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Fluid-rock interaction in retrograde granulites of the Southern Marginal Zone,Limpopo high grade terrain,South Africa

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Geoscience Frontiers 2014年第5期5卷 673-682页

作者： Jan Marten Huizenga dirk van reenen Jacques L.R.Touret Unit of Environmental Sciences and Management North-West UniversityPrivate Bag X60012520South Africa Economic Geology Research Institute(EGRU) School of Earth and Environmental SciencesJames Cook UniversityTownsvilleQLD 4811Australia Department of Geology University of JohannesburgJohannesburgSouth Africa Institut de Mineralogie.de Physique des Materiaux et de Cosmochimie(IMPMC) Sorbonne Universites UPMC Univ Paris 06UMR CNRS 7590 Museum National d'Histoire Naturelle.IRD UMR 206.4 Place Jussieu.F-75005

Fluid infiltration into retrograde granulites of the Southern Marginal Zone （Limpopo high grade terrain） is exemplified by hydration reactions, shear zone hosted metasomatism, and lode gold mineralisation. Hydration reactions include the breakdown of cordierite and orthopyroxene to gedrite ＋ kyanite, and anthophyllite, respectively. Metamorphic petrology, fluid inclusions, and field data indicate that a low H2O-activity carbon-saturated CO2-rich and a saline aqueous fluid infiltrated the Southern Marginal Zone during exhumation. The formation of anthophyllite after orthopyroxene established a regional retrograde anthophyllite-in isograd and occurred at P-Tconditions of -6 kbar and 610 ℃, which fixes the minimum mole fraction of H20 in the CO2-rich fluid phase at ~ 0.1. The maximum H20 mole fraction is fixed by the lower temperature limit （~800 ℃） for partial melting at -0.3. C-O-H fluid calculations show that the CO2-rich fluid had an oxygen fugacity that was 0.6 log10 units higher than that of the fayalite-magnetite- quartz buffer and that the CO2/（CO2＋CH4） mole ratio of this fluid was 1. The presence of dominantly relatively low density CO2-rich fluid inclusions in the hydrated granulites indicates that the fluid pressure was less than the lithostatic pressure. This can be explained by strike slip faulting and/or an increase of the rock permeability caused by hydration reactions.

关键词： GranuliteFluidLimpopoRetrogradeSouthern Marginal Zone

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Experimental and petrological constraints on local-scale interaction of biotite-amphibole gneiss with H_2O-CO_2-(K,Na) Cl fluids at middle-crustal conditions:Example from the Limpopo Complex,South Africa

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Geoscience Frontiers 2012年第6期3卷 829-841页

作者： Oleg G.Safonov Elizaveta I.Kovaleva Svetlana A.Kosova H.M.Rajesh Georgy A.Belyanin Maria A.Golunova dirk D.van reenen Institute of Experimental Mineralogy RAS ChernogolovkaMoscow Region 142432.Russia Department of Petrology.Moscow State University Moscow 119899Russia Department of Geology University of JohannesburgP.O.Box 524Auckland Park 2006JohannesburgSouth Africa Labor fur Geochronologie Department fur LithospharenforschungUniversitat WienAlthanstrasse 14A-1090 ViennaAustria Institute of Experimental Mineralogy RAS ChernogolovkaMoscow Region 142432Russia Department of Geology University of JohannesburgP.O.Box 524Auckland Park 2006Johannesburg.South Africa

Reaction textures and fluid inclusions in the -2.0 Ga pyroxene-bearing dehydration zones within the Sand River biotite-hornblende orthogneisses （Central Zone of the Limpopo Complex） suggest that the formation of these zones is a result of close interplay between dehydration process along ductile shear zones triggered by H2O-CO2-salt fluids at 750--800 ℃ and 5.5--6.2 kbar, partial melting, and later exsolution of residual brine and H2O-CO2 fluids during melt crystallization at 650--700 ℃. These processes caused local variations of water and alkali activity in the fluids, resulting in various mineral assemblages within the dehydration zone. The petrological observations are substantiated by experiments on the interaction of the Sand River gneiss with the H2O-CO2-（K, Na）Cl fluids at 750 and 800 ℃ and 5.5 kbar. It follows that the interaction of biotite-amphibole gneiss with H2O-CO2-（K, Na）CI fluids is accompanied by partial melting at 750--800 ℃. Orthopyroxene-bearing assemblages are characteristic for temperature 800 ℃ and are stable in equilibrium with fluids with low salt concentrations, while salt-rich fluids produce clinopyroxene-bearing assemblages. These observations are in good agreement with the petrological data on the dehydration zones within the Sand River olthogneisses.

关键词： Charnockite Fluid-rock interaction CO2-rich fluids NaCl-KCl brines Partial melting Experimental petrology

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