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The petrology and geochemistry of the Wilis and Lawu volcanoes, East Java, Indonesia

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Hartono, U (1994) The petrology and geochemistry of the Wilis and Lawu volcanoes, East Java, Indonesia. PhD thesis, University of Tasmania.

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Abstract

The Wilis Volcanic Complex ONVC) and Lawu Volcanic Complex (L VC)
are two volcanoes situated in the eastern part of the island of Java, Indonesia. The
volcanoes are separated by an east-west distance of about 70 km, and are part of
the chain of volcanoes which forms the eastern Sunda volcanic arc, a subductionrelated
feature that developed along the convergent boundary between the IndianAustralian
oceanic plate to the south and Eurasian continental plate to the north.
The eruptive products of the WVC are classified into five groups based on
morphostratigraphy and lithology, these are, from oldest to youngest, the precaldera
Klotok basalt and basaltic andesite, the pre-caldera Pawonsewu andesite,
the caldera Ngebel high-Si andesite and dacite, the post-caldera Jeding basaltandesite,
and the post-caldera Argokalangan andesite. Estimated temperatures of
crystallisation of WVC magmas are about 1150 0C for the basalts, about 1060 0C
for one olivine-bearing andesite, and range from 1035 0C to 950 0C for the
andesites. Water contents of the magmas seem to have increased from basaltic
through andesitic to dacitic. The bulk compositions of the WVC andesites suggest
that the magmas may have formed in a relatively deep magma chamber, at depths
of 7 - 18 km (equivalent to pressures of 2 - 5 kb), with small amounts of water.
The WVC basalts may have began to cry stallise at even greater depths,
presumably around 30 km (equivalent to pressures,.. 8 kb), and the dacites may
have fanned at about 18 km depth, possibly at slightly greater depths than the
most andesites.
The geochemical and isotopic characteristics of both the pre-caldera
Pawonsewu and post-caldera Argokalangan andesites are consistent with an origin
largely by crystal-liquid fractionation from the pre-caldera Klotok basalt parental
magma. Early fractionation of Ol+Cpx+Pl+Mg assemblages followed by
Opx+Cpx+Pl+Mg±Apt assemblages could have produced the pre-caldera
Pawonsewu and post-caldera Argokalangan andesites from Klotok-type basaltic parental magma. However, whereas both the pre-caldera Pawonsewu and postcaldera
Argokalangan andesites seem to have formed under open-system
conditions, with the magma chamber being replenished by new magmas,
assimilation of some crustal material may have been involved during the
generation of the pre-caldera Pawonsewu andesite.
The origin of the caldera N gebel high-Si andesite and dacite is more
difficult to interpret. Major, trace and REE characteristics of the dacite (including
High-Si andesite) are not consistent with an origin by simple fractionation from
andesite. They may be related petrogenetically to the pre-caldera Klotok
basalt/basaltic andesite, despite a silica-gap of about 7 wt % Si02. Crystal
fractionation involving the separation from Klotok-type basaltic parental magma
of assemblages of O l+Cpx+Pl+Mg followed by either Opx+Cpx+Pl+Mg,
Pl+Amp+Mg or Opx+Pl+Amp+Mg, could explain the major element variation
from the basalt to dacite. However, 87 Srt86sr and o18o values increase with
increasing Si02, Rb and Rb/Sr, and decrease with increasing MgO and Sr
contents, suggestive of some crustal contamination during differentiation, and
trace element modelling and the extremely high o18o (12.1%o- 13.5%o) values of
the amphibole phenocrysts from the dacite suggest that even more complex
differentiation processes may have taken place.
The L VC is divided into two groups, i.e. Old La wu which is characterised
by hornblende-bearing andesites and Young La wu which is composed of olivinebearing
andesites. Unlike Wilis, basalts are not found. The estimated
temperatures of crystallisation of the LVC andesites range from 125QOC to 9820C
for Old Lawu and from 132QOC to 9860C for Young Lawu. Amphibole-bearing
andesite may have formed at depths of 6 - 7 km, whereas the more basic andesite
could have formed at greater depths, perhaps up to 18 km. Old Lawu andesite
seems to have developed by crystal fractionation in an open-system magma
chamber with the magma chamber being replenished b y new magmas. Crystal
fractionation could also account for the geochemical variation within the Young
Lawu andesites, although contamination may have also been partly responsible for
the geochemical variations. It is not known whether both Lawu groups came from
the same parental magma, or even if their parent was basaltic. A more speculative
interpretation suggests that the Young Lawu andesite may have resulted from
fractionation of a mantle-derived basaltic parental magma, whereas the Old Lawu
andesite more probably derived from a basaltic andesite parental magma produced at the upper mantle - lower crust as a result of reaction between mantlederived
magma and anhydrous ferromagnesian phases.
Zr/Nb, 8 7sr;8 6sr and 143;144Nd values of the WVC basalts and the
87sr;86sr and 143;144Nd values of LVC andesites suggest that MORB mantle
exists in the mantle wedge beneath these volcanoes. The REE patterns of the
WVC basalts and Young Lawu andesites are inconsistent with models calling for
residual amphibole in the source region, but may be adequately modelled by
melting of spinel lherzolite.
Wheller et al (1987) identified four regional along-arc sectors in the
magmatism of the Sunda-Banda arc. They placed the transition from their West
Java to Bali sector between Lawu and Wilis and proposed that the geochemical
and isotopic differences reflected a greater involvement of crustal material in the
sources of the magmatism. It is here argued that these differences arose because
the effects of crustal assimilation are more pronounced in the Lawu Volcanic
Complex than in the Wilis Volcanic Complex, possibly because the arc crust is
thinner under Wilis than beneath Lawu.

Item Type: Thesis (PhD)
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Date Deposited: 19 Dec 2012 03:52
Last Modified: 15 Sep 2017 01:06
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