GEOLOGIA E GEOCRONOLOGIA Pb-Pb DE ROCHAS GRANÍTICAS E VULCÂNICAS ÁCIDAS A INTERMEDIÁRIAS PALEOPROTEROZÓICAS DA PROVÍNCIA PITINGA, CRATON AMAZÔNICO

Abstract

The Pitinga Province is responsible for 60% of the Brazilian tin product ion, and about 3% of the world production of this not ferrous metal. Associates to the Sn occurs important reserves of Nb,Ta and F (cryolite), beyond anomalous grades of Zr, Y, Rb, U, Th and HREE. The geological selling of the Pitinga Province consists in Paleoproterozoic, volcanic and granitic units or Uatumã Supergroup, younger granites related to the Madeira Suite and by the vulcano-sedimentary Urupi Formation. The granitic units of the Mapuera and Madeira Suite arc represented by eight plutons that are intrusive in to acid and intermediate volcanic rocks of the Iricoumé Group. Only the granites of the Madeira Suite have previous geochronologic information. This paper shows 207 Pb/ 206 Pb zircon ages of four granitic units of the Mapuera Suite and 207 Pb/ 206 Pb zircon ages of the volcanic rocks that enclose these plutons. Petrographic, geochemical and field data, allow us to distinguish two magmatic pulses in the region. The first one is responsible for the generation of volcanic and granitic rocks of ages between 1,890 and 1,876 My. The second magmatic pulse generated A type granites with ages ranging from 1,829 and 1,798 My. Based on field and remote sensor data, the volcanic rocks of the Iricoumé Group were divided in three formations, for which are proposed the names Divisor Formation, Paraíso Formation and Ouro Preto Formation. The ages of these units are around 1,89 and 1,88 Ga. The field data indicate s that the Urupi Formation shows intercalations of pyroclastic rocks. These pyroclastic rocks are interpreted as part of the Paraíso Formation; consequently it locates the Urupi Formation in the Paleoproterozoic, suggesting its correlation with Roraima Supergroup. All these lithologic units are associated with a regional, extensional, arc-shaped structure, which encloses the whole Pitinga Province. Granitic plutons of the Mapuera Suite occupy the peripheral parts of the structure, which is filled by effusive and pyroclastic rocks of the Iricoumé Group, and sedimentary rocks of the Urupi Formation. The tectonic setting suggests that the emplacement of volcanic and pyroclastic rocks of the Iricoumé Group, in the Pitinga Province, was controlled by extensional processes of caldera collapse. Subsequent reactivation of the extensional system allowed the emplacement of the Madeira Suite, in the central portion of this wide structure.