Geochronological (Rb-Sr and Sm-Nd) Studies on Intrusive Gabbros and Dolerite Dykes from parts of Northern and Central Indian Cratons: Implications for the Age of Onset of Sedimentation in Bijawar and Chattisgarh Basins and Uranium Mineralisation

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.

New 1.9-2.0 Ga, Pb-Pb (PbSL), Age of Dolomites from Vempalle Formation, Lower Cuddapah Supergroup, Eastern Dharwar Craton, India

“…Such older age is being reported for the first time from within the Cuddapah basin. Similar ages for lower Gwalior and Bijawar group of rocks were reported earlier (Absar et al, 2010 andPandey et al, 2012). These ages support the fact that most of the Purana basins as opined earlier as of Neo to Mesoproterozoic age are now considered as minimum 500 Ma older and are of Meso to Paleoproterozoic age.…”

Palaeoproterozoic Mafic magmatism in the Indian Shield: Petrologic, Geochemical and Thermal constraints on Proterozoic Mantle

“…Their 143 Nd/ 144 Nd i ratios vary between 0.5097 and 0.5102; εNd values correspondingly range from +1 to -10. Sr and Nd isotopic values of similar range are also reported from 1.88 Ga BD2 dykes in Bastar craton (Srivastava et al, 2009) and Bijawar igneous units (Pandey et al, 2012). In a Sr-Nd isotope plot (figure not shown), the Proterozoic mafic samples strikingly differ from the mantle components defined by Hart and Zindler (1989).…”

Geodynamic Significance of the Updated Statherian-Calymmian (at C. 1.65 and 1.46 Ga) Palaeomagnetic Results from Mafic Dykes of the Indian Shield

“…Assuming a similar polar wander rates during the 1.86-1. 46 Ga interval, a c. 1.65 Ga age is tentatively estimated for the steep 4 group of magnetizations. Alternatively, a c. 2.2 Ga age could be assigned to this pole because it falls between 1.86 and 2.37 Ga poles.…”

“…Further, Rb-Sr isotope study yielded a poorly defined regression line corresponding to 1656  22 Ma on one of the Bundelkhand dykes 24 . Recently, an internal isochron age of 1641  120 Ma was obtained 46 between three mineral fractions and whole rock of a dolerite in the central Indian craton. In situ 40 Ar/ 39 Ar analyses of the dykes in the Bundelkhand craton exhibit a cluster at about 1.65 Ga also 47 , while some of the ages are concentrated near a possible ~1.99 Ga emplacement age.…”