Journal of Petrology Advance Access originally published online on September 20, 2006
Journal of Petrology 2006 47(12):2433-2462; doi:10.1093/petrology/egl050
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Coexisting High- and Low-Calcium Melts Identified by Mineral and Melt Inclusion Studies of a Subduction-Influenced Syn-collisional Magma from South Sulawesi, Indonesia
1 MAX PLANCK INSTITUTE FOR CHEMISTRY, GEOCHEMISTRY DIVISION MAINZ, 55020, GERMANY
2 FACULTY OF EARTH AND LIFE SCIENCES, DEPARTMENT OF ISOTOPE GEOCHEMISTRY, FREE UNIVERSITY AMSTERDAM, 1081 HV, THE NETHERLANDS
3 DEPARTMENT OF GEOLOGY AND SOIL SCIENCE, GHENT UNIVERSITY KRIJGSLAAN 281 S8, 9000 GHENT, BELGIUM
4 SCHOOL OF EARTH SCIENCES AND CODES SRC, UNIVERSITY OF TASMANIA HOBART, TAS 7001, AUSTRALIA
5 FACULTY OF GEOSCIENCES, UTRECHT UNIVERSITY UTRECHT, 3508 TA, THE NETHERLANDS
6 DEPARTMENT OF GEOLOGY AND GEOPHYSICS, UNIVERSITY OF ADELAIDE ADELAIDE, SA 5005, AUSTRALIA
RECEIVED NOVEMBER 28, 2005; ACCEPTED AUGUST 22, 2006
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Abstract |
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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 (<4 wt %); it is in equilibrium with Cr-rich spinel with a low TiO2 content, olivine with low CaO and high NiO content (Fo90–94; 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 (Gdn/Ybn 
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.
KEY WORDS: melt inclusions; mineral chemistry; olivine; syn-collisional magmatism; ankaramites; low-Ca magma
*Corresponding author. Present address: Department of Geology and Soil Science, Ghent University, Krijgslaan 281 S8, 9000 Ghent, Belgium. Telephone: +32-9-2644566. Fax: +32-9-2644984. E-mail: Marlina.Elburg{at}Ugent.be
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