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

V. Divakara Rao ¹, T. Radhakrishna ¹, S. V. Srikantia ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad-500007, IN

Abstract

Geochemical investigations on the Jaspa granites and Rohtang gneisses from Himachal Pradesh indicate that the Rohtang gneisses are the migmatlsed/granltised products of the pre-existing metasediments and the Jaspa granite is an intrusive granite. Field relationship suggests that the Jaspa granite is a post-tectonic intrusive while the migmatisation/granltisation of the gneisses appear to be pre-and syntectonic and might have continued into the post-tectonic period.

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S. Sinha-Roy ¹, T. Radhakrishna ¹

Affiliations
1 Centre for Earth Science Studies, Trivandrum 695010, IN

Abstract

A basic dyke swarm trending ENE-WSW traverses the Agali area of north-central Kerala. The geochemistry of these dykes suggests that they are transitional in character between continental and oceanic basalts. and are akin to one of the magmatic phases of the Deccan volcanics. They have been derived from a tholeiitic magma generated by partial melting of a heterogeneous mantle containing minor amounts of phlogopite and apatite. Fractionation of clinopyroxene. magnetite and plagioclase is indicated. The present study supports one of the earlier suggestions that these dykes are related to the rifting of the western continental margin of India.

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T. Radhakrishna ¹

Affiliations
1 Centre for Earth Science Studies, Trivandrum 695010, IN

Abstract

No Abstract.

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T. Radhakrishna ¹

Affiliations
1 Centre for Earth Science Studies, Trivandrum, IN

Abstract

No Abstract.

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T. Radhakrishna ¹, G. V. S. Poornachandra Rao ², J. G. Mitchell ³, A. S. Venkatesh ²

Affiliations
1 Centre for Earth Science Studies, Trivandrum 6950 10, IN
2 National Geophysical Research Institute, Hyderabad 500 007, IN
3 School of Physics, University of New Castle Upon Tyne, Ne 17 RU, GB

Abstract

Palaeomagnetic data on the basic dyke rocks of Palachal (Cannanore district) and Agali (Palghat district) regions of Kerala in the southwestern part of India are presented. Palachal dyke has obtained stable vectors of D and I at 200Oe and possesses a single thermally discrete component with blocking temperatures at the Curie temperature of magnetite, whereas the Agali dyke has shown unstable behaviour to the AF and thermal demagnetisation tests. The Palachal dyke possesses a southwest negative NRM remanence (D = 225.5°; 1 = 48.5°; K = 48.52; α 95 = 7.24) and a pole position (44°N; 135°E) which is closer to that of Gwalior Traps (1830 ± 200m.y.) or Group IV dykes of Proterozoic age. K-Ar ages of 1668 ± 31 m.y. and 1660 ± 25m.y. have been obtained for the Palachal dyke and it is compatible with the Palaeomagnetic results. The basic dyke activity jn the southwest is not solely related to the rifting of western continental margin of India as earlier suggested but is in part of Proterozoic age.

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T. Radhakrishna ¹, John Mathai ¹, M. Yoshida ²

Affiliations
1 Centre for Earth Science Studies, P. B. No. 7250, Trivandrum 695031, IN
2 Department of Geosciences, Osaka City University, Osaka 58, JP

Abstract

The Punalur-Achankovil sector is a southwestern segment of the high-grade granulite terrain of South India. It is characterised by charnockite and garnet-biotite. Quartz-feldspar (GBQF) gneiss with minor occurrences of sillimanite and graphitebearing gneiss and basic granulite. Structurally. The terrain has undergone three phases of intense deformation and metamorphism and a late less intense deformation. The first deformation (D₁) has disrupted original depositional layering with development of quartzo-feldspathic bands; the accompanying metamorphism (M₁) is represented by granulite facies sillimanite-bearing gneiss and basic granulite. The second deformation (D₂) resulted in isoclinal folding and the corresponding metamorphism (M₂) produced large neosome now represented by the GBQF gneiss. Lithologies representing the D₃-M₃ event are the charnockite and minor granite bodies developed under high-grade metamorphic conditions; open to tight" asymmetric folds are characteristic of this deformation. The final deformation (D₄) is less penetrative and is characterised by open folds. Structural data from the area do not indicate any major shear zone features nor does the region constitute an intercratonic suture belt as suggested earlier. The topographic expression along Achankovil-Punalur represents only a neotectonic lineament.

Keywords

Structural Geology, Granulites, Punalur-Achankovil, Kerala.

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T. Radhakrishna ¹, N. R . Krishnendu ¹, G. Balasubramonian ¹

Affiliations
1 Centre for Earth Science Studies, Trivandrum-695031, IN

Abstract

Profuse Proterozoic basic magmatism is represented by mafic dykes all around the Cuddapah Basin and by lava flows/sills at different stratigraphic levels of sedimentary sequences within the basin. However, the genetic links between magmatism all around and within the basin are rather obscure. Available geochronological data have been examined in the light of our geochemical and palaeomagnetic results. The geochemical results clearly show that the dykes are sub-Alkalic tholeiitic basalts in composition and no distinctions are visible in the elemental trends with respect to field orientations or discrete age groups. However, the LIL element chemistry has been variably affected by different degrees of alteration. Such an alteration seems to have resulted in wide range in their ages, mainly by the Rb-Sr, K-Ar and Ar-Ar methods. Using palaeomagnetic constraints we recalculated the Sm-Nd isotopic data of three dykes of Mahabubnagar-Wanparti sector and estimated c. 2.4 Ga and 2.1 Ga age for the early dyke magmatism around the Cuddapah basin. Two other discrete dyke magmatic activities are assigned 1.8 and 1.65 Ga based on palaeomagnetic correlations. Thus, the sub-alkalic tholeiitic basic magmatism within lower Cuddapah Basin and around is confined to the Palaeoproterozoic times and the younger ages are attributed to disturbed isotopic systems. An interesting observation is that the dykes in the Anantapur-Gooty sector are predominantly rich in MgO, Ni and Cr and low in Zr and are dominated by olivine cumulus mineralogy. This feature evidently suggests that the dykes in this region solidified in much closer proximity to the subsurface magma chamber, retaining early-formed dense phases like olivine still as cumulates/phenocrysts. Such an interpretation provides geochemical support to the inference from Bouguer gravity anomaly for subsurface magma underplating in the southwest Cuddapah Basin. This is the first study from India to provide support from the geochemistry of dykes for subsurface magmatic body inferred from gravity anomaly.

Keywords

Mafic Dykes, Age, Petrology, Geochemistry, Proterozoic, Cuddapah Basin.

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T. Radhakrishna ¹, T. R. K. Chetty ²

Affiliations
1 Centre for Earth Science Studies, Trivandrum - 695 031, IN
2 National Geophysical Research Institute, Hyderabad - 500 007, IN

Abstract

No Abstract.

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T. Radhakrishna ¹

Affiliations
1 Centre for Earth Science Studies Trivandrum, IN

Abstract

No Abstract.

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T. Radhakrishna ¹

Affiliations
1 Centre for Earth Science Studies, Trivandrum - 695 031, IN

Abstract

South Indian granulite terrain had witnessed significant part of Precambrian mafic igneous activity in the form of episodic mafic dyke intrusions of the Palaeoproterozoic period. Strike trends of these dykes are not uniform over the region and the dykes are generally fresh, massive, black dolerites except in the Bhavani shear zone bordering the southern fringes of Nilgiri massif. In Agali-Coimbatore area of our study in the western Bhavani shear zone, the dykes appear to be penecontemporaneous with shearing. Isotopic data place age of Agali-Coimbatore dyke intrusions at about 2.1 Ga. The age of these dykes is significant to constrain an early Palaeoproterozoic age for major shearing event in the Bhavani shear zone. Other dyke emplacement ages are placed at about 1.8 Ga and 1.65 Ga based on the Ar/Ar and K-Ar isotopic results of dykes in Dharmapuri and Tiruvannamalai areas. Older ages comparable to those of the Dharwar craton are not known and in this respect future isotopic dating is vital. Geochemically, these dykes are quartz/hypersthene normative subalkalic tholeiites. An attempt is made here to provide insights into the general petrogenetic history of the Precambrian dykes. Compositional trends are explained by the fractional crystallization of ferromagnesian phases and plagioclase control is conspicuous at the advanced stages of fractionation. Geochemical characteristics suggest that the dykes have tapped Fe-rich non-pyrolite mantle sources with LIL and LREE enrichment as in many continental basalts. The data suggest that role of crustal contamination is limited in petrogenesis; crustal signatures are noticed in the more mafic end members formed in early stage of evolution suggesting that contamination was temperature controlled with most primitive high temperature magmas being most vulnerable to the process. Nd-Sr isotopic data, at present restricted to Agali-Coimbatore dykes, suggest that Palaeoproterozoic magmas tapped subcontinental lithosphere that may have stabilized in the Archaean times at about 3 Ga during the major crustal building activity in the shield region. Further work coupled with isotopic and mineral chemistry will improve our knowledge on the petrological evolution of the dyke magmas and mafic magmatism in general.

Keywords

Southern Granulite Terrain, Palaeoproterozoic, Dykes, Geochemistry, Subcontinental Lithosphere.

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