EARLY PROTEROZOIC DIKE SWARMS FROM WESTERN URUGUAY - GEOCHEMISTRY, SR-ND ISOTOPES AND PETROGENESIS

Early Proterozoic (1.86 +/- 0.12 Ga), unmetamorphosed basic dikes intr ude the Precambrian Rio de la Plata Craton of western Uruguay. The dik es define two distinct swarms of tholeiitic andesitic basalt and andes ite composition, respectively. Major- and trace-element geochemistry i ndicates that these two magma types are related via gabbro fractionati on. The incompatible trace-element patterns favour a derivation of the parent melts from melting of a garnet peridotite source. The high LIL E and LREE content is discussed in terms of crustal contamination of t he melts or of the source, or of mantle metasomatism. Mixing calculati ons do not support important crustal contamination of the melts or of the source. Mantle metasomatism is a preferred process, which operated slightly before or during melting. An anorogenic ensialic environment of emplacement is inferred.