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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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K. R. Jayaraj ¹, S. D. Khanadali ¹, T. C. Devaraju ¹, B. Spiering ²

Affiliations
1 Department of Studies in Geology, Karnatak University, Dharwad 580 003, IN
2 Mineralogisch-Petrologisches Institut, Universitat Bonn, Poppelsdorfer Schlos, 5300 Bonn 1, DE

Abstract

Hogbomite is of widespread occurrence as an accessory and minor mineral in the V-Ti-Fe ore deposits of Ubrani, Tavarekere, Masanikere, Magyatahalli and Devaranarsipur in Shimoga district and Mulemane in Uttar Kannada district. Both primary and secondary hogbomite are found. The mineral shows considerable variation in chemistry. The analytical data indicate that hogbomite structure is relatively more open than that of coexisting magnetite/ilmenite and can admit a variety of substitutions.

Keywords

Hogbomite, Electron Microprobe Analysis, Iron Ore Deposits, Karnataka.

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