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

P. O. Alexander ¹

Affiliations
1 Department of Applied Geology, University of Saugar, Sagar, (M.P.), IN

Abstract

Abundances of some rare-earth elements (REE) together with Hf, Ta and Th have been determined by neutron activation analysis on twenty-one Dhandhuka flows of known mineralogical and chemical composition. All the basaltic flows have fractionated REE patterns with enrichment of light rare-earths. La/Yb ratio varies from 7.6 to 22.2. A systematic relationship is observed between the total rare-earth abundances and the petrochemistry of the rocks. REE data of these flows are compared with the previous REE data on Deccan basalts and tholeiites from other provinces. Comparison with REE data of oceanic tholeiites of the Indian ocean and the application of partition coefficients suggests that Dhandhuka flows may have been produced by partial melting of solid type of mantle material.

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P. O. Alexander ¹

Affiliations
1 Department of Applied Geology, University of Saugar, Sagar 470003, M. P., IN

Abstract

Among the ten-flow basaltic sequence in the NE of the Deccan province, flow 5 is considered as the most 'primitive' (K₂O, 0.16%; Rb, 2 ppm; Pb, 2 ppm and ⁸⁷Sr/⁸⁶Sr, 0.7039). 190 metres of the ten-flow lava pile is singularly quartz normative and the absence of olivine and olivine tholeiites is conspicuous. The unique 'Deccan character', i.e., enrichment of iron and titania is observed but silica (48.26%), K₂O (0.2%) and coherent trace elements (Rb, 6 ppm, Pb : 6 ppm) are strikingly low. Occurrence of similar low potassic rocks are observed from elsewhere in the Deccan region. Such rocks will extend the diversity of Deccan basalts in the low potassium end of the spectrum and are shown to have a definite resemblance to the oceanic tholeiites in spite of difference in the 'environment'.

Petrogenesis of these low potassic quartz normative tholeiites, taking flow 5 as the unmodified melt is discussed in the light of the generally accepted models. Its derivation as partial melts of Upper Mantle composition analogous to garnet peridotite is not favoured. These tholeiites seem to have been derived as almost 'total melts' of possible eclogite lenses and patches in the Upper Mantle.

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P. O. Alexander ¹

Affiliations
1 Department of Applied Geology, University of Saugar, Sagar 470 003, M.P., IN

Abstract

Initial strontium isotopic ratio for the Pavagarh rhyolite is reported. The ratio, 0.7111 is of the same order of magnitude as generally met for rhyolites of continental region. Compared to rhyolites that can truly be called derivates of magmatic differentiation of tholeiitic magma only, this ratio is considerably higher. Incompatible elements like Ba, Sr, Zr, Rb, Pb, Nb and Y are similarly enriched in the Pavagarh rhyolite. The data indicate that rhyolite of Pavagarh could not have originated as a result of closed-system magmatic differentiation of basaltic magma alone and significant incorporation of sialic material into the magma is suggested for its derivation.

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J. P. Shrivastava ¹, P. O. Alexander ¹

Affiliations
1 Department of Applied Geology, University of Sagar, Sagar 470003 (M.P.), IN

Abstract

Cold extractable and total Cu, Zn and Pb in soils arc faithful pointers to concealed mineralisation. Geomicrobiological studies have failed to give positive results. Thiobacillus ferrooxidanS, T. nevellus, T. permetabolis, T. thiooxidans, T. thioparus in soils over the mineralised zones were not detected in the present study. But cultures grown in P.D.A. media have shown quantitative differences for the mineralised and non-minearalised soils. Several possible reasons for this negative geomicrobiological response are discussed which include: deficiency in Mo and organic carbon in soils. unfavourable pH of soils/water, lack of moisture and water-logging among others. More detailed study of soil geomicrobiology minerai exploration is suggested.

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