Petrogenesis and Tectonic Significance of the Paleoproterozoic Ramgarh Mylonitic Granitoids in the Baijnath Klippe, Kumaun Himalaya: Evidence for Active Continental Margin During Columbia Supercontinent Assembly

Abstract

Paleoproterozoic granitoids are important rock types all along the Lesser Himalayan belt and provide key constraints for understanding the evolution of the northern Indian continental margin and its position within the Columbia supercontinent assembly. The Paleoproterozoic Ramgarh mylonitic granitoids of the Baijnath klippe in the Inner Kumaun Lesser Himalayan Sequence show pervasive mylonitization, with augen‐shaped porphyroclasts of feldspar and quartz, which characterize the Ramgarh mylonitic granitoids throughout the Lesser Himalaya. We present petrological and whole‐rock geochemical data from the Ramgarh mylonitic granitoids of the Baijnath klippe to evaluate their petrogenesis and geodynamic implications. These monzonitic granitoids are geochemically peraluminous with high K 2 O and MgO along with enrichment in LILE (Rb, K, Pb, and Th) and depletion in Ba, P, Nb, and Ti, along with moderately negative Eu anomalies. Geochemically, the Ramgarh granitoids are classified as I‐type (Igneous arc‐type) granite. Zircon U–Pb geochronology from the Kausani body yields a crystallization age of ~1866 Ma. The granitoids are interpreted to have formed by a high degree of partial melting of underplated lower‐crustal material during the early phase of slab rollback or break‐off of a subducting slab. This magmatic event likely preceded the emplacement of the post‐collisional extensional magmatism, suggesting an evolving subduction–accretion system. The geochemical and geochronological data support an active continental margin setting along the northern Indian plate margin at ~1.8 Ga. This suggests that these oldest granitoids represent a magmatic arc developed in a subduction zone setting during the assembly of the Columbia supercontinent. Later, during the Cenozoic Himalayan orogeny, the granitoids were thrust over the Lesser Himalayan Sequence as nappes and klippen.

Bibliographic Information