SHRIMP U-Pb dating of zircons from a peralkaline S-type Lesser Himalayan granite from the Kathmandu region, Nepal indicate an age of emplacement of 475 Ma. The granites along with metasedimentary xenoliths show a similar signature of inherited detrital zircon ages ranging from Archaean to early Palaeozoic with prominent late Mesoproterozoic (Grenvillian) and Neoproterozoic (Pan-African) age peaks and have a maximum age of 500 Ma based on the youngest detrital grains. Deformation structures in xenoliths are truncated by the granite and along with the granites are assigned to a Cambro-Ordovician orogenic event, herein termed the Bhimphedian Orogeny that can be traced across the Himalaya from Pakistan to the eastern Himalaya and possibly extends west into Afghanistan. We interpret the orogeny as being related to Andean-type orogenic activity on the northern margin of the Indian continent, following Gondwana assembly. The magmatic arc was associated with andesitic and basaltic volcanism and was active from c. 530 to 490 Ma. The arc activity overlaps with, and is succeeded by, regional deformation, crustal melting and S-type granite emplacement that extended to 470 Ma. Orogenic activity was driven by coupling across the plate margin either during on-going subduction or through accretion of microcontinental ribbons, possibly represented by the Lhasa and Qiangtang blocks. It represents the termination of an orogenic cycle, termed the North Indian Orogen that commenced with Neoproterozoic rifting and passive margin development and terminated with the Bhimphedian Orogeny. This was succeeded by a return to passive margin setting along the northern Indian margin of Gondwana which continued until the Cenozoic Himalayan Orogeny.
- Early Palaeozoic