Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

S. V. G. Krishna Rao ¹, Samit Bhattacharyya ¹, K. J. Sarma ¹

Affiliations
1 Geological Survey of India, Himayatnagar, Hyderabad 500029, IN

Abstract

Manganese silicate-carbonate protore (queluzite) and a rare manganese olivine (knebelite) are being reported for the first time from India. The protore is comparable to the 'Queluzite Type' and occurs as conformable bands in close association with manganese ore body as well as different members of the Khondalite Group of rocks. So far, the knebelite is reported to occur in association with other manganese minerals, particularly in iron-manganese ore deposits and their associated skarns. It is surmised that the protore represents metamorphic equivalent of carbonate-silica-bearing manganiferous sediments.

Full Text

     

T. Kameswara Rao ¹, K. J. Sarma ¹

Affiliations
1 Geological Survey of India, Bandlaguda Complex, Hyderabad 500 660, IN

Abstract

A Kimberlite dyke, with the appearance of a conglomerate due to the presence of mega and macro-crysts of olivine resembling pebbles, is located for the first time near Kotakonda, at about 10 km SSE of the known kimberlitic occurrences of Maddur area. Its massive look, lack of country rock xenoliths, local trachytic texture and concentration of megacrysts of olivine and orthopyroxene reflect hypabyssal nature. It consists mainly of rounded and corroded mega and macro-crysts of serpentine pseudomorphs of olivine and subordinate ortho- and clinopyroxene set in a fine-grained matrix containing microphenocrysts of euhedral olivine, monticellite. perovskite, spinel, apatite, calcite, ilmenite, magnetite, chromite and pyrite. The intergranular space is occupied by phlogopite, serpentite and carbonates. The major element chemistry of Kotakonda kimberlite, with low Niggili Si and high K and Mg values and alkali- and alumina-poor and cafemic-rich nature, matches well with that of kimberlite pipes 2,3,4 and 7 of Vajrakarur and kimberlites of Russia and Greenland. Its REE data agrees with that of 'basaltic kimberlites' of south India. The Kotakonda and Maddur kimberlite occurrences, possibly representing thc northern loci of kimberlitic activity of Vajrakarur area, show a rise of + 10 mgl over the background value in the regional gravity picture and fall within the zone of intersection of NE-SW and NNW-SSE lineaments, which could be a manifestation of mantle uplift and crustal thinning, thereby opening a new vista for the search of kimberlites.

Keywords

Kimberlite, Igneous Petrology, Mahboobnagar District, Andhra Pradesh.

Full Text

     

K. J. Sarma ¹

Affiliations
1 Geological Survey of India, Map & Cartography Division, Southern Region, Hyderabad - 500 068, IN

Abstract

The granite - gneiss terrain of Dharwar Craton in Anantapur area of Andhra Pradesh is intruded by mafic dolerite dykes. These dykes are quartz or olivine-normative sub-alkalic tholeiites. Their Mg numbers range between 0.34 and 0.70 with low Ni (50-150 ppm), Cr(50-337 ppm) and Co( 41-70 ppm), indicating some degree of fractionation. The REE abundances show LREE - enriched patterns (La/Yb_N - 2.7 to 5.5). These dykes appear to be genetically related and were derived from a single magma.

Keywords

Geochemistry, Mafic Dykes, Emplacement Machanism, Anantapur, Andhra Pradesh.

Full Text