RECENT DEVELOPMENTS IN UNDERSTANDING THE EVOLUTION OF OPHIOLITES : AN OVERVIEW OF NEW GEOCHEMICAL AND PETROGENETIC MODELS FOR OPHIOLITES OF THE SW PACIFIC FROM THE POYA TERRANE, NEW CALEDONIA AND THE TANGIHUA COMPLEX, NEW ZEALAND

Cretaceous ophiolites obducted during the early -mid Tertiary onto Paleozoic-Mesozoic Gondwanan margin crust can be traced from Papua New Guinea through New Caledonia to New Zealand. Two of these ophiolites, the Tangihua Complex, New Zealand and the Poya terrane, New Caledonia, have recently been studied leading to new theories regarding their formation and emplacement.

The Tangihua Complex and Poya terrane appear very similar. They have roughly the same ages of formation (Cretaceous), rock types (predominately basalt) and stratigraphy although the Poya terrane was emplaced in the Eocene-Oligocene compared with Oligocene-Miocene for the Tangihua Complex. Early workers attributed the formation of both complexes to a large spreading centre lying eastward of New Zealand and New Caledonia. However, recent work has concluded that they could not have been formed by the same large-scale rift system as the ophiolites are geochemically distinct, and have experienced different metamorphic PT paths.

The Poya terrane basalts are predominantly P- and N-MORB tholeiites generated during the opening of a small marginal basin north-east of New Caledonia. Alteration and metamorphism in the Poya terrane shows four distinct phases, the highest of which is blueschist. The Tangihua basalts range from back arc tholeiites to calc-alkaline arc basalts suggesting formation in an arc/back-arc setting. The Tangihua Complex has three stages of alteration, the highest of which is greenschist, and contains well-preserved zeolite assemblages.

During the break up of the eastern Gondwana margin, c. 80-100 Ma, there is much uncertainty regarding specific plate movement. It is believed that in New Zealand the situation was complicated by the presence of the Phoenix micro-plate, which stalled subduction, preventing the development of a mature arc system and initiating spreading. The Poya terrane was most likely formed before the Tangihua Complex, north-eastward of New Caledonia in the opening of a small marginal basin. Hence the same overall tectonic regime resulted in two very different local settings, the history of which has taken years to unravel.