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

A. S. Janardhan ¹, N. Shadakshara Swamy ¹, G. R. Ravindra Kumar ²

Affiliations
1 Department of Geology, University of Mysore, Manasa Gangotri, Mysore 570006, IN
2 Centre of Earth Science Studies, PB No. 2235, Trivandrum 695010, IN

Abstract

Petrography and chemistry of hitherto unreported manganiferous horizons and associated recrystallised ultramafics (sagvandites) - key horizons in the Precambrian high grade terrain around Gundlupet are discussed. The occurrence of spessartine-rich garnets from an area of dominant upper amphibolite grade of metamorphism suggests that bulk chemistry was the most important controlling factor. In their mineralogy, and decrease of MnO% in garnets during subsequent migmatisation, the manganiferous horizons exhibit remarkable similarity to the khondalites of the Eastern Ghats. The sagvandites and the other lithologies, occurring now, as huge enclaves in the granite/gneissic terrain are similar to the Sargur Supracrustals.

Structural studies show that the area has suffered from three deformational episodes. The early N-S episode of the Sargurs are rotated to N 60° E and then to E-W. The dominant N 60° E episode is possibly connected with large scale granite emplacementfmigmatisation and the E-W shears are due to the prominent Moyar-Bhavani shear.

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A. S. Janardhan ¹, N. Shadakshara Swamy ², R. Capdevila ³

Affiliations
1 Department of Geology, University of Mysore, Mysore 570006, IN
2 Department of Geology, Bangalore University, Bangalore 560056, IN
3 Institute of Geology, University of Rennes, 35031-Rennes, FR

Abstract

High-grade Archaean Sargur supracrustals are made up of quartzites, pelites, carbonates, BIF and manganiferous horizons and have been metamorphosed to upper amphibolite facies. The BIF and manganiferous horizons form thin but persistent bands. Iron-manganese-carbonate association is characteristic. Ten samples of BIF and spessartine-rich manganiferous horizons were analysed for major, trace and rare earth elements. Low trace and REB abundances characteristic of these bands agree well with the data of Archaean iron-formations elsewhere. Chemistry of Sargur BIF is distinctly different from that of the Algoma and Superior types. The positive Eu anamoly shown by the iron-formation indicates oxygenic conditions. The low content of some trace elements indicates shallow water environment of deposition. The REE concentrations of manganiferous-horizons are relatively higher than that of the BIF with anomalous behaviour of Ce and Eu. The present data show that the iron and manganese separation had taken place even in the Early Archaean.

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A. S. Janardhan ¹, N. Shadakshara Swamy ², R. Capdevila ³

Affiliations
1 Department of Geology, University of Mysore, Mysore 570 006, IN
2 Department of Geology, Bangalore University, Bangalore 560 056, IN
3 Institute of Geology, University of Rennes, Rennes, FR

Abstract

Metapelites form one of the important supracrustal lithounits of the quartzite-pelite-carbonate association in the Sargur high-grade terrain of Southern Karnataka. These pelites have low ferromagnesium trace elemental concentrations, common to the pelites of the Archaean high-grade terrains elsewhere. Though concentration of Sc and U are comparable with the Archaean rocks, the Th concentration and majority of the trace and rare earth elemental ratios are more akin to post-Archaean levels. The REE pattern indicates a mixed provenance dominated by acid igneous rocks. Significant variability in the REB pattern advocates a lithologically complex but definitely a bimodal provenance and also inefficient mixing of source rocks.

Keywords

High-Grade Terrain, Karnataka, Pelite, REE, Sargur Supracrustal, Trace Element, Geochemistry.

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