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

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Lithos

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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

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 processes. 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

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 16.09 6.10 0.12 6.82 7.90 4.04 1.25 0.28 99.82 67.40 GU-07-07 64.23 0.65 17.31 4.14 0.09 1.72 5.38 3.48 2.42 0.21 99.64 66.03 GU-07-06 63.35 0.68 17.16 4.48 0.09 2.06 5.68 3.43 2.42 0.25 99.60 65.27 GU-06-11 59.33 0.77 18.18 5.32 0.10 3.83 6.93 3.54 1.53 0.22 99.75 56.16 GU-06-28 59.48 1.01 16.99 5.69 0.13 3.62 6.88 3.63 1.97 0.34 99.74 53.15 GU-06-03 62.08 0.56 15.76 3.99 0.09 2.10 9.93 3.05 2.24 0.29 100.08 48.44 GU-06-70 62.13 0.64 16.41 4.08 0.08 3.37 7.22 4.20 1.84 0.19 100.17 59.56 GU-06-73 61.85 0.68 17.50 4.66 0.10 2.38 6.64 3.10 2.39 0.27 99.59 47.68 GU-06-09 62.08 0.60 15.91 3.93 0.09 2.16 9.70 2.73 2.42 0.31 99.94 49.47 GU-06-13 62.10 0.66 16.92 4.68 0.10 2.67 6.19 4.10 2.15 0.25 99.82 50.40 GU-07-10 62.89 0.57 15.91 3.82 0.09 2.43 8.53 3.19 2.13 0.32 99.88 48.94 GU-06-32 62.06 0.68 18.06 4.24 0.09 2.10 6.48 3.36 2.19 0.30 99.56 46.82 GU-06-55 62.05 0.72 16.87 4.81 0.10 2.89 6.34 3.14 2.29 0.30 99.51 51.67 GU-06-23 62.46 0.60 17.03 3.92 0.09 2.03 7.91 3.28 2.28 0.26 99.86 48.05 GU-07-28 59.48 1.01 16.99 5.69 0.13 3.62 6.88 3.63 1.97 0.34 99.74 53.94 GU-06-37 62.45 0.62 18.05 4.47 0.10 2.23 5.98 3.41 2.01 0.25 99.58 47.10 GU-06-15 62.66 0.66 17.14 4.38 0.08 3.23 6.08 3.70 1.72 0.19 99.84 56.84 GU-06-52a 62.61 0.72 16.60 4.94 0.10 3.16 5.85 3.66 1.84 0.27 99.76 53.26 GU-06-08 62.81 0.67 17.08 3.96 0.09 2.03 5.87 5.04 2.09 0.30 99.94 47.70 GU-06-25 62.73 0.68 17.09 4.47 0.09 3.11 5.61 3.76 2.03 0.20 99.78 55.34 GU-06-48 62.95 0.63 16.32 3.91 0.07 3.55 6.47 4.21 1.73 0.21 100.04 61.78 GU-06-72 62.95 0.65 16.60 4.20 0.08 3.21 6.42 3.97 1.73 0.19 100.00 57.64 GU-06-10 62.89 0.57 15.91 3.82 0.09 2.43 8.53 3.19 2.13 0.32 99.88 53.12 GU-06-17 62.83 0.66 16.73 4.58 0.09 2.48 7.00 3.10 2.03 0.24 99.74 49.15 GU-06-49 63.02 0.62 16.71 4.14 0.08 3.18 6.14 4.09 1.81 0.19 99.97 57.83 GU-06-57 62.93 0.65 17.25 4.44 0.09 2.23 5.71 3.64 2.48 0.22 99.66 47.27 GU-07-09 63.15 0.74 16.23 4.66 0.09 2.94 5.49 4.27 2.04 0.26 99.89 52.96 GU-06-50 63.14 0.63 16.86 4.07 0.09 2.46 6.48 3.65 2.02 0.28 99.89 51.87 GU-06-01 63.39 0.56 16.18 3.78 0.08 1.96 7.76 3.64 2.31 0.26 99.94 48.11 GU-06-53 63.32 0.63 16.49 4.09 0.09 2.60 6.02 4.07 2.23 0.26 99.94 53.14 GU-06-05 63.40 0.65 16.82 4.16 0.08 3.03 5.49 4.13 1.90 0.20 99.87 56.46 GU-06-54 63.38 0.68 16.95 4.16 0.08 3.10 5.34 3.87 2.05 0.19 99.80 57.01 GU-06-51 63.25 0.65 17.12 4.40 0.10 2.27 5.83 3.23 2.43 0.26 99.80 47.95 GU-06-06 63.35 0.68 17.16 4.48 0.09 2.06 5.68 3.43 2.42 0.25 99.60 45.01 GU-06-52b 63.60 0.66 16.78 4.26 0.09 3.04 5.41 3.96 1.86 0.19 99.85 56.01 GU-06-56 63.67 0.64 16.75 3.89 0.08 2.83 5.33 4.01 2.24 0.27 99.71 56.47 GU-06-78 63.89 0.65 16.63 4.07 0.08 2.97 5.29 4.14 1.94 0.21 99.87 56.52 GU-06-12 63.84 0.63 16.96 4.17 0.09 1.97 5.53 4.01 2.31 0.23 99.75 45.67 GU-06-16 64.01 0.65 16.97 4.13 0.09 1.94 5.49 3.93 2.31 0.23 99.75 45.61 GU-06-07 64.23 0.65 17.31 4.14 0.09 1.72 5.38 3.48 2.42 0.21 99.64 42.61 GU-06-02 64.40 0.64 16.24 4.16 0.09 2.13 6.04 3.66 2.24 0.22 99.82 47.78 GU-07-12 64.56 0.63 16.24 4.36 0.09 2.63 5.29 3.94 2.05 0.16 99.94 51.83 GU-06-14 64.45 0.65 16.58 4.19 0.09 2.14 5.60 3.59 2.17 0.23 99.70 47.72 GU-06-04 64.66 0.64 16.16 4.18 0.09 2.00 5.53 3.90 2.41 0.23 99.80 46.03 GU-07-11 64.81 0.67 16.21 4.20 0.08 2.55 5.24 3.92 2.09 0.16 99.94 51.94 GU-07-38 65.52 0.56 16.42 3.89 0.08 2.29 4.91 3.83 2.21 0.16 99.86 51.18 GU-06-18 65.82 0.60 15.92 3.91 0.09 1.69 5.40 3.73 2.20 0.30 99.66 43.54 GU-07-18 66.23 0.52 15.64 3.55 0.08 2.36 4.72 4.44 2.26 0.19 99.98 54.17 GU-07-19 66.24 0.52 15.92 3.57 0.08 2.27 4.51 4.28 2.42 0.15 99.95 53.11 GU-07-05 67.91 0.28 13.47 2.17 0.06 1.21 8.70 2.73 3.83 0.08 100.45 49.90 GU-07-04 72.32 0.26 14.08 2.07 0.06 0.83 2.75 3.11 4.16 0.06 99.71 41.75 GU-07-22 72.88 0.25 14.45 1.65 0.06 0.73 2.17 4.33 3.32 0.08 99.93 44.20 GU-07-23 73.00 0.26 14.23 1.72 0.06 0.76 2.17 4.42 3.29 0.07 99.99 44.07 GU-07-01 73.19 0.27 13.78 1.92 0.06 0.79 1.88 3.15 4.51 0.07 99.62 42.47 GU-07-03 73.49 0.25 14.30 1.83 0.07 0.46 2.10 3.88 3.40 0.08 99.86 31.11 GU-07-26a 75.28 0.13 13.17 0.65 0.05 0.31 0.71 4.14 4.38 0.03 98.85 69.05 GU-07-26b 74.56 0.14 13.53 0.64 0.05 0.80 0.89 4.39 3.87 0.04 98.91 46.05 GU-07-25 75.95 0.10 13.27 0.62 0.06 0.12 0.86 4.23 3.86 0.05 99.11 25.18 Standard deviation 0.03 0.01 0.03 0.02 0.00 0.03 0.00 0.02 0.00 0.00 0.01 G. Ustunisik, A. Kilinc / Lithos 125 (2011) 984–993 987

16 groundmass. Spongy zones at the rim (Fig. 2a) and center of the phenocrystals (Fig. 2b) along with some sieve textures (Fig. 2c) are very 14 abundant in the andesites and dacites. Corroded plagioclase and 12 orthopyroxene crystals and extensive compositional and oscillatory zoning in plagioclase phenocryts are very common features of the Small 10 Hasandag andesites and dacites (Fig. 2d). Trapped melt inclusions are often observed in the rims of plagioclase phenocrysts (Fig. 2a). Thick O (wt %)

2 8 dissolution surfaces or sieve textures that cut early growth layers might 6 be an indication of reactions between the resident and intruding more O+K 2 R maﬁc magma in the system. Some dacites have resorbed quartz

Na 4 D surrounded by clinopyroxene and orthpyroxene crystals (Fig. 2e). The BA A characteristic mineralogical composition in rhyolites includes plagioclase 2 phenocrysts and Fe–Ti oxides as microphenocrysts in the groundmass. 0 The presence of minor biotite (Fig. 2e) and amphibole in the andesites 40 45 50 55 60 65 70 75 80 and dacites along with the existence of bubbles in andesites surrounded SiO2 (wt %) by ﬁne orthopyroxene and clinopyroxene crystals (Fig. 2f) suggests that the initial magma composition must have contained water. Fig. 3. Total alkali (Na2O+K2O wt.%) vs SiO2 (wt.%) diagram (Le Bas et al., 1992) for the Small Hasandag volcanic rocks. BA: basaltic andesite, A: andesite, D: dacite, and R: rhyolite. 3. Chemical composition of Small Hasandag volcanic rocks

In Table 1 we present 57 analyses of the Small Hasandag volcanic Fe–Ti oxides, ±olivine as phenocrysts, and microlitic feldspar, orthopyr- rocks, ranging in composition from basaltic andesite to rhyolite oxene, and Fe–Ti oxides as microphenocrysts in the groundmass. (Fig. 3) from the Neovolcano stage (b1 Ma). Analyses were performed Plagioclase phenocrysts have extensive compositional zoning. Some by X-ray ﬂuorescence (XRF) (Rigaku 3070 spectrometer) at the clinopyroxene crystals exhibit sector zoning. The porphyritic andesites department of Geology, University of Cincinnati. Chemical composi- and dacites show glomeroporphyrtic, intersertal, subophitic, ophitic, and tions have been reported on an anhydrous basis. For the XRF analyses, seriate textures. The characteristic mineral assemblage in the andesites each rock was chipped into fragments of approximately 1/4 of an inch and dacites includes plagioclase, clinopyroxene, orthopyroxene, sanidine, in diameter and then powdered in a tungsten carbide shatterbox. The Fe–Ti oxides, ±amphibole, ±biotite, and ±quartz as phenocrysts and powdered samples were ﬁrst heated at 110 °C and then to 1000 °C to microlitic feldspar and Fe–Ti oxides as microphenocrysts in the determine loss on ignition (LOI). 0.7 g ignited powder was mixed a b 1.5 20.0 18.0 1.0 16.0 (wt %) (wt %) 3 2

0.5 O 2 14.0 TiO Al 0.0 12.0 55 60 65 70 75 80 55 60 65 70 75 80 SiO2 (wt %) SiO2 (wt %) c d 8.0 7.0 6.0 6.0 5.0 4.0 4.0 3.0 2.0 MgO (wt %) FeO* (wt %) 2.0 1.0 0.0 0.0 55 60 65 70 75 80 55 60 65 70 75 80 SiO2 (wt %) SiO2 (wt %) ef 12.0 6.0 10.0 8.0 4.0 6.0 O (wt %)

4.0 2 2.0 CaO (wt %) K 2.0 0.0 0.0 55 60 65 70 75 80 55 60 65 70 75 80 SiO2 (wt %) SiO2 (wt %)

Fig. 4. Major oxides variation diagrams for the Small Hasandag volcanic rocks. a. SiO2 vs TiO2, b. SiO2 vs Al2O3, c. SiO2 vs FeO*, d. SiO2 vs MgO, e. SiO2 vs CaO, and f. SiO2 vs K2O. 988 G. Ustunisik, A. Kilinc / Lithos 125 (2011) 984–993

homogeneously with 3.5 g LiBO2 ﬂux and a couple of crystals of 3000, 5000, and 10,000 bars (Fig. 5). The best agreement between the ammonium iodide to prepare a fused glass disk using methods modiﬁed calculated melt compositions and the observed compositions for the from Mewe (1994). Samples were run against a set of U.S. Geological Small Hasandag volcanic rocks is attained at P=1000 bars (Fig. 5a). The Survey rock standards including AGV1, BCR1, BHVO1, BIR1, G2, JA2, JA3, 1000 bar calculations also produce mineral assemblages that agree with JB1a, JB2, JB3, JG1a, JG2, JG3, JGB1, JR1, JR2, QLO1, RGM1, W2, JCH1, JSD1, the order of crystallization observed in thin sections. The results show JSD2, and JSD3. Relative precision is based on repeated analysis of the that the simulations at 1 bar can only produce the compositional standards (Table 1) and is 1% of the amount present. spectrum of the Small Hasandag from basaltic andesites to dacites but

Harker diagrams, Fig. 4, show that TiO2,Al2O3, FeO*, MgO, and CaO not the rhyolites (Fig. 5c). At the high pressure end of our computations, decrease and K2O increases with increasing SiO2 content. These simulations at 10,000 bars showed that although computed composi- mostly linear trends can be interpreted as representing either tionscan generate the whole spectrum of rocks from basaltic andesite fractional crystallization of the parental basaltic andesite or mixing to rhyolite, the computed rhyolite compositions have much lower of parental basaltic andesite with a rhyolitic magma. Na2O+K2O values than the natural rhyolites (Fig. 5b). Compositions In order to model the evolution path of a magmatic system produced at 3000 and 5000 bars generated a liquid line of descent quantitatively it is necessary to specify initial conditions of the system similar to that of the Small Hasandag rock spectrum in the total alkali– in terms of its pressure (P), temperature (T), oxygen fugacity (fO2), silica diagram, but in corresponding MgO–TiO2,MgO–CaO, and SiO2– and water content (wt.% H2O), and then establish the mechanisms by Fe2O3 diagrams the calculated and observed rock compositions differed which the magma evolves. Accordingly, we first determined the significantly. Calculations at 1000 bars reasonably well reproduced the intensive parameters of the parental magma and then tested (1) compositions of the Small Hasandag rocks ranging in composition from isobaric fractional crystallization and (2) magma recharge followed by isobaric–isenthalpic magma mixing models using the MELTS algo- a rithm (Ghiorso and Sacks, 1995). P=1000 bars fO =QFM+1 16 2 4. MELTS calculations to constrain the initial system parameters 14 12 The MELTS algorithm of Ghiorso and Sack (1995) is widely used for 10 O (wt %) modeling magmatic systems (Asimow et al., 2001; Dogan et al., 2007; 2 8 Kress and Ghiorso, 2004). In the isobaric–isothermal closed system 6 computations, the Gibbs free energy of the system is minimized. In O+K R 2 4 D a system open to oxygen or water transfer, the MELTS algorithm 2 BA A Na minimizes the Korzhinki potential, L = GÀnO2: μO2 (Thompson, 1970), 0 and in adiabatic calculations, it minimizes the enthalpy subject to fixed P 40 45 50 55 60 65 70 75 80 and heat content. Once the starting composition, initial system SiO2 (wt %) parameters, and the constraint on magma evolution are specified, b MELTS calculates for each temperature and pressure (1) the composi- P= 10 kbars tion of crystallizing minerals, (2) the end-member compositions of solid fO2=QFM+1 16 solution minerals, and (3) the composition of the residual liquid. 14 To constrain the initial system parameters (P, T, fO ,andwt.%H O) of 2 2 12 the Small Hasandag parental magmas, we calculated the melt compo- 10 sitions that would be generated from a parental basaltic andesite magma O (wt %)

2 8 under isobaric fractional crystallization conditions in the pressure range 6 of 1 bar to 10,000 bars, an fO2 range QFM+1 to QFM+3, and a H2O O+K 4 D R 2 content of 0–4 wt.%. Compositions of calculated melts were then 2 BA A compared with those of the Small Hasandag volcanic rock series. Our Na 0 40 45 50 55 60 65 70 75 80 results show that the basaltic andesite parental magma with 2 wt.% H O 2 SiO (wt %) evolving under fractional crystallization conditions generates melts like 2 those of the eruptive products of the Small Hasandag volcano at a depth c of about 4 km below the surface (P~1000 bars). Following is a brief P= 1 bar explanation about determination of the system parameters. fO2=QFM+1 H2O=0% 16 4.1. Parental magma composition 14 12 We used a basaltic andesite (Mg#≈68, Table 1)astheanhydrous 10

ﬁ O (wt %) parental magma composition. This is the most ma cofalltherocks 2 8 collected, and represents the “most primitive” of all the rocks in the 6 R

SiO2 −Na2O+K2Oplot(Fig. 3). Although the role of various water O+K 4 D 2 concentrations ranging from 0 wt.% to 4 wt.% will be discussed in the 2 BA A Na section “Initial system water content”,weadded2wt.%watertobasaltic 0 40 45 50 55 60 65 70 75 80 andesite to ﬁx the initial system pressure and oxygen fugacity, consistent SiO2 (wt %) with the presence of biotite and amphibole in the Small Hasandag lavas.

Experiments with andesitic melts in natural systems also indicate that Fig. 5. Comparison of calculated melt compositions at, a. P=1000 bars (open black

2 wt.% of water is needed to stabilize amphibole (Eggler, 1972). triangles), b. P=10 kbars (open black circles) at 2 wt.% H2O and fO2 =QFM+1, and c. P=1 bar (open black squares) with basaltic andesite, andesite, dacite, and rhyolite 4.2. Initial system pressure (black crosses) at 0 wt.% H2O and fO2 =QFM+1 in Na2O+K2O vs SiO2 space. Calculations were carried out under closed system isobaric fractional crystallization of parental basaltic andesite (Mg#68, Table 1). Best agreement between computed melt To constrain the pressure of the parental magma, we calculated the calculations and the Small Hasandag volcanic rocks is at P=1000 bars. BA: basaltic compositions of melts generated by fractional crystallization at 1, 1000, andesite, A: andesite, D: dacite, and R: rhyolite. G. Ustunisik, A. Kilinc / Lithos 125 (2011) 984–993 989 basaltic andesite to rhyolites in total alkali–silica as well as in oxide– a 0% H2O oxide diagrams. On this basis, we concluded that 1000 bars best 6.0 represents the initial system pressure for the Small Hasandag magma. Our computational constraint for pressure (1000 bars) also agrees with 4.0 the pressure bracket of 1000–1500 bars which was experimentally determined by Dogan et al., 2007. O (wt %)

2 2.0 4.3. Initial system oxygen fugacity K 0.0 Simulations were carried out in the oxygen fugacity range of QFM+ 55 60 65 70 75 80 1–QFM+3 using the basaltic andesite parental magma with 2 wt.% SiO2 (wt %) water at 1000 bars. Fig. 6 shows the composition of melts produced at buffer conditions of QFM+1, QFM+2 and QFM+3. Comparison of b 1% H2O calculated melt compositions in MgO vs FeO* (total FeO) with the actual 6.0 rock compositions of the Small Hasandag showed that the initial oxygen fugacity is best deﬁned at QFM+1 (Fig. 6a). At two and three log units above the QFM buffer (QFM+2and QFM+3) there is increased 4.0 oxidation of iron from Fe+2 to Fe+3 and the model melts generated O (wt %) do not faithfully reproduce the observed compositions (Fig. 6b and c). 2 2.0 K 4.4. Initial system water content 0.0 55 60 65 70 75 80 Having constrained both the pressure and oxygen fugacity con- SiO2 (wt %) ditions, at P=1000 bars and fO2 =QFM+1, we next evaluated the c 2% H2O a at QFM+1 6.0 10.0

8.0 4.0

6.0 O (wt %)

2 2.0

4.0 K

FeO* (wt.%) 2.0 0.0 55 60 65 70 75 80 0.0 SiO2 (wt %) 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 MgO (wt %) Fig. 7. Comparison of calculated melt compositions at, a. 0 wt.% H2O (open black circles), b. 1 wt.% H2O (open black squares), and c. 2 wt.% H2O (open black triangles) b with basaltic andesite, andesite, dacite, and rhyolite (black crosses) at P=1000 bars at QFM+2 and fO =QFM+1 in SiO vs K O space. Best agreement between computed 10.0 2 2 2 compositions at P=1000 bars and fO2 =QFM+1 and the Small Hasandag volcano compositions is at 2 wt.% H O. 8.0 2

6.0 effect of different water concentrations, ranging from 0 wt.% to 4 wt.% 4.0 andplottedtheresultsinSiO2vs K2Ospace.Calculatedmelt

FeO* (wt %) 2.0 compositions at P=1000 bars and fO2 =QFM+1 produced compositions similar to those of the Small Hasandag magmas at a H2O 0.0 content of 2 wt.% (Fig. 7c). The results show that simulations with less 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 MgO (wt %) than 2 wt.% H2O generate melts that have higher SiO2 and lower K2O values than the Small Hasandag rocks (Fig. 7a and b). Computations c with more than 2 wt.% H2O can only produce basaltic andesite to at QFM+3 dacite part of the compositional spectrum, yet cannot produce the 10.0 rhyolites. This limits the amount of water in the Small Hasandag 8.0 magma to 2 wt.%. 6.0

4.0 5. Testing the isobaric fractional crystallization hypothesis

FeO* (wt %) 2.0 Having constrained the initial system parameters of the Small 0.0 Hasandag magma, we used a basaltic andesite composition (Mg#≈68, 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 Table 1)at1000barsandfO2 =QFM+1 with 2 wt.% water as a starting MgO (wt %) material for closed system fractional crystallization. We tested the hypothesis that isobaric closed system crystal fractionation from a Fig. 6. Comparison of calculated melt compositions at, a. fO =QFM+1 (open black 2 parental basaltic andesite magma will produce a liquid line of descent triangles), b. fO2 =QFM+2 (open black circles), and c. fO2=QFM+3 (open black squares) identical on Harker-type diagrams to the observed liquid line of descent with basaltic andesite, andesite, dacite, and rhyolite (black crosses) at 2 wt.% H2Oand ﬁ P=1000 bars in MgO vs FeO* space. Initial system fO2 is best de ned at fO2 =QFM+1. when P=1000 bars, H2O content=2 wt.% H2OandfO2 =QFM+1. 990 G. Ustunisik, A. Kilinc / Lithos 125 (2011) 984–993 ab 1.5 20.0

1.0 18.0 (wt %) (wt %) 3 16.0 2 O

0.5 2

TiO 14.0 Al

0.0 12.0 55 60 65 70 75 80 55 60 65 70 75 80 SiO2 (wt %) SiO2 (wt %) cd 8.0 7.0 6.0 6.0 5.0 4.0 4.0 3.0 2.0 2.0 MgO (wt %) FeO* (wt %) 1.0 0.0 0.0 55 60 65 70 75 80 55 60 65 70 75 80 SiO2 (wt %) SiO2 (wt %) ef 12.0 6.0 10.0 8.0 4.0 6.0

4.0 O (wt %) 2.0 2 CaO (wt %) 2.0 K 0.0 0.0 55 60 65 70 75 80 55 60 65 70 75 80 SiO2 (wt %) SiO2 (wt %)

Fig. 8. Comparison of melts generated from isobaric fractional crystallization of basaltic andesite (Mg#68, Table 1)with2wt.%H2Oat1000barsandfO2 =QFM+1 (open black triangles) with compositions of the Small Hasandag volcanic rocks (black crosses) in, a. SiO2 vs Ti2O, b. SiO2 vs Al2O3,c.SiO2 vs FeO*, d. SiO2 vs MgO, e. SiO2 vs CaO, f. SiO2 vs K2OHarkerdiagrams.

The liquidus temperature of the pertinent basaltic andesite magma at 1000 bars and fO2 =QFM+1 is approximately 1200 °C. In calculating the isobaric crystal fractionation model, the temperature of the magma is reduced 10 °C at each step of the crystallization process. The melt compositions calculated under isobaric fractional crystallization conditions are shown with the compositions of the

Small Hasandag rocks in Fig. 8. These ﬁgures show in terms of SiO2vs TiO2,Al2O3, FeO*, MgO, and CaO bivariate plots that isobaric fractional crystallization of basaltic andesite magma cannot produce a liquid line of descent that corresponds well with the chemical trend of the Small Hasandag rocks. The decrease in FeO* and MgO reﬂects crystallization of Mg rich olivine and orthopyroxene (Fig. 8c and d); decrease in TiO2 and FeO* is in response to crystallization of Fe–Ti oxides (Fig. 8a and c); decrease in Al2O3 and CaO corresponds to crystallization of plagioclase (Fig. 8b and e) and decrease in MgO, CaO and Al2O3 corresponds to crystallization of clinopyroxene (Fig. 8d and e). Although, the calculated melt compositions are mostly in agreement with the Small Hasandag rock compositions in some bivariate plots, including SiO2–TiO2, SiO2–FeO*, SiO2–CaO, and SiO2–K2O, there are still major discrepancies in the SiO2–Al2O3 and SiO2–MgO plots. In calculations involving isobaric crystal fractionation of basaltic andesite, simulated melt compositions show lower FeO* and much lower

Al2O3 at a given SiO2 and somewhat lower MgO values compared to those of the Small Hasandag (Fig. 8b). Although computations based on isobaric fractional crystallization of the parental basaltic andesite can generate the rock compositions from basaltic andesite to dacite as shown on the total alkali–SiO2 diagram, our results show that the rhyolitic compositions produced by closed system fractionation have lower total alkali (Na2O+K2O) values compared to the natural rocks (Fig. 3a). Also in the total alkali– Fig. 9. a. SiO2–MgO plot for the Small Hasandag compositions from basaltic andesite to SiO diagram, there is a gap between these higher alkali rhyolites and 2 rhyolite, b. SiO2–MgO plot for most of the data, except the very low silica values, to focus on the andesite and dacites while all rocks lie on the same trend. These the discontinuities that are mostly concentrated in the SiO2 range of 62–65 wt.%. G. Ustunisik, A. Kilinc / Lithos 125 (2011) 984–993 991 observations, coupled with previous workers' field observations of 5%) primary basaltic andesite is injected into the magma chamber. cognate inclusions and banding and with our observations of the This less evolved injected magma and the somewhat more evolved volcanic rocks in thin sections (for example reverse zoning in resident magma mix isothermally, and the product resumes differ- plagioclase), indicate that magma recharge and magma mixing may entiating under isobaric fractional crystallization conditions. This have played a significant role in the differentiation of the Small process could be repeated several times. At each injection of the less Hasandag magmas. Therefore, we reject the hypothesis that the mode evolved primary magma, the MgO content of the mixed magma of crystallization controlling the evolution of the Small Hasandag increases, and isobaric fractional crystallization subsequently causes magmas is simple closed system fractional crystallization, and now MgO to decrease. We believe it is this process of repeated magma test the magma recharge and magma mixing hypotheses. recharge that caused the MgO fluctuations in the SiO2–MgO plot shown in Fig. 9a and b. 6. Magma recharge hypothesis

In the SiO2–MgO plot (Fig. 8d), the Small Hasandag lavas show 7. Extending the hypothesis: isobaric–isenthalpic magma mixing ﬂuctuations in MgO in the SiO2 range of 62 to 65 wt.%. These oscillations of MgO as SiO2 increases can be interpreted as evidence Magma mixing can be treated as an isothermal or isenthalpic for magma recharge. Fig. 9a shows the SiO2–MgO plot for the Small process. Geologically an isothermal process is improbable because it Hasandag compositions from basaltic andesite to rhyolite and Fig. 9b requires that both the resident magma and mixing magma be at the shows the part of Fig. 9a in the SiO2 interval from 62 wt.% to 65 wt.%. same temperature. Since magma mixing by and large involves mixing There are 12 oscillations in MgO in the silica range of 62–65 wt.% and of a more primitive magma (presumably at a higher temperature) most of these oscillations are in the range of 0.5–1% of MgO or with a more differentiated magma (typically at a lower temperature), sometimes even greater than 1%. isobaric–isothermal conditions are unrealistic for magma mixing. A possible physical model for the magma recharge process at the Calc-alkaline volcanoes are open systems, which mean that period- Small Hasandag volcano might be visualized as follows. Consider a ically a less differentiated magma can enter the resident magma shallow magma chamber in the crust (the magma chamber previously chamber, causing mingling, mixing, or eruption (Sparks et al., 2000). constrained at 1000 bars) connected to a source of basaltic andesite. In our opinion, a more realistic and geologically reasonable model Initially basaltic andesite ﬁlls the shallow magma chamber and keeps the enthalpy of the system constant and equivalent to the sum undergoes some fractional crystallization under isobaric conditions. of enthalpies of the two magmas all at constant pressure (isobaric– At this stage in the evolution of the magma, a small amount (about isenthalpic conditions) (Dogan et al., 2007).

ab 20.0 8.0

18.0 6.0

(wt %) 16.0 4.0 3 O 2 14.0 2.0 FeO* (wt %) Al

12.0 0.0 55 60 65 70 75 80 55 60 65 70 75 80 SiO2 (wt %) SiO2 (wt %) c d 7.0 12.0 6.0 10.0 5.0 8.0 4.0 6.0 3.0 2.0 4.0 CaO (wt %) MgO (wt %) 1.0 2.0 0.0 0.0 55 60 65 70 75 80 55 60 65 70 75 80 SiO2 (wt %) SiO2 (wt %) e 6.0

4.0 O (wt %)

2 2.0 K

0.0 55 60 65 70 75 80 SiO2 (wt %)

Fig. 10. Comparison of calculated melts generated from isothermal recharge followed by isobaric isenthalpic mixing of dacite with rhyolite with the compositions of the Small

Hasandag volcanic rocks (black crosses) in, a. SiO2 vs Al2O3, b. SiO2 vs FeO*, c. SiO2 vs MgO, d. SiO2 vs CaO, and e. SiO2 vs K2O Harker diagrams. 992 G. Ustunisik, A. Kilinc / Lithos 125 (2011) 984–993

To illustrate our two-step model for the Small Hasandag volcano we will assume that the initial injection of basaltic andesite magma into a 7.0 shallow magma chamber is followed by repeated recharge by magma 6.0 injected under isothermal conditions and ﬁnally by the isenthalpic mixing of the last dacitic liquid with a rhyolitic magma which is common 5.0 at the Hasandag volcanic province during Neovolcano stage (b1Ma). In order to model the evolution of the Small Hasandag volcano, we 4.0 used Ghiorso and Sacks (1995) MELTS software in this two-step 3.0 differentiation process. We started at the liquidus temperature of the MgO (wt %) parental basaltic andesite (Mg#≈68, Table 1) and, after it differentiated 2.0 a little under isobaric conditions, added 5% of the parental basaltic andesite to the remaining composition. Then, we mixed 5% of a hotter, 1.0 ﬁ nal dacitic composition (at the end of the fractionation and recharge) 0.0 with a cooler 95% rhyolitic magma (Mg#≈46, Table 1) under isobaric– 56.3 62.1 62.9 62.5 63.0 62.9 63.4 63.7 64.4 65.5 72.3 75.3 isenthalpic conditions (P=1000 bars, fO2 =QFM+1). In this scenario, SiO2 (wt %) the dacitic magma cools and partially crystallizes while the rhyolitic magma is heated and some of its existing minerals may dissolve in the Fig. 12. Comparison of calculated melts generated from isothermal recharge followed – new mixed magma; thus, the heat released by the crystallization of by isobaric isenthalpic mixing of dacite with rhyolite with the compositions of the Small Hasandag volcanic rocks (black crosses) in a SiO2–MgO plot. dacitic magma is used to heat the rhyolitic magma. The MELTS algorithm calculates the composition of mixed melts and coexisting minerals as the temperature of magma decreases at constant pressure. The andesite to rhyolite. In Fig. 10, the melts generated by isothermal compositions of these calculated melts ranged from parental basaltic recharge followed by isobaric–isenthalpic mixing of dacite with rhyolite are compared with the compositions of the Small Hasandag volcanic a rocks in terms of major oxide Harker diagrams. P=1000 bars In Fig. 11 the melt compositions calculated assuming (a) simple fO =QFM+1, 2 wt% H O 2 2 isobaric fractional crystallization of the primary basaltic andesite 16 and (b) isothermal recharge followed by isobaric–isenthalpic mixing 14 of dacite with rhyolite are superimposed on the compositions of the Small Hasandag volcanic rocks total alkali–silica diagrams. The 12 agreement between the calculated melt compositions in Fig. 9band the Small Hasandag rock compositions supports the conclusion that 10 isothermal recharge and isobaric–isenthalpic magma mixing is a reasonable mechanism explaining the chemical diversity of volcanic O (wt.%)

2 8 rocks at the Small Hasandag volcano (Figs. 10 and 11b). In Fig. 12,the melt compositions computed by assuming isothermal recharge and

O+K 6 2 isobaric–isenthalpic magma mixing and the natural rock compositions R Na 4 in Small Hasandag are superimposed on a SiO2–MgO plot. The repeated D recharge of evolving basaltic andesite by less evolved magma injected 2 BA A under isobaric–isothermal conditions has successfully produced the discrete jumps in MgO content with the evolving SiO . It is perhaps 0 2 40 45 50 55 60 65 70 75 80 appropriate to discuss the rationale for isothermal mixing of less differentiated and somewhat hotter basaltic andesite magma with a SiO2 (wt %) slightly more differentiated and cooler magma. Since only 5% of less b differentiated magma is being added to 95% of slightly more P=1000 bars differentiated magma and both the recharge and differentiated fO2=QFM+1, 2 wt% H2O magma are close in composition it will not signiﬁcantly change the 16 temperature of the cooler magma. Thus isothermal recharge at the ﬁ 14 temperature of the cooler magma is justi ed. In order to compute the sequence of crystallization for minerals in the model that assumes 12 isothermal recharge with isobaric–isenthalpic mixing, and to compare the minerals generated in this model with our thin section observations 10 of the Small Hasandag rocks, we again used the MELTS algorithm, and allowed the mixed magma to crystallize. The sequence of crystallization

O (wt.%) 8 2 for minerals is a function of temperature. The major minerals appear in 6 the order olivine, orthopyroxene, plagioclase, clinopyroxene, and spinel. O+K

2 The agreement between the liquid line of descent deﬁned by the 4 R Na natural rocks and the MELTS calculations, coupled with the agreement D between the observed mineralogy of the rocks and the calculated 2 BA A sequence of crystallization support our conclusion that isothermal – 0 recharge, followed by isobaric isenthalpic magma mixing is the 40 45 50 55 60 65 70 75 80 dominant process in the differentiation of the Small Hasandag magmas. SiO2 (wt %) 8. Conclusions Fig. 11. Calculated melt compositions for, a. isobaric fractional crystallization of basaltic andesite compared to b. isothermal recharge followed by isobaric–isenthalpic mixing of dacite with rhyolite and the compositions of the Small Hasandag volcanic rocks in terms of The initial system parameters (P, T, fO2, and wt.% H2O) of the Small the total alkali–silica diagram. BA: basaltic andesite, A: andesite, D: dacite, and R: rhyolite. Hasandag magma were constrained by carrying out approximately G. Ustunisik, A. Kilinc / Lithos 125 (2011) 984–993 993

100 isobaric fractional crystallization simulations using the compo- Aydar, E., Gourgaud, A., 1998. The geology of Mount Hasan stratovolcano, Central – fi Anatolia, Turkey. Journal of Volcanology and Geothermal Research 85, 129 152. sition of the most ma c basaltic andesite with the MELTS algorithm of Blundy, J., Cashman, K., 2005. Rapid decompression driven crystallization recorded by Ghiorso and Sacks (1995) in the pressure range of 1 bar to melt inclusions from Mount St Helens Volcano. Geology 33, 793–796. Daniel, C., Aydar, E., Gourgaud, A., 1998. The Hasan Dagi stratovolcano (Central 10,000 bars, fO2 range of QFM+1 to QFM+3 and at water contents Anatolia, Turkey): evolution from calc-alkaline to alkaline magmatism in a from 0 to 4 wt.%. Comparison of the calculated melt compositions continental collision zone. Journal of Volcanology and Geothermal Research 87, with the natural rock data indicates that the initial conditions of the 275–302. Small Hasandag magma are adequately constrained at P=1000 bars, Dogan, A.U., Dogan, M., Kilinc, A., Locke, D., 2007. An isobaric–isenthalpic magma fO =QFM+1, and H O content=2 wt.%. Using these initial system mixing model for the Hasan Dagi volcano, Central Anatolia, Turkey. Bulletin of 2 2 Volcanology 70, 797–804. conditions, we used the MELTS algorithm of Ghiorso and Sacks (1995) Druitt, T.H., Bacon, C.R., 1989. Petrology of the zoned calc-alkaline magma chamber of to calculate the compositions of melts formed from a parental basaltic Mount Mazama, Crater Lake, Oregon. Contributions to Mineralogy and Petrology – andesite magma under isobaric fractional crystallization conditions 101, 21 32. Eggler, D.H., 1972. Water-saturated and undersaturated melting relations in a Paricutin (P=1000 bars). This procedure can generate a wide range of melt andesite and estimate of water content in natural magma. Contributions to compositions appropriate to the Small Hasandag volcanic rocks from Mineralogy and Petrology 34, 261–271. basaltic andesite to dacite, but not rhyolites. Moreover, in several Ercan, T., Tokel, S., Can, B., Fisekci, A., Fujitani, T., Notsu, K., Selvi, Y., Olmez, M., Matsuda, J.I., Yildirim, T., Akbasli, A., 1990. The origin and evolution of the Cenozoic Harker diagrams, the computed melt compositions are inconsistent volcanism of Hasandagi–Karacadag area (Central Turkey) (in Turkish). Bulletin of with the chemistry of the Small Hasandag volcanic rocks. For example, Geomorphology 18, 39–54. Ghiorso, M.S., Sack, R.O., 1995. Chemical mass transfer in magmatic processes IV. A in the SiO2–MgO plot (Fig. 8d), the 0.5–1% jumps in the MgO content revised and internally consistent thermodynamic model for the interpolation and with increasing silica cannot be explained by simple fractional extrapolation of liquid–solid equilibria in magmatic systems at elevated temper- crystallization. In contrast, successive isobaric fractionation and atures and pressures. Contributions to Mineralogy and Petrology 119, 197–212. magma recharge in the silica range of 62–65 wt.% (from basaltic Huppert, H.E., Sparks, S.R., 1980. The fluid dynamics of a basaltic magma chamber – replenished by influx of hot dense ultrabasic magma. Contributions to Mineralogy andesite to dacite), followed by isobaric isenthalpic magma mixing and Petrology 75, 279–289. between dacite and rhyolite compositions at 1000 bars produced melt Kress, V.C., Ghiorso, M.S., 2004. 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