Zircon-Based Comparative Geothermometry of the Adzh-Bogd Granitoid Pluton, Southern Mongolia

Abstract

The Adzh-Bogd pluton, that is located in the Transaltai Gobi area within the Hercynides of the Central Asian fold belt, is composed of rocks of two main intrusive phases: the early two feldspar granites are intruded by late alkali-feldspar granites. Zircon was chosen as the indicator mineral for interpretation of the thermal evolution of these granitoids. Two main approaches were proposed for using this mineral as a geothermometer: the zircon-saturated temperature and the Zr–Hf geothermometer. Both approaches show a similar pattern of the formation of granitoids. The early two feldspar and late alkali-feldspar granites crystallized at temperatures of 700–770 and 810–850°C, respectively. The thermobarogeochemical studies of melt inclusions in zircon from the late alkali-feldspar granites show similar temperatures of their formation (825–850°C). Our estimations of the crystallization temperature of granites of two phases of the Adzh-Bogd pluton indicate a significantly higher (by 100–120°C) temperature of the formation of alkali-feldspar granites in comparison with earlier two feldspar granites. The much higher temperature requires an additional heat source, which could be related to mantle melts. Their involvement in the formation of the Adzh-Bogd pluton is evident from the presence of mafic dikes, which divide the rocks of two granite intrusive phases. The contrasting temperature regime of nearly synchronous calc-alkaline and alkaline intrusive phases is shown for the first time in our work.