Evidence for Hydrous Metasomatism and Slab Melting in the Tabar-Lihir-Tanga-Feni Island Arc, Papua New Guinea

The Tabar-Lihir-Tanga-Feni (TLTF) island arc is located 35-75 km from the northeast Pacific coast of New Ireland Island in Papua New Guinea. These volcanoes are in an extending former forearc region of the inactive Manus-Kilinailau subduction zone, at the boundary between the Pacific and Indo-Australian Plates [1]. Volcanism shifted from New Ireland to the TLTF when Pacific Plate subduction stalled there in the Miocene, when the Ontong Java Plateau impinged on the trench [1, 2]. Fresh volcanic rocks dredged from Tubaf seamount near Lihir island in 2000 (Australian CSIRO Project SHAARC) contain abundant ultramafic xenoliths, making it one of the few locations on Earth where they can be found in a subduction setting. Several xenoliths (2-3 cm in size) and their host lavas from two dredges have been analyzed in this geochemical and petrographic study. Host lavas and xenoliths were mechanically separated, and portions of each were examined for bulk major and some trace elements using X-ray fluorescence, a suite of 29 trace elements was analyzed in the host lavas using ICP-MS, and the chemistry of individual xenolith minerals was evaluated using an electron microprobe. Olivine, ortho- and clinopyroxene crystals in the xenoliths are up to 1 mm in size, and these are crosscut by veins containing smaller crystals of pyroxene, phlogopite, and amphibole, as well as glass and open voids. These veins were previously shown to be evidence of dissolution of primary minerals to form the secondary phases via hydrous metasomatism by fluids from the subducted Pacific slab [2, 3]. The primary olivines in xenoliths have homogenous compositions both within and among the samples (e.g. olivine Mg# 80.03 - 82.42; n=22), but pyroxenes show somewhat more variation (e.g. cpx Mg# 84.33 - 89.21; n=23), with low outliers consistently near and within the metasomatic veins. The host lavas are vesicular, alkali-rich trachybasalts and trachyandesites with abundant phlogopite and amphibole phenocrysts, as well as rare olivine and pyroxene xenocrysts and microxenoliths, and glass. Their bulk compositions (and those of some other TLTF lavas) are marginally adakitic (e.g. Al2O3 > 15wt%, Sr > 300 ppm, Sr/Y > 20, Y < 18, Yb < 1.9, 87Sr/86Sr < 0.704 [4]), suggesting a slab melt component in the lavas. The Pacific slab beneath the TLTF is old and thick and would not ordinarily be a likely setting for melting [4], but its stalled condition at this inactive plate boundary may have allowed the immobile slab to heat over time and contribute a slab melt component to the composition of these forearc lavas. The role of the hydrous fluids identified in the xenoliths as a transporting agent for the adakitic signature observed in the host lavas is the subject of this ongoing study. References [1] Stracke and Hegner, 1998; [2] McInnes et al., 2001 [3] Kamenov et al., 2008 [4] Castillo, 2006.