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

B. K. Mohapatra ¹, R. Natarajan ², A. K. Paul ³, R. K. Sahoo ¹

Affiliations
1 Regional Research Laboratory, Bhubaneswar 751 013, IN
2 National Geophysical Research Institute, Hyderabad 500 007, IN
3 Department of Geology, Utkal University, Bhubaneswar 751 004, IN

Abstract

Minute discrete grains of rutile present in manganese ores of Dengura, Koira valley, Sundergarh district, exhibit imperfect columnar and crystallographic forms and an outstanding illustration of twinning. EPMA and EDAX results indicate the presence of Fe, Mn, Cr, Al, Ba, K and Ta, Nb in minor proportion, in rutile crystals. Some of these clements are either present in limited solid solution or later got introduced into its structure during late epigenetic processes. These are detrital grains, pre-existing in the host shale and later affected by solution during epigene manganese are formation.

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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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T. P. Srinivasan ¹, R. Natarajan ²

Affiliations
1 Department of Geology, Osmania University, Hyderabad 500 007, IN
2 National Geophysical Research Institute, Uppal, Hyderabad 500 039, IN

Abstract

The Fayalite syenites of Gokanakonda in Andhra Pradesh, India, are mineralogically divided into two types: (1) fayalite-clinopyroxene syenites (FC-syenites) and (2) fayalite-quartz syenites (FQ-syenites). The olivine composition ranges from Fa_90-95 and the petrochemistry suggests their successive evolution in that order. The assemblage (fayalite+magnetite+quartz) in these syenites suggests a pressure of 11.5 kb and a temperature of 900°C under constant fo_2.

Keywords

Petrology (Igneous), Fayalite-Syenite, Prakasarn District, Andhra Pradesh.

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P. Sivasubramanian ¹, R. Natarajan ², A. S. Janardhan ¹

Affiliations
1 Department of Geology, University of Mysore, Mysore 570006, IN
2 National Geophysical Research Institute, Hyderabad 500007, IN

Abstract

A new find of sapphirine-bearing granulite rocks from Perumalmalai area, 15 km from Kodaikanal is reported. These rocks, contain in addition to sapphirine, cordierite-garnet-orthopyroxene-spinel-biotite-plagioclase. Among the pairs, garnetcordierite gives pressure of 8.5 kb at temperature of 550°C; garnet-orthopyroxene gives 6 kb at 800°C and cordierite-garnet-sapphirine gives 7.2 kb at 700°C. The sapphirineforming reaction probably took place after the main charnockite event. Evidences of decompression of garnet breaking down to orthopyroxene-spinel-plagioclase-biotite at rather high temperatures indicates rapid uplift.

Keywords

Sapphirine, Perumalmalai, Palani Hills, Tamil Nadu, Geobarometry, Granulites.

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D. S. N. Murthy ¹, A. M. Dayal ¹, R. Natarajan ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad-7, IN

Abstract

Petrography and chemistry of newly discovered Chigicherla Kimberlite pipe of Andhra Pradesh, South India are presented. The textural characteristics of the rock suggest it to be more close to the hypabyssal type of kimberlite. Presence of melilite classifies these rocks as extreme variety of kimberlite. Mineralogy of the spinel harzburgite xenoliths that occur within these IGmberlite reveals two types of xenoliths:

Harzburgite xenoliths having spinel which is Cr rich (61.6%) and Al (10.29%) poor. Harzburgite xenolith having spinel which is A1 rich (4358%) and Cr (25.90%) poor.

Temperature estimates reveal that the Cr-rich type xenolith has equilibrated at 972°C while the Cr-poor ones were equilibrated at 839°C ± 31°C. The maximum pressure estimated for these xenoliths is 25 kb. Presence of chrome spinel and absence of magnesium aluminium spinel in the xenoliths, indicate that the uppermost 45 or 50 km of the mantle section has highly "depleted" peridotite composition. The study supports the contention of the steady state of thermal condition in the mantle lithosphere beneath the continental shield and has not changed appreciably through the last one billion years.

Keywords

Kimberlite, Geothermometty, Anantapur District, Andhra Pradesh.

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V. Madhavan ¹, J. Mallikharjuna Rao ², M. Srinivas ³, R. Natarajan ², Ausaf Sayeed ³

Affiliations
1 Geology Department, Kakatiya University, Warangal - 506 009, IN
2 National Geophysical Research Institute, Hyderabad - 500 007, IN
3 Geology Department, Osmania University, Hyderabad - 500 007, IN

Abstract

Rare bodies of syenites of alkaline or subalkaline character occur as intrusions in the rocks of the Cuddapah Supergroup within the Cuddapah basin. The alkali syenites (found at Racherla, Idamakallu and Giddalur) are mainly composed of riebeckite (arfvedsonite), ilmenite, orthoclase and the subalkaline syenites (located at Gundlapalle) comprise hedenbergite, and microcline-perthite. Accessory quartz is invariably present in both the syenites. These intrusive bodies display contrasting mineralogical and geochemical characteristics. The quartz- and hypersthenenormative subalkaline syenite has the attributes of a ferrosyenite and has been subjected to hydrothermal alteration which not only resulted in the formation of secondary minerals like nontronite, amphibole, biotite and magnetite- all at the expense of hedenbcrgite, but also partly converted the Fe⁺² into Fe ⁺³. These subalkaline syenites are characterized by an extreme paucity of Mg, has low to moderate contents of Ba, Sr, Rb, Zr and REE. The quartz, hypersthene and acmite- normative alkali syenites (lusitanite) on the other hand are abnormally enriched in Ti; Rb is low but Ba, Sr, Ce, Nb, Nd, Ta, Sm, La, Zr am also the ratios K/Rb, Ti/Zr, Ce/Y and HREE/LREE are conspicuously high.

The syenites of the Cuddapah basin, seem to have crystallized under contrasting conditions. While the alkali syenites crystallized under high PH₂O, the crystallization of subalkaline syenite took place under dry arid reducing conditions signifying a closed system. It is envisaged that the subalkaline ferrosyenite of Gundlapalle was formed by low-pressure crystal fractionation of a continental tholeiitic basaltic magma. The alkali syenites, on the other hand, are postulated to have crystallized from a mantle-derived, hydrous, alkaline magma enriched in Large Ion Lithophile Elements and Light Rare Earth Elements.

Keywords

Syenites, Igneous Petrology, Cuddapah Basin, Andhra Pradesh.

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J. Ratnakar ¹, C. Leelanandam ², R. Natarajan ³

Affiliations
1 Department of Geology, Post-Graduate College of Science, Osmania University, Safiabad, Hyderabad-500 004, IN
2 Department of Geology, Osmania University, Hyderabad-500 007, IN
3 National Geophysical Research Institute, Hyderabad-500 007, IN

Abstract

The electron microprobe analysis of olivines from the ferrosyenites of Sivamalai shows that they are extremely rich in iron with Fa_96.4-Fa_93.9. The restricted range of Fe²⁺:Mg:Mn values and high Fe²⁺/Mg ratios amongst the analysed samples suggest that parental liquids underwent high degree of fractionation. Association of fayalitic olivine, orthopyroxene, inverted pigeonite and quartz in the ferrosyenites of Sivamalai strongly suggests their tholeiitic nature.

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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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