Sk. Pattnaik, PETROLOGY OF THE BHELA-RAJNA ALKALINE COMPLEX, NUAPARA DISTRICT, ORISSA, Journal of the Geological Society of India, 48(1), 1996, pp. 27-40
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.