PETROLOGY OF THE BHELA-RAJNA ALKALINE COMPLEX, NUAPARA DISTRICT, ORISSA

A newly identified silica-oversaturated alkaline complex from the Bhel a-Rajna area of the Nuapara District, Orissa, comprises three major ro ck types: (i) comenditic ignimbrite (ii) quartz-porphyry (intrusive rh yolite) and (iii) riebeckite- granite, all of which were emplaced conf ormably within a linear, intra-cratonic (Bastar Craton) Proterozoic sh ear-zone and controlled by deep basement fractures. The complex is hos ted by a batholithic biotite-granite (adamellitic) body, bordering the EGMB in the east and platforming the Chhatishgarh sediments in the we st. The comenditic ignimbrite exhibits eutaxitic and rhyotaxitic textu res; the quartz-porphyry, part of which forms fissure-feeders, display s rhyotaxitic and porphyritic textures; the riebeckite-granite exhibit s hypidiomorphic and rarely porphyritic textures. Riebeckite is the mo st abundant mafic mineral in all the major rock types and the accessor y minerals are crystalline iron oxides, biotite, sphene, zircon and ep idote. The major element chemistry indicates a strong fractionation fr om a magmatic melt, implying cogenetic and comagmatic nature of the ro ck-units. The field setting, structure and lack of metamorphic signatu res indicate a late Proterozoic emplacement age for the complex.