Geology and tectonomagmatic evolution of the eastern Himalaya along the Chomolungma-Makalu transect

Abstract

Abstract The Lesser Himalayan units, the base of the Tethyan sedimentary sequence and the intervening High Himalayan Crystallines have been studied along a transect in the eastern Nepal Himalaya and southern Tibet. The lowest Lesser Himalayan tectonic unit exposed along the transect is the parautochthonous Tumlingtar Unit, a thick sequence of phyllite and quartzite of presumed Late Proterozoic age. Metamorphic grade is low (chlorite and biotite zones) and deformation much weaker than in the overlying unit, the Lesser Himalayan Crystallines, a unit including large bodies of granitic orthogneiss of probable Early Palaeozoic age. Metamorphic grade of the Lesser Himalayan Crystallines is higher than in the Tumlingtar Unit, being characterized by garnet-biotite-muscovite-kyanite-staurolite assemblages in metapelites and biotite-muscovite-kyanite in blastomylonitic orthogneiss. A belt of metasedimentary and metagranitic thrust sheets separate the Lesser Himalayan Crystallines from the overlying High Himalayan Crystallines (HHC). Its position corresponds to that of the Main Central Thrust (MCT) as defined in central Nepal. The HHC are a gneiss sequence of 6–7 km visible thickness in which metasediments of upper amphibolite facies (Barun Gneiss) are associated with large tabular bodies of granitic orthogneiss that occur especially in the middle part of the sequence and which display evidence of water-deficient partial melting. The age of the granite protolith from which the orthogneiss derives is Early Paleozoic, possibly Cambrian. The top of the HHC consists of biotite paragneiss and micaschist (Black Gneiss) hosting networks and lens-shaped bodies of tourmaline leucogranite which may attain a thickness of 1-2 km (Makalu, Baruntse) and evolve into cylindrical diapirs with gently north-plunging long axes (Nuptse). At the regional scale the top of the HHC coincides with the muscovite-in isograd and appears as a major intracrustal detachment zone which brought the HHC into contact with low-grade metasediments of the Tethyan Zone (North Col Fm) during the Miocene uplift of the Himalayan nappe pile, with attendant muscovite dehydration melting of the metasediments occurring at the top of the HHC. The top of the HHC, the overlying North Col Fm. and the leucogranite sills hosted in both, are affected by a ductile extensional tectonics which appears broadly synchronous with granite emplacement and is characterized by extension lineations trending 20° to 40°. Extensional structures occur as far south as Makalu, whereas further south ductile deformations have the same orientation but are compressional. In the area south of Makalu the ductile deformation involves some of the Miocene leucogranite bodies, suggesting that Miocene compression in the lower and middle HHC may have been synchronous with ductile extension in the upper HHC. Relations between metamorphism and deformation are inconsistent with a wholly post-metamorphic thrusting of the HHC over the Lesser Himalayan units and rather suggest that the Miocene exhumation of the HHC may be better explained by changes in the internal dynamics of the orogenic wedge produced by the Early Himalayan collision between northward-drifting India and Eurasia.