Granitic magmatism is undoubtedly the main geological feature of the Brasiliano Orogeny in the Borborema Province, northeastern Brazil. However, the petrogenesis and tectonic setting of the oldest magmatic pulse (640–620 Ma; Conceição-type granites) in the Central Subprovince is still hotly debated. The Carmo stock consists of calc-alkalic magmatic epidote-bearing Conceição-type porphyritic granodiorite and monzogranites that carry abundant mafic microgranular enclaves (MMEs). LA-ICP-MS zircon U–Pb data indicate crystallization around 615 Ma for these granitoids. Granitoids and MMEs exhibit similar chemical characteristics, both of which show high-K calc-alkalic, metaluminous and magnesian compositions. All studied rocks are enriched in large-ion lithophile elements (LILE) and light rare-earth elements (LREE), and depleted in high-field-strength elements (HFSE) and heavy rare-earth elements (HREE). Host granitoids are characterized by high initial 87Sr/86Sr ratios (0.707863–0.709677), weakly negative ε Nd(t) values (−2.1 to −3.1), and TDM model ages of 1.31–1.36 Ga. These geochemical and isotopic signatures, together with the low Ba/Th and Sm/La ratios and high La/Sm and Th/La ratios, suggest that granodiorites and monzogranites were generated from the partial melting of altered oceanic crust and subducted sediments. The MMEs have Sr–Nd isotopic compositions almost indistinguishable from their host granitoids, with initial 87Sr/86Sr ratios of 0.708895, ε Nd(t) values of −2.8 and TDM model ages of 1.40 Ga. The similarity in isotopic compositions and the petrographic evidence indicate that magma mixing and mingling played an important role in the formation of MMEs. Based on the new geochemical and isotopic data, together with regional geological features, we infer that the Carmo stock rocks, as well as Conceição-type granites, were most likely formed in a syn-collisional tectonic setting following the early break-off of a relatively weak oceanic slab.