A Paleoproterozoic intra-arc basin associated with a juvenile source in the Southern Brasilia Orogen : application of U-Pb and Hf-Nd isotopic analyses to provenance studies of complex areas

Abstract

Early Proterozoic sedimentary basins are an important record of crust generation processes and consequently a fundamental key to unravelling Earth's evolution through geological time. Sediments within the basins are typically deformed and metamorphosed by subsequent tectonothermal events, which can obliterate their links to source terranes. Nd-whole-rock and detrital zircon U-Pb and Lu-Hf isotopic analyses are among the most reliable tools to be used in provenance investigations, since zircon is a resilient mineral and the Sm-Nd system is not extensively modified during metamorphism. These methods have been applied to a study of the provenance and tectonic setting of the São Vicente Complex, preserved in a Neoproterozoic passive margin related allochthon within the Southern Brasilia Orogen. The complex consists of siliciclastic and calc-silicate gneisses with mafic and minor ultramafic rocks, which were deformed and metamorphosed during late Neoproterozoic collision between the Paranapanema Block and the São Francisco-Congo plate. Detrital zircons indicate derivation from a juvenile Paleoproterozoic source terrane (peaks of crystallisation ages of ca. 2130 Ma, 2140 Ma and 2170 Ma; ɛHft between +0.1 and +6.0; NdTDM = 2.31-2.21 Ga; ɛNdt = +1.6 to +2.8), with a minor contribution from older continental crust. Interlayered amphibolite rocks, with juvenile signatures (ɛHft = +5.8 to +8.2; NdTDM = 2.14 and 2.30 Ga; ɛNdt = +2.2 and +3.2), yielded similar ages of 2136 ± 17 and 2143 ± 14 Ma, suggesting syn-sedimentary magmatism. Thus, the maximum age of deposition at around 2130 Ma represents the best estimate of the depositional age of the complex. The dominance of detrital zircons ages close to the age of deposition, along with syn-sedimentary magmatism, imply a convergent margin basin tectonic environment for the São Vicente Complex, with similarities to fore arc basin and trench deposits. Amphibolite and meta-sedimentary rocks point to important juvenile magmatism around 2.14 Ga. Juvenile Rhyacian (ca. 2.1 Ga) granite-granodiorite-tonalite orthogneisses with arc-related geochemical signatures (Pouso Alegre Complex) that override the São Vicente Complex, are the probable main source of detritus within the complex. Both basin and source were part of the southern edge of the São Francisco plate during the assembly of West Gondwana, and served as sources for early Neoproterozoic passive margin related basins. The age of intrusive anorogenic A-type Taguar granite indicates that by 1.7 Ga the São Vicente Complex was in a stable tectonic environment.