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

V. Madhavan ¹, M. Srinivas ², J. Mallikharjurna Rao ³, N. V. Chalapathi Rao ⁴

Affiliations
1 Department of Geology, Kakatiya University, Warangal-506 009, IN
2 Department of Geology, Osmania University, Hyderabad-500 007, IN
3 National Geophysical Research Institute, Hyderabad-500007, IN
4 Department of Earth Sciences, Bullard Laboratories, Madingley Rise; Cambridge, CB2 3EQ, IN

Abstract

The paper presents the mineral analyses and the major, trace and rare earth element geochemistry of the subalkaline syenites from Vikurthi and Kotappakonda in the Guntur district of Andhra Pradesh. A comprehensive model is proposed wherein these and other similar rocks from the Cuddapah intrusive province are envisaged as the differentiated products of an intermediate magma of trachybasaltic composition, which, in turn, has been derived by the differentiation of a parental gabbro magma.

Keywords

Geochemistry, Syenites, Igneous Petrology, Guntur 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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V. Madhavan ¹, J Mallikharjuna Rao ², M. Srinivas ³

Affiliations
1 Department of Geology, Kakatiya University, Warangal-506009, IN
2 National Geophysical Research Institute, Hyderabad-500007, IN
3 Department of Geology, P.G. College of Science, Saifabad, Osmania University, Hyderabad-500004, IN

Abstract

In the Dharwar craton, the recently recognized Cuddapah intrusive province hosts an array of mid-Proterozoic alkaline and non-alkaline intrusives; the latter are represented by two manifestly voluminous magma types: gabbro and granitoid, which are marked by a bimodal association. Here we present the tectono-magmatic framework of the Cuddapah intrusive province and in the light of it, the petrology, geochemistry and genesis of alkaline rocks are discussed to highlight the mid-Proterozoic intraplate alkaline magmatism in the eastern Dharwar craton.

Keywords

Igneous Petrology, Alkaline Magmatism, Mid-Proterozoic, Cuddapah Province, Eastern Dharwar Craton.

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V. Madhavan ¹, K. David ¹, J. Mallikharjuna Rao ², N. V. Chalapathi Rao ³, M. Srinivas ⁴

Affiliations
1 Department of Geology, Kakatiya University, Warangal, A.P. - 506 009, IN
2 National Geophysical Research Institute, Hyderabad- 500 007, IN
3 Department of Applied Geochemistry, Osmania University, Hyderabad- 500 007, IN
4 Department of Geology, Osmania University, Hyderabad- 500 007, IN

Abstract

The mid-Proterozoic lamprophyric rocks broadly represented by kimberlites, lamproites and lamprophyres (sensu stricto) are emplaced in and adjacent to the intracratonic Cuddapah basin in the eastern Dharwar craton of India which was recently recognized as the Cuddapah (intrusive) province (CIP). While the kimberlite pipes and lamprophyre dykes are located outside but close to the western and eastern margins of the Cuddapah basin respectively, the lamproites are hosted within the basin. Lamprophyres occur in seven locations of the Prakasam district of Andhra Pradesh. EIchuru, where more than one hundred lamprophyre dykes are found, represents a prominent centre of lamprophyre magmatism. Purimetla comes next to Elchuru with a tally of twenty lamprophyre dykes. At other places lamprophyres are numerically insignificant. The CIP lamprophyres encompass different varieties and they cut across an equally wide-ranging variety of host rocks; these lamprophyres come under 1) feldspathoidal or non- feldspathoidal and 2) plagioclase- or orthoclase-bearing categories. With a single exception, all the lamprophyres are micaceous and distinctly alkaline. The genesis of CIP lamprophyres as a manifestation of alkaline magmatism is discussed.

Keywords

Petrology, Geochemistry, Lamprophyres, Cuddapah Province, Andhra Pradesh.

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