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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. KAr 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 KAr 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 KAr 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 KAr 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 KAr 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 KAr 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.

An important characteristic of Neogene magmatism (20.0–1.6 Ma) in the Bolivian Andes is the presence of numerous pyroclastic flows. Samples collected along a west-to-east transect (Western Cordillera, Altiplano, Eastern Cordillera) have been dated radiometrically, and this information provides the basis for interpretation of these rocks within the context of the late compressive phase of the Quechuan orogeny. In the Western Cordillera , the Abaroa Formation gave Miocene ages (20.8±0.5 to 13.5±0.4 Ma). The Pliocene to latest Miocene age of Cerke volcanism has been confirmed (5.7±0.5 Ma). In the Altiplano, ash-flow deposits are clearly assigned to the Pliocene (5.2±0.3 Ma). Along the eastern boundary of the Altiplano, a widespread tuff deposit was dated as late Pliocene (2.8±0.4 Ma). In the La Paz Basin, the Patapatani tuff is Pliocene in age (2.8±0.1 Ma), and its intercalation in glacial deposits comfirms the presence of a coeval glacial event. The absence of the Quechuan phase Q2 has been confirmed. In the La Paz Basin, the Quechuan phase Q4 takes place between 2.8±0.1 Ma and 1.6±0.1 Ma.