Geochemistry of the Topuk Pluton associated with the Kozbudaklar W-skarn deposit (Western Anatolia, Turkey): Implication for crystallization conditions

Abstract

The Kozbudaklar scheelite-bearing skarn deposit in the Tavşanlı Zone, western Turkey, occurs at the contact between Eocene Topuk pluton and Triassic _Inönü marble of calcic character. The Topuk pluton is medium-coarse grained, granodiorite in composition and has a hypidiomorphic equigranular texture. The host rock contains mafic microgranular enclaves (MME) of monzodiorite-monzogabbro composition and is interrupted by porphyritic granodiorite and granite-aplite vein rocks. The pluton is calk-alkaline, metaluminous and composed of I-type melt character. d18O and dD compositions of silicate minerals from granodioritic host rock are 5.9-10.6‰ and 77.0 to 71.4‰ and conformable with the range of unaltered I-type granites. Trace element contents indicate that pluton is crystallized from mantle-derived magma interacted with continental crust in a volcanic arc or subduction related setting. Major and trace element concentrations of Topuk pluton are quite consistent with geochemical patterns of Cu-skarn granitoids. Results of mineral chemistry analysis of the pluton yield that plagioclases are of oligoclase-andesine, amphiboles are of magnesio-hornblende and biotites are of ferro-magnesian composition. Amphiboles and biotites of granodioritic host rock are represented by calc-alkaline, I-type melt composition evolved in a subduction environment. Based on the results of plagioclase-Al in hornblende and amphibole chemistry data from the pluton, two different stages are proposed for the magma crystallization. The first stage was developed in a relatively deeper environment (>15 km) under high pressure (>4 kbar) and low log fO2 (> 17.6) conditions which reflect fractional crystallization and magma-mixing depth of basaltic magma and these conditions are not correlated with scheelite mineralization. The second crystallization stage of magma which proceeded at shallow depths (<6 km) was also developed in two separate phases with respect to P-T conditions. The first phase associated with scheelite mineralization is characterized by high temperature (788-854 oC), relatively high pressure (1.20-1.62 kbar), shallow depth (5-6 km) and high log fO2 (>-12.9 to -11.0) values are accompanied by high H2O contents (5.39-6.88 wt.%). High water content of melt gave rise to magma to ascend to shallower depths (4-3 km) and crystallization to proceed under low pressure (~1.00 kbar), high temperature (751-859 oC) and log fO2 (-13.3 to -11.0) conditions with lower water contents (4.55-5.50 wt.%).