Biotite and Hornblende Composition Used to Investigate the Nature and Thermobarometry of Pichagchi Pluton, Northwest Sanandaj-Sirjan Metamorphic Belt, Iran

Pichagchi pluton is situated in the northeastern part of the Soursat complex in Sanandaj-Sirjan metamorphic belt of Iran. The pluton with the age of 74.20 Ma is composed mainly granodiorite, diorite, and tonalite in which mafic minerals are biotite and amphibole. In this study, composition of minerals used to describe the nature of the granitic magma and estimate the pressure and temperature at which Pichagchi pluton is emplaced. Based on chemistry of biotites Pichagchi pluton formed from calc-alkaline magma. This type of magma is typically produced in subduction environments. It means that the pluton could have formed in an orogenic suit from calc-alkaline magma in subduction zone. The results obtained based on biotite chemistry compare well with the previous interpretations on the Sanandaj-Sirjan belt. Temperatures of emplacement calculated with the hornblende-plagioclase thermometer range from 615 °C to 691 °C. Aluminum-in-hornblende geobarometry indicate that the Pichagchi pluton was emplaced at pressure of 3.80-4.24 Kbar.