Geochronology of the Pre-Cambrian rocks of Singhbhum and adjacent regions, Eastern India

Abstract

Tectonic implications of 32 new potassium-argon age determinations of minerals and rocks of Singhbhum and adjacent regions are discussed along with those of recent structural, stratigraphic and petrologic studies, and some pre-existing geochronologic data. Three distinct orogenic cycles with closing dates at c. 3200, 2700 and 850 m.y. respectively have been recognized. The earliest cycle is represented by the Older Metamorphic Group of metasediments with associated basic intrusives and biotite granodiorite gneiss; they occur mainly as relics within the huge Singhbhum granitic complex. Emplacement of the latter (c. 2700 m.y.) marks the culmination of the Iron Ore orogenic cycle which is characterized by the low-grade metamorphic Iron Ore Group sediments and volcanics and associated basic intrusives. Along the northern and eastern edges of the stabilized Iron Ore orogenic belt, thick sedimentation of the Singhbhum and Gangpur Groups (?2000–1700 m.y.) was followed by large scale basic volcanic activity (1700–1600 m.y.); these rocks were folded and regionally metamorphosed in several phases between c. 1550 and c. 850 m.y. (Singhbhum orogenic cycle); extensive thrusting and granitization occurred in the later part of the cycle.