Petrological conditions and thermobarometry of Trachyandesites in Lar Region, Southwest of Damavand Volcano

Damavand volcano is a large composite cone (≥ 400 km3) with height elevation (5678m) above sea level, in Central Alborz. This volcano consists of two buildings including old lavas between 1.8 to 0.8 Ma in the northern and eastern part of the current cone and young lavas with about 0.4 to 7 ka years in the south and west part. Trachytic and trachy-andesitic lavas are the most abundant lavas of this Quaternary volcano. The current Damavand cone, which is about 600,000 years old, is located above the old and eroded building which include periodically of andesitic-trachyte lava flows and pyroclastic flows with small eruptions of mafic lavas. Geological studies of this volcanic cone have been mostly based on geochemistry and petrogenesis, and a little data on mineral chemistry and termo-barometry are available. Geologically, the Damavand volcano bed rock has been formed since the Precambrian deposits, mainly on Mesozoic deposits (Shemshak and Lar Formations), and formed in the youngest geological period (Quaternary), about 10,000 years ago to the present. Late Cretaceous volcanic activity in Alborz is covered by shallow and progressive Eocene sediments in most places, but the Middle Eocene activity with Tofit composition is significant. The intensity of Alborz volcanic activity continues during Tertiary, but is not uniform, as its maximum is in the late Eocene and Oligocene, and after a period of relative inactivity, activity re-intensifies in the Pliocene.

After initial microscopic studies, suitable samples of thin section were prepared and sent to Iran Mineral Processing Research Center for analysis. The analysis performed in this center is performed by the electronic microprocessor model CAMECA-SX 100 made by the French company Cameca. This device is equipped with a spectrometer with an electron diode receiver and works automatically based on a high accuracy of 1% and the simultaneous operation of several diode detectors and electron beam stability with a carbon coating.

Volcanic activities in the middle part of the Central Alborz Mountain were started about 1.8 million years ago with the eruption of lava and pyroclastic rocks, which has caused the formation of the current Damavand cone, in the northern part of of Tehran. Damavand trachyandesites in Lar region have hyaloporphiric and microlitic porphiric texture with main phenocrysts include feldspar (oligoclase to labradorite), clinopyroxene and amphibole, and mica, apatite and opaque titanomagnetite to ilminite minerals. Mineral chemistry studies of clinopyroxenes show that these minerals with a percentage (Wo14-47, En0.1-47, Fs7-85) show the composition of diopside, which are in the range of calcium pyroxenes, magnesium-iron and alminodiopside. High Magnesium Number (80-79 = Mg #) of studied clinopyroxenes, shows that trachyandesites of Lar region generated from the low evolution and low-silica parent magma. The zonation of clinopyroxenes with Fe3 + fluctuations indicates the oxidant environment of the magmatic reservoir of these rocks, which is also confirmed by the high amounts of Fe3 + mica. Changes in the amounts of calcium and silica from the core to the rim of these minerals indicate crustal contamination of these rocks. The amphiboles composition is magnesium hostengite and mica show the composition of phlogopite. Phlogopites are characterized by Fe # <0.33, TiO2 6.03 - 7.7% and high Mg # number. Mineral chemistry studies indicate the sub-alkaline nature of the host magma. Thermo-barometric studies of clinopyroxenes show these minerals form at temperature range between 1112 to 1191 (± 50) ° C and pressure range of 6.2 to 6.9 kb, at depths of 20 to 22 km, with a water content of about 10%. Amphibole has a water content of about 4 to 7% and high oxygen fugacity range from -5.83 to -6.15. Thermal anomalies related to geothermal activity have been revealed based on remote sensing models that theoretically confirm the existence of hot zones containing melt at shallow depths. According to other researches, there is an unexposed igneous mass at a depth of about 22 to 35 km, which is somewhat consistent with the results of geophysical models based on a high-velocity P-wave mass at a depth of about 20 km below Damavand volcano (old cone area). Existence of non-equilibrium textures such as co-occurrence of aqueous and anhydrous minerals, reactive margins of phlogopites and amphiboles, non-equilibrium corrosion texture in feldspar, oscillatory and normal zoning and oscillation of iron oxides between the core to the rims of clino-pyroxene minerals indicate that Magmatic evolution of Lar lavas take place in an open system with different pressure, temperature, oxygen fugacity, and water content. Oxygen fugacity and water content of magma have increased during ascent to higher sections. Multiple magmatic chambers at different depths can be a justification for the local thickening of the crust under Damavand volcano.

The composition of phlogopites with Fe # <0.33, TiO2 6.3 to 7.7% and magnesium number (Mg #) is high and the formation temperature is 912 to 995. Mineral chemistry indicates the sub-alkaline nature of the host magma. Earth-temperature barometric studies of clinopyroxenes show that these minerals are formed in the temperature range of 1112 to 1191 (± 50) ° C and pressure of 6.2 to 6.9 kg, at depths of 20 to 22 km, with a water content of about 10%. Amphibole water content is about 4 to 7% and its fugacity is -5.83 to -6.15.