Petrology of the Bhela-Rajna Alkaline Complex, Nuapara District, Orissa

A newly identified silica-oversaturated alkaline complex from the Bhela-Rajna area of the Nuapara District, Orissa, comprises three major rock types: (i) comenditic ignimbrite (ii) quartzporphyry (intrusive rhyolite) and (iii) riebeckite- granite, all of which were emplaced conformably within a linear, intra-cratonic (Bastar Craton) Proterozoic shear-zone and controlled by deep basement fractures. The complex is hosted by a batholithic biotite-granite (adamellitic) body, bordering the EGMB in the east and platforming the Chhatishgarh scdiments in the west.

The comenditic ignimbrite exhibits eutaxitic and rhyotaxitic textures; the quartz-porphyry, part of which forms fissure-feeders, displays rhyotaxitic and porphyritic textures: the riebeckite-granitc exhibits hypidiomorphic and rarely porphyritic textures. Riebeckite is the most abundant mafic mineral in all the major rock types and the accessory minerals are crystalline iron oxides, biotite. sphene, zircon and epidote. The major element chemistry indicates a strong fractionation from a magmatic melt, implying cogenetic and comagmatic nature of the rock-units. The field setting, structure and lack of metamorphic signatures indicate a late Proterozoic emplacement age for the complex.