similar to those of arc basalts.
Basalts from Buldt Telor in central Sumatra are rich in mantle xenoliths (lherzolite)
and resemble the high-Ti basalts of Sukadana, but are overall more primitive and
more alkaline, and probably represent the composition of the unmetasomatised mantle
wedge at some distance from the volcanic arc. Isotopically, they resemble basalts of
the Ninetyeast Ridge and of some Indian Ocean MORB. In general, the basalts from Sukadana and Bukit Telor have trace element and isotopic
characteristics similar to those of tholeiitic, to mildly alkaline basalts from Kerguelen,
the Ninetyeast Ridge, and from the eastern Australian volcanic province. Similar
magmatism may also occur within the southeast part of the Eurasian plate
(Karimunjawa Islands, and Quaternary volcanics in central and western Kalimantan).

Item Type: Thesis (PhD)
Keywords: Volcanism
Copyright Holders: The Author
Copyright Information:

Copyright 1993 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (Ph.D.)--University of Tasmania, 1994. Includes bibliographical references (p. 280-312)

Date Deposited: 08 Dec 2014 23:57
Last Modified: 11 Mar 2016 05:55
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