In-situ chemistry of plagioclase and amphibole phenocrysts of Mt. Lamington volcano in Papua New Guinea: Evidence for influence of Woodlark spreading ridge to Papuan arc

Abstract

Mt. Lamington is a Quaternary volcano located on the Papuan Peninsula, SE Pacific. Texture studies and in-situ chemistry analyses on mineral phenocrysts are performed on andesite and its enclaves and pre-erupted shoshonite to better understand the petrogenesis and tectonic settings of the volcano. Both amphibole and plagioclase phenocrysts have variable zoning textures in the three major lithologies. Although the andesite and enclaves have similar bulk 87Sr/86Sr ratios (~0.7039), their plagioclase phenocrysts show varied in-situ87Sr/86Sr: nging from 0.70360 to 0.70519 in andesite (An26–58) and from 0.70373 to 0.70393 in enclaves (An31–61). The shoshonite shows a bulk 87Sr/86Sr ratio of ~0.7037, and plagioclase phenocrysts in it show in-situ87Sr/86Sr ratios ranging from 0.70386 to 0.70409 (An27–57). A synthetic analysis of in-situ plagioclase strontium isotopes of the volcano, as well as bulk strontium and neodymium isotopes data from adjacent areas indicates three components: one high-143Nd/144Nd and low-87Sr/86Sr depleted mantle source represented by the most depleted samples of the Woodlark oceanic crust, one low-143Nd/144Nd and medium-87Sr/86Sr enriched mantle source represented by arc-type volcanics, and a third relatively low-143Nd/144Nd and high-87Sr/86Sr source represented by melts from arc crustal precursors. A spatial negative correlation of 143Nd/144Nd ratios of volcanics from east (Woodlark Basin) to west (Mt. Lamington) indicates that the magma sources have changed from depleted mantle to recently enriched lithospheric mantle plus contamination of the Papuan arc crust. This relationship agrees with a hypothesis that Mt. Lamington volcano represents the west extension of the Woodlark spreading ridge onto the Papuan arc, which in turn was a result of subduction from north.