The role of fractional crystallization, magma recharge, and magma mixing in the differentiation of the Small Hasandag volcano, Central Anatolia, Turkey

During the last 7 Ma, eruptions of the Small Hasandag composite volcano in Central Anatolia, Turkey, have produced calc-alkaline lavas 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 previous studies 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 Sack (1995) to determine the initial system parameters in terms of temperature (T), pressure (P), oxygen fugacity (fO2), and water content (wt.% H2O) and then evaluated the consequences of magma differentiation under closed system fractional crystallization, magma recharge, and magma mixing conditions separately. In order to determine the initial system parameters, we carried out approximately 100 isobaric fractional crystallization simulations of the parental basaltic andesite magma (Mg#68) in the pressure range of 1 bar to 10,000 bars, an 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 rocks was achieved at P = 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 are very similar to observed lava 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 total alkali (Na2O + K2O) values lower than those of the Small Hasandag rhyolites. Furthermore, natural rock compositions in the silica range of 62–65 wt.% show discrete cycles of sudden increase and decrease in the MgO content in the range of 0.5–1 wt.%, suggesting magma replenishment. This study shows that fractional crystallization and magma recharge in the composition range of basaltic andesite to dacite, followed by isobaric–isenthalpic mixing of dacite with the most differentiated rhyolite (Mg#46) generate melt compositions that most closely resemble the entire compositional range of the Small Hasandag lavas, including the rhyolites. The agreement between the liquid line of descent defined by the natural lavas and MELTS calculations, and the agreement between the observed mineralogy of the rocks and the calculated order of crystallization support this conclusion.