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

S. M. Hussain ¹, S. M. Naqvi ¹, T. Gnaneshwar Rao ¹

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

Abstract

The ultramafic rocks of Holenarasipur schist belt show micro-spinifex texture, deformed pillow lava-like structures and contain interbedded fuchsite-quartzite. Compositionally the mafic-ultramafic rocks are similar to peridotitic, pyroxenitic and basaltic komatiites. Available data exhibit a gap between 20 and 30% MgO content, which suggests discontinuity in their composition. A two stage melting model is projected to explain this compositional gap. The abundance of komatiitic mafic-ultramafic rocks supports the view that the Holenarasipur schist belt is a true greenstone belt.

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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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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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