The Sierras Pampeanas of Argentina: Paleozoic Building of the Southern Proto-Andes

RAPELA, C.W.

Tectonic Evolution of South America

31st IGC

Año: 2000; p. 381 - 387

Lower Paleozoic magmatism in South America is most clearly recorded in outcrops in central and northwestern Argentina, extending into southern Bolivia, northeastern Chile and the coast of southwestern Peru. Igneous and metamorphic rocks of this age are well exposed in the geological provinces of central and northwestern Argentina (Fig. 1), and its characteristics are considered representatives of the Paleozoic evolution of southwestern Gondwana. Outcrops of Lower Paleozoic rocks are much more scattered S of 33’S, where they appear as the basement rocks of the Patagonian massifs and are also recognized in drill cores as far S as the Strait of Magellan. This distribution indicates that the basement of the southwestern sector of the South American Continent was mainly developed during the Early Paleozoic being the Pampean and the Famatinian the best recognized orogenic episodes (Pankhurst and Rapela, 1998). These large Paleozoic regions are the basement that structurally controlled the Meso-Cenozoic Andean magmatism associated with the Pacific subduction. Within the classic geological-morphostructural provinces recognized in the Argentinean geology, there are several that include extensive sectors composed mainly of igneous and metamorphic rocks of Early and Middle Paleozoic age. The best exposed and studied are the provinces of the northwestern sector: the Sierras Pampeanas, the Famatina System, the Precordillera, the Eastern Cordillera and Puna; and in Patagonia, the North Patagonian Massif (Fig. 1). The Early Paleozoic tectonic evolution of southern South America described here is based on new geochemical, isotope and petrological data from a 500 km traverse at 31”S - 32°S across the Eastern Sierras Pampeanas, the southern extension of the Famatina System, the Western Sierras Pampeanas, and the Precordillera (Pankhurst et al., 1998; Rapela et al., 1998a, b, 1999). Pre-Silurian metamorphic and magmatic history in this transect (Fig. 1) has been inferred from: (1) dating by conventional U/Pb on abraded zircon, U/Pb SHRIMP analyses, and whole-rock Rb/Sr and KIAr; (2) thermo-barometry based on microprobe mineral analyses; and (3) Nd and Sr isotopes, and major element and trace element geochemistry of (he magmatic suites. Recent accounts on the Early Paleozoic history of the provinces of (he northwestern Argentina, and different viewpoints on its geodynamic evolution are presented in the book The proto-Andean Margin of Gondwana, Pankhurst and Rapela (eds), 1998. Pioneering contributions and research oil the geology of these provinces, back in time up to 1978, can be consulted in the Segundo Simposio de Geología Regiona1Argeitina, volume I, Turner (ed), 1979.