No abstract

The Dargawan gabbros intrusive into the Moli Subgroup of Bijawar Group, yielded Rb-Sr whole rock isochron age of 1967 ± 140 Ma. Based on the oldest age from overlying Lower Vindhyan (1.6Ga) and the underlying youngest basement ages (2.2 Ga), the time range of Bijawar sedimentation may be assigned as 2.1-1.6 Ga (Paleoproterozoic). Sm-Nd Model ages (TDM), obtained, for Dargawan gabbros, is c. 2876 - 3145 Ma. High initial 87Sr/ 86Sr ratio of 0.70451 (higher than the contemporary mantle) and negative εNdi (at 1.9 Ga) value of -1.5 to - 4.5, indicate assimilation of Archaean lower crustal component by the enriched mantle source magma at the time of gabbroic intrusion. The dolerite, from Damdama area, which is intrusive into the basement and overlying sediments of Chandrapur Group in the central Indian craton, yielded Rb-Sr internal isochron age of 1641 ± 120 Ma. The high initial 87Sr/86Sr ratio of 0.7098 and εNdi value of -3.5 to -3.7 (at 1.6 Ga) is due to contamination of the mantle source magma with the overlying sediments. These dolerites have younger Sm-Nd Model ages (TDM) than Dargawan gabbros as c. 2462 - 2675 Ma, which is similar to the age of the Sambalpur granite, from which probably sediments to this part of Chattisgarh basin are derived. Hence mixing of sediments with the Damdama dyke during its emplacement, gives rise to high initial 87Sr/86Sr and low initial 143Nd/144 ratios for these dykes. The c. 1600 Ma age indicates minimum age of onset of the sedimentation in the Chandrapur Group of Chattisgarh basin. Both the above mafic intrusions might have taken place in an intracratonic rift related (anorogenic) tectonic setting. This study is the first reliable age report on the onset of sedimentation in the Chandrapur Group. The total minimum time span of Chandrapur and Raipur Group may be 1.6 Ga to 1.0 Ga (Mesoproterozoic). The unconformably underlying Shingora Group of rocks of Chhattisgarh Supergroup thus indicates Paleoproterozoic age (older than 1.6 Ga). Most part of the recently classified Chattisgarh Supergroup and Bijawar-Vindhyan sequence are of Mesoproterozoic-Paleoproterozoic age and not of Neoproterozoic-Mesoproterozoic age as considered earlier. Petrographic study of basic dykes from Damdama area (eastern margin of Chattisgarh Supergroup) indicated presence of primary uranium mineral brannerite associated with goethite. This is the evidence of mafic intrusive providing geotherm and helping in scavenging the uranium from the surrounding and later alterations causing remobilisation and reconcentration of pre-existing uranium in host rocks as well as in mafic dyke itself otherwise mafic rocks are poor source of uranium and can not have primary uranium minerals initially. It can be concluded that mafic dykes have role in uranium mineralisation although indirectly.

The Barabazar granite, exposed at the northern margin of Singhbhum craton, Eastern India, occurs along the South Purulia Shear Zone (SPSZ) and is emplaced into the Palaeoproterozoic metapelites and felsic volcanics of Singhbhum Group. Geochemical, petrographical and geochronological studies on the Barabazar granite addressed in the work have wide implications on understanding the geodynamics of SPSZ during Palaeoproterozoic to Mesoproterozoic. Geochemically, Barabazar granite displays limited range of major oxides, alkali enrichment and highly fractionated features (SiO 2 > 75%; Eu/Eu* = 0.16-0.33; enrichment of K, Rb, Th, U and Nb; depletion of Ba, Sr, P and Ti). It is predominantly peraluminous (molar Al 2 O 3 /CaO+Na 2 O+K 2 O (A/CNK) =1.14-144) and contains abundant alkali feldspar, perthite, and minor plagioclase, biotite and accessory minerals. Geochemical and petrological data indicates that it is A-type granite, which formed in 'Within plate granite' tectonic set up. The Barabazar granite was emplaced at ca. 1771 Ma (Pb-Pb) in rift related environs and evolved by partial melting of stabilized lower/middle crust (initial 87 Sr/ 86 Sr = 0.7302 ± 0.0066 and µ 1 = 8.5 ± 0.5). Subsequently, the shear zone (SPSZ) developed during the closure of the riftogenic basin and was reactivated during the Grenvillian orogeny (Ca. 900-1300 Ma), resulting in rehomogenisation of the strontium isotopes and thereby yielding younger whole-rock Rb-Sr isotope age of c. 971 Ma for the Barabazar granite. Probably during this tectonic event, the Singhbhum craton (Southern India Shield) would have finally juxtaposed with Northern Indian Shield along Central Indian Tectonic Zone (CITZ) during the global Grenvillian orogeny.

Uranium mineralised and non-mineralised dolomites from Vempalle

The basic dykes in Mikir Hills have gabbroic composition. The prominent trends of these dykes are E-W in Southern part of Mikir Hills and NE-SW trend in the Northern Part of Mikir Hills. The geochemical characterization of dyke rock shows that they are sub-alkaline to alkaline basalt in nature, medium to high potassic, high titanium, Fe enriched tholeiites. The evolved nature of magma is represented by low Mg# 39.34 to 49.76 and are characterized by relatively low abundances of MgO, Cr, Ni, Zr, Y and high Rb, Ba, Th, Nb, indicating the within plate basalt (WPB) and continental rift basalt (CRB) chemical affinity. This dyke activity may be related to the extensional tectonics related to Proterozoic rifting hence, showing characteristics of continental rift basalt (CRB) type. The nine samples of gabbro define a five-point Rb-Sr isochron age of 1200±67 Ma with 87Sr/86Sr ratio of 0.70477±0.00042 with MSWD of 0.85. The Sm-Nd model ages (TDM) varies from 1600 to 2018Ma (εNd +0.11 to -2.04) indicates that the protolith from which these gabbroid rocks are derived is mostly Palaeo- to Meso-Proterozoic in age. The isotopic evolution of initial Sr, Nd and Pb reveals crustal contamination.