Magma plumbing system processes in fast − spreading back − arc basin: records of phenocrysts in volcanic rocks from the Eastern Manus Basin

Abstract

The Eastern Manus Basin is one of the fastest expanding back − arc basins in the world and it is the site of recent volcanism and hydrothermal activity. The role of magma mixing in the origins of the volcanic rocks in this region, as well as the modeling of its magma plumbing system, are still unclear. In this study, we have clarified the magma plumbing system processes of the Eastern Manus Basin by analyzing the petrography and geochemical characteristics of whole rocks and minerals of basaltic andesite, andesite, and dacite in this region. The analyses reveal that basaltic andesite has experienced high undercooling and intense degassing, while both andesite and dacite samples have experienced magma mixing during their formation. The mineral assemblages in andesite are derived from basaltic, dacitic, and mixed melts, with the mixed melt comprising a 2:8 ratio of the former two. Dacite samples contain three mineral assemblages derived from andesitic, rhyolitic, and mixed melts, showing multiple injections of more primitive melts, as indicated by phenocryst textures and chemical zoning patterns. Moreover, they may have experienced the capture of mafic wall rocks. The performance of different mineral − based thermobarometers has been assessed by constructing the experimental datasets applicable to this study, and the best − performing thermobarometers are all from Putirka (2008). Calculations show that the pre − eruption storage temperatures for basaltic andesitic, andesitic, and dacitic magmas are 1090 ± 13 °C, 1032 ± 9 °C, and 938 ± 10 °C, respectively, with storage pressures not well constrained at 4.3 ± 1.4 kbar, 2.8 ± 1.3 kbar, and 2.5 ± 1.3 kbar, respectively. This study provides evidence that magma mixing plays a significant role in the origins of andesite and dacite from the Eastern Manus Basin and that its complex magma plumbing systems provide materials and potential heat sources for the volcanism and hydrothermal activity.