Estimating of Crystallization Temperature of Mard-Abad (Karaj) Granitic Intrusion Using Mineralogy, Geochemistry and Morphology of Zircon Crystals

Abstract

Mard-abad calc-alkaline granite formed along the margin of active pull-apart or intra-continental plates, and it is post-collision granite type. Granophyric, perthitic and myrmekitic are main textures that could form from crystallization of hypersolvus source magma, which occurred under water vapor pressure of approximately 2 kb. During this study the temperature of formation of these granites has been estimated as high as 750 to 800 C° based on mineralogical and geochemical evidences. However, the thermometric measurement on zircon crystals has also been applied to achieve this goal. Zircon is of great importance because of its certain features such as resistance to chemical and mechanical processes, high melting point, and the ability of recording various geological events in its external structure as well as its internal texture. Crystallization of zircon is a function of temperature, chemical composition and the amount of water of the magma. The morphology of zircon crystals in Mard-abad granite points hybrid origin of granitic magma, which is in accordance with the geochemical evidence of the associated rocks. Such morphology also indicates to calc-alkaline and dry nature of the magma. The calculated crystallization temperature of these granite bodies is 700 to 800 C° based on morphology of zircons, 772.2 to 792 C° according to zircon saturation thermometry, and 760 to 800 C° based on the results from chemical analyses.