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

Early Proterozoic (1.86 +/- 0.12 Ga), unmetamorphosed basic dikes intrude the Precambrian Rio de la Plata Craton of western Uruguay. The dikes define two distinct swarms of tholeiitic andesitic basalt and andesite composition, respectively. Major- and trace-element geochemistry indicates that these two magma types are related via gabbro fractionation. The incompatible trace-element patterns favour a derivation of the parent melts from melting of a garnet peridotite source. The high LILE and LREE content is discussed in terms of crustal contamination of the melts or of the source, or of mantle metasomatism. Mixing calculations 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.