Coexisting high- and low-calcium melts identified by mineral and melt inclusion studies of a subduction-influenced syn-collisional magma from South Sulawesi, Indonesia

Mineral and melt inclusions in olivines from the most Mg-rich magma from the southern West Sulawesi Volcanic Province indicate that two distinct melts contributed to its petrogenesis. The contribution that dominates the whole-rock composition comes from a liquid with high CaO (up to 16 wt %) and low Al₂O₃ contents (CaO/Al₂ O₃ up to 1), in equilibrium with spinel, olivine (Fo_85-91; CaO 0.35-0.5 wt %; NiO 0.2-0.30 wt %) and clinopyroxene. The other component is richer in SiO₂ (>50 wt %) and Al₂O₃ (19-21 wt %), but contains significantly less CaO (<4 wt %); it is in equilibrium with Cr-rich spinel with a low TiO₂ content, olivine with low CaO and high NiO content (Fo_90-94; CaO 0.05-0.20 wt %; NiO 0.35-0.;5 wt %), and orthopyroxene. Both the high- and low-CaO melts are potassium-rich (>3 wt % K₂O). The high-CaO melt has a normalized trace element pattern that is typical for subduction-related volcanic rocks, with negative Ta-Nb and Ti anomalies, positive K, Pb and Sr anomalies, and a relatively flat heavy rare earth element (HREE) pattern. The low-CaO melt shows Y and HREE depletion (Gd_n/ Yb_n ≤41), but its trace element pattern resembles that of the whole-rock and high-CaO melt in other respects, suggesting only small distinctions in source areas between the two components. We propose that the depth of melting and the dominance of H₂O- or CO₂-bearing fluids were the main controls on generating these contrasting magmas in a syn-collisional environment. The composition of the low-CaO magma does not have any obvious rock equivalent, and it is possible that this type of magma does not easily reach the Earth's surface without the assistance of a water-poor carrier magma.