Outcrop-scale silicate liquid immiscibility from an alkali syenite (A-type granitoid)-pyroxenite asociation near Puttetti, Trivandrum Block, South India

The Pan-African (572 Ma) Puttetti syenite (A-type granitoid)-pyroxenite association intrudes the high-grade metamorphic terrain of the Trivandrum Block, South India. Field evidence indicates the contemporaneous nature of syenitic and pyroxenitic liquids. The occurrence of a mixed-rock (∼70% syenite and 30% pyroxenite) with an emulsion-like texture, and the occurrence of pyroxenite globules in syenite, is interpreted as relies of immiscible magmas. Both syenite and pyroxenite show similar mineral assemblage (with major minerals having overlapping compositions), but the relative proportions differ. Major element and trace element partitioning trends, parallel REE patterns, and similar Sr initial isotope compositions are in accord with behavior either predicted or measured for immiscible melts in experimental and/or natural systems. The more pronounced Eu anomalies and LREE/HREE ratios of syenite and pyroxenite (relative to the mixed-rock) is related to fractionation caused by immiscible separation. The proposed origin of the Puttetti pluton involves the intrusion of a magma whose bulk composition is that of the mixed-rock. This melt behaved immiscibly and split into two fractions, which produced the syenite and pyroxenite magmas.