The origin of medium-K ankaramitic arc magmas from Lombok (Sunda arc, Indonesia): Mineral and melt inclusion evidence

Abstract

High-calcium, nepheline-normative ankaramitic basalts (MgO>10 wt.%, CaO/Al2O3>1) from Rinjam volcano, Lombok (Sunda arc, Indonesia) contain phenocrysts of clinopyroxene and olivine (Fo(85-92)) with inclusions of spinel (Cr# 58-77) and crystallised melt. Olivine crystals have variable but on average low NiO (0.10-0.23 wt.%) and high CaO (0.22-0.35 wt.%) contents for their forsterite number. The CaO content of Fo(89-91) olivine is negatively correlated with the Al2O3 content of enclosed spinel (9-15 wt.%) and positively correlated with the CaO/Al2O3 ratios of melt inclusions (0.9-1.5). Major and trace element patterns of melt inclusions are similar to that of the host rock, indicating that the magma could have formed by accumulation of small batches of melt, with compositions similar to the melt inclusions. The liquidus temperature of the magma was similar to 1275 degrees C, and its oxygen fugacity <= FMQ + 2.5. Correlations between KO, Zr, Th and LREE in the melt inclusions are interpreted to reflect variable degrees of melting of the source; correlations between Al2O3, Na2O, Y and HREE are influenced by variations in the mineralogy of the source. The melts probably formed from a water-poor, clinopyroxene-rich mantle source.