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

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Progressive Charnockitization of a Leptynite-Khondalite Suite in Southern Kerala, India- Evidence for Formation of Charnockites Through Decrease in Fluid Pressure?


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

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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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  • Progressive Charnockitization of a Leptynite-Khondalite Suite in Southern Kerala, India- Evidence for Formation of Charnockites Through Decrease in Fluid Pressure?

Abstract Views: 216  |  PDF Views: 2

Authors

C. Srikantappa
Department of Geology, University of Mysore, Manasa Gangotri, Mysore 570006, India
M. Raith
Mineralogisch-Petrologisches Institut, Universitat Bonn, Poppelsdorfer Schloss, 5300 Bonn, Fed Rep, Germany
B. Spiering
Mineralogisch-Petrologisches Institut, Universitat Bonn, Poppelsdorfer Schloss, 5300 Bonn, Fed Rep, Germany

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.