Ar-40-Ar-39 and Rb-Sr geochronology of the Uruguayan dike swarm, Rio de laPlata Craton and implications for Proterozoic intraplate activity in western Gondwana

The Uruguayan dike swarm intrudes country rocks of the Rio de la Plata Crat on that were deformed and metamorphosed to high grade during the 2200-1900 Ma Transamazonian orogeny. Slow cooling of the continental crust following this event is suggested by the Ar-40-Ar-39 plateau ages of hornblende (2016 +/- 11 Ma) and biotite (1817 +/- 10 Ma) from the Pintos post-tectonic gran odiorite. There are seven Ar-40-Ar-39 age determinations on mafic dikes and felsic ve ins from the swarm between 1727 and 1700 Ma. One dike yielded concordant pl ateau ages for igneous hornblende (1727 +/- 10 Ma) and biotite (1725 +/- 10 Ma). These ages are taken as the best estimate for the emplacement of the Uruguayan swarm. A Rb-Sr whole rock best fit line calculated for 15 dikes y ielded 1766 +/- 124 Ma, in agreement (within error) with the Ar-40-Ar-39 pl ateau ages. The origin of the Uruguayan swarm is tectonically related to the emplacemen t of the Illescas and Minas de Corrales rapakivi granites that crop out in the Nice Perez domain. The tectonic setting of these units is also compatib le with bimodal magmatism and basin formation associated with extensional e vents in the Uruguayan and Brazilian shields. On the whole, the geologic fr amework suggests that the Uruguayan dike swarm marks a transcontinental int raplate episode within the South America continent which occurred shortly a fter the Transamazonian orogeny. The results of initial release portions of Ar-40-Ar-39 spectra (dikes and f elsic veins) coupled with a Rb-Sr mineral isochron from one felsic vein are between 1370 and 1170 Ma. Eight K-Ar ages range from 1600 to 1170 Ma, sugg esting variable argon loss in the systems. These isotopic disturbances were probably favored by low-grade hydrothermal overprints, exemplified by diff erences in the Ar-40-Ar-39 spectra from one felsic vein (muscovite and biot ite), and suggested also by the secondary mineralogy that may be present in the dikes. These post-emplacement ages could result from Mesoproterozoic c rustal shortening, recognized in the Nice Perez domain, interpreted to be t ectonically associated with the Namaqua orogeny of Southern Africa. This in ference is important in understanding the Late Mesoproterozoic evolution of Western Gondwana and brings potential implications for a paleogeographic r econstruction between the paleocontinent Plata and Amazonia at the time of the Grenville orogeny. (C) 1999 Elsevier Science B.V. All rights reserved.