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Understanding the accretion of an oceanic island arc to a craton is a major issue in unravelling the evolution of the continents. P-T-t conditions registered in metamorphic rocks, the boron isotopic compositions of tourmaline provide essential information to delimit the accretionary processes. The Tonian-Cryogenian Sao Gabriel Terrane is a prominent juvenile structure in the southern Brasiliano Orogen that records key information about accretionary processes. Limited previous studies have investigated the Sao Gabriel Terrane, with little presently known about the metamorphic conditions. To pursue the objective, we combined multiple methods, including pseudosection modeling, U-Pb garnet geochronology and δ11B isotopes of tourmaline applied to one key sample from the Serrinha Formation of the Sao Gabriel Terrane. The recorded peak P-T conditions occurred at 4.5–5.1 kbar and 536–555 °C. U-Pb dating of low-U (

Integrated study of Tonian ophiolite melanges and intra-oceanic arc of the Sao Gabriel juvenile terrane exposes a key geotectonic unit in the evolutionary history of Gondwana during Brasiliano Orogeny. LA-ICP-MS U-Pb geochronology in rutile and zircon were combined with Lu-Hf and rare earth elements in zircon. We report the first rutile dating of ultramafic rock in the Brazilian Shield of 787.6 ± 2.6 Ma for the Cambaizinho ophiolite magnesian schist. Dating of rutile corresponds to the metamorphic age of the mantle section of the ophiolite. Sao Gabriel arc infrastructure is represented by the Cambai Complex, including the Vila Nova Suite metatonalite with zircon U-Pb age of 724.6 ± 3.2 Ma and late Sanga do Jobim Suite monzogranite intrusions with two main pulses at 698.9 ± 4.2 Ma and 673.9 ± 6.8 Ma. Zircon eHf(t) values are positive for both rocks and range between +10.1 and +7.4 in metatonalite and +9.2 to +2.5 in monzogranite. Detrital zircon from metasedimentary rocks support superstructure arc investigation. Ages range from 817 to 650 Ma, with strong concentration between 750 and 730 Ma. Positive eHf(t) values and Rare Earth Elements (REE) signature point to main source of metasedimentary rocks in the arc, with ophiolitic contribution. Therefore, geotectonic processes in the Brasiliano Orogeny include the construction of Sao Gabriel terrane with Tonian oceanic crust evolving to intra-oceanic subduction until 650 Ma to final development along the margin of continental crust. Overthrusting of the Sao Gabriel terrane onto the Rio de La Plata Craton occurred at 650–620 Ma. Our data correlate with Tonian intra-oceanic arcs from Brasilia belt (Goias arc ca. 862–630 Ma) and Ribeira belt (Serra da Prata arc – 856–838 Ma; Rio Negro arc – 790–620 Ma). Oceanic crust generation combined with subduction to form Tonian arcs along the Brasiliano Orogen were most significant for construction of West Gondwana.

Juvenile Neoproterozoic dioritic, tonalitic, trondhjemitic and granodioritic gneisses in the Sao Gabriel block, southern Brazil, have been identified by geochronologic studies. Age proposals for associated (ultra-)mafic metavolcanic and metasedimentary rocks, however, range from Archean to Neoproterozoic. Whole rock Sm–Nd analyses presented here support a Neoproterozoic age for these rocks. TDM model ages of the (ultra-)mafic metavolcanic rocks range between 0.65 and 1.35 Ga with ɛ Nd( t ) positive values between 3.16 and 6.87; TDM model ages of metasedimentary and metavolcanoclastic rocks vary between 0.77 and 1.19 Ga with ɛ Nd( t ) values between 1.2 and 6.23; tonalitic calc-alkaline gneisses show ɛ Nd( t ) values of 4.34 and 6.3 and TDM model ages of 0.89 and 0.72 Ga, respectively. A late-kinematic granite (Santa Zelia granite) display slightly negative ɛ Nd( t ) values (−1.6) and a higher TDM model age of about 1.4 Ga. These data support the existence of Meso/Neoproterozoic juvenile oceanic crust and island arc rocks during the Brasiliano orogenic events. The main source rocks of the metasedimentary units are previously formed juvenile rocks. The data also indicate minor assimilation of older crustal material and/or contamination of the melts by radiogenic Nd released from older rocks on the subducting slab. Existence of widespread old sialic crust in the subduction zone environment, however, can be ruled out indicating important orogenic accretion between 0.9 and 0.7 Ga. A geotectonic model for the Sao Gabriel block and the eastern margin of the Rio de la Plata craton comprises eastward subduction and following accretion of an intra-oceanic island arc between 0.9 and 0.8 Ga and a subsequent westward subduction with formation of an active continental margin at the eastern margin of the Rio de la Plata craton between 0.8 and 0.7 Ga. We postulate that the juvenile rocks of Sao Gabriel block represent relics of a Neoproterozoic ocean between the Rio de la Plata craton and a continental block (Encantadas block) possibly derived from the Kalahari craton. Subduction and arc accretion began roughly coeval with the initial stages of the break-up of Rodinia (0.9 Ga) and indicate a peripheric Rio de la Plata craton in relation to the Rodinia supercontinent with evolution from a passive margin to an active margin in the beginning of the Neoproterozoic Brasiliano orogenic events.