Manam Island, Papua New Guinea: Petrology and geochemistry of a low-TiO₂ Basaltic Island-arc volcano

Abstract

Manam volcano consists of relatively mafic and compositionally similar tholeiitic basalts and low-SiO2 andesites that are characterized by notably low (mainly 0·3-0·35 weight per cent) TiO2 contents. These rocks provide an ideal opportunity to investigate both the extent of depletion in their peridotite magma-source regions (which are evidently similar in many respects to the highly depleted sources of boninitic magmas), and the interplay of the high-level processes of magma mixing, crystal fractionation, and upper crustal contamination, in an island-are volcano. Manam rocks have pronounced enrichments in Rb, Ba, K, and Sr relative to the light rare-earth elements and, especially, to the high-field-strength elements (Sr/Ti values are exceptionally high). However, there is no compelling evidence that these enrichments were caused by addition of a hydrous, slab-derived component to the peridotite source region. Nd and Sr-isotope ratios plot within the oceanic mantle array; 207Pb/204Pb values are only slightly higher than those for oceanic rocks; and the absence of hydrous minerals, the early crystallization and modal preponderance of plagioclase over pyroxene, high estimated quenching temperatures, and low water contents in the Manam rocks, are all evidence that the magmas crystallized under markedly water-undersaturated conditions. Unusually anorthite-rich plagioclase phenoerysts in the more diffrentiated rocks may correspond to crystallization under higher water-vapour pressures, possibly caused by influxes of groundwater, or they may be accidental xenocrysts. Fractionation of olivine, clinopyroxene, and spinel (early chromite followed by magnetite) has dominated the evolution of the magma series. However, clear correlations between incompatible trace-element ratios, 87Sr/86Sr, and 100 Mg/(Mg 3 Fe23) values are convincing evidence for an accompanying mixing process-either of (1) two basaltic magma types (one more fractionated and lower in 87Sr/86Sr than the other), or (2) pristine magmas and contaminant from basaltic conduit and reservoir wall rocks. Wall-rock contamination is the less likely process, and is the more difficult one to identify, particularly if it accompanied magma mixing.