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D. V. Subba Rao ¹, S. M. Naqvi ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad-500007, A.P., IN

Abstract

The cluster of enclaves at Kalyadi, Hassan district includes ultramafic-mafic metavolcanic rocks and minor metasedimentary rocks such as chert, cherty quartzite and pelite. Occurrence of pillow and vesicular structures in the ultramafic rocks is being reported for the first time from this area. The average composition of the ultramafic rocks is : SiO₂ 46.05%, TiO₂ 0.18%, MgO 31.81%, K₂O 0.15%. Cr 3409 ppm and Ni 1610 ppm with a CaO/Al₂O₃) ratio close tol. The composition, relict igneous structures and metasedimentary rock association are suggestive of a komatiitic affinity for these rocks, which possibly accumulated under subaqueous conditions.

Keywords

Igneous Petrology, Geochemistry, Komatiites, Pillow Structures, Karnataka.

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M. N. Qureshy ¹, S. C. Bhatia ¹, D. V. Subba Rao ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad, IN

Abstract

The regional gravity survey in the area bounded by Lat. 21°N to 23°N and Long. 85°E to 87°E has brought out a broad gravity' low' over the granite batholith south of the Singhbhum thrust zone. The amplitude of gravity' low' is more in the southern portion (about 30 mgal near Keonjhargarh) than that in the northern portion of the granite batholith (about 20 mgal near Hatgamaria). This gravity 'low' in general suggests that the granite body may continue to a considerable depth.

A broad gravity 'high' has also been observed east of Singhbhum granite where one or two outcrops of basic rocks are observed. The areal distribution of this' high' would seem to suggest the existence of a substantial amount of basic rocks in this area which may represent the Simlipal and the Dhanjori basins filled with high density material as suggested by Iyengar and Alwar.

This correlation of gravity to surface geology can be advantageously used as an aid to structural mapping and in getting the third dimensional configuration of the major rock types.

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S. C. Bhatia ¹, D. V. Subba Rao ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad, IN

Abstract

No Abstract.

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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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R. Srinivasan ¹, S. M. Naqvi ¹, B. Uday Raj ¹, D. V. Subba Rao ¹, V. Balaram ¹, T. Gnaneshwar Rao ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad 500007, IN

Abstract

Turbidite greywackes of the northwestern part of the Chitradurga Schist Belt constitute a part of >2.6 Ga Cbitradurga Group of Dharwar Supergroup. They consist of a detrital assemblage of mono- and poly-crystalline quartz, microcline and plagioclase feldspar. and volcanic rock fragments. Quartz content and Na₂O/K₂O ratios show their quartz intermediate character. In this respect and also in their high FeO(t)+ MgO content they are similar to other Archaean greywackes. The TiO₂ content. K₂O/Na₂O, Al₂O₃/CaO+Na₂O and Al₂O₃/SiO₂ ratios plotted against Fe₂O₃(t)+MgO, although assign an island arc environment for these greywackes, do not unequivocally discriminate between continental and oceanic island arc setting. The Chitradurga greywackes are highly enriched in Zr, Cr and Ni indicating a mixed felsic and mafic source. The contribution from contemporary volcanism in the basin could be significant. The QFL proportions indicate a dissected arc and/or recycled orogenic nature of the provenance. The chemical index of alteration varying from 58 to 63 and Al₂O₃/Na₂O ratio less than 6, indicate relatively unweathered nature of source rock and chemical immaturity of the sediment respectively. The REE patterns show wen-defined negative europium anomalies which reflect granitoid upper crust in the provenance during the Archaean.

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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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D. V. Subba Rao ¹, S. Nirmal Charan ¹, S. Moeen ¹

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

Abstract

Sapphirine is found in the pelitic gneisses of Kosavankovilpatti near Usilampatti in Tamil Nadu. It is intimately associated with melt products and the associated minerals are garnet, biotite, orthopyroxene, plagioclase, mesoperthite and K-feldspars. The sapphirine-bearing rock is characterised by (i) enrichment of MgO, depletion of SiO₂ (ii) absence of coronas and symplectites, (iii) presence ofmesoperthites (iv) high X_Mg values of the associated minerals and (v) high P-T conditions (855±20°C and 8.0±O.3 Kb) in contrast to the adjacent sapphiri ne absent rocks (750-800° and P of 6.5-7.5 Kb). This occurrence of prismatic sapphirines is in contrast with the reported sapphirine occurrences where it occurs as symplectites and coronas defining a decompression path (T= 700°C and p 5-7 Kb) in the adjacent Kodaikanal massif. It is suggested that the sapphirine being reported is of prograde origin.

Keywords

Petrology, Precambrian, Sapphirine, Metamorphism and Geothermobarometry.

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S. H. Jafri ¹, D. V. Subba Rao ¹, S. M. Ahmad ¹, Ramavati Mathur ¹

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

Abstract

Spinifex textured ultramafic rocks occur at Bhaktarahalli in Nuggihalli schist belt and as an enclave within the Peninsular Gneiss near Dodda Gudda, Holenarasipur belt in Hassan District. They have Komatiitic chemistry pointing to ultramafic volcanism in the Dharwar Craton during the Archaean Era.

Keywords

Petrology, Geochemistry, Spinifex Texture, Komatiite, Karnataka.

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D. V. Subba Rao ¹, S. N. Charan ¹

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

Abstract

A lense shaped gabbro-norite pluton emplaced into the Archaean migmatitic Peninsular gneiss and intruded in turn by 2.6 Ga Banavar granite is exposed in the vicinity of Kalyadi, Hassan district, Karnataka. The norite is metamorphosed to upper greenschist-lower amphibolite facies. The parental magma of this suite is noritic (high Mg and Si rich) and may have been derived from an enriched mantIe source at higher degrees of partial melting.

Keywords

Igneous Petrology, Geochemistry, Gabbro-Norites, Karnataka.

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D. V. Subba Rao ¹

Affiliations
1 National Geophysical Research Institute Hyderabad, IN

Abstract

No Abstract.

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D. V. Subba Rao ¹, M. W. Y. Khan ², D. N. Sridhar ¹, K. Naga Raju ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad - 560007, IN
2 Pt. Ravishankar Shukla University, Raipur (C. G)., IN

Abstract

Younger and fine-Grained unmetamorphosed dolerite dykes intrude the Raipur Group sediments in the central part of the Meso-Neoproterozoic Chhattisgarh Basin in Central India. Petrologically these dykes consist of unaltered microphenocrysts of intermediate pigeonite (X_Mg=0.40), augite (X_Mg=0.595), calcic plagioclase (X_ca=O.64), titanomagnetite and ilmenite. These rocks show high Fe₂O₃^T (14.43 to 15.19%), TiO₂ (2.75 to 2.87%) and P₂O₅(0.27 to 0.29%) contents with an unusual Continental Flood Basalt (CFB) affinity. Significantly these dykes show lower K₂O content (0.43%). The trace element abundance such as Ba, Sr, Rb, Zr, Nb and trace element ratios Sm/Nd, Ba/Nb, Zr/Nb, Y/Nb, Th/Yb, Ta/Yb and K/Ba in these rocks chemically resemble the least contaminated Deccan Basalts. Chondrite/OIB normalized REE patterns of these rocks also show striking similarities with the Deccan tholeiitic basaltic flows. The available petrological, mineralogical, major, trace and rare earth element data indicate that these within basin basaltic dykes of Raipur are intraplate basalts, which were derived from an enriched mantle source.

Keywords

Basaltic Dykes, Chhattisgarh Basin, Geochemistry, Continental Flood Basalts.

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D. K. Sinha ¹, D. V. Subba Rao ²

Affiliations
1 Scientific Officer/G, AMD, Plot No 188, Laxmi Nagar, Niwaru Road, Jhothwara, Jaipur - 302 012, IN
2 National Geophysical Research Institute, Hyderabad - 500 007, IN

Abstract

No Abstract.

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P. V. Sunder Raju ¹, S. Nirmal Charan ¹, D. V. Subba Rao ¹, B. Uday Raj ¹, S. M. Naqvi ¹

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

Abstract

Gold mineralisation occurs at Chikkasiddavanahalli (C S Hall!) in Archaean volcano-Sedimentary sequence of the Ingaldhal Formation, in Chiradurga Group, Dharwar craton. The C S Hal1i ridges and low mounds are about 1 2 km long and 100 in wide with N-S strike and vertical to steep easterly dips comprising mixed sulphide/oxide facies banded iron formations (MSOBIF) and associated ferruginous phyllites, metavolcanics and minor carbonates. Several horizons of BIF's occur as parallel to subparallel linear brands and extend over a strike length of 20-40 km with widths ranging from 2-25 m. These BIF s are highly sheared, deformed and metamorphosed to upper greenschist facies. The shears in BIF's are occupied by several syn-Tectonic quartz carbonate veins of various dimensions and found to be favourable loci for Gold-Sulphide mineralisation. Petrological studies show major mineral constituents with pyrite, arsenopyrite and subordinate grunerite, hornblende, chlorite, muscovite, ankerite, calcite, magnesian siderite and quartz. Most gold deposits world over are hosted in second order and subsidiary shear zones. The primary gold in BIFs is enriched with intense fracturing controlled by major shear systems extending from Gadag in the north and Doduguni in the south Active convergent margin settings related to accretionary processes and fluid activity are suggested to have led to Epigenetic gold mineralisation in BIFs at C S Halli area with an average gold grade of 2 08 g/t.

Keywords

Au Mineralisation, Structural Controls, Shear Zones, C S Halli, Chitradurga Schist Belt, Karnataka.

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V. Balaram ¹, D. V. Subba Rao ¹

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

Abstract

No Abstract.

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D. Srinivasa Sarma ¹, R. H. Sawkar ², S. N. Charan ², D. V. Subba Rao ², S. M. Naqvi ¹

Affiliations
1 National Geophysical Research Institute, Uppal Road, Hyderabad - 500 007, IN
2 292, 10th Main, Jayanagar, Bangalore - 560 041., IN

Abstract

No Abstract.

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D. V. Subba Rao ¹, V. Balaram ¹, K. Naga Raju ¹, D. N. Sridhar ¹

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

Abstract

Boninite-Like rocks represented by high-Ca boninitic dykes, melanogabbro dykes, recrystalized plagioclase bearing high MgO dykes and high-Mg nonte suites occur at few places in the vicinity of Meso to Neoproterozoic Chhattisgarh sedimentary basin in the northern Bastar craton in Central India. These rocks are formed in an intracratonic setting, not at convergent margin, similar to Archaean bonmitic rocks reported from intracratonic settings such as Mallina Basin, Northwest Australia and Abitibi, Optica regions of Canada. These high-MgO mafic dykes show a strong boninitic affinity with high SiO₂ (>52%), high MgO (9-15%), low TiO₂ (0 30-0 54 wt%) and strong LILE enrichment. These unusual dykes show distinct mineralogical, petrological and geochemical charactenstics and are totally different to that of the normally occurring abundant metadoleritic and metagabbroic dykes in Chhattisgarh region. The generation of bonmite magmatism requires unique thermal conditions such as shallow melting, elevated geothermal gradient and subducted slab flux. On the basis of field, geological, petrological and geochemical inferences on these Chhattisgarh boninitic and nontic dykes, a two-stage melting model and derivation from a strongly depleted mantle source, enriched later by metasomatic events is suggested.

Keywords

Boninitic-Noritic Dykes, Chhattisgarh, Geochemistry, Bastar Craton.

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D. V. Subba Rao ¹, M. Satyanarayanan ¹, D. Srinivasa Sarma ¹, K. S. V. Subramanyam ¹, K. Venkateswarlu ¹, M. Hanuma Prasad ²

Affiliations
1 CSIR-National Geophysical Research Institute, Hyderabad 500 007, IN
2 Geomysore Services (India) Pvt Ltd, Bengaluru 560 037, IN

Abstract

We report here the recent occurrence of native gold and platinoids, associated with Fe-Ni-Cu-Pb-Zn sulphides and tungsten-bearing phases in the gabbroic intrusions emplaced in the Betul-Chhindwara Belt in the Central Indian Tectonic Zone (CITZ). The ore minerals are represented mainly by native gold, pentlandite, chalcopyrite, pyrite, bowieite and rare phases of tungsten, cadmium and palladium, which were identified by scanning electron microscopy. The whole rock geochemistry shows anomalous Ni-Cu and platinum group elements (PGE) in which palladium is always higher in concentration than in platinum. The inter-element relationships of PGE such as Pd versus Ir and Pt versus Ir indicate their fractionation by a sulphide phase and also suggest that the parental magmas are primitive in nature having high-Mg, low- Ti characteristics, which indicate the boninitic/picritic nature of magmas. A thorough investigation is essential for exploring the Betul mafic-ultramafic intrusions for proving their potential as the host rocks for Au and Ni-Cu-PGE ores.

Keywords

Gabbro Intrusions, Geochemistry, Citz, Native Gold, Platinum Group Elements.

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S. M. Naqvi ¹, C. Manikyamba ¹, T. Gnaneshwar Rao ¹, D. V. Subba Rao ¹, M. Ram Mohan ¹, D. Srinivasa Sarma ¹

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

Abstract

Metavolcanics of 20 km wide Sultanpura volcanic block of the Neoarchaean Sandur (greenstone) schist belt are divided into tholeiitic basalts, high Mg basalts, Al-depleted and Al-undepleted komatiitic ultramatic schists. Metabasalts are metamorphosed to amphibolite facies, but still preserve their pillow structures. Ultramasic komatiitic rocks are transformed to actinolite-tremolitc schists with no recognizable original textures or mineralogy. Mctabasalts and ultramafic komatiitic schists are interbedded with thin layers of sulphidic banded iron formations, argillaceous carbonate rocks and carbon phyllites that are interprcied as metamorphosed pclagic sediments of the deep ocean. No terrigenous sediments are found in Sultanpura block indicating that eruption of these submarine volcanic rocks took place in the deeper part of the ocean, away from the western and eastern shelf parts of the Sandur belt, where terrigenous sediments are abundant. SuItanpura block in its west and east is discordantly boundcd by thrust, subduction complex and shclf lhcies sedimcnts. These observations are interpreted to indicate that Sultanpura block is a telescoped prolo-oceanic part between the two shelves and island arc complexes. MgO of melavolcanic rocks varies from 6 to 30%, with a gap between 16 to 22%. Al₂O₃/TiO₂ shows characteristic variation for tholeiites (10-15), high Mg basalts (13-21), Al-unclepleted ultramafic komatiitic schists (9-23) and Al-depleted ultramafic komatiitic schists (11-20). CaO/Al₂O₃, of tholciites and high Mg basalts is ∼1, whereas for the ultramafic komatiitic schists, this ratio exhibits a range between 0.5 to 2, as a consequence of CaO mobility.

REE, HFSE and ¹⁴³Nd/¹⁴⁴Nda ta from Sultanpura volcanic rocks vary between CHUR (Chondrite Uniform Rcservoir), primitive mantle and depleted manilc, but appear to be derived from primitive mantle and have been probably contaminated by continental crust. Although the abundance of REE varics from 2 to 12 chondrite, the patterns are smooth and flat with small negative or positive Eu anomalies as artifacts of alteration. Generally positive, but in few samples negative Nb anomalies are also found, with (Ce/Sm)_N, and (Gd/Yb)_N, being near chondritic. Ti/V, Ti/Zr, Zr/Y, Sc/Y, Nb/La, Nb/Th, Nb/U, MgO/TiO₂, MgO/FeO and Al₂O₃/TiO₂, also for many samples are ncar chondritic, εNd=+0.8649±0.00024 resembling CHUR. ThMb, NblU and some other ratios are near to those of Ontong Java and Gorgana plateaus (0.80 Ga) and the tholeiite-komatiitic sequence found in 2.7 Ga Southern volcanic zone, Abitibi belt of Canada and 2.1 Ga Birimian belt of West Africa. Collectively, thcsc data indicate that a mantle plume. derived from an enriched mantle, possibly played an important role in the oceanic volcanic sequence of Sultanpura block. Some of the HFSE follow the olivine control line, whereas other elements following the olivine control line define a narrow array tube. Formation of such array tubes on the plots of some HFSE elements and their ratios, and the scatters of HFSE/REE ratios, probably suggest dynamic melting of the plume during ascent. Entrenchment, mixing of Archaean ocean ridge basalts (AORB), crustal contamination and subduction of such a plume-fed slab may have generated the compositional heterogeneities observed in the Sultanpura metavolcanic rocks.

Keywords

Archaean Plumes, Greenstone Belts, Geochemistry, Tectonic Evolution, Sultanpura Volcanic Block, Sandur Schist Belt, Dharwar Craton.

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