Hydrous metasomatism of oceanic sub-arc mantle, Lihir, Papua New Guinea: petrology and geochemistry of fluid-metasomatised mantle wedge xenoliths

Ultramafic, mafic and sedimentary xenoliths have been recovered from a rece ntly erupted, shoshonitic submarine cinder cone (Tubaf and Edison volcanoes ) from the Tabar-Lihir-Tanga-Feni island are, located in the New Ireland ba sin of Papua New Guinea. These samples represent a proxy drill hole that ca n be re-assembled into an 'ophiolite-type model of oceanic lithosphere. Pet rographic and geochemical examination of the gabbroic and depleted mantle x enoliths indicates that the New Ireland fore-are lithosphere is a fragment of ancient Pacific Plate generated at a mid-ocean ridge spreading centre an d transported to the Pacific-Australian Plate margin. Convergent margin pro cesses subjected the harzburgitic mantle wedge to hydrofracturing and hydra tion metasomatism at T = 790-1030 degreesC as a consequence of dewatering o f a subducted slab. Advection of a high-density, H2O-rich fluid containing a substantial dissolved component (alkali aluminosilicate melt and aqueous carbon and sulphur species) through these mantle fractures caused a net tra nsfer of soluble elements from the lower to upper mantle wedge and created a network of oxidised (Delta FMQ approximate to1.8-2.0) metasomatised perid otite enriched in orthopyroxene, clinopyroxene, phlogopite, amphibole, magn etite, and Fe-Ni sulphides. The vein mineral assemblage magnetite+sulphide indicates precipitation from a hydrous fluid with high SO2/H2S, consistent with the hydrous fluid bring derived from dehydration of subducted, altered oceanic crust. Preferential partial melting of these metasomatically enric hed mantle wedge regions could account for the highly oxidised, sulphur- an d alkali-rich nature of the high-K calc-alkaline volcanoes of the Tabar-Lih ir Tanga-Feni island chain, (C) 2001 Elsevier Science B.V. All rights reser ved.