LATE PROTEROZOIC GRANITIC MAGMATISM IN SOUTHWESTERN GOIÁS, BRAZIL

Late Proterozoic to Early Palaeozoic granitoids intrude metavolcano-sedimentary sequences of probable Late Proterozoic age and older gneiss terrain in soulhwestem Goias, Brazil. Five types of granitoid bodies are recognized: i) small, deformed, and metamorphosed gabbro to granite bodies intruding the supracrustal pile; ii) elongated fault-bounded mylonitic granite- granodiorite bodies; iii) late-teetonic balholilh of megacrystic granitoids ranging from gabbro and quartz diorite to monzogranite; iv) post-teetonic pink-to red-coloured granite batholiths and smaller plutons; v) hornblende-bearing post-tectonic granite stocks. Ages range between 692 +_ 110 Ma for the Serra do Tatu mylonitic granite and 462 +_ 28 Ma for the hornblende-bearing Israelândia granite. Low initial 87 Sr/ 86 Sr ratios suggest an important event of addition of juvenile mantle-derived material to the continental crust. Two geochemically different major groups of granitoid rocks were identified. Syn- to late-tectonic bodies are similar to calc-alkaline granitoids of orogenic regions, whereas the post-tectonic intrusions display features of alkali-rich granites emplaced during the latest stages of orogenic events. The geochemical differences probably reflect space-time trends of increasing are maturity, as the result of the subduction of Late Proterozoic oceanic lithosphere and the subsequent collision between the Amazonian Craton and a small plate represented by lhe Goias massif.