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

V. Natarajan ¹, S. Rajagopalan Nair ¹

Affiliations
1 Geological Survey of India, Andhra Pradesh Circle (N), Hyderabad 500029, IN

Abstract

In the northeast part of the Palnad Basin, Krishna district, Andhra Pradesh, interesting structural features are noticed in Precambrian sedimentary rocks. A prominent thrust which has brought the Cumbums over the younger Kurnools, and other primary and secondary structures in the rocks are significant and help in deciphering structural events. The thickness of the Narji limestones based on the structural studies is estimated to be 300 m and not 6400 m as suggested by Heron.

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P. Perraju ¹, V. Natarajan ¹

Affiliations
1 Geological Survey of India, Hyderabad, IN

Abstract

The' Peninsular Gneiss' in the northern parts of Andhra Pradesh can be described briefly as a group of composite gneisses, mostly banded or layered, formed as a result of migmatisation of the older meta-pelites (predominantly biotite-schists), metapsammites, and meta-basics (predominantly amphibolites and hornblende schists), by a grey quartzo-feldspathic injection. The resultant migmatitic gneisses enclose inclusions of all shapes and sizes of the older rocks and exhibit different degrees of interaction and assimilation of them. This complex was later intruded by a 'Pink Granite' the effect of which is also seen as a ' second period of migmatisation '. The two major injections appear to be fairly well separated in time.

These two granitic injections and their corresponding migmatisation effects on the older group of diverse rocks has produced a heterogeneous complex of rock types which show wide variations in composition as well as in texture over the region. Both granites have their own groups of aplite, pegmatite and quartz veins. The white quartz and pegmatite veins traversing the grey porphyroblastic biotite gneisses in Karimnagar district carry molybdenite and sulphide mineralisation. All the rocks were later injected by quartz, epidote and at places by fluorite veins. Fluorite is also found as a component mineral of the' pink granites' in Karimnagar district. Dolerite dykes which cut across all the above, mark the last phase of igneous activity.

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V. Natarajan ¹, S. Rajagopalan Nair ², J. K. Nanda ³, H. Sarvothaman ⁴

Affiliations
1 3-4-756 A/I, Barkatpura, Hyderabad, IN
2 10-2-317/22, Vijayanagar Colony, Hyderabad, IN
3 12-2-111/A, Muradnagar, Mehdipatnam, Hyderabad, IN
4 3-4-448,2/1, Narayanguda, Hyderabad, IN

Abstract

A number of laterite cappings have been demarcated over the Deccan Trap basaltic rocks in parts of Medak and Nizamabad districts of Andhra Pradesh. Favourable lithology of traps and a geomorphic surface with almost gentle unidirectional slope from south to north appear to have strongly influenced the lateritisation process in the area that resulted in thick lateritic cappings.

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V. Natarajan ¹, J. K. Nanda ¹

Affiliations
1 Geological Survey of India, Southern Region, Hyderabad 500001, IN

Abstract

Large scale basin and dome structures with intense buckling in some layers are recorded in the Archaean high grade metamorphic-migmatitc complex of the Eastern Ghats near Visakhapatnam, South India. Two major phases of deformation have been deciphered. The earlier phase was regionally continuous, and resulted in nonplunging, tight and isoclinal (overturned) folds, as well as gently plunging elongate structures. The axial trends of these folds vary from NE-SW to ENE-WSW. Gneissic layering of rocks, development of axial plane schistosity and tight isoclinal angular parasitic minor folds, and large scale shearing and faulting in the axial regions, are correlated to this event.

The later phase of deformation was discontinuous and variable in nature. Southerly/southeasterly plunging, open to tight, normal, similar folds belong to this event and they were superposed on the earlier structures. They modified the pattern and plunges of the latter, and also produced intense buckling or gentle warping locally at several places. Development of warp-like open minor folds, and groove and mineral lineations are correlated to this deformation.

On the basis of critical synthesis of the available structural data, the evolution of the Balacheruvu basin and Madhuravada dome has been attributed to a single deformation of the earlier phase of folding.

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N. P. Nathan ¹, S. Sundara Raghavan ¹, V. Natarajan ¹, R. Raman ¹

Affiliations
1 Geological Survey of India, Op : Tamil Nadu & Pondicherry, Madras - 600032, IN

Abstract

Occurrence of cordierite bearing metapelites of Khondalitic suite of rocks is reported from Nartamalai area, Pudukkottai district for the first time. The cordierite bearing gneiss occurs in close association with chamockite and pyroxene granulite in the vicinity of the Nartamalai granites. The mineral assemblages seen in the metapelites are cordierite-almandine-sillimanite-plagioclase-K-feldspar- biotite and quartz. Zircon, magnetite and spinel are the accessory minerals. The texture of the metapelites is generally gneissose except in the eastern part of the area where it simulates a hornfels. The Nartarnalai cordierite contains 59% Mg-cordierite and 41 % Fe-cordierite. The Fe₊₂/Fe₊₂+Mg ratio of the Nartamalai cordierite is 0.414 which is strikingly higher than that of the cordierites of regional metamorphism and it is comparable with the Fe₂/Fe₊₂+Mg ratios of the cordieritcs described from hornfels assemblage. The coexisting garnet is an almandine-pyrope with very minor andradite and spessartitc, which is comparable with the garnets of regional metamorphic assemblage. The tie-line joining the composition of the coexisting cordierite and garnet indicates a regional metamorphic environment. An overprint of thermal metamorphism, attributable to the intrusion of Nartamalai granites, on the pre-existing regional metamorphic fabric is suggested.

Keywords

Petrography, Petrochemistry, Cordierite, Nartamalai, Tamil Nadu.

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