Indonesian magmatism


	Magmatism in Indoenesia is caused by the northward subduction of the Australian plate underneath the Indonesian plate. Two volcanically active areas, the Sunda and Banda arcs, are separated by an inactive area, where the collision with the Australian continent has halted subduction. The geochemistry of the magmatic rocks shows a clear along-arc change towards the extinct area.

	Map of the Indonesian arc. The area to the south of the Weber thrust, consisting of the islands Alor, Atauro, Wetar and Romang, have become extinct between 3.5 (Alor-Wetar) and 1.8 Ma (Romang), as a result of the collision with the Australian contintent (to which Timor, geologically, belongs).
	Along-arc variation in ²⁰⁶Pb/²⁰⁴Pb ratios. The highest values are measured on the north-east coast of the island of Alor, after which the vlues suddenly drop of on Alor's south coast, Wetar en Romang. This has been interpreted to reflect mixing between upper crustal and lower crustal components of Australian derivation in the magma's source (Elburg et al., 2004; 2005).
	Along-arc variation of potassium-enrichment, expressed as K₂O(wt.%)/(SiO₂(wt.%)-40). This shows that the trend in Pb isotopes seen above is not related to potassium enrichment. The samples from the extinct sector have medium- to high-K characteristics, and do not show the shoshonitic enrichment like Batu Tara or the volcanoes on Sumbawa do.
	Melt Inclusion Studies and Ankaramitic Magmas
	The Indonesian arc shows a distinct lack of basaltic magmas with MgO > 6 wt.%. Quite a few magma's that have high MgO contents trend towards high CaO/Al₂O₃ ratios. These calcic magmas are characterised by a large number of olivine and clinopyroxene crystals, and can therefore be classified as ankaramitic magmas. The trend for the whole arc is similar to that seen in Lombok (red squares), and two of the ankaramitic samples from this island (at 11% MgO) were selected for more detailed studies of their mineral phases and melt inclusions in olivine.

	Melt inclusion in olivine from sample LB43, after homogenisation at 1250 ºC.
	CaO (wt.%) versus mg# for olivine from sample LB43. The variation in CaO is larger than what would be expected for olivine crystallised from a homogeneous magma.
	The CaO/Al₂O₃ ratio of the melt inclusions are correlated with the CaO content of the olivine in which they are enclosed. This suggests hat the olivines may have crystallised from melts with a composition similar to that found as inclusions within the olivines. It is likely that the whole rock composition reflects an amalgamation of individual batches of melts and crystals.


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