Petrological Evidences of Impact-induced Shock Metamorphism in the Basement Granitoids and Rhyolitic Melt Breccia of Mohar Area, Shivpuri District, Madhya Pradesh

摘要

The circular structure at Mohar (Dhala structure) in the western part of Bundelkhand Gneissic Complex, is marked by a prominent outlier of Kaimur sediments surrounded by low lying concentric sequence of sediments of Dhala Formation and basement granite breccia. This has been interpreted as a volcanic eruption related cauldron structure and meteoritic impact crater structure by various authors, on the basis of absence or presence of shock indicators in the clasts of a rhyolite-like rock that crops out scantily in the north western part of the structure. During the course of extensive sub-surface uranium exploration in this structure, the geoscientists of Atomic Minerals Directorate for Exploration and Research observed unequivocal and rampant evidences of shock metamorphic features for the first time in drill core samples of basement granitoids which constitute the bed rock for the rhyolite-like melt breccia, which overlies it. Published data of shock metamorphic features from this area are largely confined to the surface samples of the rhyolitelike melt rock, exposed in sparse outcrops. The shock metamorphic features recorded in the sub-surface granitoid bed rock samples during the present study, comprise planar deformation features (PDF) in quartz, feldspar, apatite and zircon, toasted, diaplectic, ladder-textured feldspars, selectively shock-melted feldspars and melt-veined quartz. The shock metamorphic features recorded in surface and sub-surface samples of the melt rock include ballen quartz, PDF in quartz clasts, toasted and diaplectic feldspar clasts shocked basic rock fragments with isotropised feldspars. Both the shocked bedrock granitoid and the melt rock bear uncharacteristic geochemical signatures with elevated K_2O, MgO and depleted CaO. The study also observes that the melt breccia overlying the granitoid bedrock also occurs as pocket-like patches at various depths within the granitoids. Thus, the present findings have helped in understanding the attributes of the basement granitoid and associated melt breccia, thereby linking the genesis of the latter by selective melting of the former, due to the process of impact. It reinforces the already propounded theory of impact as the likely cause for the development of the structure in the basement Bundelkhand granitoid that was later filled by sediments standing out presently as a mesa.