Advances in SHRIMP geochronology and their impact on understanding the tectonic and metallogenic evolution of southern Brazil

Significant improvements, both in understanding the evolution of zircons an d in understanding the geotectonic and metallogenetic evolution of the comp lex terrain of southern Brazil, are obtained from a SHRIMP geochronology st udy and reviewed in this paper. The use of backscattered electron and catho doluminescence images, prior to SHRIMP isotopic determinations, proved of e normous fundamental value for this technique. Zircon is a domainal open-sys tem mineral in many geological conditions; very old domains may be preserve d, but the same crystal may show ages of younger tectonic events. Zircons m ay recrystallise inwards from the rims or outwards from the cores, and also along euhedral high-U or metamict thin zones. Zircons also may be recrysta llised during gold-related hydrothermalism, phyllic alteration of granitic rocks. The precise dating of amphibolite dykes can be achieved by the ident ification and dating of magmatic zircons. Precambrian orogenies are identif ied along with the intervening intracratonic tectonic cycles of supercontin ents in southern Brazil from 3300 to 470 Ma. Granulite protoliths were form ed during the Jequie Orogeny (ca 2600 Ma), but extensive arc accretion occu rred in the Palaeoproterozoic (ca 2250 Ma) Encantadas Orogeny. Late in the Transamazonian Cycle, granites were formed by crustal melting at about 2000 Ma in the Camboriu Orogeny. Both accretionary and collisional orogenies ar e also identified in the Neoproterozoic Brasiliano Cycle. These are the acc retionary Passinho Orogeny (ca 900 Ma) and Sao Gabriel Orogeny (ca 700 Ma), that were succeeded by the collisional Dom Feliciano Orogeny (ca 600 Ma). Base-metal and gold deposition occurred in juvenile island arcs and in late orogenic porphyry-copper-type magmatic-hydrothermal settings during the Ne oproterozoic.