Petrology and geochemistry of the Cyclops ophiolites (Irian Jaya, East Indonesia): consequences for the Cenozoic evolution of the north Australian margin

The Cyclops massif (Irian Jaya - Western Indonesia) displays all components of an ophiolitic sequence including residual mantle peridotites (harzburgi tes and dunites), cumulate gabbros, dolerites, normal mid-oceanic ridge bas alts (N-MORB) and minor amounts of boninitic lavas. This ophiolitic series tectonically overlies high temperature (HT)-high pressure (HP) mafic rocks metamorphosed during the Miocene. Mineral chemistry and bulk rock rare-earth element (REE) abundances of the peridotites are characteristic of highly residual mantle rocks. The high Cr # [Cr# = 100*Cr/(Cr+Al)] of spinel (up to 60) and very low heavy rare-earth element (HREE) concentrations of peridotites (<0.1 time the chondritic val ues) are in agreement with residues of 25 to 35% melting as expected for pe ridotites from supra-subduction zone environments. Ti-enrichments in spinel s and secondary clinopyroxenes (up to 1%, and 0.5%, respectively) are likel y a consequence of reaction between mantle-derived melts and the host perid otites. High light rare-earth element (LREE) concentrations reaching up to chondritic values and high field strength element (HFSE) anomalies suggest that the initial composition of the residual peridotites has been previousl y modified by the passage of boninitic melt(s). The associated basalts and related cumulate rocks display major and trace element contents with Nb-neg ative anomalies typical of back-are magmas. New K-40/Ar-40 isotopic ages obtained from the back-are basin basalts (BABB - 29 Ma) and boninites (43 Ma) combined with the geochemical signatures of the rocks studied here, indicate that the Cyclops Mountains may have forme d in a single supra-subduction environment. This implies southward plunging subduction of the Australian oceanic lithosphere beneath the northern part of the Australian margin. The ultramafic rocks and related lavas (boninite s) likely formed during the Eocene in a foreare environment, before their s outhward obduction onto the island are crustal welt during the early Miocen e. The Pliocene back-thrusting event has led to the slicing of the back-arc basin series onto the arc and fore-are sequences.