Study and Comparison of Petrology and Petrogenesis Plio-Quaternary basaltic lavas of Kaleybar, Varzeqan- Ahar- northwestern of Iran

Abstract: The study areas are part of the Iran-Turkey Plateau and are located in the central part of the Arabian-Eurasian collision zone. The basaltic volcanic units studied are located in northwestern of Iran and East Azerbaijan Province and are of Plio-Quaternary age according to stratigraphic evidence. These lavas are composed of basalt, basaltic andesite, trachybasalt, basalt-trachyandesite and trachyandesite. Their main minerals include plagioclase, pyroxene, olivine, amphibole, and minor minerals include apatite, quartz, and opac minerals. In rock samples, the textures of the rocks are diverse and include microlithic porphyritic, hyaloporphyritic, porphyritic and trachytic textures. Clinopyroxenes are of the augite and diopside types and are alkaline and subalkaline in nature, and all of them are located in the igneous rocks range. The environment in which clinopyroxenes formed had high oxygen fugacity and they formed at low to moderate pressures. The amount of magmatic water at the time of clinopyroxene formation was moderate to low and these minerals formed at less than 10 kbar water barometric pressure. The mineralogical composition of plagioclase phenocrysts is andesine, bitonite, labradorite and oligoclase. The amphiboles in the study areas are of the pargasite and schermakite type. All olivines in the study areas are of the chrysolite type and the iron-titanium oxides are of the magnetite and titanomagnetite type. Thermobarometric calculations show a temperature of 1016.23-1226.07 °C and a pressure of 3-13 kbar for the pyroxene mineral, a temperature of 1176.85-1249.90 °C and a pressure of 10.15-30.86 kbar for the plagioclase mineral, a temperature of 733.37-954.87 °C and a pressure of 5.13-10.06 kbar for the amphibole mineral, and a temperature of 1261.97-1337 °C for the olivine mineral. The results of barometric calculations of pyroxene and amphibole indicate crystallization at medium pressures and equivalent to the middle-lower crust, and barometric calculations of plagioclase indicate crystallization at greater depths and equivalent to the Earth's mantle. The studied samples are enriched in K, Ba, Rb, U and Cs. Also, the enrichment in LILE elements and depletion in HREE elements indicate that these rocks originate from an asthenosphere mantle with a garnet source and low relative melting point. The reason for the presence of positive anomalies of K, Ba, Rb, U and Cs and negative anomalies of Ta, Nb and Ti can be interpreted in relation to crustal contamination. Based on the distribution coefficients of La and Y elements, the source rock of the studied samples contains 6 to 30% garnet. Examination of the tectonic structure of the samples has shown post-collisional volcanic arcs for these areas. Keywords: Pyroxene, plagioclase, amphibole, olivine, geochemistry, petrology, thermobarometry, post-collisional, Plioquaternary, northwestern of Iran.