The deep crust beneath island arcs: inherited zircons reveal a Gondwana continental fragment beneath East Java, Indonesia

Abstract

Inherited zircons in Cenozoic sedimentary and igneous rocks of East Java range in age from Archean to Cenozoic. Thedistribution of zircons reveals two different basement types at depth. The igneous rocks of the Early Cenozoic arc, found along thesoutheast coast, contain only Archean to Cambrian zircons. In contrast, clastic rocks of north and west of East Java containCretaceous zircons, which are not found in the arc rocks to the south. The presence of Cretaceous zircons supports previousinterpretations that much of East Java is underlain by arc and ophiolitic rocks, accreted to the Southeast Asian margin duringCretaceous subduction. However, such accreted material cannot account for the older zircons. The age populations of Archean toCambrian zircons in the arc rocks are similar to Gondwana crust. We interpret the East Java Early Cenozoic arc to be underlain by acontinental fragment of Gondwana origin and not Cretaceous material as previously suggested. Melts rising through the crust,feeding the Early Cenozoic arc, picked up the ancient zircons through assimilation or partial melting. We suggest a WesternAustralian origin for the fragment, which rifted from Australia during the Mesozoic and collided with Southeast Asia, resulting inthe termination of Cretaceous subduction. Continental crust was therefore present at depth beneath the arc in south Java whenCenozoic subduction began in the Eocene. 2007 Elsevier B.V. All rights reserved.