COMPARATIVE GEOTHERMOMETRY OF THE AJ-BOGD GRANTITOID MASSIF (SOUTHERN MONGOLIA) BASED ON ZIRCON STUDY

Abstract

The Aj-Bogd massif, located in the Trans-Altai Gobi within the Hercynides of the Central Asian fold belt, is composed of rocks from two main intrusion phases. The rocks of the early phase are represented by two-feldspar granites. while the late phase is characterized by alkali-feldspar granites. During the reconstruction of the thermal history of these granites zircon was chosen as a mineral-indicator of the temperature regime of their formation. Two main approaches to using this mineral as a geothermometer have been proposed: the zircon saturation index and the zirconium-hafnium geothermometer. Both approaches demonstrate a similar thermal picture of the formation of granites from Aj-Bogd massif. Alkaline feldspar granites of the late intrusion phase crystallized at temperatures of 810–850°C while two-feldspar granites of the early phase formed at temperatures of 700–770°C. According to thermobarogeochemical studies of melt inclusions in zircon of late-phase alkali-feldspar granites, a similar temperature range of their formation was defined (825–850°C). All obtained data of the formation temperatures of studied granites from Aj-Bogd massif indicate a significantly higher (by 100–120°C) temperature of formation of alkali-feldspar granites compared to earlier two-feldspar granites. The significantly higher crystallization temperature of late alkali feldspar granites allow us to suggest the appearance of an additional heat source that could be mantle derived melts. The evidence of such melt participation in the formation of Aj-Bogd massif are mafic dikes that separate granites of two intrusion phases. The contrast temperature regime of the calc-alkaline and alkaline intrusion phases close in time was determined for the first time in this work.