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.

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C. Srikantappa ¹, M. Raith ², B. Spiering ²

Affiliations
1 Department of Geology, University of Mysore, Manasa Gangotri, Mysore 570006, IN
2 Mineralogisch-Petrologisches Institut, Universitat Bonn, Poppelsdorfer Schloss, 5300 Bonn, Fed Rep, DE

Abstract

The crustal segment south of the Achankovil zone in southern Kerala is built up by two litho-tectonic units: The Nagerkoil unit is represented by a series of acid to intermediate magmatogenic gneisses and charnockites associated with layered anorthositic to noritic complexes. The Pon Mudi unit consists of interlayered garnet-biotite gneisses (leptynites), garnet-biotite-sil1imanite gneisses (khondalites) and garnet-biotite-sillimanite-cordierite metatexites which represent a series of intensely deformed psammitic and pelitic sediments metamorphosed to upper amphibolite grade (700-750°C; 6-8 k bars). Subsequent to regional metamorphism in the POD Mudi unit, the leptynites have been partly transformed to massive orthopyroxene-garnet-bearing charnockites along a conjugate set of fractures (s1: N30E ilDd s2: S70E) and the foliation planes of the rocks (N30-60W).

The geochemical data and the results of geothermobarometry show that charnockitization was essentialJy isochemical and occurred at 750 ± 50°C and 6 ± 1 kbars lithostatic pressure. A complex but conformable development of tbe fluid phase composition during metamorphism is indicated by the occurrence of at least four generations of fluid inclusions (almost pure H₂O and CO₂ inclusions, mixed CO₂-H₂O inclusions and CH₄-N₂ inclusions) in both the leptynites and the charnockites. The common presence of graphite+pyrrhotite+ilmenite furthermore suggests internal buffering of the fluid composition at oxygen fugacities below those defined by the quartz-magnetite-ferrohypersthene assemblage and XCO₂>0.5.

The results of tbe present study indicate that charnockitization probably was not caused by the influx of CO₂-rich fluids of deep-seated origin, tbe presently favoured model of granulite genesis (Newton, 1984), but rather was induced by an isothermal decrease of fluid pressure relative to lithostatic pressure, due to migration of the pore fluid into the network of fractures and ascent into higher crustal levels.

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C. Srikantappa ¹, M. Raith ², B. Spiering ²

Affiliations
1 Department of Geology, University of Mysore, Mysore 570006, IN
2 Institute for Mineralogy and Petrology, University of Bonn, Poppelsdorfer Schloss, Bonn, 5300, West Germany, DE

Abstract

No Abstract.

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I. Braun ¹, M. Raith ¹

Affiliations
1 Mineralogisch-Petrologisches Institut, Universitat Bonn, Poppelsdorfer Schlobb, D-53115 Bonn, DE

Abstract

This paper presents the first description of fluorapatite in rocks of the Kerala Khondalite Belt in southern India. The fluorapatites occur either dispersed or aggregated in granitic and pegmatitic veins which intruded into high-grade metamorphic gamet-biotite gneisses. Microtextures indicate an exclusively magmatic origin for the fluorapatites and the strong zoning of F and OH points to a change in fluid/melt composition during apatite crystallization. The P-T conditions of melt crystallization are estimated to be in the range of 750-800°C and ≤5 kbar. The origin of the fluorapatite bearing melts is poorly known. It is suggested that they formed either through fluid-absent incongruent melting of a F-rich biotite or during a late stage of fractional crystallization of a granitic parent magma. Additionally, we report a single occurrence of a monazite-rich quartzmonzodiorite vein. Field observations indicate an emplacement subsequent to the fluorapatite bearing granites and pegmatites. The occurrence of magmatic andalusite and cordierite in this rock points to a crystallization at low pressures (P ≤ 3 kbar).

Keywords

Petrology, Fluorapatite, Monazite, Pegmatite, Kerala Khondalite Belt.

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