Dacite Agglomerate in Nandgaon Group, Dongargarh Supergroup: Implications for Palaeoproterozoic Felsic Volcanism in Bastar Craton, Central India

Located to the south of the Central Indian Tectonic Zone, the Palaeoproterozoic Dongargarh Supergroup, constituted mainly by a volcano-sedimentary sequence, is an important geological domain in Bastar Craton, Central India. Through this paper, we bring to light a significant event of dacite volcanism in the Dongargarh Supergroup of rocks in Bastar Craton, Central India. Field studies reveal the presence of an intermittently exposed linear zone of volcaniclastic rocks within the Nandgaon Group in the northern part of the Dongargarh Supergroup. The dacite eruption zone is marked by the presence of volcaniclastic rocks that are perhaps associated with proximal volcanic vents in the study area around Ganjesarra-Murkutta-Parsahi-Dulhapur areas. The volcanic bombs in the dacite agglomerate are oval to semi-oval in shape and range in size from 2 to 6 cm. The finegrained matrix is felsic in composition. Petrographic studies reveal the presence of quartz, K-feldspar, plagioclase, biotite and amphibole, while zircon, titanite and ilmenite are noticed as accessory phases. Plagioclase in the fine-grained matrix is oligoclase to andesine in composition. Absence of eutaxitic fabric in the field outcrops and absence of typical volcanic glass indicate the volcaniclastic nature of the rock. In the IUGS TAS diagram, both matrix and pyroclastic fragments fall in the field of dacite. In the incompatible trace element plots, the agglomerate exhibits negative Nb and Ti anomalies, a subduction-setting geochemical signature. Chondrite-normalised Rare Earth Element (REE) patterns of the arc lavas show fractionated LREE, a negative Eu anomaly and flat HREE signatures. The linear zone of felsic agglomerate of dacite composition in the Nandgaon Group signifies the presence of a major event of explosive pyroclastic volcanism during the evolution of the Palaeoproterozoic Dongargarh Supergroup, Central India.