Evolution of ca. 2.5 Ga Dongargarh volcano-sedimentary Supergroup, Bastar craton, Central India: Constraints from zircon U-Pb geochronology, bulk-rock geochemistry and Hf-Nd isotope systematics

The amalgamation of five Paleo- to Mesoarchean protocontinent blocks [Singhbhum, Bastar, Dharwar, Aravalli and Bundelkhand] resulted in the present-day tectonic configuration of India. The Dongargarh Supergroup is a major volcano-sedimentary sequence in the Bastar craton, central India. For nearly six decades, it was presumed that the extensive lava pile in the Dongargarh Supergroup has accumulated over a protracted duration of magmatic eruptions spanning from early Paleoproterozoic (similar to 2462 Ma) to mid-Mesoproterozoic (similar to 1367 Ma). However, in the absence of any age information, this time-window remained speculative. The detrital zircon U-Pb depositional ages acquired for the three intercalated sedimentary units do not provide any such evidence of significantly prolonged magmatism in the Dongargarh Supergroup. The close proximity between the depositional ages and the similarities between the geochemical attributes of the basalts in the four volcanic formations [Pitapani, Sitagota, Mangikhuta and Kotima] of the Dongargarh Supergroup, suggest contemporaneous evolution spanning few tens of million years around ca. 2.5 Ga. The negative initial epsilon Nd-(t=2.5 Ga) isotopic compositions in the bulk-rock basalt samples of Pitapani (-0.36 to + 0.41) and Mangikhuta (-0.62 to -0.49) suggest interaction with similar to 3.5 Ga continental crust in the Bastar craton. On the contrary, near chondritic to positive initial epsilon Nd-(t=2.5 Ga) isotopic compositions in the Kotima (+0.01 to + 0.92) and Sitagota (+ 1.75 to + 2.24) basalts are inconsistent with any significant crustal assimilation. Overall, the initial epsilon Hf-(t=2.5 Ga) isotopic compositions in the four volcanic formations are positive and range from + 0.31 to +6.26, which is consistent with their generation from juvenile mantle sources. On the basis of Nb concentrations and rare earth element systematics, the basalts in the Dongargarh Supergroup can be subdivided into two geochemical groups comprising of Sitagota + Kotima with Nb < 4.5 ppm, and Pitapani + Mangikhuta with Nb > 4.5 ppm. The Zr/Nb and Nb/Yb systematics in these basalts suggest that their mantle sources were relatively fertile and enriched in high field strength elements, which is comparable to the source region of mid-oceanic ridge basalts, and basalts that are generated in the back arc basins. Collectively, the Dongargarh basalts exhibit negative Nb and Ti anomalies on a primitive mantle normalized incompatible trace element variation diagram. The geochemical signatures in combination with the initial isotopic compositions in these basalts are inconsistent with their origin in a continental-rift setting, or from a mantle plume, as hypothesised by the previous evolutionary models proposed for the Dongargarh Supergroup. Moreover, it cannot be attributed to crustal assimilation sense strict. Instead, we interpret the evolution of the volcanic sequences in the Dongargarh Supergroup by the subduction of an oceanic spreading ridge beneath the continental lithosphere, presumably in a back-arc or marginal sea basin that has witnessed contemporaneous sedimentation, which led to the development of an Andean-type magmatic arc around ca. 2.5 Ga in the Bastar craton, central India.