PETROLOGY OF LATE PROTEROZOIC MAFIC DIKES IN THE NICO-PEREZ REGION, CENTRAL URUGUAY

Tholeiitic basaltic and basaltic andesite dikes of Brasiliano (or PanA frican) age (similar to 600 Ma) intrude the basement of the Nice Perez region, Uruguay. Major and trace element geochemistry of the basalts indicates that they suffered fractionation in shallow magma chambers. The variation in element ratios, which remain virtually unchanged duri ng fractionation (K/Rb, Rb/Ba, Ba/Nb, La/Nb, Zr/Nb and Ti/Zr), indicat e that the dikes are not strictly comagmatic. However, they have certa in features in common: LILE and LREE enrichment with respect to HFSE a nd HREE; high Rb/Ba(> 0.9) and Rb/Sr (> 0.08); low K/Rb (< 214); negat ive Nb and Ti anomalies (La-n/Nb-n > 2; Ba/Nb > 22, Ti/Zr < 60). Sr-87 /Sr-86 and Nd-143/Nd-144 at 665 Ma are in the range 0.7052 - 0.7119 an d 0.51158 - 0.51177, respectively. The lack of correlations between is otope and trace element variations indicate that these characteristics are not controlled by crustal contamination of the melts. They are in terpreted as being due to the melting of an enriched mantle (C1) under the influence of a fluid-rich component (C2) which stabilized a Nb-re taining titanate phase in the residuum. Although this process may be r elated to a subduction environment, it is also possible that it occurr ed in an ensialic region by the interaction of deep mantle fluids with the lithospheric continental mantle. The parent mantle underlying the early Proterozoic (1.8 Ga) Florida region had isotope and geochemical characteristics which could evolve to the values observed in the adja cent Nice Perez region. It is therefore proposed that mantle enrichmen t took place in the early Proterozoic and that this mantle melted unde r the influence of fluids in the late Proterozoic to derive the Nice P erez dikes.