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

P. Rama Rao ¹, A. Narasinga Rao ²

Affiliations
1 National Geophysical Research Institute, Hyderabad, IN
2 Andhra University, Waltair, IN

Abstract

Hornblendic rocks (hornblende schists, amphibolites, and hornblendites) occur in intimate association with metasedimentary schists and granitic rocks in the Kanigiri Gogulapalle area of Andhra Pradesh, India. Field studies indicate that a majority of these rocks are possibly concordant intrusives, some of them being metasedimentary. Petrographic study shows that most of them consist of hornblende and plagioclase as essential minerals, whereas some are characterized by tremolite and plagioclase with accessory calcic minerals. Absence of detrital minerals and pyroxenic cores in the amphiboles characterize all the rocks.

Geochemical studies reveal that the rocks are most probably magmatic intrusives judged from the low Niggli k and high TiO₂ values and the presence of high Sr, Cr, V, and Ni amongst the trace elements.

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S. M. Naqvi ¹, B. L. Narayana ¹, P. Rama Rao ¹, S. M. Ahmad ¹, B. Uday Raj ¹

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

Abstract

The lowermost member of the Javanahalli schist belt consists of meta-arkoses/ paragneisses with interbedded epidote-quartzite and calc-silicate bands. These meta-arkoses exhibit relict current-bedding, arkosic texture and grain-to-grain variation in anorthite content within the same sample. Plagioclase is subordinate to K-feldspar and is relatively more weathered and sericitized. Orthoclase predominates over microcline in the total K-feldspar content and the quartz/feldspar ratio is generally very low. The meta-arkoses appear to be first-cycle sediments.

The composition of these paragneisses is unusual for Archaean metasediments. They are enriched in K, Si, Fe, Mg, Th, Rb, Ni, Cr and Co and depleted in Na, AI, Sr and often in Ca. The meta-arkoses of Javanahalli schist belt may be the product of strong and/or rapid chemical disintegration of Archaean tonalitic gneisses and basic and ultrabasic rocks. all of which were prominent components of the continental crust at the time the Javanahalli belt formed. If the meta-arkoses have not undergone major metasomatism, then their high abundance of K and Rb may indicate intense chemical leaching of these tonalitic source areas. Most of the saIt of the primitive oceans might have been derived from this intense leaching of the ancient cratonic tonalite gneisses.

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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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P. Rama Rao ¹, V. Diyakara Rao ¹

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

Abstract

Hornblende, biotite and feldspar rich gneisses (HG, BG and FG respectively), in that order of formation are associated with the Krishnarajpet schist belt in Dharwar craton, a greenstone belt coeval with, and similar to, the Holenarasipur and Nuggihalli schist belts. These gneisses show compositional differences with SiO₂, K₂O and Rb increasing and TiO₂, Al₂O₃, Fe₂O₃, CaO, MgO, Sr, Zr and Ni decreasing from HO to SO and FG. The overall REE is more in BG (ΣREE = 332.90), while it is similar in HG (ΣREE = 173.36) and FG (ΣREE = 159.03) with comparable LREE/HREE ratios (BG = 12.71; HG = 10.26; FG = 10.95). The overall chemistry of the three phases suggests a single source of origin for all the phases. While the field characteristics suggest successive origit from HG to BO and FG, the chemical signatures also support this mode of origin from HG to FG through BG. Available geochronological data on the gneisses indicate that probably the HG, a partial melt of the mafic schists of the greenstone belt, wns recycled at lower crustal levels in Late Archaean foming BG and FG. The time gap between the phases appears to be less but no detailed age data exist on the different phases to support this.

Keywords

Metamorphic Petrology, Krishnarajpet Belt, Dharwar Craton, Karnataka.

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V. Divakara Rao ¹, M. V. Subba Rao ¹, P. Rama Rao ¹, G. V. C. Pantulu ¹

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

Abstract

The homophanous coarse-grained granite near Perur, Chittoor district, Andhra Pradesh ranges in composition from granite (normative) to granite-quartzmonzonite (chemical) and records a Rb-Sr whole rock isochron age of 2549 ± 39 Ma. Variation observed in the major and trace element concentrations (SiO₂ = 67.09-73.83%; Al₂O₃ = 12.76-14.51%; total iron = 2.31-4.97%; MgO = 1.03-3.66%; Cr = 7-102 ppm; Rb = 169-223 ppm and Sr = 128-423 ppm) and the low CaO (less than 2%) in the granite suggest that it is a crustal remelting (CR) type granite formed by the anatectic remelting of an intermediate source rock. The high total REB (201.38-245.63 ppm), their well fractionated nature (La_N/Yb_N= 17.83-22.32) and the negative Eu anomaly of the granite support that the source rock was intermediate in composition having hydroxyl-bearing minerals like biotite and hornblende.

The Rb-Sr whole rock age (2459 ± 39 Ma) of the granite is similar to that of Closepet, Lepakshi and Hyderabad granites suggesting that this granite has formed along with the other K-rich granites by extensive crustal remelting during the Archaean-Proterozoic Transition (APT).

Keywords

Geochemistry, Granites, Chittoor District, Andhra Pradesh.

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P. Rama Rao ¹

Affiliations
1 H No C-2 & 3, Flat No 203, Raghavendra Golden Heights, Durgabhai Deshmukh Colony, Hyderabad - 500 007, IN

Abstract

No Abstract.

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P. Rama Rao ¹

Affiliations
1 H.No. 5-62/6, St. No.8, Habsiguda Hyderabad - 500 007, IN

Abstract

No Abstract.

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P. Rama Rao ¹

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

Abstract

No Abstract.

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T. Ramamohana Rao , P. Rama Rao

Abstract

No Abstract.

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