Tonian juvenile metagabbros of the Passinho Arc, São Gabriel Terrane, southern Brazil: U–Pb geochronology, geochemistry, and Sr–Nd isotopic constraints on early arc construction

The São Gabriel Terrane in southern Brazil represents an early Neoproterozoic record of juvenile intra-oceanic subduction that preceded its incorporation into the orogenic systems involved in Western Gondwana assembly. Within this domain, the ca. 920–830 Ma Passinho Arc records primitive intra-oceanic magmatism in the Charrua oceanic domain, though its early evolution remains poorly constrained. To reconstruct this arc system, we investigate the Tabuleiro region metagabbros using petrography, whole-rock geochemistry, Sr–Nd isotopes, and U–Pb zircon geochronology. The rocks are composed mainly of plagioclase and amphibole, with relics of clinopyroxene, and show restricted compositions (SiO 2 = 48.9–51.1 wt.%; MgO = 6.9–7.3 wt.%), weak Eu anomalies (Eu/Eu* = 0.9–1.1), flat REE patterns (La/Yb ratios ∼1.45), high LILE (Ba, Sr, Pb) and depleted HFSE (Nb, Ti, Zr), consistent with a supra-subduction-zone, intra-oceanic arc affinity. U–Pb zircon dating yields a magmatic age of 902 ± 7 Ma, slightly younger than the ca. 909 Ma Três Vendas eclogite protolith, interpreted to record subduction within the Charrua Ocean, while predating more evolved ca. 890 Ma Passinho Arc metadiorites. The metagabbros present ƐNd t of +1.6 to +4.5 and elevated 87 Sr/ 86 Sr t up to 0.7041, consistent with a predominantly juvenile source, although limited assimilation of older crustal material cannot be excluded. These data constrain an early stage of Passinho Arc construction within the Charrua oceanic domain, prior to its incorporation into the São Gabriel accretionary system, and document an early juvenile subduction system subsequently integrated into Western Gondwana.