Geologia USP. Série Científica (Apr 2008)
Nd-Sr Isotopic Geochemistry and U-Pb Geochronology of the Fé Granitic Gneiss and Lajedo Granodiorite: Implications for Paleoproterozoic Evolution of the Mineiro Belt, Southern São Francisco Craton, Brazil
Abstract
The Fé granitic gneiss and Lajedo granodiorite belong to a voluminous felsic-mafic plutonism, tectonically linked toPaleoproterozoic magmatic evolution of the Mineiro Belt, southern portion of the São Francisco Craton, central-easternBrazil. The Fé pluton is located north of the Lenheiros shear zone and is intrusive with respect to the Rio das Mortesgreenstone belt and pyroxenite - gabbroic bodies, as indicated by xenoliths of gneiss and amphibolite, in the first case, andpyroxenite in the latter. The Lajedo granodiorite is located south of the Lenheiros shear zone and cuts the metamafic rocks ofthe Forro peridotite – pyroxenite and mafic and intermediate rocks of the Nazareno greenstone belt, as evidenced by xenolithsfrom the latter unit. The modal composition of the Fé granitic gneiss lies within the ranges of monzogranite and syenogranite.It is peraluminous and shows a large variation in K2O content, which implies a middle-K calc-alkaline to high-K calc-alkalinetendency. The Lajedo modal composition is consistent with granodioritic and tonalitic compositions. It indicates a predominantlyperaluminous composition and calc-alkaline character. The U-Pb zircon crystallization age of the Fé granitic gneiss is 2191± 9 Ma, whereas the Lajedo granodiorite yields 2208 ± 26 Ma. The Nd/Sr characteristics of the Fé and Lajedo plutons areconsistent with mixtures of enriched mantle (EMI-type), DMM and crustal components during magma genesis in a plutonicarc setting, while the low 87Sr/86Sri ratios point to contribution of mafic rock protoliths during magma genesis. This is also inaccordance with the characteristic xenoliths observed within the investigated plutons from the Nazareno and Rio das Mortesgreenstone belts. The Fé granitic gneiss and Lajedo granodiorite show tectonic characteristics which are comparable to thoseof nearby coeval plutons: Brito quartz-diorite (2221 ± 2 Ma), Brumado de Cima granodiorite (2219 ± 2 Ma), Brumado deBaixo granodiorite (2218 ± 3 Ma), Gloria quartz-monzodiorite (2188 ± 29 Ma), Cassiterita Trondhjemite/Tonalite (2162± 10 Ma), Rio Grande diorite (2155 ± 3 Ma), Brumado diorite (2131 ± 4 Ma) and Ritápolis granite (2121 ± 7 Ma). As a whole,the integrated data and geologic setting of the plutons in the Mineiro belt define a well-constrained pre- to syntectonic phaseof the Mineiro Belt to the period between 2221 and 2121 Ma.
