Abhinaba Roy

Evolution of the Bhandara-Balaghat granulite belt along the southern margin of the Sausar Mobile Belt of central India

The Bhandara-Balaghat granulite (BBG) belt occurs as a 190 km long, detached narrow, linear, NE-SW to ENE-WSW trending belt that is in tectonic contact on its northern margin with the Sausar Group of rocks and is bordered by the Sakoli fold belt in the south. The Bhandara part of the BBG belt is quite restricted, comprising a medium to coarse grained two-pyroxene granulite body that is of gabbroic composition and preserves relic igneous fabric. The main part of the belt in Arjuni-Balaghat section includes metasedimentary (quartzite, BIF, Al- and Mg-Al metapelites) and metaigneous (metaultramafic, amphibolite and two-pyroxene granulite) protoliths interbanded with charnockite and charnockitic gneiss. These rocks, occurring as small bands and enclaves within migmatitic and granitic gneisses, show polyphase deformation and metamorphism. Geochemically, basic compositions show tholeiitic trend without Fe-enrichment, non-komatitic nature, continental affinity and show evolved nature. Mineral parageneses and reaction textures in different rock compositions indicate early prograde, dehydration melt forming reactions followed by orthopyroxene stability with or without melt. Coronitic and symplectitic garnets have formed over earlier minerals indicating onset of retrograde IBC path. Evidences for high temperature ductile shearing are preserved at places. Retrogressive hydration events clearly post-date the above paths. The present study has shown that the BBG belt may form a part of the Bastar Craton and does not represent exhumed oceanic crust of the Bundelkhand Craton. It is further shown that rocks of the BBG belt have undergone an earlier high-grade granulite metamorphism at 2672 ± 54 Ma (Sm-Nd age) and a post-peak granulite metamorphism at 1416 ± 59Ma (Sm-Nd age, 1380 ± 28Ma Rb-Sr age). These events were followed by deposition of the Sausar supracrustals and Neoproterozoic Sausar orogeny between 973 ± 63Ma and 800 ± 16Ma (Rb-Sr ages).

Late Archaean mantle metasomatism below eastern Indian craton: Evidence from trace elements, REE geochemistry and Sr—Nd—O isotope systematics of ultramafic dykes

Trace, rare earth elements (REE), Rb-Sr, Sm-Nd and O isotope studies have been carried out on ultramafic (harzburgite and lherzolite) dykes belonging to the newer dolerite dyke swarms of eastern Indian craton. The dyke swarms were earlier considered to be the youngest mafic magmatic activity in this region having ages not older than middle to late Proterozoic. The study indicates that the ultramafic members of these swarms are in fact of late Archaean age (Rb-Sr isochron age 2613 ± 177 Ma, Sri ∼ 0.702 ± 0.004) which attests that out of all the cratonic blocks of India, eastern Indian craton experienced earliest stabilization event. Primitive mantle normalized trace element plots of these dykes display enrichment in large ion lithophile elements (LILE), pronounced Ba, Nb and Sr depletions but very high concentrations of Cr and Ni. Chondrite normalised REE plots exhibit light REE (LREE) enrichment with nearly flat heavy REE (HREE; (ΣHREE)N ∼ 2–3 times chondrite, (Gd/Yb)N ∼ 1). The εNd(t) values vary from +1.23 to -3.27 whereas δ18O values vary from +3.16‰ to +5.29‰ (average +3.97‰±0.75‰) which is lighter than the average mantle value. Isotopic, trace and REE data together indicate that during 2.6 Ga the nearly primitive mantle below the eastern Indian Craton was metasomatised by the fluid (± silicate melt) coming out from the subducting early crust resulting in LILE and LREE enriched, Nb depleted, variable εNd, low Sri(0.702) and low δ18O bearing EMI type mantle. Magmatic blobs of this metasomatised mantle were subsequently emplaced in deeper levels of the granitic crust which possibly originated due to the same thermal pulse.

Low Pressure Metamorphism, Deformation and Syntectonic Granite Emplacement in the Palaeoproterozoic Mahakoshal Supracrustal Belt, Central India

Porphyroblast assemblage of andalusite, garnet, staurolite and cordierite in the metasedimentary rocks of the Palaeoproterozoic Mahakoshal supracrustal belt suggests low pressure/medium temperature metamorphism. These porphyroblasts exhibit both dynamic (syn-kinematic) and static (post-kinematic) growth. Field and petrographic studies indicate that the growth of andalusite and garnet porphyroblasts initiated during the early stages of D2 (S2 cleavage) deformation and outlasted it. The staurolite porphyroblasts appeared during the late stages of D2 deformation and continued after it. Restricted occurrence of these syn-kinematic porphyroblasts to the vicinity of the shear zone suggest that intense deformation and repetitive cleavage development, apart from the elevated temperature conditions, enhanced the growth of the porphyroblasts. The cordierite porphyroblasts, whose growth is controlled essentially by the lithological composition, exhibit static growth, post-dating D1 (S1 cleavage) deformation. The mineralogical assemblages in the metasedimentary rocks suggest a peak P and T around 3.5 kb and 550–600°C respectively, which was attained during the D2 deformation. The D2 deformation was coeval with the development of reverse-oblique slip ductile shear zone and emplacement of linear, syn-kinematic granites along the southern margin of the belt. This indicates a contractional tectonic regime, wherein rising granitic plutons caused advective heat transfer to the middle and upper crust and created relatively higher temperature conditions at lower pressure.

Palynostratigraphy and age correlation of subsurface strata within the sub-basins in Singrauli Gondwana Basin, India

In the study area, changes in the facies of sediments and spores-pollen content appear to be all causally linked with the depositional set-up. Here, the qualitative and quantitative changes observed in the spores-pollen assemblages have led to recognize 10 Assemblage-zones representing from that earliest Permian in the Talchir Formation to that latest Late Triassic in the Parsora Formation. These spores-pollen assemblages are obtained from the wider parts in the Singrauli Gondwana Basin that includes (i) Moher sub-basin (boreholes SSM-1 and 2), and (ii) Singrauli main sub-basin (boreholes SMJS-2, 3 and SMBS-1). The progressively changing spores-pollen content infer the hiatuses of varied magnitude in the sedimentary sequences during the extended time interval of Permian and Triassic.