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

B. L. Narayana ¹

Affiliations
1 School of Studies in Geology, Vikram University, Ujjain, IN

Abstract

Amphibolites occur towards south and south-east of Devgad Baria town in the Panchmahals district of Gujarat State, in the form of disconnected bands showing concordant relationship with the rocks of the Banded gneissic complex and passing into striped varieties, and also as caught-up patches in the granitic intrusive bodies. Mineralogically, the former type is a quartz and plagioclase bearing amphibolite, while the latter is only plagioclase-bearing.

The quartz-amphibolites are shown to be para-amphibolites on the basis of their intimate association with the impure calcareous metamorphosed sedimentary rocks, lack of relict igneous textures, absence of sphene and plagioclase, high modal percentage of quartz, and low weight percentage of TiO₂ and Al₂O₃ showing a near concordance with the alumina content of marbles of the area. On the other hand, the plagioclase-amphibolites show, in addition to the disconnected sill-like form, relic sub-ophitic texture, high proportion of sphene and high weight percentage of TiO₂ and Al₂O₃ indicating their primary igneous nature.

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B. L. Narayana ¹, S. M. Naqvi ¹

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

Abstract

Spinifex-textured peridotitic komatiites (STPK) of Ghatti Hosahalli have low CaO/Al₂O₃ ratio (< 1), low TiO₂, K₂O and high MgO. These STPK have comparable Ni and Co contents with those of the Australian and Canadian shields. These characters support the view that the spinifex-texture observed in these rocks is primary and not the result of later metamorphism. The Al₂O₃/TiO₂ ratios are comparable to those of Chondrites indicating that the STPK are products of partial melting of the Archaean mantle. The STPK of Ghatti Hosahalli arc overlain by exhalative chemical precipitates and argillaceous metamorphosed sediments like bedded-barytes, fuchsite-quartzites and kyanite-mica schists. It is suggested that Ghatti Hosahalli band is a relic of an early Archaean greenstone belt.

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S. M. Naqvi ¹, B. L. Narayana ¹, P. Rama Rao ¹, S. M. Ahmad ¹, B. Uday Raj ¹

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

Abstract

The lowermost member of the Javanahalli schist belt consists of meta-arkoses/ paragneisses with interbedded epidote-quartzite and calc-silicate bands. These meta-arkoses exhibit relict current-bedding, arkosic texture and grain-to-grain variation in anorthite content within the same sample. Plagioclase is subordinate to K-feldspar and is relatively more weathered and sericitized. Orthoclase predominates over microcline in the total K-feldspar content and the quartz/feldspar ratio is generally very low. The meta-arkoses appear to be first-cycle sediments.

The composition of these paragneisses is unusual for Archaean metasediments. They are enriched in K, Si, Fe, Mg, Th, Rb, Ni, Cr and Co and depleted in Na, AI, Sr and often in Ca. The meta-arkoses of Javanahalli schist belt may be the product of strong and/or rapid chemical disintegration of Archaean tonalitic gneisses and basic and ultrabasic rocks. all of which were prominent components of the continental crust at the time the Javanahalli belt formed. If the meta-arkoses have not undergone major metasomatism, then their high abundance of K and Rb may indicate intense chemical leaching of these tonalitic source areas. Most of the saIt of the primitive oceans might have been derived from this intense leaching of the ancient cratonic tonalite gneisses.

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B. L. Narayana ¹, D. V. Subba Rao ¹, P. K. Govil ¹

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

Abstract

High alumina, magnesium and iron-bearing sediments represented by cordierite-anthophyllite rocks occur in association with amphibolites. calc silicate rocks and banded magnetite quartzites, within the Javanahalli schist belt, Karnataka. Their chemical composition is anomalous. The abundance levels of elements like Fe, Mg, AI, Ni, Cr, Co, Rb and Sr indicate that probably they represent metamorphosed chemical sediments generated by volcanogenic exhalative activity.

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D. Rameshwar Rao ¹, B. L. Narayana ¹, S. N. Charan ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad 500007, IN

Abstract

Detailed mineralogical studies of amphibolite to granulite facies transition zone rocks and granulite facies rocks in Dharmapuri district, Tamil Nadu, were carried out in order to know the gneiss-charnockite relations, nature of protolith, and metamorphic conditions.

The studies show that the rocks are of magmatic origin. and show a gradual decrease in modal composition of hornblende and biotite and gradual increase of hypersthene and iron oxide in a north to south traverse. Breakdown of hydrous minerals to anhydrous minerals is evident with the release of iron oxide suggesting progressive transformation of gneisses to charnockite, Biotite occurs in two generations, an earlier one is of massive type and breaks down to anhydrous minerals, while the latter one occurs as tabular flakes and likely to have formed during the waning stages of amphibolite to granulite facies metamorphism, and forms a zone of concentration just above the stage or CO₂ migration. The mineralogy of pyroxene, plagioclase, hornblende, biotite and iron oxides also suggests that the pressure-temperature conditions in the transition zone are lower than those of the granulite zone.

Keywords

Petrology, Igneous and Metamorphic, Charnockites, Dharmapuri District, Granulites, Tamil Nadu.

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D. V. Subba Rao ¹, S. Nirmal Charan ¹, B. L. Narayana ¹, S. M. Naqvi ¹

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

Abstract

Occurrence of fine-grained, fingerprint-like symplectic intergrowths of cordierite, K-feldspar and quartz formed due to the retrogressive, garnet breakdown reactions following decompression during uplift, is reported here from metapelites of Usilampatti area. These metapelites consisting of cordierite, garnet, hypersthene, biotite, hercynitic spinel, sillimanite, K-feldspar assemblage were metamorphosed at 6.5-7.5 Kb and 750-800°C. The symplectites were formed at 600-670°C and 5 Kb pressure. The textural and mineralogical evolutions in these metapelites indicate a rapid decompressional P-T evolution. Petrological and geochemica1 data suggest that these metasediments were derived mostly from an evolved basement source of granodiorite-granite composition with a minor basic component.

Keywords

Metamorphic Petrology, Gneiss, Tamil Nadu.

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H. K. Gupta ¹, R. K. Chadha ¹, M. N. Rao ¹, B. L. Narayana ¹, P. Mandal ¹, M. Ravi Kumar ¹, N. Kumar ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad - 560 007, IN

Abstract

The Jabalpur earthquake of magnitude 5.8 occurred on May 22, 1997 along seismically active Narmada-Son Lineament (NSL) in central India, with its epicenter about 30 km southeast of Jabalpur. A maximum intensity I_o = VIII on MM scale was observed in Jabalpur and surrounding region. The isoseismal map prepared for this earthquake shows NE-SW trend which in general agrees with the focal mechanism solution indicating thrust faulting along a ENE-WSW fault with minor strike-slip component. Several ground cracks mapped in meizoseismal area show a dominant N75° E trend. The damage pattern of the constructions clearly shows a strong correlation with lithology, structures and soil cover of the area. There are no foreshocks reported for this earthquake and only a few aftershocks are recorded. which warrant a close monitoring of this region.

Keywords

Earthquake, Seismology, Jabalpur.

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H. C. Tewari ¹, V. Divakar Rao ¹, B. L. Narayana ¹, M. M. Dixit ¹, N. Madhav Rao ¹, A. S. N. Murty ¹, B. Rajendraprasad ², P. R. Reddy ¹, N. Venkateswarlu ¹, V. Vuaya Rao ¹, D. C. Mishra ¹, S. B. Gupta ¹

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

Abstract

Lithological, gravity, magnetic, and seismic data within 100 Km corridor of the 400 km long seismic reflection profile are compiled to constitute the NW-SE Nagaur-Jhalawar Geotransect. The transect sequentially cuts across the Neo-Proterozoic Marwar Basin (MB) on the northwest, the Palaeo/Mesoproterozoic Delhi Fold Belt (DFB), the middle/late Archaean Bhilwara Gneissic Complex (BGC) and the MesolNeoproterozoic to early Palaeozoic Vindhyan Basin (VB) at the southeast. The BGC and DFB belts show polyphase deformation and metamorphism.

The BGC within the transect, consists of Sandmata Granulite Complex, followed by amphibolite facies Mangalwar Complex and Greenschist facies Hindoli/Sawar groups. The BGC show evidence of crustal reworking at c.3.0 Ga. The DFB is represented by amphibolite facies metavolcanic-metasedimentary shallow marine sequences and is tectonically highly disturbed. The DFB deposits (c. 2.0 - 1.5 Ga.) were subjected to tectonic deformation during Delhi orogeny (c. 1.5 Ga.), which is marked by syntectonic granitic plutonism. Both, the BGC and OFB also appear to have been affected by Neoproterozoic thermal events and granitic plutonism. The Neoproterozoic MB consists of clayevaporite sequences of shallow oscillatory basin deposits.

Seismic, gravitylmagnetic and magneto-telluric techniques could delineate a number of shallow to deep faults, intrusive bodies and a high conductivity zone. The total magnetic intensity shows a regional increase towards southeast. The Bouguer anomaly values show a steep rise of upto 80 mGal towards the boundary of OFB and BGC. Based on the seismic studies, doubling of the crust under the OFB and vertical intrusion of high density material under the BGC are inferred. The upper crust is, in general, transparent in its reflectivity while the lower crustal reflectivity is high in the transect area, except in the BGC and the VB. A thrust boundary, dipping NW, is present at the eastern margin of the BGC and could be traced up to 30 km depth. The Moho is at a depth of 36-38 km under the MB. Multiple Moho reflections are identified in the DFB crust, the deepest being at 45-50 km depth. In some part of the BGC the Moho can not be identified but in parts it is traced at about 50 km depth, with southeast up dip, before becoming subhorizontal at depth of 41-42 km. It becomes shallower to about 30 km depth at the SE end under the VB.

Keywords

Delhil Aravalli Fold Belt, Geotransect, Northwest India.

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S. H. Jafri ¹, S. Moeen ¹, N. Charan ¹, B. L. Narayana ¹, V. Divakara Rao ¹

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

Abstract

Presence of several morphologies of quench olivine crystals, which provide important clues on cooling rate(s) of magma, are reported here in a basaltic dyke, from the Precambrian gneissic terrain of Schirmacher Oasis, Queen Maud Land, East Antarctica. The dyke, composed of plagioclase, clinopyroxene, olivine, magnetite and glass, shows intergranular texture. It is alkali basalt in composition with low silica (SiO₂ = 46.20-47.20) and negative Nb anomaly. The morphologies of quench olivine crystals suggcst rapid cooling of the basaltic magma approximately at 15-80°C/hour. Further, the delicate nature of these quench olivine crystals, confined mostly to the glassy margins of the dyke, also suggest in situ crystallisation.

Keywords

Basaltic Dyke, Quench Olivine Crystals, Petrography, Geochemistry, Schirmacher Oasis, East Antarctica.

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