Petrogenesis of volcanic rocks from Eastern Manus Basin: indications in mineralogy and geochemistry

Abstract

To understand the petrogenesis and magma evolution history in the Eastern Manus Basin (EMB), geochemistry of pyroxene and plagioclase mineral phenocrysts in basaltic andesites and dacites were reported. The plagioclase-melt thermometry showed that, plagioclase in dacites crystalized in 1 027.2–1 028.5°C under 3.37–5.08 kbar, whereas in basaltic andesite was 1 181.5–1 187.0°C under 1.79–4.46 kbar. Pyroxene compositions and invariable La/Sm vs La values of whole rock powders indicate that lavas were erupted in rapid cooling rate and mainly controlled by fractional crystallization (FC). In addition, oscillatory zonings in plagioclase and pyroxene phenocrysts indicated small local perturbations and degassing episodes in the magma chamber. Some high Mg# clinopyroxene antecrysts were found in EMB lavas. The highest Mg# of parental EMB melts is 69, which falls into the range of initial partial melts from upper mantle peridotite source (68–75). In terms of isotopic compositions, the EMB lavas most likely originated from Indian-type MORB mantle which was influenced by subduction components. In details, the subduction components are mainly derived from the dehydration of a subducted altered oceanic crust, and the contribution of sediment influence is minor. The Pb isotopic compositions and end member modeling further suggest that the source of subduction components is more likely from the Pacific Plate instead of the Solomon Plate.