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The Role of Fractional Crystallization, Magma Recharge, and Magma Mixing in the Differentiation of the Small Hasandag Volcano, Central Anatolia, Turkey

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The Role of Fractional Crystallization, Magma Recharge, and Magma Mixing in the Differentiation of the Small Hasandag Volcano, Central Anatolia, Turkey Lithos 125 (2011) 984–993 Contents lists available at ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos The role of fractional crystallization, magma recharge, and magma mixing in the differentiation of the Small Hasandag volcano, Central Anatolia, Turkey Gokce Ustunisik a,⁎, Attila Kilinc b a Department of Geosciences, SUNY at Stony Brook, Stony Brook, NY, 11794-2100, USA b Department of Geology, University of Cincinnati, Cincinnati, OH, 45221-0013, USA article info abstract Article history: During the last 7 Ma, eruptions of the Small Hasandag composite volcano in Central Anatolia, Turkey, have Received 9 November 2010 produced calc-alkaline lavas ranging in composition from basalt to rhyolite. Published research on this Accepted 28 May 2011 volcano suggests that crystal fractionation and magma mixing are the two important processes controlling the Available online 15 June 2011 differentiation of the Small Hasandag magmas. The shortcomings of previous studies are that neither the intensive variables (P, T, fO ) nor the constraints under which the presumed parental magmas evolved have Keywords: 2 been quantitatively evaluated. Small Hasandag volcano Magma recharge In this study, we have used the MELTS algorithm of Ghiorso and Sack (1995) to determine the initial system Isobaric–isenthalpic magma mixing parameters in terms of temperature (T), pressure (P), oxygen fugacity (fO2), and water content (wt.% H2O) Isobaric–isothermal magma mixing and then evaluated the consequences of magma differentiation under closed system fractional crystallization, MELTS algorithm 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. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Previous studies on the petrology and geochemistry of the Hasandag volcanics have provided a detailed geological map of the Hasandag and The Small Hasandag volcano is one of three calc-alkaline volcanoes in the Small Hasandag volcanoes, as well as descriptions of the regional Central Anatolia, the others being the Hasandag volcano and the Erciyes stratigraphy, of lava flows and pyroclastic materials, and some age volcano (Fig. 1). It has erupted many times during the past 7 Ma, determinations based on K/Ar isotopes (Aydar and Gourgaud, 1998; covering a large part of this region with lava flows and pyroclastic Daniel et al., 1998; Ercan et al., 1990). Three evolutionary stages; stage 1: materials (Daniel et al., 1998). It is an excellent example of a subduction paleovolcano, 7 Ma, stage 2: mesovolcano, 1 Ma and stage 3: neovolcano, zone calc-alkaline volcano, and provides a well-exposed suite of rocks b1 Ma; have been recorded in the history of Hasandag complex (Aydar ranging in composition from basalt to andesite, dacite, and rhyolite. and Gourgaud, 1998). In their interpretations of the petrogenesis of the Hasandag volcanics, previous researchers emphasized different process- es. For example, to explain the wide range of compositional diversity ⁎ Corresponding author. exhibited by the volcanic rocks, Aydar and Gourgaud (1998) suggested E-mail address: [email protected] (G. Ustunisik). that fractional crystallization was the governing process for evolution of 0024-4937/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2011.05.013 G. Ustunisik, A. Kilinc / Lithos 125 (2011) 984–993 985 Fig. 1. Simplified tectonic context of Turkey and the distribution of Central Anatolian Volcanics (CAV) including Hasandag, Small Hasandag, and Erciyes (modified from Aydar and Gourgaud, 1998). the Hasandag magma. Their conclusions were based on decreasing MgO shown in Harker diagrams may suggest crystal fractionation, they do and TiO2 and increasing Na2OandK2O with increasing SiO2. On the other not give information about the initial state of the system. Even if hand, Daniel et al. (1998) used least-square calculations between basaltic crystal fractionation was the controlling process at the Small and rhyolitic end members to suggest that magma mixing dominates the Hasandag volcano, the initial state of the system that produced the petrogenesis of intermediate compositions (i.e. andesites and dacites). In rocks under fractional crystallization constraint is not known. addition to being contradictory, these studies do not provide a Consideration of fluid dynamics and the thermodynamics of magma quantitative evaluation of the proposed processes. mixing suggests that magma evolution is a very complicated process; An implication of the fractional crystallization model, which Aydar even bringing two magmas together in a magma chamber does not and Gourgaud (1998) based on linear trends shown in the Harker necessarily result in mixing of the two magmas (Huppert and Sparks, diagrams, is that crystal fractionation processes follow the same path 1980; Russell, 1990; Sparks and Marshall, 1986). Magma mixing under different initial system conditions and constraints. If this calculations as modeled by the least-square calculations of Daniel assumption were true, fractional crystallization taking place under et al. (1998) are based only on mass transfer of oxides between a mafic different initial state variables for the system would produce melts of and a felsic magma to generate the intermediate hybrid compositions. In the same composition. This is clearly not true because crystal such calculations the intensive variables of mixing (e.g. P, T, and fO2)are fractionation can take place under isobaric (Druitt and Bacon, 1989) not specified, yet these variables can alter the results of the calculations or polybaric (Kuritani, 1999), or isentropic (Blundy and Cashman, significantly. Most importantly, in a quantitative analysis of magma 2005), or even isochoric (constant volume) conditions and in each mixing, not only mass transfer but also heat transfer between two case the path of magma evolution is different. Although the patterns magmas must be considered. Magma mixing in nature involves mixing Fig. 2. Photomicrographs under cross polarized light showing some of the important textures and features from the Small Hasandag andesite and dacites. Plag: plagioclase, Qz: quartz, Bi: biotite, Opx: orthopyroxene, Cpx: clinopyroxene, and MI: melt inclusion. 986 G. Ustunisik, A. Kilinc / Lithos 125 (2011) 984–993 of a more primitive magma at higher temperature with a more constrained the initial state of the system, we then calculated magma differentiated magma at lower temperature. Thermodynamic principles evolution paths for three models: (a) isobaric fractional crystalliza- applied to this process require that the total heat content of the system tion, (b) isothermal magma recharge and (c) isobaric–isenthalpic must be constant and equal to the sum of the enthalpies of the two magma mixing. magmas. Therefore, a realistic and geologically tractable modeling of magma mixing at a given depth must be carried out under isobaric– 2. Mineralogy of Small Hasandag volcanic rocks isenthalpic conditions, assuming that the heat loss to the wall rock is negligible (Dogan et al., 2007). The porphyritic basaltic andesite from the Small Hasandag volcano In this study, we first constrained the initial intensive variables for shows glomeroporphyrtic and intersertal textures. The characteristic the Small Hasandag parental magma, P, T, wt.% H2O, and fO2. Having mineral composition includes plagioclase, clinopyroxene, orthopyroxene, Table 1 Chemical composition of Small Hasandag volcanic rocks (basaltic andesite to rhyolite) in terms of weight percent of oxides (reported on anhydrous basis). Rocks have been analyzed by the XRF method. Standard deviation indicates the analytical precision based on the repeated analyses of standards. Sample number SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2OK2OP2O5 Total Mg number GU-07-08 56.28 0.93
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