Provenance and age delimitation of Quadrilatero Ferrifero sandstones based on zircon U-Pb isotopes

摘要

The Quadrilatero Ferrifero has some of the largest iron and gold deposits in the world and is a major geotectonic unit of the Sao Francisco Craton in Brazil. U-Pb zircon SHRIMP geochronology of six detrital sedimentary and metasedimentary rocks (114 zircon crystals, 118 spot analyses) has improved the understanding of the sedimentary processes and provenance ages of both rocks and the associated iron formation. The age of deposition of the iron formation is constrained between 2.58 and 2.42 Ga. The presence of an old Paleoarchean crust is dated in detrital zircon crystals, including the oldest zircon in South America (3809 + 3 Ma). Only high-Th/U, magmatic zircon crystals are present in the dated sedimentary rocks, and these indicate that the crust of the region was formed mostly during the Jequie cycle (six age peaks between 3055 and 2635 Ma). This time span of ～ 420 m.y. is similar to the duration of a long-lived Wilson cycle. Most of the Rio das Velhas Basin was filled during approximately 30 m.y. between 2746 and 2717 Ma, though volcanism probably started earlier. The youngest detrital zircon age from the Minas Supergroup indicates that the intracratonic basin fill, including the iron formation, was deposited after 2580 Ma. Therefore, the crust was cratonized shortly after the intrusion of minor granitic bodies at around 2.62-2.58 Ga. A large gap in orogenic activity is indicated by the absence of zircon ages of 2580-2182 Ma.