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

T. Ramamohana Rao ¹, C. Borreswara Rao ¹

Affiliations
1 Department of Geology, Andhra University, Waltair, IN

Abstract

Eight trace elements Co, Cu, Ga, Ge, Li, Ni, Pb and V have been determined semi-quantitatively with the Hilger spectrograph in 14 minerals and 18 rocks belonging to the Singareni series and the Gneissic complex of the early Precambrian and the Pakhals of the late Precambrian. The Pakhal phyllite is higher in Co, Ga, Ge, Li and Ni and Lower in Cu, Pb and V than the average shale. The pelitic schist of the Singareni Series broadly shows the same order of trace element distribution as that of the Pakhal phyllite. Metamorphism (amphibolite facies) of the Pakhals has not affected any redistribution of the trace elements in them. The granite-gneisses which are the granitised derivatives of the Singareni series show a decrease in Co, Ni and V. The trace elements Co, Cu, Li and Ni follow the major elements Fe, Mg and are preferentially fixed in the ferromagnesian minerals forming the essential constituents of the rock.

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T. Ramamohana Rao ¹, Ch. Shanmukha Rao ¹, K. Sanyasi Rao ¹

Affiliations
1 Geology Department, Andhra University, Waltair 530003, IN

Abstract

Textural analysis reveals that the black sand samples are well sorted. negatively skewed, mostly mesokurtic and unimodal in distribution with the Mean Size of the grains falling mostly within the fine sand. The opaques and non-opaque heavies in a weighted average sample constitute 78.1% and represent the full range of minerals found in the rocks of the Eastern Ghats. The marked preference of a particular heavy mineral to a certain sieve fraction can be related to the grain size of the heavy mineral in the source rocks. The occurrence of thin layers of black sand in the stream channels inland. the non-disappearance of any unstable heavy mineral in the deposits, suggest that the black sands are derived from the highland area.

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T. Ramamohana Rao ¹, B. V. Satyanarayana Raju ¹

Affiliations
1 Department of Geology, Andhra University, Waltair 530003, IN

Abstract

The Chimalpahad layered anorthosite at the border zone of Archaean supracrustal rocks of Khammam and Eastern Ghats shows normal stratification predominantly in the form of rhythmic layering and sometimes in the form of zebra layering. Cross-stratification showing clear truncation by the top normal stratification from the layered anorthosite of Chimalpahad is one of the very few such occurrences reported from layered rocks of the world. The cross-stratification could have originated due to flow of magma current and fluctuation in its velocity across the floor of the magma chamber. The structure is useful as a way-up criterion.

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T. Ramamohana Rao , P. Rama Rao

Abstract

No Abstract.

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T. Ramamohana Rao ¹

Affiliations
1 Department of Geology, Andhra University, Waltair, IN

Abstract

Electron probe microanalyses of 47 chlorites from the Lower Proterozoic rocks, , Porphyroidal' rocks and from magnetite lode structures of Tennant Creek field are presented. Recrystallisation of chlorite, progressive in Warramunga rocks and retrogressive in 'Porphyroidal' rocks was contemporaneous and took place under lower greenschist facies conditions during regional deformation. Chlorite in the magnetite lode structures had formed ahead of the sulphide mineralisation in the Warramunga rocks subsequent to regional metamorphism and was due to metasomatic alteration of the wall rocks. Chlorite occurs with muscovite in all the three rock groups studied.

Structurally, the majority of the analysed chlorites have an octahedral cation total short of the stipulated 6.0 per half unit cell. This deficiency is systematically related to the difference between Al^vI and AI^IV; the former is half the latter and confirms Foster's (1962) conclusion. The generalised relationship is (R³⁺ - Al^IV) : (6.0 - Al^IV- R²⁺) : : 2 : 3. Most of the analysed chlorites have Al^vI greater than Al^IV and the value of 1.5 reported here for (Al^vI - Al^IV) may be the limit in natural chlorites. The majority of the analysed chlorites fall on the composition field of ripidolite or brunsvigite or diabantite. The occurrence of diabantite variety in the Tennant Creek field is the first report of its kind from any low grade metamorphic terrain.

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