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

S. Ravi ¹, T. K. Rau ¹, N. S. Reddy ¹, S. S. Nayak ¹

Affiliations
1 Geological Survey of India, Southern Region, Hyderabad – 500 068, IN

Abstract

The diamondiferous Upper Proterozoic conglomerates and the associated sandstone beds display sedimentary structures that are suggestive of provenance in the northwest, i.e. towards the Tungabhadra river drainage basin. Multidisciplinary surveys in this region have led to the discovery of a kimberlite body (350 m x 470 m), concealed under alluvial soil spread on the left bank of the Tungabhadra River near Mantralayam. The study area, represented by the granitoids of Peninsular Gneissic Complex with remnants of Gadwal schist belt, presents a domal upwarp that plunges into the river. The domal surface displays intense fracturing with prominent NW-SE/WNW-ESE and subordinate NE-SW trends, which show marked preponderance and dilation on the northern low angle plunge parts. Traverses coupled with extensive stream sediment sampling and the study of specific mineral chemistry is suggestive of presence of kimberlite, which is supported by actual recovery of micro-diamonds from the yellow ground. The kimberlite contains mantle nodules of phlogopite-bearing websterite and siliceous mica-garnet-chrome diopside bearing lherzolite; the selective non-silicification of interstitial chrome diopside in the latter being enigmatic. The silicification of these mantle nodules could be due to the emplacement of kimberlite into a surface water body and subsequent action of silicasaturated groundwater.

Keywords

Kimberlite, Microdiamonds, Mantle Nodules, Tungabhadra River, Kurnool District, Andhra Pradesh.

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N. S. Reddy ¹

Affiliations
1 Petrology Division, Geological Survey of India, Mukharamjahi Road, Hyderabad 500 001, IN

Abstract

Pillow structures are noticed in the spilitised basalts occurring within the shales of Tadpatri Formation indicating subaqueous eruption as against the traditional notion of subaerial volcanism in Cuddapah basin. The basalt is enriched in alkalis and silica and depleted in calcium and magnesium. The pillows are separated by a black siliceous inter-pillow material. The pillowed basalt is under1ain by a massive Java with a thin intervening bed of aphanitic rock and nodular limestone. This pillowed and massive lava flow appears to extend for long distances in the Cuddapah basin.

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M. Ramakrishnan ¹, M. Santosh ¹, N. S. Reddy ²

Affiliations
1 Centre for Earth Science Studies, Trivandrum 695 031, IN
2 Geological Survey of India, Hyderabad 500 001, IN

Abstract

Patches and oriented zones of 'pseudo-charnockites', similar in appearance to incipient charnockites from the granulite transition zones, are reported for the first time from the pink granites of Gadwal and Kurnool areas in the low-grade metamorphic terrain of northeast Dharwar craton. With a mineral assemblage of quartz, feldspars, micas and amphiboles showing varying degrees of alteration to chlorite and sericite, these zones preserve the signature of hydration following peak metamorphism. Though aqueous brine inclusions (average 6 wt % NaCl equivalent) constitute the dominant fluid phase in the granite and the retrograde zones, the occurrence of coexisting monophase inclusions with a near-pure carbonic fluid (density 0.70-0.80g/cm3) in the retrograde zones invoke special attention. Though carbonic inclusions generally typify dehydration reactions, it is suggested that retrograde reactions of the type which produced the pseudo-charnockite horizons could also result in the entrapment of nearpure CO₂ of moderate to low densities. The carbonic inclusions have apparently resulted from the immiscible separation of a mixed carbonic-aqueous fluid which exsolved into brine and CO₂ upon decompression following the rapid uplift of the Eastern Block along the Chitradurga boundary fault, concomitant with the copious intrusion of younger granitoids.

Keywords

Andhra Pradesh, Dharwar Craton, Fluid Inclusion, Greenstone Terrain, Pseudo-Charnockite.

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