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  

作　　者：Oleg G.Safonov Elizaveta I.Kovaleva Svetlana A.Kosova H.M.Rajesh Georgy A.Belyanin Maria A.Golunova Dirk D.Van Reenen 

机构地区：[1]Institute of Experimental Mineralogy RAS,Chernogolovka,Moscow Region 142432.Russia [2]Department of Petrology.Moscow State University,Moscow 119899,Russia [3]Department of Geology,University of Johannesburg,P.O.Box 524,Auckland Park 2006,Johannesburg,South Africa [4]Labor fur Geochronologie,Department fur Lithospharenforschung,Universitat Wien,Althanstrasse 14,A-1090 Vienna,Austria [5]Institute of Experimental Mineralogy RAS,Chernogolovka,Moscow Region 142432,Russia [6]Department of Geology,University of Johannesburg,P.O.Box 524,Auckland Park 2006,Johannesburg.South Africa

出　　处：《Geoscience Frontiers》2012年第6期829-841,共13页地学前缘（英文版）

基　　金：supported by Russian Foundation for Basic Research(project 10-05-00040 to OGS);Russian President Grants for Young Scientists(MD-222.2012.5 to OGS);grant from the National Science Foundation of South Africa(GUN:20531 92 to DDvR);University of Johannesburg as a part of the Russian South African scientific collaboration

摘　　要：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.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 

分 类 号：P597.3[天文地球—地球化学]