DEEP FLUIDS AND ELECTRICAL-CONDUCTIVITY O F THE LOWER CONTINENTAL-CRUST

Current notions concerning the composition of the deep continental cru st support the possible existence of two types of crust. A granulitic crust which appears highly developed occurs in the Precambrian shields , whereas an eclogitic type corresponds to roots of collision orogens. The P-T values of the granulitic metamorphism are defined. A model fo r the granulitic crust formation is elaborated that assigns an essenti al role to deep synmetamorphic intrusions. Data on fluid inclusions re veal the importance of two fluids in particular: CO2 introduced locall y during peak metamorphism (carbonic metamorphism) and brines (aqueous fluids with high concentrations of dissolved salts, especially NaCl) that are distant remnants of premetamorphic fluids. Although most deep continental domains would have formed in the Precambrian, the continu ing importance of the fluids even up to the Present is affirmed by ele ctrical conductivity anomalies almost always found at these levels. Th ere are two possible sources for these anomalies: graphite, resulting from CO2 reduction, and brines. The latter appears the most plausible, and in any case the most generalized. The brines could be derived fro m vestiges of old premetamorphic fluids preserved in inclusions, lixiv iated by deep infiltration of surface fluids at the end of the metamor phic evolution of the continental domain.