EXHUMATION AND COOLING HISTORY OF GREATER HIMALAYAN ROCKS IN THE EASTERN HIMALAYA: A U-PB AND 40AR/39AR THERMOCHRONOLOGY STUDY FROM CENTRAL AND EASTERN BHUTAN

Within the active Himalayan orogen, the Greater Himalayan (GH) unit consists of mid- to lower-crustal rocks that were exhumed during ongoing convergence between the Indian and Asian plates. Within the eastern Himalayan kingdom of Bhutan, the GH has been divided into two structural units, the upper- and lower-GH, by an intra-GH shear zone, the Kakthang Thrust (KT). Within central and eastern Bhutan, high-temperature geochronology and thermobarometry suggest that the GH was constructed as a composite tectonic unit by progressive ductile underplating from ca. 27 Ma in the upper-GH to ca. 15 Ma in the lower-GH. The ductile underplating likely occurred along intra-GH shear zones, including the KT. The GH rocks remained at >550 ºC until 13 Ma, indicating that the GH was not exhumed to shallow crustal levels during underplating. To investigate the final exhumation and cooling history of the GH within Bhutan, titanite and rutile were analyzed via split-stream LA-ICPMS to obtain U-Pb ages and trace-element analyses from samples collected along two transects within central and eastern Bhutan. In addition, ⁴⁰Ar/³⁹Ar muscovite cooling ages were obtained. Zirconium-in-titanite and Zr-in-rutile temperatures range from ~700–825 ºC and 645–735 ºC, respectively, in central Bhutan and Zr-in-rutile temperatures of 625–725 ºC were obtained in eastern Bhutan, suggesting high-temperature crystallization of these phases. The U-Pb rutile dates, however, likely record cooling. The titanite in the upper-GH from central Bhutan (re)crystallized at ca. 19 Ma, whereas U-Pb rutile cooling ages from central and eastern Bhutan ranged from 11.4 to 10.4 Ma and 9.5 to 8.6 Ma, respectively, from all structural levels. Furthermore, ⁴⁰Ar/³⁹Ar muscovite cooling ages indicate that the GH cooled through ~425 ºC from ca. 10.5 to 9.5 Ma in central Bhutan and from 10.1 to 7.7 Ma in eastern Bhutan. The ⁴⁰Ar/³⁹Ar muscovite cooling ages from the Lesser Himalaya (LH), located structurally below the GH, are coeval with cooling ages from the GH, suggesting that their exhumation is dynamically linked. Duplexing within the LH from ca. 20 to 8 Ma likely caused uplift of the composite GH unit. This structural elevation resulted in focused erosion and the final exhumation of the GH and LH rocks to the near-surface.