| Popis: |
The Campo Formoso and Carnaiba granites belong to a suite of middle Proterozoic magmatic rocks located in the northern part of the Sao Francisco craton. They intrude the Archaen basement and Lower Proterozoic Jacobina volcanosedimentary series. Emerald-bearing K-metasomatites in the mining districts of Campo Formoso and Carnaiba are developed within serpentinites at the contact with granite-related pegmatitic veins. KAr and 40 Ar/ 39 Ar measurements were performed on biotites and deuteric muscovites from these two granites, and phlogopites from the K-metasomatites. For the Campo Formoso granite, the biotites yield ages between 1875 ± 45 Ma and 1908 ± 47 Ma (2 σ ) and the muscovites yield ages of 1897 ± 34 Ma and 2040 ± 24 Ma (2 σ ). For the Carnaiba granite, the biotites and muscovites fit isochrons with ages of 1888 ± 32 and 1979 ± 28 Ma (2 σ ), respectively. In contrast, phlogopites from emerald-bearing metasomatites display KAr ages that spread between 1900 and 2000 Ma with an isochron of 1973 ± 20 Ma (2 σ ) for Carnaiba. Generally, the youngest biotite and phlogopite ages occur for specimens where these minerals are chloritized. 40 Ar/ 39 Ar step heating release spectra are complex but give integrated ages in good agreement with the KAr ages. The least disturbed spectrum permits assignment of an age of 2032 ± 10 Ma (2 σ )for the first granitic pulse of the emplacement of the Campo Formoso composite pluton. Since in Carnaiba, deuteric muscovites and chlorite-free phlogopites give similar KAr ages, 1979 ± 28 and 1973 ± 20 Ma (2 σ ) respectively, we conclude that emerald mineralization is contemporaneous with the pervasive muscovitization of the granite. The 1979 ± 28 Ma (2 σ ) age obtained by KAr on muscovite represents the best estimate of the Carnaiba granite cooling age. A model invoking the pervasive alteration of the upper part of the granitic cupola along the pegmatite veins and serpentinites by the muscovitizing fluids is proposed for the formation of emerald-bearing K-metasomatites. The disturbances of 40 Ar/ 39 Ar release spectra testify to the existence of a hydrothermal heating that overprinted the KAr clock of biotite and to a less extent phlogopite. This event is clearly subsequent to the Transamazonian granitization and emerald mineralization and consequently younger than 1973 ± 20 Ma. Due to the lack of structural evidences for a Braziliano event (700-500 Ma) in this region, we tentatively propose a Transamazonian age (1900 Ma) for the thermal overprint. |
