Tectonic evolution of Chitradurga Schist Belt, Western Dharwar Craton, South India
摘要
In Chitradurga Schist Belt (CSB), the late Archaean low to medium grade Dharwar; namely as basal Bababudan Group (cross-bedded quartz arenite-pelitic sediments-subaerial basic volcanics association) as shallow platformal sequence unconformably over the PGC-I basement is restricted to the western, northwestern and southern parts. Thick differentiated volcanics of massive – pillowed – pyroclastic character along with co-magmatic discontinuous basic-acidic intrusives of overall arc affinity, and thin volcaniclastic–nonclastic sediment intercalation along the contact of intrusive granitoids (PGC-II) is delineated as the deep water facies of middle Ingaldhal Group in central, northern and eastern parts of CSB, respectively. The Ingaldhal Group’s occurrence in the down dip of shallow platformal Bababudan of southern part suggests the transformation of initial shallow platformal basin as a linear mid-continental rift zone for accumulation of deep water ocean plate lithologies with a gravity high in resemblance with subduction zonal facies and as the suture package. Thus, the basal shallow platformal Bababudan and middle deepwater Ingaldhal facies are found in foreland and continental arc set-ups with unconformable cover of thick greywacke-argillite suite of upper Chitradurga Group as the overall litho-stratigraphic record. The unusual occurrence of pillowed-spinifex textured ultramafic lenses without any geochemical affinity with the adjoining volcanics in the western shallow platformal Bababudan is considered as the allochthonous unit. The generation of these as an extrusive oceanic crust from the eastern Ingaldhal subduction zone and its transfer – accumulation towards the continental margin by abduction mechanism in a ophiolitic setting is envisaged analogous with a Meso-Cenozoic plate tectonic model. The junction of Bababudan and Ingaldhal dissimilar environmental facies is marked with an intrusive alkali-feldspar granite emplacements along the CSB’s western and eastern margins respectively. The basal rudacesous-arenaceous upper Chitradurga Group with underlying Bababudan-Ingaldhal clasts association suggests the upliftment-reworking of the later in a fault-controlled sedimentation coupled with turbidity environment as an orogenic cycle. The middle non-clastic sediments ± manganese association and upper thick greywacke-argillite suite indicate a short period of tectonic quiescence and resurgence of orogenic cycle similar to the basal part respectively. The overall CSB as a close outcrop of F1 synclinorium; and southern Sirankatte dome, middle Chitradurga antiform and regional Dodguni synform; the overall as superposed non-cylindrical F2 folds is differentiated. The structurally controlled-closed outcrops of Ingaldhal Group within the upper Chitradurga Group of sediments are delineated as the middle Dharwar.
Research highlightsA three-fold lithostratigraphic classification for the late Archean Dharwar Schist in Chitradurga Schist Belt (CSB), Western Dharwar Craton is worked out. The initial basic volcanic fissure eruptions within a wide shallow platformal basin with cross bedded quartz arenite-parallel laminated sediments assemblage; and the middle differentiated volcanics-comagmatic intrusives-volcanic sediments of gravitational settling; overall as subduction zonal facies in a linear rift basin is differentiated in the CSB during its evolutionary stages. The unusual occurrence of ultramafic lenses within initial shallow platformal sequence of western CSB with these derivation from the eastern subduction zone of differentiated volcanics and the transfer of same to the continental margin as ophiolite by abduction mechanism analogous with Meso-Cenozoic plate tectonic setting mechanism is envisaged during the evolutionary history of overall basal CSB. The upper greywacke-argillite suite of resedimented facies in a fault-controlled sedimentation is highlighted. The overall CSB is differentiated as a NNW-SSE closed outcrop with the effects of both F1 and F2 folds.