Duration of the Trans-Amazonian Cycle and Its Correlation within South America Based on U-Pb SHRIMP Geochronology of the La Plata Craton, Uruguay

Abstract

The La Plata craton of Uruguay consists mostly of Paleoproterozoic terranes that are ideal for the investigation of the Trans-Amazonian Cycle evolution because, unlike the terranes of southeastern Brazil, they are unaffected by the Brasiliano collisional orogenies (640-590 Ma). The U-Pb isotopic dating of zircons from four Uruguayan rocks by the Sensitive High-mass Resolution Ion Micro Probe (SHRIMP II) shows an evolution from 2224 to 2056 Ma. We also extend the discussion of the cycle to the entire South American continent to provide a broad overview of the processes and belts involved. The Trans-Amazonian Cycle of South America consists of four main orogenies, of which three are present in the Uruguayan Piedra Alta (Isla Mala, Paso Severino, and Soca units) and Nico Pérez (Rivera and Valentines samples) terranes. All four Trans-Amazonian Cycle orogenies described in the classical Trans-Amazon orogen of the type-area (São Luís cratonic fragment, Gurupi belt) and in the Amazon craton are present in the La Plata craton. In Uruguay the second (2180-2120 Ma) and the third (2080-2050 Ma) Trans-Amazonian Cycle orogenies are well developed, whereas the oldest orogeny (2260-2200 Ma) is identified by only one inherited zircon (2224 Ma, Valentines granulite) and the fourth orogeny (2020-2010 Ma) has to be detected, but it is present in neighboring southern Brazil (Itapema, Camboriú, and Santa Maria-Chico units). The Rivera meta-trondhjemite was formed at 2140 ± 6 Ma, an epoch of granitoid-greenstone formation along all the Trans-Amazonian Cycle belts of South America, and by the Paso Severino felsic volcanics, formed at 2146 ± 7 Ma. The Rivera trondhjemite was metamorphosed at 2077 ± 6 Ma, a period characterized by intrusion of post-tectonic potassic granitoids and regional high-grade metamorphism. The metamorphism of the Valentines granulite is slightly younger (2058 ± 3 Ma) and this rock contains zircon with several inherited ages, such as 2224, 2163 (second orogeny), 2535, and 2619 Ma. The Soca charnockite is the youngest known Trans-Amazonian Cycle rock in Uruguay (2056 ± 6 Ma) and represents a continent-wide period of emplacement of post-tectonic, evolved granites and charnockites, such as the Calçoene charnockite (2059 Ma) of Amapá, Brazil. The data presented improve our understanding of tectonic processes active in South America during the Rhyacian Trans-Amazonian Cycle. However, the evolution of large segments of Paleoproterozoic crust and the Trans-Amazonian Cycle subdivision remain uncertain in many regions where U-Pb data are scarce or not available. The dating and characterization of these terrains require further investigation.