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Umberto G. Cordani, Institute of Geosciences, Univ. São Paulo (Brazil)
Colombo C.G. Tassinari, Institute of Geosciences, Univ. São Paulo (Brazil)
Wilson Teixeira, Institute of Geosciences, Univ. São Paulo (Brazil)
José Moacyr V. Coutinho, Institute of Geosciences, Univ. São Paulo (Brazil)
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The basement of the Rio Apa cratonic fragment, in the central part of South America, is covered by the Neoproterozoic deposits of the Corumbá and Itapocumi Groups and is bound to the east by the southern portion of the Paraguay-Araguaia belt. In its northern part, schists, quartzites and paragneisses predominate. In the central part, slightly foliated homogeneous orthogneisses are widespread, and are intruded by the undeformed granitic bodies of the Alumiador Suite. The mineralogy of all of these granitoid rocks is very simple, with quartz, microcline, oligoclase and biotite as main components. Felsic volcanics of the Amoguijá Group occur at the Brazil-Paraguay border. Further south, in Paraguay, several additional granite bodies are described. Some ages by the Rb-Sr and K-Ar methods were already available for the Brazilian portion of the area. In this work, additional determinations by the U-Pb, Rb-Sr, Sm-Nd and Ar-Ar methods were obtained, some of them in Paraguay.
The most relevant outcomes for the regional geologic history are eight U-Pb SHRIMP zircon ages. A basement gneiss yielded an age of 1940 Ma, and a monzogranite of the Alumiador Suite about 1830 Ma. Four orthogneisses showed similar U-Pb results between 1700 and 1760 Ma, indicating the occurrence of a major orogenic magmatic process. Finally, very discordant ages were obtained on two granitoid rocks from Paraguay, where their zircons showed unusually high uranium contents and were affected by multi-stage Pb loss. About 40 Rb-Sr whole-rock analyses are available for the regional orthogneisses, and for the intrusive granitoids. With a few exceptions, all these analytical points are remarkably aligned in a Rb/Sr isochron diagram.
The inclination of the calculated best-fit line corresponds to about 1700 Ma, with a 87Sr/86Sr initial ratio of ca. 0.706, confirming the possibility of the major orogenic episode already indicated by the U-Pb zircon work. A few Sm-Nd whole-rock determinations showed Sm-Nd TDM model ages between 2000 and 2800 Ma, and most of them yielded slightly negative values of eNd(T). Eight Ar-Ar ages were also obtained for the regional rocks. Five biotites indicated very precise and concordant ages close to 1300 Ma., indicating the effect of a pervasive regional heating. Two amphiboles resulted slightly older, and one of them showed signs of excess argon 40. Only one biotite yielded a younger apparent age of about 1060 Ma. A few less precise K-Ar ages were roughly of the same order of magnitude.
Taking into account all the available data, the tectonic evolution of the Rio Apa Craton basement rocks seems to be related to a series of magmatic arc complexes, whose original magmas involve some crustal component. They correlate well with the SW portion of the Amazonian Craton, where rocks of the Rio Negro-Juruena tectonic province, with ages between 1600 and 1800 Ma, are reworked by the Rondonian-San Ignacio orogeny, which show a strong metamorphic imprint at about 1300 Ma.
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