Informatics Publishing Limited

Anil Kumar ¹, K. Gopalan ¹, K. R. P. Rao ², S. S. Nayak ²

Affiliations
1 National Geophysical Research Institute, Uppal Road, Hyderabad - 500 007, IN
2 Geological Survey of India, Bandlaguda, Hyderabad - 500 068, IN

Abstract

Acid-Leached phlogopite Rb-Sr isochron ages of the Kotakonda and Mudalbid kimberlites in the Narayanpet kimberlite field northwest of the Cuddapah basin, south India are 1085±14 and 1099±12 Ma, respectively. These concordant ages are much younger than the recently reported K-Ar and ⁴⁰Ar-³⁹Ar ages of 1363±48 and 1402±5 Ma, respectively for the former kimberlite, and support our earlier conclusion that the south Indian kimberlites erupted episodically close to 1090 Ma ago. Three lamproites-one each from Ramannapeta at the northeastern margin and Chelima and Zangamarajupalle within the Cuddapah basin-give distinctly different Rb-Sr ages of 1224±14, 1354±17 and 1070±22 Ma, respectively. While the Rb-Sr age of the Ramannapeta lamproite is also younger than an earlier reported K-Ar age of 1381±18 Ma, it is the same as that of many spatially close alkaline complexes like at Elchuru and Kunavaram. The ages for the Chelima and Zangamarajupalle rocks are only tentative, as these rocks contain a large secondary carbonate component with its Sr isotopic composition genetically unrelated to that of their phlogopites.

Keywords

Kimberlites, Lamproites, Narayanpet Kimberlite Field, Rb-Sr Ages, Dharwar Craton.

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K. R. P. Rao ¹, T. A. K. Reddy ¹, K. V. S. Rao ¹, K. S. B. Rao ¹, N. V. Rao ¹

Affiliations
1 Geological Survey of India, Operation: Andhra Pradesh, Hyderabad 500 068, IN

Abstract

Apart from the well known occurrence of kimberlites in south India at Wajrakarur in Anantapur district, kimberlites were located by Geological Survey of India (GSI), during 1984-85 around Maddur and Narayanpet areas in Mahabubnagar district of Andhra Pradesh. Landsat imagery and photo-interpretation studies followed by intense geological traverses including stream sediment surveys, launched during 1994-96, led to the discovery of 20 additional kimberlites in the western part of Mahabubnagar district, Andhra Pradesh and in the adjoining Gulbarga district, Karnataka, thus reporting kimberlite for the first time from the state of Karnataka. A total of 30 kimberlites have been reported in this field named as "Narayan pet Kimberlite Field" (NKF).

The kimberlites are emplaced along E-W and NW-SE trending major faults and associated NE-SW tear faults in the Peninsular Gneissic Complex carrying enclaves of Dharwar greenstones of Gadwal schist belt. These kimberiites occur both as small bodies of a few metres across and also as dykes with a strike length of about 2 km. The characteristic mineral assemblages and textures indicate that these bodies correspond to hypabyssal facies or ischolar_main zone kimberlites. The indicator mineral suite present in these rocks are olivine, chrome diopside, magnesian ilmenite, chrome spinel, ai1d rare pyrope garnet Mantle xenoliths are rare, nevertheless spinel Iherzolite; abundantly found in two kimberlites.

All the kimberlites of NKF are depleted in SiO₂ (∼ 35%), potassic (K₂O >Na₂O) and magnesium rich (MgO >20%). The REE abundance of these kimherlites is consistent with their derivation by partial melting of mantle and subsequent fractional crystallisation of the melt. The similarity of petrological and geochemical characters of these kimberlites indicate that all the kimberlites of NKF are derived from similar source in the upper mantle.

Preliminary processing and testing of some of the kimberlite bodies have not yielded any diamonds so far. However, the reported stray diamond finds in the Krishna and Bhima River gravels indicate the presence of diamondiferous kimberlites further south and southwest of the area.

Keywords

Economic Geology, Geochemistry, Kimberlite, Andhra Pradesh, Karnataka.

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K. R. P. Rao ¹, S. Viswanathan ², A. Vasanthi ³, K. Mallick ³

Affiliations
1 Geological Survey of India Training Institute, Hyderabad - 500 068, IN
2 10, Bapuji Apts., Rajendra Prasad Road, Dombivli (East) Mumbai - 421 201, IN
3 NGRI, Hyderabad - 500 007, IN

Abstract

No Abstract.

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