Abstract

The Pleistocene-Recent calcalkaline suite from the active volcano, Rindjani, on the island of Lombok in the east Sunda Arc, Indonesia, is composed of a diverse range of lavas. These include: ankaramite, high-Al basalt, andesite, high-K andesite and dacite. Sr-isotopic and geochemical constraints suggest that this suite was derived from the sub-arc mantle. Geochemical models suggest that fractional crystallization is an important process in the suite's differentiation, although the series: ankaramite-high-Al basalt-andesite-dacite does not represent a continuously evolving spectrum of liquids. The compositional variation within the high-Al basalt group can be modelled in terms of fractionation of its olivine-clinopyroxene-plagioclase phenocryst assemblage, but crystallization of this assemblage does not yield andesitic residual liquids. Compositional variation within the andesite group cannot be reproduced by fractionation of the plagioclase-hypersthene-clinopyroxene-magnetite assemblage of these lavas, although removal of this assemblage does yield residual liquids.

If the andesites are products of fractional crystallization of high-Al basalt parent magmas, the trends established by variation in K2O, Rb and Sr concentrations and K/Rb and Mg/(Mg + Fe) ratios, require fractionation of an assemblage: amphibole + plagioclase + magnetite ± olivine ± clinopyroxene ± apatite. Phase relations in the high-Al basalt-water system place limitations on conditions of andesite formation. Optimum conditions under which the amphibole-plagioclase-magnetite assemblage may co-exist, closest to the basalt liquidus, occur with 3 per cent H2O, between 6 and 10 kb and oxygen fugacity between those of the NNO and HM buffers. Under these conditions this assemblage may occur up to about 1050 °C, about 50 °C below the basaltic liquidus.

Thus for the Rindjani lava suite to yield andesitic residual liquid from basaltic parent magmas, the latter must cool at depths of 20–35 km, there crystallizing amphibole-gabbro assemblages which may be underplated on the base of the arc's crust.

In this model the arc grows both subaerially by eruption of lavas and downwards by accretion at its base and continues to have a basaltic bulk composition.

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