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

G. V. S. Poornachandra Rao ¹, J. Mallikharjuna Rao ¹, K. Jaya Prasanna Lakshmi ¹

Affiliations
1 Palaeomagnetism Laboratory, National Geophysical Research Institute, Hyderabad - 500 007, IN

Abstract

Laboratory investigations aimed at inferring the nature of magnetic grains (magnetic granulometry) are reported for nine basaltic flows with thicknesses between 15 and 75 meters encountered in a borehole drilled at the recent Latur earthquake site near Killari village in Latur district, Maharashtra. Five to eight samples were studied for saturation remanent hysteresis, low-field hysteresis, susceptibility variation with temperature and Lowrie-Fuller test, apart from the alternating field (AF) and thermal demagnetizations. Magnetite and titanomagnetite with coercive forces (Hc) in excess 15 mT in mixed domain states of single, multiple, cation-deficient and superparamagnetic grains are inferred to be responsible for remanence in these rocks. These rocks exhibit very stable behaviour during alternating field and thermal demagnetization studies with median destructive fields >20 mT and blocking temperatures between 400 and 580°C.

Keywords

Rock Magnetism, Hysteresis, Magnetic Susceptibility, Demagnetization, Rayleigh Loop, Deccan Traps.

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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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G. V. S. Poornachandra Rao ¹, M. S. Bhalla ¹

Affiliations
1 Palaeomagnetism Division, National Geophysical Research Institute, Hyderabad - 500 007, IN

Abstract

In the absence of conventional radiometric dating and fossil evidence, magnetostratigraphy is considered to be a very powerful tool to correlate rock formations. Often the magnetozones are used as bench marks in correlation of rocks as the geomagnetic field reversals are ubiquitously synchronous. The Vindhyan sedimentation in the Indian stratigraphy represents a very important time period between 1400-400Ma with lithounits quite suitable for recovering the geomagnetic field signatures. With the recently obtained results from the Senui Group, palaeomagnetic field during the main Vindhyan Groups namely the Semri, Kaimur, Rewa and Bhander periods is available. It is quite interesting to note that all the formations investigated from these groups reveal both normal and reversed polarities of the palaeomagnetic field. Using this information a magnetic polarity time scale (GPTS) is constructed for the Vindhyan Supergroup during the late Proterozoic. When correlated with the Russian magnetostratigraphic scale for the Riphean period, it is noticed that the geomagnetic field during the late Proterozoic is similar to that of the Phanerozoic with superchrons at some periods and frequent reversals at others throughout the Precambrian.

Keywords

Magnetostratigraphy, Palaeomagnetism, Vindhyan Basin.

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G. V. S. Poornachandra Rao ¹, J. Mallikharjuna Rao ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad - 500 007, IN

Abstract

The Bijawar Group of rocks unconformably overlie the 2555 Ma Bundelkhand Granite massif and contain in its lower sub-group the Kawar Volcanic Formation exposed near the Bhusor village (24° 39'N; 79° 51' 3O"E) SW of Panna.

The Kawar volcanics are fine grained, magnesium rich tholeiites, basalt to basaltic andesite in composition, with spinifex texture. The immobile trace elements suggest that these volcanics were emplaced in a continental environmental setting in rapid succesion with a low residence time. The REE pattern reveals that these rocks were derived from a depleted mantle source. A comparison of average chemical compositions of Mahakoshal and Gwalior basalts with a Kawar volcanics suggest that these were originated in different environmental conditions although they had previously believed to be the same.

Detailed AF and thermal demagnetization of 20 oriented block samples from 4 sites revealed a characteristic stable remanence in the Kawar volcanics with a declination of 254.9 and inclination of -64.0 (K = 70.75, α₉₅, = 8.32, N = 4). The corresponding VGP, located at p = 27.6° N, Lp = 129.3° E (δp = 6.29, δm = 13-26), appears to be approximately 2000 Ma when compared with other Indian Precambrian data. This age is in agreement with the stratigraphic position above the Bundelkhand granite massif.

Keywords

Kawar Volcanics, Geochemistry, Palaeomagnetisrn, Central India.

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G. V. S. Poornachandra Rao ¹, S. B. Singh ¹, K. J. Prasanna Lakshmi ¹

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

Abstract

The Jodhpur Sandstone (Marwar Supergroup) is investigated from three sites in western Rajasthan, India, using thermal and AF demagnetization techniques. The Characteristic Remanent Magnetic (ChRM) direction isolated is having a normal polarity of Dm=19.59°, Im=+46.27° (K=206.19, α₉₅= 5.63°, N=3) and yielded a Virtual Geomagnetic Pole (VGP) at λp=32.4° S, Lp = 51.3° E (δp=4.63°, δm=7.22°). The palaeomagnetic data of the Jodhpur Sandstone is in good agreement with that of the Upper Rewa Sandstone of the Vindhyan Supergroup rocks that are correlated with the Jodhpur Sandstone on lithological similarities. The Malani Rhyolite overlain by the Jodhpur Sandstone with well constrained radiometric and palaeomagnetic results thus assign a maximum age of < 745±10 Ma to the Jodhpur Sandstone and in turn to the Rewa Sandstone of the Vindhyan Supergroup which were correlated with them. This correlation helps in constraining the age of the Rewa Group rocks in the Upper Vindhyans, which are devoid of any fossil evidence and suitable radiometric material to date them.

Keywords

Palaeomagnetism, Neoproterozoic, AF Demagnetization, Rodinia, Jodhpur Sandstone, Marwar Supergroup, Rajasthan.

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B. S. Paliwal ¹, G. V. S. Poornachandra Rao ², S. B. Singh ², K. J . Prasanna Lakshmi ²

Affiliations
1 Department of Geology, Jai Narain Vyas University, Jodhpur - 342 005, IN
2 NGRI, Hyderabad, IN

Abstract

No Abstract.

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J. Mallikharjuna Rao ¹, G. V. S. Poornachandra Rao ¹, K. P. Sarma ²

Affiliations
1 National Geophysical Research Institute, Uppal Road, Hyderabad - 500 007, IN
2 Department of Geology, Guwahati University, Guwahati, IN

Abstract

The Archaean gneissic basement of Shillong plateau has been traversed by number of mafic dyke swarms. At least two suites of dykes are identified in the region represented by Proterozoic Khasi greenstone related dolerites and younger Cretaceous dolerite dykes in addition to mafic alkaline dykes. The older Khasi greenstone dolerites are altered and have undergone low-grade metamorphism compared to fresh Cretaceous dykes, which are well exposed in the West Garo Hills region. All the Khasi greenstone dolerites are tholeiite in composition and range from basalt to basaltic andesite in composition and show olivine or quartz normative character. Most of the dykes show continental nature of emplacement with some overlapping oceanic tectonic setting of origin. Petrochemical study suggests that they were derived from picrites that subsequently undergone low- pressure fractionation. Palaeomagnetic study of the older Khasi greenstone related dolerites show a direction of magnetization of Dm=17, Im=+57 (α₉₅= 23.34; K=31.5; N=24) with a palaeolatitude of 29.7°N to the Indian subcontinent that clearly support the Proterozoic dyke/dyke swarm emplacement in the region. The magnetic carrier as inferred from K-T studies is in multi domain (MD) size and cation deficient (CD) domain states.

Keywords

Khasi Greenstone, Dykes, Mafic Magmatism, Shillong Plateau, Meghalaya.

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