40Ar/39Ar, pressure, temperature and fission track constraints on the age and nature of metamorphism around the main central thrust in the eastern Bhutan Himalaya

Abstract 40 Ar/ 39 Ar data from a profile across the Main Central Thrust in the eastern Bhutan Himalaya indicate muscovite cooling ages of 14.1±0.2 Ma from a sample in the immediate hanging wall of the thrust and 11.2 Ma from about 400 m structurally higher in the hanging wall. These two ages are repeated by two samples from 2.1 and 4.7 km vertical distance from the thrust within the hanging wall, respectively. A single apatite fission track age from the immediate hanging wall of the thrust gives an age of 3.1±0.6 Ma. Pressure–temperature estimates give temperatures around 650°C and 6.5 kbar for the highest sample collected. Samples closer to the Main Central Thrust give also temperatures between 600 and 650°C at the same pressure, indicating possibly a slight temperature decrease with proximity to the thrust. However, uncertainties are large and the parageneses are thermodynamically too highly variant to place much significance on their interpretation. The 40 Ar/ 39 Ar cooling age data are consistent with a repetition of the sequence in the hanging wall of the thrust. They confirm the data of Davidson et al. (1997; Metamorphic reactions related to decompression and synkinematic intrusion of leucogranite, High Himalayan Crystallines, Bhutan. Journal of Metamorphic Geology 15, 593–612) and are consistent with a more rapid exhumation of deeper levels towards the centre of the High Himalayan Crystalline Complex. Despite the large uncertainties, the PT data shown here are also consistent with this interpretation. The apatite fission track results reveal low-temperature cooling and final exhumation of the Main Central Thrust at the same time as in Nepal.