International Journal of Earth Sciences and Engineering

R. Mallikarjuna Reddy ¹

Affiliations
1 Department of Geology, Kakatiya University, Warangal-506009, Telangana, IN

Abstract

The highly deformed eastern side of the Cuddapah basin led to the formation of numerous rifts, faults and major lineaments which in turn facilitated the Meso Proterozoic miaskitic alkaline magmatism. Co spatial association of mafic and felsic alkaline rocks within this zone is found in a row trending in NE-SW direction, all the plutons, constitute the Prakasam alkaline province later renamed as Cuddapah intrusive province. The above mentioned association appears pronouncedly at Elchuru, designated as Elchuru alkaline complex, where mafic and felsic (feldspathoidal) alkaline syenites i.e. shonkinite, malignite, nepheline syenite, lamprophyre are present in adequate representation to correlate the genesis of these rocks. Petrographically shonkinite shows the predominance of clinopyroxene, biotite and amphibole as mafics and on the other hand, nepheline and perthitic feldspar constitutes the felsics. This phenomenon is reverse in the case of nepheline syenites, which predominantly consists of subhedral nepheline along with the K-feldspar (perthitic) with subordinate aegirine augite, ferrohastingsite and discrete biotite. Predominance of mafics in the former and dearth of those in the later denote the fractionation of nepheline syenites from the shonkinitic magma. Geochemically, the percentages of FeO, MgO, and CaO are higher in shonkinite and low in nepheline syenite indicating the fractionational crystalisation process that was involved in the genesis of nepheline syenites. Subsequent increment of SiO₂, Na₂O and K₂O with the magmatic differentiation from the early formed shonkinites to later formed nepheline syenites is also noticed. Geochemical discrimination diagrams and the plots in the Petrogeny’s residua system at 1 Kb PH₂O clearly illustrates the evolution of nepheline syenites from the K-rich hydrous mafic magma. Enrichment of transition metals such as Cr, V, Ni, Zn and LFSE in the shonkinites and lack of them in the nepheline syenites further substantiate that the genesis of nepheline syenites is undoubtedly from an undersaturated shonkinitic magma at Elchuru alkaline complex.

Keywords

Cuddapah Intrusive Province, Elchuru Alkaline Complex, Fractional Crystallization, Shonkinite, Nepheline Syenite.

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R. Mallikarjuna Reddy ¹

Affiliations
1 Department of Geology, Kakatiya University, Warangal-506009, Telangana, IN

Abstract

The paper deals with the petrographic details of the subalkaline ferrosyenites which are emplaced in the Cuddapah intrusive province (CIP) of peninsular India. These leucocratic rare rock types (colour index ranges 7 to 19 vol%) are found emplaced at Gundlapalle and Gokanakonda in the Guntur district and at Uppalapadu in the Prakasam district of Andhra Pradesh. All the ferrosyenites show equigranular, hypidiomorphic texture but mineralogically they are different, even though they are uniformly subalkaline in character. Ferrohedenbergite and its altered product called nontronite are found in Gundlapalle ferrosyenite, but fayalite along with clinopyroxene are found in Gokanakonda and Uppalapadu ferrosyenite. Again, at Gokanakonda, a second type of ferrosyenite is found with fayalite along with quartz.

Keywords

Ferrosyenite, Hypidiomorphic Texture, Cuddapah Intrusive Province.

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K. Saikrishna ¹, R. Mallikarjuna Reddy ¹, D. Purushotham ², T. Brahmaiah ³

Affiliations
1 Department of Geology, Kakatiya University, Warangal-506009, Telangana, IN
2 National Geophysical Research Institute, Hyderabad-500007, IN
3 Department of Geology, Sri Venkateswara University, Tirupathi, IN

Abstract

Ferrosyenites, which are unique rock types, are emplaced in Cuddapah intrusive province of peninsular India. These rare rock types are found to emplace at Gundlapalle and Gokanakonda in the Guntur District and at Uppalapadu in the Prakasam District of Andhra Pradesh. These syenites show considerable variation in mineralogical and geochemical composition. For an example the Gundlapalle syenites are devoid of olivine whereas in the other plutons olivine is essentially present with or without quartz. Normatively, these syenites are characterised by quartz, hypersthene or quartz+hypersthene combination. There are no indications of alkaline character in these rocks. As the name of these rocks indicates Mg is found to depleted whereas Fe+2 is enriched due to the presence of ferrohedenbergite and fayalite. The rocks are derived from anhydrous gabbroic magma which is evidenced by the absence of hydrous ferromagnesium minerals and the magma is subjected to closed system fractionation. The ferrosyenite from Uppalapadu alkaline complex is dated by U-Pb Zircon as 1352 Ma.

Keywords

Ferrosyenites, Cuddapah Intrusive Province.

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