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

B. L. Narayana ¹, D. V. Subba Rao ¹, P. K. Govil ¹

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

Abstract

High alumina, magnesium and iron-bearing sediments represented by cordierite-anthophyllite rocks occur in association with amphibolites. calc silicate rocks and banded magnetite quartzites, within the Javanahalli schist belt, Karnataka. Their chemical composition is anomalous. The abundance levels of elements like Fe, Mg, AI, Ni, Cr, Co, Rb and Sr indicate that probably they represent metamorphosed chemical sediments generated by volcanogenic exhalative activity.

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P. K. Govil ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad 500007, IN

Abstract

Major and minor oxides: SiO₂, Al₂O₃, TiO₂, CaO, MgO, Na₂O, K₂O, MnO, P₂O₅. Fe₂O₃(T) and a few selected trace elements Zr, Y, Sr, Rb, V, Ba, Cr. Ni, Co, Cu, Zn in the IWG standards AN-G. BE-N and MA-N have been determined relative to USGS geochemical reference samples and synthetic standards by using a PW1400 XRF analyser with P851 computer system in an attempt to get usable values for the standards.

Samples prepared in duplicate were repeatedly (ten times) analysed to check the reproducibility of the data which is within the range of ± 1.0 % of reported values for all the major and minor elements, ± 5% for trace elements and ± 10% for the trace elements at concentrations below 10 ppm. The average values are found to be in good agreement with the proposed values. The results on MA-N granite were found to be very reproducible and MA-N may be used as a replacement of USGS-Gl or G2 which are no longer available.

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P. K. Govil ¹, S. M. Ahmad ¹

Affiliations
1 National Geophysical Research Institute, Hyderahad 500 007, IN

Abstract

As a part of standard rock sample preparation program, NGRI has prepared a standard rock sample of granite for chemical analysis of granite rock samples using instrumental methods of analysis and to calibrate the instruments. International granite rock standards prepared by United States Geological Survey are no available and, thererore, it was felt necessary to prepare our own standards for different rock types.

The sample is homogeneous and gives reproducible values for all the major and minor oxides and ten trace elements. Replicate analysis at NGRI using XRF, AAS, Spectrophotometer and at other laboratories in USA and West Germany have given confident results. Proposed values for nine oxides and seven trace elements are given in this paper and the sample is offered to be used as a geochemical reference standard for granite samples.

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P. Rama Rao ¹, S. M. Naqvi ¹, P. K. Govil ¹, V. Balaram ¹

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

Abstract

The Sigegudda trondhjemites contain moderate Zr, high Ni and Cr and therefore appear to have formed by partial melting and/or fractional crystallization of Ni- and Cr-rich rocks like amphibolites. Lack of fractionation of Y, Zr and TiO₂ and comparatively lower LREE/HREE ratio in these rocks perhaps indicate that garnet was not present in the magma source. The trondhjcmites were probably emplaced during the stabilization of the western Dharwar craton around 3.0 b.y. ago.

Keywords

Trondhjemites, Geochemistry, Sigcgudda, Karnataka.

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R. M. k. Khan ¹, P. K. Govil ¹, S. M. Naqvi ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad 500007, IN

Abstract

The Banded Iron Fonnation is interbedded with 2.9 Ga old mafic and acid volcanics. It mainly consists of magnetite and quartz; calcite is also present in some mesobands. Thin silicate rich layers with abundant actinolite, cummingtonite, grunerite, biotite, riebeckite and aegirine occur between the quartz and magnetite rich rhythmites. Mixed oxide. silicate and carbonate facies (MOSCF) BIFs are found which arc prcmctamorphic cherty and shaly types. The distribution of Al₂O₃ , CaO, MgO, K₂O, Na₂O, Ni, Cr, Co, Rb, Sr, Zr, Cr/MgO and REEs indicate a combination of hydrothenna1, volcanogenic and continental sources. The relationship between Cr/Zr, Zr/Cr+Co+Ni, ZeN and Nd/Yb reflect that certain fine silicate-rich bands between the microbands arc made up of volcanic ash, whereas others are made up of shales of continental source or a mixture of both. LA enrichment, depletion in total REE, positive Eu anomalies, Nd/Yb, La/Lu and LREE/HREE ratios in Fe-rhythmites indicate that Fe, Si and REEs of the BIF were supplied by hydrothennal and fumarolic activity accompanying submarine volcanism. The enrichment in total REE and obliteration of La signature is probably a consequence of mixing of non-chemogenic component and ambient sea water with the chemical precipitates. Absence of negative Ce anomalies is consistent with most of the Archaean BIFs of other localities. Deposition of BIF appears to have taken place below wave base and photic zone of the shallow shelf region. It is inferred that oxygen was generated at some other place in the basin. Ocean circulation and upwelling brought FeO and 0₂ together resulting in precipitation of iron along with fine grained argillites.

Keywords

Geochemistry, Banded Iron Formation, Kudremukh Schist Belt, Kamataka.

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D. S. N. Murthy ¹, A. M. Dayal ¹, R. Natarajan ¹, V. Balram ¹, P. K. Govil ¹

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

Abstract

Newly discovered kimberlite pipe 11 of Chigicherla area of Andhra Pradesh consists of two generations of olivine with phlogopite, spinel. perovskite, melilite etc., A comparative study of chemical characterstics of pipe 11 with those of pipe 1,2,3,4 and 5 of Wajrakarur- Lattavaram area and pipe 10 of Chighicherla indicate that Zr-Hf, Nb-Ta, Th-U have a high degree of correlation. The ratio of Ba/Nb( <10), Z/Hf( -50) and Nb/Zr (>0.5) are similar to the Group-I kimberlites. The chondri te normalised REE pattern of pipe 11 is similar to the other kimberlites and shows enrichment in LREE. Spider diagram of incompatible elements indicate similarity with those of many oceanic island alkalic basalts or other types of alkalic intraplate volcanics. They are highly enriched in incompatible elements. Major elements, Ce/Yb and La/Yb ratios suggest that the formation of kimberlite is due to low degree of partial melting of ultrabasic source along with an earlier subducted oceanic lithosphere.

Keywords

Petrology, Kimberlite, Phlogopite, Andhra Pradesh.

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