The Plio-Quaternary Ambon arc, Eastern Indonesia

Plio-Quaternary lavas and granites have been collected from Ambon, Seram, K elang, Haruku, Saparua, Ambelau and Banda Api islands, Eastern Indonesia. T hey include low-K calc-alkaline basalts, andesites, dacites and rhyolites a nd high-K calc-alkaline andesites, dacites, rhyolites and granites. All the se rocks present the usual chemical characteristics of island-are magmas. T he high-K suite of Ambon is mostly represented by cordierite-bearing dacite s (known as ambonites) and granites. Low-K and high-g magmas were emplaced in neighbouring islands or even in the same island (Ambon), often concomita ntly, during two magmatic pulses at 5-3.2 Ma and 2.3-1 Ma, respectively. We propose that the low-K suite results from the evolution of basaltic magmas derived from mantle melting above the Western Irian Jaya plate which subdu cts along the Seram trough. Intermediate and acidic rocks of the high-K sui te (e.g. ambonites) are thought to derive from low-K mafic magmas through m assive assimilation of the Seram-Ambon continental crust, as originally pro posed by Van Bemmelen in 1949. The timing of magmatic events and the geoche mical features of the studied lavas are clearly different from those of the southern part of the Banda are, in which the low-K suite is lacking. In ag reement with earlier seismic evidence for two different slabs subducting be neath the Seram-Ambon continental block and beneath the southern Banda are (from Wetar to Manuk), respectively, we propose to recognise a new Plio-Qua ternary island are, i.e. the Ambon are, extending west-east from Ambelau to the Banda Archipelago active low-K volcanoes through Kelang, southwestern Seram, Ambon, Haruku and Saparua. (C) 1999 Elsevier Science B.V. All rights reserved.