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

Abhijeet Mukherjee ¹, Ch. Sravan Kumar ¹, K. Krishna Reddy ¹, S. N. Siddiquie ¹, Lokesh Chaturvedi ¹

Affiliations
1 National Mineral Development Corporation Ltd., Khanij Bhavan, Masab Tank, Hyderabad – 500028, IN

Abstract

Two Kimberlite pipes in Bommaganapalli area, Kalyandurg, Anantapur District are reported based on stream sediment sampling yielding HIM's viz., garnets ('G 9' lherzolitic, 'G 10' harzburgitic and eclogitic with Na₂O (>0.07 wt%), Cr-diopsides (from garnet peridotites), spinels and ilmenites. Ground magnetic survey indicated magnetic anomaly of about 280 nT in KL-5 and an anomaly of100 n T over KL-6.The anomaly over KL-5 was dipolar. The 'Maturation Index' (MI) of 9.8 and MgO content 8.08 of the calcrete over this area indicate their kimberlitic nature. Analysis indicates that the chrome spinels fall in the diamond stability field. Garnets suggest they are of the nature of "G 10" harzburgitic garnets.

Keywords

Kimberlite, Stream Sediment Sampling, Magnetic Survey, Diamond Indicator Minerals.

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C. Leelanandam ¹, K. Krishna Reddy ²

Affiliations
1 Department of Geology, Osmania University, Hyderabad 500007, IN
2 Geological Survey of India, Patna 800 013, IN

Abstract

The Uppalapadu alkaline pluton is predominantly composed of hypersolvus nepheline syenites containing biotite as the sole ferromagnesian mineral. Hornblendesyenites and orthopyroxene-bearing hornblende-syenites are the associated rock types. The pluton is enveloped by a variety of rock types such as amphibolites, gabbros, and granitic rocks.

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P. S. N. Murthy ¹, K. Krishna Reddy ¹

Affiliations
1 National Mineral Development Corporation, Donimalai Township, 583118, Bellary District, IN

Abstract

Confirmed stromatolites from a limestone band from Sandur greenstone belt of Karnataka Craton are reported for the first time. The stromatolites are oval to elliptical in shape in gently dipping bedding plane and conical in cross section. Indirect radiometric age data indicates the rocks in which the structures occur are 2900 m.y. old. The discovery of stromatolites demonstrates that biological processes were operative in the early Archaean basins of the Karnataka Craton.

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K. Krishna Reddy ¹, J. Ratnakar ², C. Leelanandam ³

Affiliations
1 Geological Survey of India, Bandlaguda, Hyderabad-500 660, IN
2 Department of Geology, Post-Graduate College of Science, Osmania University, Hyderabad-500 004, IN
3 Department of Geology, Osmania University, Hyderabad-500 007, IN

Abstract

The Proterozoic Uppalapadu alkaIine complex (1348±41 Ma) in the Prakasam province of Andhra Pradesh is predominantly composed of nepheline syenite, hornblende syenite and quartz syenite, all of which represent an alkaline series. The pluton is closely associated with tholeiitic gabbro and ferrosyenite representing a sub-alkaline series. Geochemical characters suggest that the rocks of the alkaline series were formed by the differentiation of the hydrous alkaline basaltic magma under high P_H2O, whereas those of the sub-alkaline series were formed by differentiation of an anhydrous olivine tholeiitic magma under low fO₂ (reducing) conditions. Quartz syenite (an offshoot in the alkaline series) is formed either by fractionation of K-feldspar, ferrohastingsite and/or biotite under high P_H2O conditions or by crustal contamination of critically saturated syenitic liquids.

Keywords

Petrochemistry, Proterozoic, Alkaline Complex, Andhra Pradesh.

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