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Maibam B.,Physical Research Laboratory | Maibam B.,Manipur University | Goswami J.N.,Physical Research Laboratory | Srinivasan R.,Geomysore Services India Pvt. Ltd
Journal of Earth System Science | Year: 2011

207Pb-206Pb ages of zircons in samples of metasediments as well as ortho- and para-gneisses from both the western and the eastern parts of the Dharwar craton have been determined using an ion microprobe. Detrital zircons in metasedimentary rocks from both yielded ages ranging from 3.2 to 3.5 Ga. Zircons from orthogneisses from the two parts also yielded similar ages. Imprints of younger events have been discerned in the ages of overgrowths on older zircon cores in samples collected throughout the craton. Our data show that the evolution of the southwestern part of eastern Dharwar craton involved a significant amount of older crust (>3.0 Ga). This would suggest that crust formation in both the western and eastern parts of the Dharwar craton took place over similar time interval starting in the Mesoarchaean at ca. 3.5 Ga and continuing until 2.5 Ga. Our data coupled with geological features and geodynamic setting of the Dharwar craton tend to suggest that the eastern Dharwar craton and the western Dharwar craton formed part of a single terrane. © Indian Academy of Sciences. Source

Ugarkar A.G.,Karnatak University | Solankar S.N.,Karnatak University | Vasudev V.N.,Geomysore Services India Pvt. Ltd
Journal of the Geological Society of India | Year: 2013

In the Kolar greenstone belt of the Dharwar craton, felsic metavolcanics are encountered prominently in its eastern region around Surapalli and Marikoppa. These felsic volcanic rocks are essentially homogeneous and their bulk mineralogy is almost the same. They consist of phenocrysts of quartz and feldspar, set in a fine-grained quartzo-feldspathic groundmass. They are calc-alkaline rhyolite in composition, and are characterized by high SiO 2 (av. 75.74 wt.%), moderate Al2O3 (av. 11.84 wt.%), Na2O (av. 3.55 wt.%), K2O (av. 3.26 wt%) contents and low Mg# (av. 6.07), Cr (av. 8 ppm), Ni (av. 8 ppm), Sr (av. 331 ppm.), Y (av. 7 ppm), Yb (av. 0.87 ppm) and Nb/Ta (av. 6.40) values, suggesting Tonalite-Trondhjemite- Granodiorite (TTG) affinity for these felsic volcanics. They are strongly fractionated [(La/Yb)N- = 14.41-48.70] with strong LREE enrichment [(La/Sm)N = 2.50-3.59] and strong HREE depletion [(Gd/Yb)N = 1.34-2.77] with positive Eu anomaly. The regional geological set-up, petrographic and geochemical characteristics suggest that these felsic volcanics probably were derived by partial melting of a subducting basalt slab at shallow depth without much involvement of mantle wedge in an island arc geodynamic setting. © 2013 Geological Society of India. Source

Sarangi S.,Indian School of Mines | Sarkar A.,Indian Institute of Technology Kharagpur | Srinivasan R.,Geomysore Services India Pvt. Ltd | Patel S.C.,Indian Institute of Technology Bombay
Journal of Asian Earth Sciences | Year: 2012

Carbon and oxygen isotopic compositions of carbonates from auriferous quartz carbonate veins (QCVs) of two orogenic gold deposits - Ajjanahalli and Guddadarangavvana Halli (G.R. Halli) - from the Neoarchean Chitradurga schist belt of the Dharwar craton, southern India are examined to understand the origin of the mineralizing fluids. The average carbonate carbon (δ 13C pdb) and oxygen (δ 18O smow) isotope compositions of QCVs of Ajjanahalli are -5.5±1.3‰ and 14.1±2.7‰, respectively. The same ratios for the QCVs of G.R. Halli are -6.2±1.9‰ and 14.1±0.5‰. The corresponding average fluid δ 13C and δ 18O compositions are -5.81±1.14‰, 13.78±5.1‰ for Ajjanahalli and -4.64±0.7‰, -6.50±0.6‰ for G.R. Halli. The δ 13C pdb of syn-sedimentary carbonates of BIF of Ajjanahalli (-1.8±0.1‰), carbonated metabasalts of Ajjanahalli (-1.4‰) and G.R. Halli (-1.3‰) fall in the compositional range of marine carbonates (0±2‰). As dissolution/decarbonation reactions during metamorphism of pre-existing carbonate/carbonated rocks produce CO 2 with δ 13C values similar to or more enriched than parent rock, the carbonate or fluid δ 13C ratios of the QCVs (which fall in the compositional range of mantle/magmatic derived CO 2 or carbonates) obtained in this work cannot be the result of metamorphism. It is proposed that gold mineralizing fluids were derived from juvenile magmatic melts and were channeled through crustal scale shear zones to give rise to the gold deposits. © 2012 Elsevier Ltd. Source

Rekha S.,Indian Institute of Technology Kharagpur | Upadhyay D.,Indian Institute of Technology Kharagpur | Bhattacharya A.,Indian Institute of Technology Kharagpur | Kooijman E.,University of Munster | And 3 more authors.
Precambrian Research | Year: 2011

The Central Indian Tectonic Zone (CITZ) is a ∼700. km long orogenic belt in north-central India that demarcates the accretion zone between the Northern and Southern Indian crustal domains. However, the timing of accretion remains speculative. The Chottanagpur Gneiss Complex (CGC), the North Singhbhum Mobile Belt (NSMB) and the Singhbhum Craton (SC) are three crustal units located at the eastern extremity of the CITZ. The CGC, dominated by blastoporphyritic granitoids intrusive into amphibolite- to granulite facies gneisses, is juxtaposed against the greenschist-facies northern metaflysch belt in the NSMB (N-NSMB) north of the Dalma Ophiolite Belt (DOB), along the Northern Shear Zone (NSZ). Both south CGC and N-NSMB exhibit correlated increase in strain and prograde metamorphism towards the NSZ. Laser ablation inductively coupled plasma mass spectrometry U-Pb zircon and U-Th-Pb monazite chemical ages from the southern parts of the CGC lie between 1.1 and 0.9. Ga. Monazite from N-NSMB are of similar age. In contrast, zircon from central parts of the CGC away from the NSZ preserves older concordant ages (up to 2.6. Ga), and the 1.0-0.90. Ga age population is scarcely represented. South of the DOB, the ∼1.0. Ga ages are absent in S-NSMB. Instead, monazites from all samples in S-NSMB record a tightly constrained 1.59-1.56. Ga high-P metamorphic event, arguably a result of northward subduction of the metaflysch belt. These two terranes were in turn accreted to the CGC at 1.0-0.90. Ga. This accretion at the leading edge of the amalgamated SC-S-NSMB block led to the emplacement of expansive granitoids in the CGC. Apparently, the polychronous CITZ orogen marks the gradual closure of a major ocean basin separating the crustal domains of the Northern and Southern Indian blocks during the Meso- and the Early Neoproterozoic era. © 2011 Elsevier B.V. Source

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