PETROLOGY AND TRACE ELEMENT GEOCHEMISTRY OF THE CYCLOPS OPHIOLITE (IRIAN JAYA - EAST INDONESIA):

CONSEQUENCES FOR THE EVOLUTION OF THE NORTH AUSTRALIAN MARGIN DURING CENOZOIC

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Christophe Monnier*, Jacques Girardeau

UPRES-EA 2156, Laboratoire de Pétrologie Structurale, Université de Nantes, BP 92208, 44322 Nantes, Cedex 3, France

Manuel Pubellier

URA 1316, Laboratoire de Géologie de ENS, 75321 Paris Cedex 05, France

Mireille Polvé

UMR 5563, Laboratoire de Géochimie, Université Paul Sabatier, 31400 Toulouse Cedex, France

Hervé Bellon

UMR 6538, Laboratoire de Pétrologie, Université de Bretagne Occidentale, B.P 809, 29285 Brest Cedex, France

and Haryadi Permana

UPRES-EA 2156, Laboratoire de Pétrologie Structurale, Université de Nantes, BP 92208, 44322 Nantes, Cedex 3, France

*e-mail (Monnier): monnier@chimie.univ-nantes.fr

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ABSTRACT

The Cyclops Mountain (Irian Jaya - Western Indonesia), display all terms of an ophiolitic sequence including residual mantle peridotites (harzburgites and dunites), cumulate gabbros, dolerites, lavas of normal mid-oceanic basalt (N-MORB) composition and rare boninitic rocks. This series overlies basic rocks corresponding to the infra-ophiolitic sole metamorphosed during Miocene.

Mineral chemistry and bulk rock rare earth element abundance of peridotites are characteristic by highly residual mantle rocks. The high Cr# of spinel (up to 60) and very low heavy rare earth element (HREE) concentrations (<0.1 time the chondritic values), together are in agreement with residues of 20 to 35% melting as expected for peridotites from supra-subduction zone environment. Nevertheless, Ti-enrichments in spinels and secondary clinopyroxenes (up to 1% and 0.5%, respectively), both is a likely consequence of large reactions between mantle-derived melts and mantle whole-rocks. However, the high light rare earth element (LREE) contents reaching up to the chondritic values and the high field strength element (HFSE) anomalies, strongly suggest that the composition of the peridotites has been previously modified by the passage of boninitic melt(s). On the other hand, major and trace elements data on basalts and related cumulate rocks, and particularly the Nb negative anomalies compared to neighbouring incompatible elements, strongly suggest a back-arc basin environment for their formation.

According to new 40K/40Ar isotopic ages obtained on BABB and boninites, and geochemical data from the ophiolitic series, we propose that the Cyclops Mountains have formed in a single supra-subduction environment likely associated with the south plunging subduction zone of the Pacific plate beneath the northern Australian margin. The ultramafic rocks and related lavas (boninites) are probably formed at least during Eocene (43Ma) in a fore-arc environment before their obduction onto the island arc crustal welt during Miocene (20Ma). On the other hand, the crustal part from the ophiolitic series which includes gabbros, dolerites and lavas would represent a part of the back-arc basin accreted during early Oligocene (30Ma).