Mineral chemistry and Thermobarometry of Talkh-rud Sub-volcanic Intrusion, Damavand Volcano

Damavand volcano is situated in the central part of the Alborz mountain range along the southern margin of the Caspian Sea, North of Iran and also located about 50 km NE of Tehran. The basement of the volcano is made up of Paleozoic to Cenozoic sediments. Damavand stratovolcano consists mainly of trachyandesite-trachyte lava flows and pyroclastics formed in the past two Ma (Allenbach, 1966; Davidson et al., 2004). The erosion processes have formed valleys on the flanks of volcano like Talkhrud valley where a subvolcanic body of syenodiorite composition crops out that is considered as probably feeder dike of lateral vents. An investigation on the texture and mineral chemistry of this subvolcanic body has carried out to decipher crystallization processes and thermobarometry based on mineral-mineral equilibria, since it has been shown that minerals can record the entire range of pressure, temperature and composition of melts in which they develop during storage, ascent and emplacement. It is necessary to mention that petrography and geochemistry of the study subvolcanic body has recently been published by Aliakbari et al. (2020).

Minerals of two representative syenodiorite rock samples were analyzed on polished thin sections by electron microprobe at Oslo University in Norway, using a CAMECA SX100 instrument with analytical conditions of 15 kV accelerating voltage and 10 nA beam current. Excel and Minpet 2.02 software were used to calculate structural formulae of minerals and to plot their diagrams. A total of 66 point analyses of different minerals are used in this research.

The study subvolcanic intrusion, comprising syenodiorite, is cropped out in Talkh-rud valley located in the northeast of Damavand volcano flank. This sub-volcanic intrusion possesses porphyritic texture and consists of plagioclase, alkali feldspars, phlogopite, amphibole, clinopyroxene, apatite and titanomagnetite. Feldspars show signs of disequilibrium as coarse and fine sieve textures in their cores and rims. Mica and amphibole phenocrysts have breakdown reaction rims with opaque replacement. Amphibole crystals are highly opacitized and show replacement by brown mica. The intrusion is geochemically alkaline and shoshonitic and tectonically plots in the realm of within plate volcanics.Electron microprobe analyses show that the composition of plagioclase (An23.20- 41), is oligoclase-andesine, alkali feldspar (Or38.70-61), is sanidine, following the feldspar ternary diagram of Deer et al., (1992), brown micas K1.76 (Mg3.90 Fe1.21) [Ti0.67 Si5.58 Al2.24O20] (OH)4, on the classification diagram of Speer (1984), plot in the phlogopite field. Amphibole Ca1.79 (Mg3.21 Fe0.27 Al1.76) [Ti0.42 Al1.76 Si6.16 O22](OH)2, following the nomenclature of Leake et al., (1997) is magnesiohastingsite, and clinopyroxenes (Wo46.61-47.41 En42.38-44.79 Fs8.28-10.20), following Morimoto et al. (1988) are classified as diopside (Figures 5 to 9). Phlogopites show high TiO2 (5.9–6.3% wt) and a minor variation of MgO (16.3–19.6 wt %). In comparison with phlogopites that are in equilibrium with lower crust and mantle (Mg >5 apfu), the Talkhrud phlogopites have lower Mg (3.6–4.2 apfu). The experimental studies showed that the solubility of Ti in phlogopites increases with temperature and decreases with pressure for a given bulk TiO2 content (Tronnes et al., 1985). Magnesiohastingsite amphibole crystals are opacitized and show replacement by phlogopite. Composition of this secondary phlogopite is not different from primary ones (Table 3). The reaction rims of phlogopites and amphiboles indicate open system processes (Rutherford, 2008) and have been suggested as related to a slow ascent of magma (Rutherford and Hill, 1993).Numerous thermobarometers are employed to estimate the ranges of pressure and temperature of crystallization. Based on geothermobarometry calculations, clinopyroxene crystals formed from 1000 to 1184 0C and pressure ranges from 0.6 to 8.4 kb. Amphibole barometry shows crystallization temperature at 1000 0C and pressures ranging from 6.2 to 6.4 kb. Phlogopite also formed at 837 0C and pressures from 0.22 to 1.16 kb. Composition of phlogopite and amphibole indicate high oxygen fugacity during their crystallization. In addition, Mg number of amphiboles is higher than 0.7 indicating their mantle origin.

There are no tangible differences between chemical compositions of Talkhrud subvolcanic minerals with Damavand ones, for instance, titanium oxide values in the study amphiboles (magnesiohastingsite) are barely less than the reported amounts from volcanic amphiboles (kaersutite). In addition, subvolcanic pyroxene composition is diopside and alkali feldspar, sanidine. Furthermore, chemical compositions of amphiboles plot in within plate field of tectonic discrimination diagram of amphiboles. Hornblende-Plagioclase equilibrium temperature shows a range of 998.5 to 923.6 0C and plagioclase-alkali feldspar thermometry is in the range of 907.5 to 1018.3 0C. The presence of hydrous and anhydrous minerals, disequilibrium textures in plagioclase, reaction rim of phlogopite and amphibole may imply magmatic evolution in an open system, multistage crystallization and accumulation and storage in different levels of the crust.