Records of the evolution of the Himalayan orogen from in situ Th–Pb ion microprobe dating of monazite: Eastern Nepal and western Garhwal

Abstract

In situ Th–Pb monazite ages from rocks collected along two transects (the Dudh Kosi-Everest, eastern Nepal and the Bhagirathi River, Garhwal Himalaya, India) perpendicular to the Main Central Thrust (MCT) suggest a striking continuity of tectonic events across the Himalaya. The youngest age reported in this study, 5.9±0.2Ma (MSWD=0.4), from matrix monazite grains collected beneath the MCT in the Garhwal region is consistent with several age data from rocks at similar structural levels in central Nepal, providing support for widespread Late Miocene MCT activity. The lateral parallelism of orogenic events is further manifested by the 20.7±0.1Ma age of a High Himalayan leucogranite from an injection complex that outcrops along the Dudh Kosi-Everest transect, resembling ages of these bodies reported elsewhere. The youngest monazite grain analyzed along the Dudh Kosi-Everest transect is 10.3±0.8Ma. The absence of 7–3Ma monazite ages in eastern Nepal may reflect a different nappe structure, which obscures the reactivated ramp equivalent exposed in the Garhwal and central Nepal. Garnets from the MCT hanging wall (Greater Himalayan Crystallines) and footwall (Lesser Himalaya) display different major element zoning, and the patterns are useful for constraining the location of the thrust system that separates the two lithologies. Pressure–temperature paths for two upper Lesser Himalayan garnets that contain monazite inclusions indicate the utility of an in situ methodology to constrain the metamorphic evolution of the shear zone. Along the Dudh Kosi-Everest transect, upper Lesser Himalayan monazite grains from three rocks record a clear signal at 14.5±0.1Ma (MSWD=8.4), and the ∼23Ma age that characterizes the hanging wall is notably absent. Monazites collected within a large-scale Greater Himalayan Crystallines fold yield the ∼14Ma age, consistent with the structure forming due to MCT-related compression. Paleo-Mesoproterozoic (1407±35Ma) matrix monazite grains are found within an augen gneiss unit located beneath the MCT, whereas Cambro-Ordovician (436±8;548±17Ma) inclusions are preserved within garnets of the Greater Himalayan Crystallines. The presence of 45.2±2.1Ma grains from lower structural levels of the Greater Himalayan Crystallines indicates the unit realized conditions conducive for monazite growth during the Eocene.