Zircon U–Pb geochronology of Paleoproterozoic Statherian intraplate A-Type magmatic associations of the Lagoa Real Uranium Province, São Francisco Craton (Bahia, Brazil)

Abstract

The Lagoa Real Complex, in the Lagoa Real Uranium Province, is a manifestation of extensive intraplate A-type magmatism that occurred in the São Francisco Craton during the Paleoproterozoic. U–Pb dating of zircon by LA-MC-ICP-MS shows that granites were emplaced during a Statherian, rift-related, 1.76–1.72 Ga magmatic episode, coeval with emplacement of the A-type Borrachudos suite and felsic metavolcanic rocks from the southeastern, central and northern Espinhaço Igneous Province of the São Francisco Craton. The Lagoa Real Complex involves at least three co-genetic pulses, starting with the initial emplacement of the alkali-feldspar-bearing Lagoa do Barro granite, which crystallized at 1762 ± 9 Ma. This was followed by emplacement of the Juazeirinho granite at 1755 ± 6 Ma, and finally the intrusion of the São Timóteo granite, which represents the main magmatic peak of the Lagoa Real Uranium Province, at 1741 ± 4 Ma. Although the São Timóteo granite has some characteristics that suggest it may be a rapakivi granite, there is no classical cogenetic association with basic rocks. This study confirms that the charnockites of the Lagoa Real Complex have an age of 2047 ± 5 Ma. They are therefore not part of the evolution of the 1.76–1.74 Ga magmatic episode. Also, rocks within the Lagoa Real Uranium Province that were previously mapped as leucodiorites are, in fact, albite-gneiss with ages of 1743–1740 Ma, which originated through the gneissification processes that affected the São Timóteo granite and can also be classified as albitites. The highly negative values of ϵNd(0) from −27 to −33 and high initial Sr ratio are compatible with the Lu–Hf isotopic data of previous studies. These results suggest the Lagoa Real Complex has a crustal derivation with minimum mantle involvement. The formation of the Lagoa Real Complex granites is inferred to be a result of fractional crystallization. All studied granites have a similar geochemical signature in terms of trace elements including REEs, which indicates that the granites come from the same magmatic source and are related to rifting and the initial stages of the Espinhaço Supergroup evolution. The source of the Lagoa Real Complex would be reworked Archean crustal material, probably in the mid to deep crust. Finally, the studied granites, although predominantly metaluminous, have U contents (maximum 12 ppm) below the world average for A-type granites (20 ppm). The fractionation values of Th in relation to U, and the absence of metamictization of the minerals (mostly allanite) that potentially host uranium confirms this observation. Therefore, the granites of the Lagoa Real Complex, despite being the protolith of bodies that host U mineralization, should not be considered as the principal source of U in the U mineralization found in the Lagoa Real albitites. However, the provenance of an important part of the uranium could be associated with Statherian meta-rhyolites and meta-rhyodacites, contemporaneous with the São Timóteo granite, and occurring to the west of the Lagoa Real Complex.