Multiple accretion at the eastern margin of the Rio de la Plata craton: the prolonged Brasiliano orogeny in southernmost Brazil

Abstract

The Neoproterozoic-Eoplalaeozoic Brasiliano orogeny at the eastern margin of the Rio de la Plata craton in southernmost Brazil and Uruguay comprises a complex tectonic history over 300 million years. The southern Brazilian Shield consists of a number of tectono-stratigraphic units and terranes. The São Gabriel block in the west is characterized by c.760-690 Ma supracrustal rocks and calc-alkaline orthogneisses including relics of older, c. 880 Ma old igneous rocks. Both igneous and metasedimentary rocks have positive eNd(t) values and Neoproterozoic TDM model ages; they formed in magmatic arc settings with only minor input of older crustal sources. A trondhjemite from the São Gabriel block intruding dioritc and tonalitic gneisses during the late stages of deformation (D 3 ) yield an U-Pb zircon age (LA-ICP-MS) of 701 ± 10 Ma giving the approximate minimum age of the São Gabriel accretionary event. The Encantadas block further east, containing the supracrustal Porongos belt and the Pelotas batholith, is in contrast characterized by reworking of Neoarchean to Palaeoproterozoic crust. The 789 ± 7 Ma zircon age of a metarhyolite intercalated with the metasedimentary succession of the Porongos belt provides a time marker for the basin formation. Zircons of a sample from tonalitic gneisses, constituting the Palaeoproterozoic basement of the Porongos belt, form a cluster at 2,234 ± 28 Ma, interpreted as the tonalite crystallization age. Zircon rims show ages of 2,100-2,000 Ma interpreted as related to a Palaeoproterozoic metamorphic event. The Porongos basin formed on thinned continental crust in an extensional or transtensional regime between c. 800-700 Ma. The absence of input from Neoproterozoic juvenile sources into the Porongos basin strongly indicates that the Encantadas and São Gabriel blocks were separated terranes that became juxtaposed next to each other during the Brasiliano accretional events. The tectonic evolution comprises two episodes of magmatic arc accretion to the eastern margin of the Rio de la Plata craton, (i) accretion of an intra-oceanic arc at c. 880 Ma (Passinho event) and (ii) accretion of the 760-700 Ma Cambaí/Vila Nova magmatic arc (São Gabriel event). The latter event also includes the collision of the Encantadas block with the Rio de la Plata craton to the west. Collision and crustal thickening was followed by sinistral shear along SW-NE-trending orogenparallel crustal-scale shear zones that can be traced from southern Brazil to Uruguay and have been active between 660 and 590 Ma. Voluminous granitic magmatism in the Pelotas batholith spatially related to shear zones is interpreted as late-to post-orogenic magmatism, possibly assisted by lithospheric delamination. It marks the transition to the post-orogenic molasse stage. Localized deformation by reactivation of preexisting shear zones continued Electronic supplementary material The online version of this article (

The Rio de la Plata craton and the adjoining Pan-African/brasiliano terranes: Their origins and incorporation into south-west Gondwana

“…12). The model combines our results with those of recent contributions (e.g., Goscombe and Gray, 200B;Gaucher et aI., 2009;Frimmel et aI., 2010;Oyhantc;:abal et al, 2010b;Saalmann et al, 2010).…”

“…11. The possibility that the basement inliers of the Dom Feliciano belt, such as the Punta del Este terrane and the Encantadas complex (Leite et al, 2000;Saalmann et al, 2010 and references therein), as well as the exotic Luis Alves microplate ( Fig. 2; Basei et al, 2009) and the Betara region in southwestern Brazil (Siga et al, 2011) were all part of the same collage is a hypothesis well worth testing in future studies.…”

“…3a) (Basei et al, 2008a). Data from the metavolcano-sedimentary Porongos Group contain a Neoproterozoic fraction indicating a post-620 Ma deposi tional age for these rocks (Basei et al, 2008a), which conflicts with the c. 780 Ma age inferred for the succession (Saalmann et al, 2010). Nevertheless, this composite Schist Belt detrital pattern is important because it shows Mesoproterozoic (1200 and 1500 Ma), Late Palaeoproterozoic (1800-1900 Ma) and Archaean zircon populations, which are also present in the Punta Mogotes Formation (Fig.…”

“…This has been largely overcome by new geochronological data for Neoproterozoic extensional magmatism in the Dom Feliciano Schist Belt (Oyhantc;abaJ et aI., 2009;Saalmann et aI., 2010 and references therein). Most of these magmatic rocks are felsic; some show intraplate signatures (838 ± 9 Ma, mylonitic A-type granite, Brusque Group, Basei et al, 2008b), but many are acid metavolcanic rocks intercalated with the metasedimentary rocks of the Schist Belt (783 ± 6 Ma to 789 ± 7 Ma, Porcher et al, 1999;Chemale, 2000;Saalmann et al, 2010). This volcanism shows Palaeoproterozoic to Neoarchaean Nd TOM model ages as well as negative ENd (t= 780 Ma), suggesting re-melting of the basement during the extensional basin formation (Chemale, 2000;Saalmann et al, 2007Saalmann et al, , 2010.…”

The Río de la Plata Craton: a review of units, boundaries, ages and isotopic signature

“…4; for details see Saalmann et al 2010, this volume) comprises Fig. 4 Geological map of the Nico Pérez Terrane and the contiguous Dom Feliciano Belt (after Preciozzi et al 1985;Bossi and Ferrando 2001;Mallmann et al 2007;Sánchez Bettucci et al 2010) Paleoproterozoic gneisses of tonalitic and trondhjemitic composition and minor associated amphibolites and metaultramafic rocks and extends in Rio Grande do Sul from Pinheiro Machado in the southeast to Porto Alegre in the northeast Philipp et al 2008).…”