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

C. Manikyamba ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad - 500 007, IN

Abstract

Granular iron formations (GIF) which are abundant in some Proterozoic, BIF bearing sedimentary basins are reported for the first time from the Archaean Sandur schist belt. These beds show lateral and vertical transition and repetition with oxide facies BIFs. Graded bedding and angular fragments are found in them. They consist mainly of iron and silicon oxides. Their trace clement and REE patterns itre similar to the BlFs from which they have been derived as a consequence of submarine erosion on the shelf through transgfession-regression cycles. Changes in sea level indicated by these structures were probably caused by volcanic activity and associated seismic tremors.

Keywords

Sedimentology, Granular Iron Formation, Sea Level Changes, Archaean, Sandur Schist Belt, Karnataka.

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V. Balaram ¹, C. Manikyamba ¹, S. N. Charan ¹, M. satyanarayanan ¹

Affiliations
1 National Geophysical Research Institute, Uppal Road, Hyderabad-500007, IN

Abstract

No Abstract.

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G. Lakshminarayana ¹, C. Manikyamba ², Tarun C. Khanna ², Prachiti P. Kanakdande ², K. Rajub ³

Affiliations
1 Geological Survey of India, Bandlaguda, Hyderabad - 500 068, IN
2 National Geophysical Research Institute (Council of Scientific and Industrial Research), Uppal Road, Hyderabad - 500 606, IN
3 National Geophysical Research Institute (Council of Scientific and Industrial Research), Uppal Road, Hyderabad - 500 606, IN

Abstract

The Rajahmundry Traps of the Krishna Godavari Basin (K-G Basin) consist of three distinct basalt flows interbedded with two intertrappean sedimentary horizons, which in turn are underlain by the late Cretaceous fossiliferous limestone bed (infratrappean) and overlain by the Cenozoic Rajahmundry Formation (Conglomerate/Sandstone). Among the three, the lower flow is characterized by the presence of the physical volcanological features such as ischolar_mainless cones, tumuli and dyke like forms along with single to multitier columnar and radial jointing. The middle and upper flows are simple, massive and vesicular and exhibit spheroidal weathering. Physical volcanological features and lithological attributes indicate that the lower flow was formed by an explosive volcanic activity in hydrous environment, followed by sub aerial eruption to form the middle and upper flows. The fossiliferous limestone bed is a representative horizon for the K-T boundary mass extinction caused due to intense volcanism. Intertrappean sediments exhibit weathered soil profiles (palaeosols) with limestone beds denoting a distinct time gap during various phases of lava eruption. Evaluation of the palaeogeographic scenario of the Krishna and Godavari Rivers does not provide any evidence for the existence of Cretaceous palaeovalley which would have provided pathway for lava transportation from the Deccan volcanic province of western India to the K-G Basin situated along the east coast. The present study opens up an alternative approach to explain the origin of basalt flows at Rajahmundry. In all probability the lavas could be intrabasinal. NW-SE and NESW faults or their intersection zones are probable pathways for lava eruption in the K-G Basin.

Keywords

Rajahmundry Traps, Intertrappeans, Infratrappeans, K/T Boundary, Krishna-Godavari Basin.

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C. Manikyamba ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad - 500 007, IN

Abstract

Mixed oxide-silicate facies banded iron formations are found as thin bands in the Eastern Volcanic Block of the Sandur schist belt They consist of haematite, magnetite, cummingtonite-grunerite and chert/quartz. Chemical constituents are SiO₂, Fe₂O₃ and FeO with no appreciable concentration of any other oxide. This composition is reflected in their trace and REE data, such as high order depletion in geochemical twins like Zr-Hf, V-Sc and Nb-Ta. The REE patterns are highly depleted (ΣREE 0.96 to 9.98 ppm) with Nd/Yb varying between 2 and 5. Cherty BIFs from this belt, metamorphosed to lowest greenschist facies. show almost identical geochemistry. The available data from this belt and other places indicate that unless impurities from terrigenous and volcaniclastic sources are present, metamorphism from lowest greenschist facies to granulite facies does not make an appreciable change in their trace and REB constituents.

Keywords

Geochemistry, Petrology, BIF, Sandur Schist Belt, Karnataka.

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C. Manikyamba ¹

Affiliations
1 National Geophysical Research Institute, Uppal Road, Hyderabad - 500 007, IN

Abstract

No Abstract.

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S. M. Naqvi ¹, C. Manikyamba ¹, T. Gnaneshwar Rao ¹, D. V. Subba Rao ¹, M. Ram Mohan ¹, D. Srinivasa Sarma ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad - 500 007, IN

Abstract

Metavolcanics of 20 km wide Sultanpura volcanic block of the Neoarchaean Sandur (greenstone) schist belt are divided into tholeiitic basalts, high Mg basalts, Al-depleted and Al-undepleted komatiitic ultramatic schists. Metabasalts are metamorphosed to amphibolite facies, but still preserve their pillow structures. Ultramasic komatiitic rocks are transformed to actinolite-tremolitc schists with no recognizable original textures or mineralogy. Mctabasalts and ultramafic komatiitic schists are interbedded with thin layers of sulphidic banded iron formations, argillaceous carbonate rocks and carbon phyllites that are interprcied as metamorphosed pclagic sediments of the deep ocean. No terrigenous sediments are found in Sultanpura block indicating that eruption of these submarine volcanic rocks took place in the deeper part of the ocean, away from the western and eastern shelf parts of the Sandur belt, where terrigenous sediments are abundant. SuItanpura block in its west and east is discordantly boundcd by thrust, subduction complex and shclf lhcies sedimcnts. These observations are interpreted to indicate that Sultanpura block is a telescoped prolo-oceanic part between the two shelves and island arc complexes. MgO of melavolcanic rocks varies from 6 to 30%, with a gap between 16 to 22%. Al₂O₃/TiO₂ shows characteristic variation for tholeiites (10-15), high Mg basalts (13-21), Al-unclepleted ultramafic komatiitic schists (9-23) and Al-depleted ultramafic komatiitic schists (11-20). CaO/Al₂O₃, of tholciites and high Mg basalts is ∼1, whereas for the ultramafic komatiitic schists, this ratio exhibits a range between 0.5 to 2, as a consequence of CaO mobility.

REE, HFSE and ¹⁴³Nd/¹⁴⁴Nda ta from Sultanpura volcanic rocks vary between CHUR (Chondrite Uniform Rcservoir), primitive mantle and depleted manilc, but appear to be derived from primitive mantle and have been probably contaminated by continental crust. Although the abundance of REE varics from 2 to 12 chondrite, the patterns are smooth and flat with small negative or positive Eu anomalies as artifacts of alteration. Generally positive, but in few samples negative Nb anomalies are also found, with (Ce/Sm)_N, and (Gd/Yb)_N, being near chondritic. Ti/V, Ti/Zr, Zr/Y, Sc/Y, Nb/La, Nb/Th, Nb/U, MgO/TiO₂, MgO/FeO and Al₂O₃/TiO₂, also for many samples are ncar chondritic, εNd=+0.8649±0.00024 resembling CHUR. ThMb, NblU and some other ratios are near to those of Ontong Java and Gorgana plateaus (0.80 Ga) and the tholeiite-komatiitic sequence found in 2.7 Ga Southern volcanic zone, Abitibi belt of Canada and 2.1 Ga Birimian belt of West Africa. Collectively, thcsc data indicate that a mantle plume. derived from an enriched mantle, possibly played an important role in the oceanic volcanic sequence of Sultanpura block. Some of the HFSE follow the olivine control line, whereas other elements following the olivine control line define a narrow array tube. Formation of such array tubes on the plots of some HFSE elements and their ratios, and the scatters of HFSE/REE ratios, probably suggest dynamic melting of the plume during ascent. Entrenchment, mixing of Archaean ocean ridge basalts (AORB), crustal contamination and subduction of such a plume-fed slab may have generated the compositional heterogeneities observed in the Sultanpura metavolcanic rocks.

Keywords

Archaean Plumes, Greenstone Belts, Geochemistry, Tectonic Evolution, Sultanpura Volcanic Block, Sandur Schist Belt, Dharwar Craton.

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