Petrology and trace element geochemistry of the Papuan Ultramafic Belt

Abstract

New petrologic and geochemical data are presented for a suite of rocks from the Papuan Ultramafic Belt (PUB), Papua New Guinea. Tectonite harzburgites at the base of the ophiolite have extremely refractory, uniform mineralogy, and are exceptionally depleted in lithophile elements. These features are consistent with the proposed origin of these rocks as 'depleted' upper mantle, residual after extraction of a basaltic melt. The tectonite peridotites are overlain by a thick sequence of layered ultramafic and mafic cumulates containing olivine, orthopyroxene, clinopyroxene and plagioclase as the major cumulus phases. Early cumulates are characterized by magnesian olivine Mg90, orthopyroxene Mg90 and calcic plagioclase An86, and exhibit cryptic variation towards more iron-rich and sodic compositions. Abundances of 'incompatible' elements in the cumulates are extremely low which, together with the nature of the cumulus phases, points to a magnesian olivine-poor tholeiite or magnesian quartz tholeiite parent magma(s) strongly depleted in 'incompatible' elements. Highly fractionated iron-rich products of this parent magma type are represented by the LREE-depleted lavas in the overlying basalt sequence which, although resembling the most depleted mid-ocean ridge basalts (MORB) in terms of their low abundances of 'incompatible' elements, have higher abundances of transition metals and lower abundances of Ti, HREE and other high valence cations compared to common MORB of similar Mg/(Mg+Fe) ratio. Eocene tonalites intruding the PUB are genetically unrelated to the ophiolites, and appear to be related to the Ti-poor high-Mg andesites of Cape Vogel and similar andesites and dacites at the northern end of the PUB. These rocks are considered to represent the early stages of island-arc magmatism associated with a northeastward-dipping subduction zone in the early Eocene immediately prior to emplacement of the PUB.