Genesis of mugearites and benmoreites of Nemrut Volcano, eastern Turkey: Magma mixing and fractional crystallization of trachybasaltic melt

Abstract

Presents detailed mineralogical and phase characteristics of trachybasalt, mugearite, trachydacite, and comendite samples from Nemrut Volcano in eastern Turkey, estimates of mineral formation conditions, and analyses of glasses from melt inclusions in olivine and rock matrix. Based on the analysis of the mineralogy and geochemistry of the rocks and mass balance calculations, the most feasible models of mugearitic and benmoreitic magma formation were proposed. The crystallization conditions of olivine, feldspar, and Fe-Ti oxides (titanomagnetite and ilmenite) were determined. Titanomagnetite and ilmenite were formed under the following conditions: 960-922°C, $\Delta \log _{f_{O_2 } }$ NNO from −1.54 to −0.73 in the mugearite, 940-890°C, $\Delta \log _{f_{O_2 } }$ NNO from −1.46 to −0.79 in the benmoreite, 870-845°C and $\Delta \log _{f_{O_2 } }$ NNO from −2.11 to −1.82 in the trachydacite, and 705-667°C, $\Delta \log _{f_{O_2 } }$ NNO from −2.48 to −2.18 in the comendite. Feldspars crystallized at 1150-950°C in the trachybasalt, 920-800°C in the benmoreite, and 760-720°C in the comendite. The temperature of melt inclusion entrapment in olivine (Fo _75-40) from the trachybasalt, mugearite, benmoreite, and trachydacite was estimated as 1270-860°C. Except for the trachybasalt, partly resorbed phenocrysts and/or xenocrysts were observed in all the samples, which indicates their formation under nonequilibrium conditions. Mass balance calculations for rock compositions (FC, AFC, and FCA models) and mineralogical observations suggest that the magmas or melts of mugearitic and benmoreitic compositions could be produced by the fractional crystallization of trachybasaltic melt (mass fraction of melt F = 0.63–0.79), which assimilated a small amount of crustal material, as well as by the mixing of trachybasaltic (F = 0.16–0.45) and trachydacitic (F = 0.45–0.58) magmas in the presence of excess olivine, plagioclase, magnetite, and apatite (totaling 10–24 wt %). Pre-caldera comendites are enriched in Fe (4–5 wt % FeO_tot) and trace elements compared with post-caldera comendites (2–3 wt % FeO_tot). The analysis of the geochemical data and mass balance calculations indicated that the post-caldera benmoreitic magma could not be produced by the fractionation of trachybasaltic melt. This magma and corresponding post-caldera benmoreites have anomalously low Ba (46–54 ppm) and Sr (203–269 ppm) contents, which could not be obtained in the models of fractional crystallization of trachybasaltic melt accompanied by crustal assimilation. The compositions of post-caldera benmoreites and hybrid rocks of trachydacitic composition showing evidence for magma mixing (presence of xenocrysts from benmoreitic and comenditic magmas and compositionally variable glasses) were best reproduced by mixing trachybasaltic (F = 0.7-0.5) and low-Fe comenditic (F = 0.3–0.5) melts. Magma chambers with low-Fe comenditic melts appeared during the post-caldera stage owing to the fractional crystallization of pre-caldera trachytic and trachyte-comenditic magmas. Perhaps, the repeated eruptions of low-Fe comendites in the caldera and “rift” zone of Nemrut Volcano were related to the injection of benmoreitic magma into these chambers.