Evidence of Ediacaran transitional arc magmatism in the pelotas terrane, southernmost Brazil: Insights into the source and magmatic processes of the Pinheiro Machado Complex

The Dom Feliciano Belt in southern Brazil records successive accretionary processes of the Brasiliano/Pan-African cycle that contributed to the assembly of Western Gondwana. This study investigates the Pinheiro Machado Complex (PMC), the main Neoproterozoic unit of the Pelotas Terrane, through whole-rock geochemistry and Sr–Nd isotope analyses. PMC lithotypes range from diorites to granites, with SiO 2 contents of 53.9-73.9 wt.% and MgO of 0.6-6.1 wt.%, reflecting compositional diversity from mantle- to crust-dominated sources. Trace-element spidergrams show enrichment in LILE (K, Th, Ce) and negative Nb–Ta–Ti anomalies, consistent with subduction-related arc magmatism. Chondrite-normalized REE patterns indicate LREE (Ce/Yb N = 2.5 - 55.73) enrichment and subtle negative Eu anomalies. Sr–Nd isotope ratios define a mixed mantle–crustal signature, with 87 Sr/ 86 Sr (i) = 0.7087 - 0.7104, εNd (t) = -6.7 to -4.5, and T DM ages of 1.4 - 1.7 Ga, suggesting significant assimilation of Paleoproterozoic orthogneiss basement. Geochemical modeling indicates that PMC magmas evolved predominantly through decoupled assimilation-fractional crystallization (AFC), involving ∼20% assimilation of gneissic crust. In comparison, the mafic members were generated by 5-10% partial melting of a spinel lherzolite mantle. These results, discussed in the context of the Pelotas Terrane, demonstrate that PMC magmatism formed in an oblique continental-arc environment during the closure of a V-shaped oceanic basin. The geodynamic model involves subduction, slab failure, and subsequent collision, recorded in the multiple intrusive pulses and structural associations of PMC rocks. This study refines the petrogenetic framework of the Pelotas Terrane and contributes to understanding the Neoproterozoic evolution of Western Gondwana. • Pinheiro Machado Complex is a series of dioritic to granitic multiple intrusions. • 87 Sr/ 86 Sr (i) of 0.708705 to 0.710407, εNd (t) of -6.74 to -4.49, Nd-T DM 1.4 to 1.7 Ga. • The main magmatic process is the decoupled assimilation and fractional crystallization. • Two distinct sources: spinel lherzolite mantle and an orthogneiss basement. • This complex represents an oblique continental magmatic arc with the closure of a V-shaped basin.