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

M. N. Viswanathiah ¹, M. A. Shankara ¹, A. S. Janardhan ²

Affiliations
1 The Mineralogical Institute, Manasa Gangotri, Mysore - 570006, IN
2 Department of Geology, Manasa Gangotri, Mysore-570006, IN

Abstract

The green garnets found in the calc-silicate horizons around Bandihalli, is a chrome grossular (∼7% Cr₂O₃) with appreciable amount of Mn content. Thus, a partial miscibility between uvarovite and spessartite end members appears possible. The occurrence of thin band of graphite bearing quartzite within the calc-silicate horizons points to its metasedimentary origin. The presence of another variety of garnet (pyrope-almandine rich) in the associated cordierite bearing rock of the area shows that the composition of garnet may not be wholly dependant on grade of metamorphism.

Full Text

   

A. S. Janardhan ¹, M. Jayananda ², M. A. Shankara ¹

Affiliations
1 Department of Geology, University of Mysore, Manasagangothri, Mysore-570006, IN
2 Department of Geology, Bangalore University, Bangalore 560 056, IN

Abstract

The granulites of the Biligiri Rangan (BR) and Niligiri Hill ranges are characterised by distinct field, geochemical and isotopic signatures. The B.R. Hill granulites contain a significant metasedimentary component (pelite-BIF-Mn-horizons), whilst metasediments are rare in the Niligiri granulite terrain. Lithologic and structural continuity between the two terrains cannot be demonstrated as they are separated by E-W running Moyar shear zone. Isotopic data suggest that B.R. Hills and Niligiri granulites have had different pre-crustal histories. Sm-Nd model TDM ages and U-Pb zircon concordia upper intercept age suggest that the protoliths of B.R.Hill quartzo-feldspathic charnockites accreted at 3.4Ga. On the other hand, Sm-Nd model TDM ages indicate that the magmatic protoliths of Niligiri granulites differentiated from mantle at 2.6 Ga.

Geochemical data from the B.R. Hills charnockites, particularly low-Mg numbers (0.2-0.3), low content of transition and HFS elements and strongly fractionated REE patterns with HREE depletion, suggest that their magmatic protoliths were derived by partial meltingof young, hydrated oceanic crust, in which garnet and amphibole were important residual phases. The high content of transition elements together with low total LREE and less-or unfractionated REE patterns of the B.R. Hills mafic granulites imply the derivation of their protoliths from long term depleted mantle sources. On the other hand the high Mg numbers (0.5-0.6), high content of transition and HFS elements, moderately fractionated REE patterns with HREE enrichment suggest that the protoliths of the Niligiri granulites are interpreted to have formed by melting of metasomatized mantle wedge with olivine and spinel as residual phases.

The Niligiri terrain was accreted onto the Dharwar Craton at C.2.6 Ga and the two terrains witnessed a common major granulite event at 2.51 Ga.

Keywords

Granulites, Isotopes, Geochemistry, Tectonics, B. R. Hills, Niligiri Hills, Karnataka, Tamil Nadu.

Full Text

     

Fareeduddin ¹, M. A. Shankara ², B. Basavalingu ², A. S. Janardhan ²

Affiliations
1 H.408, Ramesh marg, C Scheme, Jaipur 302 001, IN
2 Department of Geology, Univeisity of Mysore, Manasagangotri, Mysore-570 006, IN

Abstract

A brief account of the petrology and structure of medium grade pelitic granulites and associated charnockites occurring as slivers in the Anasagar migmatitic gneiss is given. These granulite slivers have been traced along strike up to 40 km and may cover a much wider area. Cordierite bearing pelitic granulites associated with charnockites around Chinwali were chosen for estimating metamorphic P-T conditions. The pelitic granulites are represented by the assenlblage opx-cord-gt-bio--silli-plag-k felds- qtz. Biotite-fibrolite intergrowths grow on cordierite which has opx inclusions. Garnet is often smeared with graphite. Metamorphic conditions of petite show peak pressures of 7.5 kb at 640°C. Charnockites generally record retrogressive pressure conditions of 5.7 - 4.0 kb. The mineralogy of the Chinwali granulites and P-T condition compare well with that of Sandmata granulites. The presence of these granulites, north of Ajmer, in basement migmatitic gneisses suggest the presence of an older deep crustal fragment (ca. 1.7 Ga) along the northwestern margin of the Aravalli Delhi Mobile Belt (ADMB) and belies the prevailing concepts of eastward crustal thickening and westward age polarity in the western Indian shield.

Keywords

Granulite, Charnockite, Petrology, Ajmer, Chinwali, Rajasthan.

Full Text