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Título

New U-Pb Geochronological Insights for the Santa Bárbara and Guanhães Complexes, Eastern Quadrilátero Ferrífero, Brazil

Texto do resumo

The Southern São Francisco craton (SSFC) is an Archean to Paleoproterozoic crustal segment in the southeastern part of Brazil. Its basement complexes encompass Paleo- to Neoarchean gneisses that are commonly intruded by granitoids of varied ages and compositions, including late orogenic potassic granitoids, with subordinated remnants of ultramafic and mafic rocks. On its eastern margin, the Santa Barbara Complex has been considered one of the oldest segments of TTG (tonalite-trondhjemite-granodiorite) crust in the SSFC. It defines a domal structure belonging to the Quadrilatero Ferrifero mineral province, but its relations and limits with the Guanhães Complex, another important cratonic inlier, are still poorly constrained. To better understand the evolution of these basement rocks, we used field mapping, petrography, geophysics and geochronology in the Floralia quadrangle (SE.23-Z-D-IV-3-SE). The detailed geological map produced for this area indicates that the Santa Bárbara Complex outcrops in a smaller extension than previously suggested, being restricted to the east of the Floralia district. It is mainly composed of coarse to medium-grained tonalitic to granodioritic plutonic rocks, foliated and migmatized in variable degrees. On the other hand, the Guanhães Complex occupies a larger area than formerly proposed, outcropping in the eastern portion of the Floralia quadrangle and around the Statherian Peti Pluton (to the north), with distinct geophysical signatures (especially in airbone gamma spectometric maps) when compared to the Santa Bárbara Complex. New U-Pb zircon ages (LA-ICP-MS) were obtained for four samples of the studied complexes. A tonalite sample from the Pedreira Santa Bárbara quarry, previously included in the Santa Bárbara Complex and located near the homonymous town, yielded a U-Pb crystallization age of 2.636 ± 11 Ma. This age, coupled with field data, suggests that these rocks can be individualized in a younger igneous body, namely Campos Lacerda. A lower intercept age of 552 ± 24 Ma was obtained for this sample, which corresponds to metamorphism during the Brasiliano event, related to the assembly of West Gondwana. The Guanhães Complex was sampled in three different locations. In the northern part of the Floralia quadrangle, near the Peti Damn, a leucocratic metagranitoid yielded an age of 2.950 ± 27 Ma, interpreted as the crystallization age. In the central area, close to the Reforgio district, a mylonitic TTG gneiss provided a crystallization age of 2.742 ± 16 Ma, with a lower intercept age of 539 ± 17 Ma, interpreted as the metamorphic age. To the north, within the São Gonçalo do Rio Abaixo quadrangle (SE.23-Z-D-IV-3-NE), a metasyenogranite provided a crystallization age of 2.889 ± 23 Ma. The novel U-Pb data suggest that the Guanhães Block records an older crust generation than previously reported in the literature, ranging from 2.95 Ga to ca. 2.7 Ga. Also, the age of the Campos Lacerda metagranitoid reinforces that the expressive Neoarchean magmatism observed in the southern part of the Santa Bárbara dome, was also relevant in its northern extension. As Neoarchean ages progressively outnumber Paleoarchean ages in the Santa Bárbara Complex, a new viewpoint on the geological evolution of this crustal segment takes place, reducing the spatial extent of the preserved Paleoarchean continental crust.

Palavras Chave

Southern São Francisco craton; Quadrilátero Ferrífero; Santa Bárbara Complex; Guanhães Complex; U-Pb LA-ICP-MS.