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The petrology of some young volcanic rocks from Lombok and Sumbawa, lesser Sunda Islands

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Foden, JD (1979) The petrology of some young volcanic rocks from Lombok and Sumbawa, lesser Sunda Islands. PhD thesis, University of Tasmania.

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Abstract

Five volcanoes have been studied from the eastern Sunda arc, Indonesia:
Rindjani on Lombok island and G. Sangenges, Tafubora, Soromundi and Sangeang
Api on Sumbawa island. Rindjan-i, Tambora and Sangeang Api are all active.
G. Sangenges and Soromundi became inactive -in the Quaternary. These
volcanoes all occur 165-190 km above the active, north dipping Benioff
Zone.
With respect to other parts of the Sunda arc, the Bali-Lombok-Sumbawa
sector is unique in being flanked both to the north and south by oceanic
crust. It is also part of the young (post Miocene) eastern Sunda arc, which
is a much more re.cent feature than the western Sunda arc. These factors
suggest that the lavas of these volcanoes have had minimal opportunity for
contamination by differentiated crustal material.
The volcanoes of this sector of the arc have erupted a diverse range
of lavas. They range from the ankaramite-high-Al basaH--andesite-dacite
suite of Rindjani, through the moderately potassic ne-tlAachybasalttrachyandesite
suites from Sumbawan volcanoes, Tambora and Sangeang Api,
to highly undersaturated, leucite-bearing types from G. Sangenges and
Soromundi. The K20-content of these suites shows no correlation with the
depth to the Benioff Zone.
Rindjani is a large, compound stratovolcano. Its lavas are similar to
the calcalkaline suites erupted by many circum-Pacific volcanoes, though
they show exceptional composit·ional divers-ity ranging from ne- to Qnormati
ve.
The Sr-isotopic composition of all these lavas are the same (average
0.7039), which suggests a s-ingle source. Fractional crystallisation appears
to play an important role in their differentiation, but cannot account for
all aspects of geochemical differentiation. It seems likely that more than one primary liquid must be involved. The andesites cannot be derived from
the hi gh-AJ. basalts by fract·i onati on of the observed phenocryst assemb 1 ages~
nor can the compositional range within the andesite group be modelled by
mixtures of phenocrysts and groundmasses. The dacites may be liquids in
equilibrium with the plagioclase-dom·inated phenocryst assemblages of the
andesites and resemble their residual glasses.
For the andesites to be the products of fractional crystanisat·ion of
high-A1 basalt parent magmas, constraints placed by variation in K2o, Rb
and Sr and K/Rb and ~1g/~~g+Fe, suggest that such a connection may result
from the crystallisation of an assemblage; amph+plag+mag~cpx~ol~apatite.
Phase relations in the system high-A1 basalt + water suggest that
optimum condit·ion's under which the assemb'lage, amph+plag+mag may crystallise
closest to the basalt liquidus, occur with 3% H20, between 7 and 10 kb and
f02 between the NNO and HMN buffers. Under these conditions this assemblage
may coexist up to about 1050-1060°C, which is still 50°C below the basalt
liquidus. To yield andesitic differentiates, mantle derived parent basalts
must cool markedly in the pressure-range 7-10 kb, a pressure corresponding
to the probable depth to the Moho in this sector of the Sunda arc.
The elimination of amphibole from andesitic derivative magmas at low
pressures and the rapid increase in the liquidus temperature of plagioclase,
results in the replacement of this early amphibole-rich assemblage by a lowpressure
plagioclase-r·ich assemblage. The dacites may represent the residual
liquid formed as a result of this re-equilibration.
The suites from the other four volcanoes (G. Sangenges, Soromundi,
Tambora and Sangeang Api) are almost all undersaturated. Like the Rindjani
lavas, most of the ne-normative trachybasalts and trachyandesites from
Tambora and Sangeang Api are p'Jagiophyric, but are also distinguished by
the presence of groundmass K··fe l clspar. Leucitites from Soromundi and G. Sangenges are highly undersaturated and are plagioclase-poor or -free.
Leucite occurs as phenocrysts and in the groundmasses.
Sangeang Api volcano has also erupted a suite of mafic to ultramafic
nodules as xenoliths and blocks. These range from olivine-clinopyroxenites
and magnetite clinopyroxenites to hornblende-anorthite gabbros.
Highly undersaturated, potassium-rich leucitic suites are unusual in
island arcs and where they do occur are considered to be late-stage products
and to be spatially most distant from the trench, above the deepest Benioff
zone region. Yet in the Sunda arc, these rocks occur in a relatively young
arc-sector, at a similar height above the Benioff zone to laterally adjacent
calcalkaline suites.
The sequenc~ from calcalkaline lavas, through ne-normative trachybasalts
and trachyandesites to leucitites is marked by a progressive increase in K2o,
Rb, Sr, and LREE concentrations, increasing K20/Na2o and decreasing K/Rb.
The isotopic composition of Sr also becomes more radiogenic. By contrast,
Zr, Ti, Nb and Hf and to a lesser extent P, show much less marked enrichment.
This between-suite variation must reflect variation in the composition
of primary magmas, which ·j n turn are considered to reflect a heterogeneous
mantle-source which is variably enriched in K-type LIL incompatible elements.

Item Type: Thesis (PhD)
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Date Deposited: 17 Mar 2014 05:15
Last Modified: 15 Sep 2017 01:06
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