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

P. Raase ¹, M. Raith ², D. Ackermand ¹, M. N. Viswanatha ³, R. K. Lal ⁴

Affiliations
1 Meneralogisch-Petrographisches Institut, der Universitat Olshausenstr, 40-60, D 2300 Kiel, DE
2 Mineralogisch-Petrologisches Institut, der Universitat Bonn, Poppelsdorfer Schloss, D 5300 Bonn, DE
3 Geological Survey of India, Southern Region, Hyderabad 500001, IN
4 Department of Geology, Banaras Hindu University, Varanasi 221005, IN

Abstract

The mineralogy of chromiferous quartzites from Archaean greenstone belts of southern Peninsular India has been studied. The majority of the quartzites are metamorphosed clastic sediments derived from a sialic source with enclaves of ultramafic chromite-bearing rocks. For the quartzites from the Ghatti Hosahalli belt, however, a volcanic-exhalative origin is most likely. The chromium in the sediments was fixed in the detrital chromite and/or the clay minerals.

The typical mineral assemblages developed in a wide range of metamorphic conditions from greenschist to amphibolite facies comprise fuchsite, quartz, kyanite and accessory rutile, tourmaline and Cr-spinel. Microprobe analyses show high chromium contents for fuchsite (2-4wt. % Cr₂O₃), kyanite (up to 6wt. % Cr₂O₃), rutile (1.0-2-6wt. % Cr₂O₃) and tourmaline (2.5-7.6 wt. % Cr₂O₃). Cr-spinels are essentially solid solutions between hercynite and chromite (Mg/(Mg+Fe²⁺)=0.060.15; Cr/(Cr+Al)=0.42-0.75). Comparison with igneous chromites indicates that compositional reequilibration during metamorphism was characterised by replacement of Mg by Fe²⁺ and, to a lesser extent, of Cr³⁺ by Al. The micas of chromiferous quartzites from the Chitradurga area have high Cr and Ba contents (1.4-3.9wt. % Cr₂O₃; 0.5-8.5 wt. % BaO) and represent solid solutions extending from muscovite to Cr-oellacherite, A baryte-bearing quartzite from the Ghatti Hosahalli belt contains the rare assemblage quartz, baryte, celsian (Cs₈₅Or₁₁Ab₄), fuchsite (15 wt. % Cr₂O₃; 8.5 wt. % BaO), uvarovite (Uv₇₁Gro₂₈) and tremolite.

The regular distribution of chromium between mica and associated Cr-spinel, rutile and tourmaline indicates equilibration during metamorphism.

Full Text

     

V. K. Gairola ¹, D. Ackermand ²

Affiliations
1 Department of Geology, Banaras Hindu University, Varanasi 221005, IN
2 Mineralogy and Petrography Institute, 40-60, Olshausen Str., 2300 Kiell, West Germany, DE

Abstract

Pelitic schists, gneisses and amphibolites from the Central Crystallines of the Garhwal Himalaya display evidence of multiple deformation (D₁, D₂ and D₃) and polymetamorphism (M₁, M₂ and M₃)' During the M₁ episode of regional metamorphism which represents a thermal peak, the pelitic schists and gneisses developed the assemblage: sillimanite-kyanite-garnet- biotite-plagioclase-K-feldspar-quartz±garnet. Widespread development of garnet and sillimanite (locally kyanite) in muscovite-bearing schists and gneisses, hornblende in amphibolites, and diopside in calcschists indicate prograde metamorphism in the sillimanite-muscovite zone of the amphibolite facies. During the M₂ and M₃ episodes the higher grade rocks of the M₁episode retrograded under PT conditions of greenschist facies.

The electron microprobe analyses of coexisting minerals from the schists. gneisses and amphibolites have been used to estimate the PT conditions of metamorphism during the Ml episode through the pertinent models of geothermobarometry. These models of geothermobarometry suggest temperatures of 645°C±50°C and pressures of 7.5±0.5 kb during the thermal peak of metamorphism (M₁ episode).

Full Text