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/Al2O3 up to 1), in equilibrium with spinel, olivine (Fo85–91; 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 ( 3w t %K 2O). 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 (Gdn/ Ybn � 41), but its trace element pattern resembles that of the wholerock 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 waterpoor carrier magma.

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