Massive Assimilation of Continental Crust by Low-K Subductionrelated Magmas: The Plio-Quaternary Ambon Arc (Eastern Indonesia)

Abstract

The Ambon Plio-Quaternary arc includes the islands of Ambelau, Kelang, Seram, Ambon, Haruku, Saparua, and Banda Api, Eastern Indonesia. It results from the subduction of the Western PapuaNew Guinea continental shelf along the Serum trough. Its magmatic rocks present the common petrologic and chemical characteristics of island-arc magmas. They include low-K calc-alkaline basalts, andesites, dacites and rhyolites together with high-K calc-alkaline andesites, dacites, rhyolites and granites. Low-K and high-K magmas were emplaced, often concomitantly, during two events which occurred at c. 5-3.2 Ma and 2.3-1 Ma. The low-K suite parent magmas likely derived from the melting of a depleted mantle wedge having incorporated a continental component as described by Vroon et al. (1993) for the Banda arc, possibly through contribution of subducted continent-derived sediments. The high-K suite is represented in Ambon by cordierite-bearing dacites (ambonites) and granites. Both of them contain numerous Al-rich sanidinite-facies xenoliths which display various stages of thermal metamorphism, melting and assimilation of cordierite-sillimanite-garnet-bearing migmatites from the substratum of the arc. The highK suite is characterised by a strong continental crust