Origin of the Mussau Trench in the Western Pacific: Geochemical and mineralogical constraints from basalts and serpentinized peridotites

Abstract

The Mussau Trench is apparently an incipient subduction zone. However, petrologic and geochemical evidence for the nature and origin of the Mussau Trench is lacking. We present the first mineralogical and geochemical analyses of basalts and serpentinized peridotites from the Mussau Trench. All sampled Mussau Trench basalts yield MORB-like trace-element patterns and lack the geochemical characteristics of typical arc lavas. The Dupal-type isotopic compositions of the MORB-like lavas confirm that the overriding plate shows more affiliation to the Caroline Plate than to the Pacific Plate. The Mussau Trench could have formed along a pre-existing transform fault in the Caroline Plate. Peridotites from the Mussau Trench are highly serpentinized and can be divided into two groups. Group I peridotites have low MgO/SiO2 ratios and contain talc and quartz, suggesting that they were metasomatized by SiO2-rich fluids. Group II peridotites have higher Al2O3/SiO2 ratios at a given MgO/SiO2 ratio and rarely contain talc or quartz. The peridotites in the Mussau Trench differ from typical fore-arc peridotites in the Mariana Trench; for example, the Mussau peridotites are less depleted in trace elements and have lower spinel Cr# (<0.6) than those from the Mariana Trench, suggesting that they originated in the lithosphere of the Caroline plate. The temperatures (<688 °C) and pressures (<7.3 kbar) estimated using amphibole compositions (dominantly edenite and pargasite) suggest that the peridotites were serpentinized by high-temperature slab-derived fluids. Although the limited plate convergence at the Mussau Trench was insufficient to cause arc magmatism, subducted slab-derived fluids caused serpentinization of the overriding lithospheric mantle in an infant subduction channel. Therefore, the serpentinized peridotites and basalts in the Mussau Trench provide important information on the marks and processes of intra-oceanic subduction initiation.