Geology, geochemistry and Sr Nd isotopes of the Rio Branco Suite, Nova Brasilândia belt in southwest of the Amazon Craton: Evidence of a Rodinia pre-assembly accretionary phase (ca. 1137 and 1106 Ma) during the evolution of the Nova Brasilândia orogeny

Abstract The tectonic affinity of the Rio Branco Suite magmatism in the Nova Brasilândia belt, southwest of the Amazon Craton, is important in the understanding of Mesoproterozoic orogenetic events associated with the Rodinia supercontinent pre-assembly. The Rio Branco Suite (ca. 1119–1106 Ma) is composed of coeval mafic–felsic rocks (metagabbro, metatroctolite, massive metabasalt, metadiabase, amphibolite, metagranite, felsic metavolcanic rocks, and metatrondhjemite) exposed as rounded or oval intrusions, sills, and dikes. Whole-rock geochemical data show that the metamafic rocks of the Rio Branco Suite are sub-alkaline, medium- to low Ti tholeitic, with composition and predominant overlay signature of island arc tholeiite and back-arc basin basalt. In multi-element diagrams, the rocks show negative anomalies of Nb and Ta, positive anomalies of Pb, and gradual enrichment in Th relative to Nb and Ta. The following features are also present: Ti/V = 10–50, La/Yb = 1.74–12.33 and Th/Yb = 0.10–3.56; low to high Ce/Y (4.12–24.67) ratios; and low-ratios Ce/Pb (3.3–20.5), Nb/U (3.2–16.25), and Th/La (0.01–0.30). All these signatures are characteristic of mafic rocks generated in a subduction environment. Sr Nd isotope composition showed eNd with slightly negative to positive values between −2.89 and + 1.66 (t = 1120 Ma) close to CHUR and varied 87Sr/86Sr ratios (0.7024–0.7138) and low 143Nd/144Nd ratios (~0.51235), suggesting crustal contamination and metasomatism of the mantle wedge. Advanced geological data, such as geochemical and isotopic signatures of the Rio Branco Suite metamafic component shown in discriminated tectonic diagrams and trace elements diagrams, are considered effective in the diagnosis of subduction. The results demonstrate that the Rio Branco Suite metamafic rocks were generated in an arc–back-arc environment developed by intra-oceanic subduction associated with the accretionary phase of the Nova Brasilândia orogeny between ca. 1137 and 1106 Ma.