Petrology of the upper proterozoic mafic dikes in the Nico Perez region, Central Uruguay

Tholeiitic basaltic and basaltic andesite dikes of Brasiliano (or PanAfrican) 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 during fractionation (K/Rb, Rb/Ba, Ba/Nb, La/Nb, Zr/Nb and Ti/Zr), indicate that the dikes are not strictly comagmatic. However, they have certain features in common: LILE and LREE enrichment with respect to HFSE and HREE; high Rb/Ba(> 0.9) and Rb/Sr (> 0.08); low K/Rb (< 214); negative 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 and 0.51158 - 0.51177, respectively. The lack of correlations between isotope and trace element variations indicate that these characteristics are not controlled by crustal contamination of the melts. They are interpreted as being due to the melting of an enriched mantle (C1) under the influence of a fluid-rich component (C2) which stabilized a Nb-retaining titanate phase in the residuum. Although this process may be related to a subduction environment, it is also possible that it occurred 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 adjacent Nice Perez region. It is therefore proposed that mantle enrichment took place in the early Proterozoic and that this mantle melted under the influence of fluids in the late Proterozoic to derive the Nice Perez dikes.