Lesser Himalayan sequences in Eastern Himalaya and their deformation: Implications for Paleoproterozoic tectonic activity along the northern margin of India

摘要

Substantial part of the northern margin of Indian plate is subducted beneath the Eurasian plate during the Caenozoic Himalayan orogeny, obscuring older tectonic events in the Lesser Himalaya known to host Proterozoic sedimentary successions and granitic bodies. Tectonostratigraphic units of the Proterozoic Lesser Himalayan sequence (LHS) of Eastern Himalaya, namely the Daling Group in Sikkim and the Bomdila Group in Arunachal Pradesh, provide clues to the nature and extent of Proterozoic passive margin sedimentation, their involvement in pre-Himalayan orogeny and implications for supercontinent reconstruction. The Daling Group, consisting of flaggy quartzite, meta-greywacke and metapelite with minor mafic dyke and sill, and the overlying Buxa Formation with stromatolitic carbonate-quartzite-slate, represent shallow marine, passive margin platformal association. Similar lithostratigraphy and broad depositional framework, and available geochronological data from intrusive granites in Eastern Himalaya indicate strikewise continuity of a shallow marine Paleoproterozoic platformal sequence up to Arunachal Pradesh through Bhutan. Multiple fold sets and tectonic foliations in LHS formed during partial or complete closure of the sea/ocean along the northern margin of Paleoproterozoic India. Such deformation fabrics are absent in the upper Palaeozoic–Mesozoic Gondwana formations in the Lesser Himalaya of Darjeeling-Sikkim indicating influence of older orogeny. Kinematic analysis based on microstructure, and garnet composition suggest Paleoproterozoic deformation and metamorphism of LHS to be distinct from those associated with the foreland propagating thrust systems of the Caenozoic Himalayan collisional belt. Two possibilities are argued here: (1) the low greenschist facies domain in the LHS enveloped the amphibolite to granulite facies domains, which were later tectonically severed; (2) the?older deformation and metamorphism relate to a Pacific type accretionary orogen which affected the northern margin of greater India. Better understanding of geodynamic evolution of the northern margin of India in the Paleoproterozoic has additional bearing on more refined model of reconstruction of Columbia. Graphical abstract Display Omitted Highlights ? Relook at pre-Himalayan deformation of extensive Proterozoic passive margin sequences. ? New constraints on Paleoproterozoic deformation in LHS of Eastern Himalaya. ? Deformation kinematics gleaned from new data on microstructures. ? Distinct compositional range of snowball garnets in metapelites below MCT. ? Subduction-accretion during the Columbia assembly influenced Northern Indian margin.