The central ophiolite belt of Irian Jaya (Indonesia): Petrological and geochemical evidence for a back-arc basin origin.

Since the Mesozoic, the north-south oblique convergence between the Austral ian and Pacific plates produced the obduction of pieces of oceanic lithosph eres now incorporated into the New Guinea Island. The largest one correspon ds to Central Ophiolite Belt (COB), located in the central part of Irian Ja ya, that displays well-preserved peridotites, gabbros, dolerites and basalt s. The ultramafic rocks, characterized by the abundance of harzburgites, sh ow residual characteristics implying residues of a 20 to 25 % melting. The associated basalts are MORE-like, but display significant Nb and Ta depleti ons suggesting a subduction zone environment for their origin. These data s uggest that COB was formed in a backarc environment, along the Australian c ontinental margin rather than within a large oceanic domain. (C) 2000 Acade mie des sciences / Editions scientifiques et medicales Elsevier SAS.