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

Saumitra Misra ¹

Affiliations
1 Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur - 721 302

Abstract

The Precambrian granite-Greenstone terrain of eastern Indian shield includes Singhbhum-Orissa craton, Singhbhum Mobile Belt supracrustals (Singhbhum Group) along its northern, eastern and western margins, and Chotanagpur Gneissic Complex towards further north. Recent isotopic ages along with other geological considerations show that these three crustal units perhaps constitute a single cratonic block, which grew in sequence from -3 55 to 1 00 Ga. The center of crustal growth gradually migrated with younging from the present south to north. Prior to the onset of this crustal growth, there was a still older crust in this region at -3 60 Ga, the remnants of which are preserved only as detrital zircons at present. However, the Singhbhum-Orissa craton represented the nucleus of the presently existing cratonic block, which had formed between -3 55 and 3 12 Ga through two successive supracrustal-granite cycles, well separated by an erosional unconformity. The first cycle included Older Metamorphic Group supracrustals, Older Metamorphic Tonalite Gneiss and Singhbhum Granite, phase-I, Singhbhum Granite, phases II, Chakradharpur Granite Gneiss and Nilgiri Granite that grew in sequence from -3 55 to 3 30 Ga. The second cycle included Iron Ore Group supracrustals followed by emplacement of Bonai Granite and Singhbhum Granite, phase III, and had an evolutionary history ranging from -3 30 to 3 16 Ga or up to -3 12 Ga. The supracrustal-Granite cycles began with formation of supracrustal sequences in anorogenic setting, followed successively by major folding during following orogeny and final emplacement of orogenic granites into these deformed supracrustals. The emplacement of anorogenic Mayurbhanj Granite pluton with Mayurbhanj Gabbro along the northeastern margin of the craton at -3 09 Ga marked the stabilization of Singhbhum-Orissa craton. Emplacement of Singhbhum Granite, phase II, Singhbhum Granite, phase III and Mayurbhanj Gabbro recorded metamorphism on the older rock units of Singhbhum-Orissa craton. In the late Meso- to Neoarchaean period Singhbhum-Orissa craton grew along its northern, western and eastern margins by formation of a supracrustal sequence of syn-Rift nature including mostly clastic sediments and minor basic and acid volcanics, which followed upward by major basic volcanisms. These supracrustals included Singhbhum Group, which unconformably overlay the Singhbhum Granite, phase-III and equivalent granite basements and was subjected to two major phases of folding, before being overlain by undeformed. Simlipal volcano-Sedimentary basin occurring at the eastern part of the craton. The evolution of Singhbhum Group and Simlipal basin took place between -3 12 and 3 09 Ga, before being intruded by anorogenic Mayurbhanj Granite( -3 09 Ga) and was followed by the formation of overlying riftogenic Dalma Group and adjacent Dhanjori Group of basic volcanics and some minor acid plutonics at the marginal part of the craton at -2 80 Ga and their subsequent major metamorphism at -2 50 Ga. A critical review of present available geological and radiometric age data suggests that Chotanagpur Gneissic Complex was evolved from a supracrustal precursor, some of them were formed at least before -2 3 Ga. The Singhbhum Group supracrustals and adjacent Chotanagpur Gneissic Complex perhaps had evolved from a common sedimentary precursor along with volcanics, which were deposited in a marine basin that existed at the present north of Singhbhum-Orissa craton during late Mesoarchaean period. The syn-rift sedimentary assemblage of the Singhbhum Group was deposited along the margin of Singhbhum Orissa craton, whereas the post-rift stable shelf sedimentary precursor of the Chotanagpur Gneissic Complex was deposited towards further north. In the Neoarchaean period between - >3 09 Ga and -2 5 Ga the nature of deformation along the marginal part of Singhbhum-Orissa craton was extensional type. In the following Palaeo- to Mesoproterozoic period, the protolith of Chotanagpur Gneissic Complex grew in sequence between - > 2 3 and 1 0 Ga through two time-Separated supracrustal granite cycles where the first and second cycles ended at -1 60 and 1 00 Ga respectively. Radiometric ages suggest that Chotanagpur Gneissic Complex along with Singhbhum Group experienced major magmato-metamorphic activities during Palaeo- and Mesoproterozoic periods at -2 5 or >2 3 Ga, 1 6 Ga, 1 0 Ga and 0 9 Ga. During this period Singhbhum-Orissa craton recorded basic volcanism (Jagannathpur and possibly Malangtoli Volcanics) at -2.25 Ga, followed by deposition of clastic sediments (Kolhan Group) and finally emplacement of mafic dykes and sills (Newer Dolerite) in three successive sequences at -2.0 Ga, 1.6 Ga and 1.0 Ga, where all these phenomena took place under anorogenic setting. The Singhbhum Shear Zone, which marked a tectonic boundary between Singhbhum-Orissa craton and Singhbhum Mobile Belt to north, was reactivated several times during geological past, the oldest being at -3.09 Ga, followed by emplacement of Soda Granite pluton along this shear zone at -2.22 Ga, copper mineralization at -1.77 Ga, shearing at - 1.67-1.63 Ga, uranium mineralisation at -1.58- 1.48 Ga and the final phase of shearing at - 1.0 Ga.

Keywords

Precambrian Chronostratigraphic Evolution, Singhbhum Mobile Belt, Singhbhum Shear Zone, Chotanagpur, Gneissic Complex, Singhbhum-Orissa Craton.

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Saumitra Misra ¹, Somnath Dey ²

Affiliations
1 Geological Studies Unit, Indian Statistical Institute, Kolkata - 700 035, IN
2 Department of Geology, Presidency College, Kolkata - 700 073, IN

Abstract

A group of small isolated domical to phacolithic granite plutons of 2 to 5 km in diameter have intruded the metasedimentary rocks of Bihar mica belt (BMB), representing north-eastern extremity of the Satpura orogenic belt of central and eastern India. The metasedimentary rocks of BMB rest on the Chotanagpur gneissic complex (CGC) with an erosional base and have recorded three phases of folding (F₁, F₂ and F₃), where the first two phases are the most dominant. A re-examination of structural, petrological and geochemical data, as well as study on new Rb-Sr whole rock isotopic ages of the BMB granite plutons, suggest that they are post-orogenic in nature in relation to the evolution of the basement (Chotanagpur gneissic complex). These plutons were emplaced between the second phase of compressive deformation (F₂,) of the Satpura orogeny, and before the establishment of post-compressional extension, when the regional stress pattern was likely to be transpressional or transextensional and/or mildly extensional type. The new Rb-Sr whole rock ages provided by Geological Survey of India suggest that the BMB plutons were formed between - 1.2 and 1.0 Ga. Though the parent magma of the plutons was earlier suggested to have been generated by partial melting of the tonalitcgranodiorite component of the Chotanagpur gneissic complex, a subordinate amount of BMB metasedimentary country rock is found to be involved in the genesis of the plutons. The pressure and temperature of emplacement as well as formation of a large number of BMB granite plutons is estimated around 5 kbar and slightly above 1000°C in a relatively anhydrous condition. The pegmatites intruding the BMB metasedimentary rocks are found to be emplaced either before F₁, folding or after F₂ folding of the host metasediments and are not strictly time correlative with the emplacement of the BMB granite plutons. It appears that the necessary fluids for the formation of pegmatites were generated by muscovite dehydration reaction(s) of the metasedimentary country rocks during their metamorphism.

Keywords

Post-Orogenic Granite Plutons, Bihar Mica Belt, Chotanagpur Gneissic Complex, Satpura Orogeny.

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N. Sulekha Rao ¹, Saumitra Misra ²

Affiliations
1 Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur - 721 302, IN
2 School of Geological Sciences, University of Kwazulu-Natal, Durban 4000, ZA

Abstract

This paper intends to explore whether there is an important source for monazite beach placer of the Gopalpur- Chhatrapur-Rushikulya coast, Orissa, in the adjacent Eastern Ghat Mobile Belt (EGMB). Petrographic and mineralogical studies were conducted on all the rock types constituting the EGMB exposed over a stretch extended up to ~20 km landward from the estuary of the River Rushikulya that is believed to transport the major bulk of sand to the Gopalpur- Chhatrapur-Rushikulya beach. Heavy mineral population was concentrated using bromoform and percentages of each heavy mineral constituting the population were estimated for all the potential source rock types. Isodynamic separation and XRD techniques were deployed for precision identification of every heavy mineral present. The study identified the granitoid (or migmatite) basement rock as by far the major contributor of monazite to the Chhatrapur beach sand. The study also reveals that charnockite is an important contributor of orthopyroxene as well as garnet, although the sillimanitegarnet- quartz schist (khondalites) is also an important source for the latter. On the other hand, garnet-quartz schist and garnet-biotite-quartz schist may also contribute substantial quantity of pyroxene and garnet. The high grade metasedimentary rocks, in general, could be the major sources for rutile, while ilmenite, magnetite and zircon in the beach sand have their sources perhaps in all the varieties of rocks constituting the EGMB.

Keywords

Gopalpur-Chhatrapur-Rushikulya Beach Placer, Monazite, Rushikulya River, Isodynamic Separation, XRD Analysis, Eastern Ghat Mobile Belt, Orissa.

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