Five 10Ar/39Ar mineral age determinations were performed on two gneissic rocks in the state of Rondônia, Brazil, in the southwestern part of Amazonian Craton. These samples were taken from the crystalline basement of the Río Negro-Juruena province, which is characterized by medium- to high-grade metamorphic rocks overprinted by polyphase deformation and retrograde metamorphism. This was intruded later by rapakivi granites and associated mafic rocks, as welt as by late tin-bearing granite phases over the time interval 1.57 to 1.0 Ga. Reconnaissance Rb-Sr studies on the country rocks from Rondônia yielded a whole-rock Rb-Sr isochron age of 1539 ± 36 Ma, which is interpreted as the timing of the regional metamorphism. Granitoid gneisses from the area dated by the SHRIMP U-Pb method on zircons reveal that their crystallization occurred at 1750 ± 17 Ma and 1570 + 17 Ma, The Younger Granites of Rondônia and associated mafic rocks in the area have Rb-Sr and U-Pb dates that range from 1.08 to 1.0 Ga. The new 40Ar/39Ar plateau ages on hornblendes-1156 ± 36 Ma (RO-12) and 1149 ± 35 Ma (RO-13)-and biotites-1001 ± 33 Ma (RO-12) and 912 ± 30 Ma (RO-13)-together with the 968 ± 33 Ma integrated date obtained on the RO-12 feldspar, record a slow cooling rate following an inferred Mesoproterozoic regional metamorphic episode. These results are consistent with the K-Ar ages available for the Younger Granites of Rondônia (1.08 to 1.0 Ga). In addition, the 40Ar/39Ar dates are in agreement with the approximate period (1150 to 910 Ma) of tectonomagmatic episodes recorded in the adjacent Rondonian and Sunsás provinces, as well as within the Garzon Santa Marta and Medellin inliers of the Andean Belt in Colombia. Reconnaissance paleomagnetic studies based on five consistent poles (gneissic rocks and diabase dikes r3a, 13a, and 13b) compare well with poles from Mesoproterozoic rocks of the Grenville province, North America and an overprinting episode in the Imataca complex (Venezuela). The new paleomagnetic evidence, coupled with the 40Ar/39Ar radioisotopic pattern obtained, suggest a wide distribution for the Mesoproterozoic metamorphism, formerly limited in the Rondonian/Sunsás provinces. The likely scenario implies that during such an episode the area was positioned near the Grenville province (at ca. 1.0 to 1.1 Ga), which reinforces Dalziel's model (1992a) of a geometric fit between Grenvillia and Amazonia during Proterozoic times; additional evidence for such a model recently was reported by Sadowski and Bettencourt (1995), Tosdal et al. (1994), and Tosdal (1995). The new paleomagnetic results permit a tectonic correlation between the Amazonian craton and Eastern Laurentian shield and reveal new possibilities for metallogenic and exploration models in both regions.
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