Eoarchean to Mesoarchean crustal evolution of the northern São Francisco Craton, Brazil: geochronological and isotopic perspectives. Rafael GORDILHO BARBOSA1, Rommulo V. CONCEIÇÃO1, Felipe P. LEITZKE1, Johildo S.F. BARBOSA2, Jonas TUSCH3, Carsten MÜNKER3,Alanielson C.D. FERREIRA4 1 Institute of Geosciences, Federal University of Rio Grande do Sul, Av. Bento Gonçalves, 9500, Porto Alegre, RS, Brazil. 2 Basic Geology Nucleus, Federal University of Bahia, Rua Barão de Jeremoabo, s/n, Ondina, Salvador, BA, Brazil. [email protected] 3 Institut für Geologie und Mineralogie, Universität zu Köln, Zülpicher Straβe 49b, 50674 Cologne, Germany 4 Department of Earth Sciences, Stellenbosch University, Private Bag X1, Matieland, 7602, Stellenbosch, South Africa INTRODUCTION STUDY AREA The Earth’s oldest cratons represent the relicts of Eoarchean to Paleoarchean continental crust formed by The research was focused on the northern part of the SFC, specifically in the areas where Eo-to- tonalite-trondhjemite-granodiorite (TTG) rocks associated with Na-rich granitic leucosome. In this context, Mesoarchean rocks outcrop, i.e: the major question is what the sources and available heat at that time are and, more importantly, how did the ▪ Gavião Block (GB) – It represents most of the northern SFC. Three generations of TTGs occur in the GB early-Earth tectonic processes and the crust generation operate in Hadean/Eo-to-Mesoarchean times. with U-Pb ages of 3642-3529 Ma (Oliveira et al., 2020), 3403-3378 Ma (Bastos Leal et al., 2003) and 3259- In South America, the São Francisco Craton (SFC) is one of the most representative cratonic areas and 3240 Ma (Santos Pinto et al., 2012). the oldest dated rock within the continent outcrops on its northern portion (i.e., 3642 Ma gneisses of the ▪ Serrinha Block (SB) – The Archean rocks of this Block are represented by the Santa Luz Complex, in Mairi Complex). Despite a long history of geochronological and isotopic studies there is still strong which occur orthogneisses and migmatites with U-Pb ages between 3162 and 2983 Ma. uncertainty about the sources and Hadean/Archean crustal evolution mechanisms of the northern SFC. ▪ Uauá Block (UB) – In this Block occur Mesoarchean TTGs with U-Pb ages between 3150 and 3125 Ma, Likewise, the SFC exposes Archean rocks formed at different crustal levels and offers a rare opportunity which are crosscut by Neoarchean mafic dykes. to understand the timing, mechanisms and conditions of the 3.64 to 3.15 Ga Paleoarchean processes: (i) TTG high-grade metamorphic terranes, which represent Archaean mid-continental crust; (ii) Low-grade granite-greenstone belts related to Archaean upper continental crust; (iii) Evolution by gravity-driven within the crust (e.g., sagduction, peel-back; Chowdhury et al., 2020); (iv) Multiple partial melting processes; (v) Thrust faults analogous to “horizontal” Phanerozoic-style and (vi) Amphibolite- to granulite facies transitions. THE OBJECTIVE OF THIS STUDY is to compile robust literature data (up to present- day) about geochronological and isotopic studies performed in the Eo-to- Mesoarchean rocks of the northern SFC, in order to provide an overview on the crustal evolution processes that occurred in the studied region. Fig. 1 – Major tectonic compartments of the São Francisco Craton. GB – Gavião Block; SB – Serrinha Block; UB – Uauá Block. Modified from Gordilho Barbosa et al., (2021). METHODOLOGY ▪ A comprehensive literature review was undertaken in order to properly identify peer-reviewed articles about geochronological and isotopic studies performed in the Eo-to-Mesoarchean rocks of the northern São Francisco Craton. ▪ This portion of the craton was chosen because the oldest rocks of South America dated until present-day outcrop in the region (e.g. 3642 Ma gneisses of the Mairi Complex, Oliveira et al., 2020) ▪ As a result, 88 different references about the northern SFC, published over more than four decades, were compiled. ▪ From the obtained Hf, Nd and Sr isotopes data it was possible to build the diagrams presented in the Results section. RESULTS Fig. 5 - Archean Hf isotopic data for the São Francisco Craton. Data indicate that the ancient crust of the Mairi Gneiss TTG complex, in the northern portion of the GB, reworked at 3.6-3.5 Ga, while it is possible to verify minor juvenile contribution, associated with crustal reworking, to the 3.42 Ga Sete Voltas TTG Fig. 2 - Comparison of the Archean crustal and the 3.30 Ga Contendas and Mundo Novo Sequences. evolution of Gavião, Serrinha and Uauá Blocks. Fig. 4 - Sr isotopic data for the Archean São Francisco blocks. A – Rb/Sr vs age diagram for the northern SFC, suggesting high initial Rb/Sr ratios for the 3.30 Ga rhyolitic-granitic systems, a linear enrichment evolution for the 3.45 Ga TTGs and low initial Rb/Sr ratios for the 3.1-3.0 Ga tholeiitic rocks. Fig. 3 - Nd isotopic data for the Archean São Francisco blocks. A - εNd(t) vs age B – 87Sr/86Sr(i) vs age diagram for the rocks of the diagram for the southern Gavião Block, indicating the occurrence of three different northern SFC. The lower 87Sr/86Sr(i) for the 3.5-3.4 periods of juvenile and crustal reworking processes. B - εNd(t) vs age diagram for Ga tonalite residue and 3.2-3.0 Ga granodioritic to the northern Gavião Block, suggesting the occurrence of juvenile and crustal granitic leucosome may indicate anatexis of the reworking processes in two different periods. C - εNd(t) vs age diagram for the lower crust, while high 87Sr/86Sr(i) in high-Si granitic 3.42 Ga Sete Voltas TTG, the 3.3 Ga Contendas rhyolite and the 3.17-3.14 Ga magmatism suggest partial melting of high K/Na Sete Voltas granitic leucosome. D - εNd(t) vs age diagram for the western Gavião rocks in the upper continental crust. Block, indicating the occurrence of three different periods of crustal reworking and juvenile processes. E - εNd(t) vs age diagram for the Uauá Block, suggesting two different periods of juvenile and crustal reworking processes. F - εNd(t) vs age diagram for the Serrinha Block, indicating essentially juvenile processes for the origin of this crustal segment. Fig. 6 - Timing of juvenile crust and reworking events for the Archean blocks of the northern São Francisco Craton. Circles: northern and southern GB, triangles: western GB, diamonds: SB, squares: UB. Fig. 7 - Archean crustal evolution correlations between São Francisco, Zimbabwe, Kaapvaal and Pilbara Cratons. CONCLUSIONS & PERSPECTIVES ▪ The Nd isotope data suggest that the Eo-to-Mesoarchean crustal evolution in the northern SFC was associated to juvenile and crustal reworking processes. ▪ The lower 87Sr/86Sr(i) for the 3.54 Ga tonalite residue and 3.2-3.0 Ga granodioritic to granitic leucosome may indicate anatexis of the lower crust, while high 87Sr/86Sr(i), for the 3.3 Ga, high-Si granitic magmatism suggests partial melting of high K/Na rocks in the upper continental crust. ▪ The Hf isotope data suggest an evolution of ca. 4.3-3.9 Ga crustal sources to the Mairi Complex (northern Gavião Block) and a second large period of crust generation between 3.9 and 3.5 Ga, with restricted contributions of juvenile sources. At 3.3 Ga and, mainly, at 3.0 Ga, occurs an intense mixing of the crusts between 4.3-3.5 Ga with juvenile magmas. ▪ The northern part of the SFC shows similar crustal evolution with Kaapvaal and Zimbabwe Cratons, in southern Africa, and Pilbara Craton, in western Australia suggesting that, in the Archean, the same processes occurred in different parts of the Earth’s primitive crust. ▪ Despite these conclusions, some questions remain not answered: (i) What is the nature of the source(s) of the 3.64-3.42 Ga TTGs?; (ii) What are the early-Earth processes that led into the generation of these magmas? In order to answer these questions our future works intend to use tools such as short-lived isotopes (e.g. 142Nd and 182W) to better constrain the early-Earth processes that occurred in the northern SFC. REFERENCES: 1. Bastos Leal, L.R., Cunha, J.C., Cordani, U.G., Teixeira, W., Nutman, A.P., Leal, A.B.M., Macambira, M.J.B. 2003. SHRIMP U-Pb, 207Pb/206Pb zircon dating, and Nd isotopic signature of the Umburanas greenstone belt, northern São Francisco Craton, Brazil. J. S. Am. Earth Sci. 15, 775-785. 2. Gordilho Barbosa, R., Lana, C.C., Zincone, S.A. 2021. Paleoproterozoic granitic magmatism in the northern São Francisco Craton, NE Brazil: New perspectives from geochemistry, U-Pb geochronology and Hf isotopes. J. S. Am. Earth Sci. 105, 103004. 3. Oliveira, E.P., McNaughton, N., Zincone, S.A., Talavera, C. 2020. Birthplace of the São Francisco Craton, Brazil: Evidence from 3.60-3.64 Ga Gneisses of the Mairi Gneiss Complex. Terra Nova 32, 281-289. 4. Santos Pinto, M., Peucat, J.J., Martin, H., Barbosa, J.S.F., Fanning, C.M., Cocherie, A., Paquette, J.L. 2012. Crustal evolution between 2.0 and 3.5 Ga in the southern Gavião block (Umburanas-Brumado-Aracatu region), São Francisco Craton, Brazil: A 3.5-3.8 Ga proto-crust in the Gavião Block? J. S. Am. Earth Sci. 40, 129-142..