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

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.