Role of Isobaric Fractional Crystallization and Isobaric-Isenthalpic Magma Mixing in the Differentiation of the Small Hasandag Volcano, Central Anatolia, Turkey

During the last seven million years, eruptions of Small Hasandag volcano in Central Anatolia, Turkey have produced calc-alkaline rocks ranging in composition from basalt to rhyolite. Published research on this volcano suggests that crystal fractionation and magma mixing are the two important processes controlling the differentiation of the Small Hasandag magmas. The shortcomings of these researches are that neither the intensive variables (P, T, fO₂) nor the constraints under which the presumed parental magmas evolved have been quantitatively evaluated. In this study, we have used the MELTS algorithm of Ghiorso and Sacks (1995) to determine the initial system parameters in terms of temperature, pressure, oxygen fugacity (fO₂), and water content and then evaluated the consequences of magma differentiation under fractional crystallization and magma mixing conditions separately. In order to determine the initial system parameters, approximately 100 isobaric fractional crystallization simulations of the parental basaltic andesite magma (Mg#=68) were carried out in the pressure range of 1 bar to 10 Kbars, fO₂ range of QFM+1 to QFM+3 and at water contents from 0 to 4 wt%. The best agreement between the computed melt compositions and the natural rock data was achieved at 1000 bars, fO₂=QFM+1, and 2 wt% water. Computations with parental basaltic andesite at these initial system conditions and under isobaric fractional crystallization generated melt compositions from basaltic andesite to dacite that is very similar to observed rock compositions. Compositions more evolved than dacites however, cannot be produced by closed system fractional crystallization alone. This is because rhyolites generated by closed system fractional crystallization have lower total alkali values compared to the Small Hasandag rhyolites. On the other hand, the isobaric-isenthalpic mixing of parental basaltic andesite with the most differentiated rhyolite (Mg#=46) generated melt compositions most closely resembling the entire compositional range of the Small Hasandag rocks, including the rhyolites.