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

U. K. Pandey ¹, Tikam Chabria ¹, P. Krishnamurthy ¹, R. Viswanathan ¹, B. Kumar ²

Affiliations
1 Atomic Minerals Directorate for Exploration and Research, Department of Atomic Energy, Begumpet, Hyderabad - 500 016, IN
2 National Geophysical Research Institute, Hyderabad - 500 007, IN

Abstract

Carbon isotope studies on calcite - graphite system yield narrow Δ Cat - Gr values, varying from 3.9 to 4.3, indicating equilibrium between coexisting calcite and graphite. Calcite - graphite geothermometry has assigned temperatures ranging from 630° to 700°C for the calc - granulites found to the NE of Usilampatti area. The inferred temperatures correspond to the retrograde temperatures obtained by cation exchange thermometry on the granulites from other areas in the Madurai Block. These temperatures may correspond to the late isobaric cooling event. X-ray diffraction studies indicate well ordered nature of these graphites and corroborate the findings of isotope geothermometric inferences.

Keywords

Geothermometry, Calc-Granulite, Calcite-Graphite, Carbon-Oxygen Isotopes, X-Ray Diffraction, Usilampatti, Tamil Nadu.

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S. M. Naqvi ¹, B. S. Venkatachala ², Manoj Shukla ², B. Kumar ¹, R. Natarajan ¹, Mukund Sharma ²

Affiliations
1 National Geophysical Research Institute, Hyderabad 500007, IN
2 Birbal Sahni lnstitute of Palaeobotany, Lucknow 226007, IN

Abstract

Silicified cyanobacterial remains are recorded from the black cherts interbedded in the Donimalai Formation of the Archaean Sandur Schist Belt and amorphous organic matter in the stromatolites underlying Deogiri Formation of the same belt. The presence of elemental carbon at the fringes of the cyanobacteria is confirmed by microprobe analysis. Carbon (δ¹³C= -1.4 ± 0.5% vs PDB) and oxygen (δ¹⁸O= + 10.6±0.5% vs SMOW) isotopic ratios and the presence of cyanobacteria extend the existence of photosynthetic activity into the Archaean.

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B. Kumar ¹, S. Das Sharma ¹, M. Shukla ², M. Sharma ²

Affiliations
1 National Geophysicai Research Institute, Uppal Road, Hyderabad - 500 007, IN
2 Birbal Sahni Institute of Palaeobotany, Lucknow - 226 007, IN

Abstract

Carbon isotope records on carbonates from the Bhima basin are characterized by ¹³C enriched values (up to 4‰ PDB). These data are integrated with the available palaeontological records to suggest a Terminal Proterozoic (post - Varanger) age for the Bhima sediments. The carbon isotope compositions of Katamadevarhalli and Shahabad carbonates analysed in the present study exhibit two distinct trends. While the Katamadevarhalli carbonates show a positive trend, those from Shahabod display a negative trend from bottom to top, probably indicating long-term temporal trend of carbon isotope values.

Keywords

Chronostratigraphy, Carbonates, Carbon and Oxygen Isotopes, Bhima Basin, Terminal Proterozoic, Karnataka.

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B. Kumar ¹

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

Abstract

No Abstract.

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S. R. Sharma ¹, G. Parthasarathy ¹, T. R. Ravindran ², A. K. Arora ², B. Kumar ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad - 500 007, IN
2 Materials Science Division, Indira Gandhi Centre for Atomic Research, Kalpakkam- 603 102, IN

Abstract

We present here the results of Laser-Raman Spectroscopic studies on graphite samples from the Archaean metasedimentary rocks of Dharwar craton to correlate the structural parameters of graphite with the metamorphic grade of the host rocks. The present studies show that the first order Raman peak (O) of the graphite samples appears in the wave number range from 1581 to 1585 cm^-1; the second order Raman peak (S) appears in the wave number range from 2699 to 2721 cm^-1; and the disorder Raman (D) peak appears in the wave number range from 1349 to 1356 cm^-1. It is suggested that the intensity ratios of the Rarnan peaks (D/O)_a are the inverse function of in-plane crystallite size of the graphite. The observed values of intensity ratio (D/O)_h, varies from 0.095 to 0.591, (D/D+O)_a varies from 0.18 to 0.46 and (S/O)_h, varies from 0.21 to 0.32. This range of values indicates that the graphite samples are well crystallized. A correlation of (D/O) area ratios along with the in-plane crystallite size La (50 to 193 Å) and the metamorphic grade shows that the metamorphic grade of the host rocks of the studied samples lies in the range between upper chlorite zone and lower sillimanite zone. The estmated peak metamorphic temperatures of the host rocks are in the range of 575 to 625°C and they are in excellent agreement with temperatures obtained by the Powder X- ray diffraction studies and conventional geothermometry studies.

Keywords

Raman Spectroscopy, Graphite, Carbon Isotopes, Metamorphic Temperature, Dharwar Craton, Karnataka.

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S. R. Sharma ¹, G. Parthasarathy ¹, B. Kumar ¹

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

Abstract

We report here., the results of powder x-ray diffraction studies on six graphite samples from the Archaean metasedimentary rocks of Dharwar craton, with the motivation of correlating the graphitization process of carbonaceous matter during metamorphism with metamorphic grade. Our studies show that interplanar spacing d(002) and crystallinity L_c(002) for all the studied samples are very close to that of well crystallized graphite.

The observed decrease in full width at half maximum (FWHM) of the (002) diffraction peak from 0.20 to 0.102 Å. increase in crystallite size, L_c(002) from 408 to 800 Å and graphitizing degree from 86 to 362 indicate different metamorphic grades. Our results indicate that the rocks from Hampapura are of low metamorphic grade and those from Arjun Bettahalli are highly metamorphosed. The peak metamorphic temperatures of metasedimentary rocks are estimated to be around 600±50°C. The temperature estimates based on graphite crystallinity studies and the temperatures estimated by silicate mineral geothermometry studies of the coexisting mineral assemblages were found to be consistent.

Keywords

Mineralogy, Graphites, X-Ray Diffraction, Metamorphic Grade, Peak Metamorphic Temperatures, Dharwar Craton.

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D. J. Patil ¹, Das Sharma ¹, B. Kumar ¹, A. M. Dayal ¹, M. Shukla ²

Affiliations
1 National Geophysical Research Institute, Uppal Road, Hyderabad - 500 007, IN
2 Birbal Sahni Institute of Palaeobotany, Lucknow - 226 007, IN

Abstract

Carbon isotopic compositions of carbonate rocks from Narji and Koilkuntla Formations belonging to the Kurnool Group range frorn -1.5 to +2.2%0 and +3.1 to +4.3%0 respectively. 13C values on coexisting organic carbon are around -26%0. ⁸⁷Sr/⁸⁶Sr ratios of limestones from Koilkuntla Formation (0.7086 - 0.7104) exhibit less variation than those belonging to Narji limestones (0.7073 - 0.7132). Bivariant plots of ⁸⁷Sr/⁸⁶Sr and δ¹³C against Mn/Sr and δ¹⁸O show open system diagenetic trends for the carbonate rocks of Narji Formation. Similar plots for Koilkuntla Formation do not reveal any definite alteration trends. Comparison of carbon isotope data coupled with the least altered ⁸⁷Sr/⁸⁶Sr values of Narji and Koilkuntla limestones shows similarity in the isotopic ratios exhibited by other worldwide Late Neoproterozoic carbonate successions. This finding is corroborated by available age information on the underlying Cuddapah Supergroup of rocks as well as palaeontological evidences on the rock formations of Kurnool Group.

Keywords

Carbonate Rocks, Carbon and oxygen isotopes, ⁸⁷Sr/⁸⁶Sr, Kurnool Group, Andhra Pradesh.

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