Petrogenetic insights of Bezerros and Serra Negra plutons: evidence of syn/post-collisional shoshonitic magmatism in the Caruaru-Arcoverde Batholith, Rio Capibaribe Domain, Northeastern Brazil

Potassic magmatism in the Transversal Zone Domain of the Borborema Province is represented by high-K to shoshonitic granitic suites, commonly associated with syn-to post-collisional settings. The Bezerros (BZP) and Serra Negra (SNP) plutons are located in the eastern portion of the Caruaru–Arcoverde composite Batholith, one of the largest granitic bodies emplaced during the Brasiliano/Pan-African orogeny. They have an elliptical shape oriented NE–SW, controlled by three shear zones. Two of these shear zones, located at the southeastern margin and central part of the intrusions, exhibit dextral kinematics, while the third, situated on the northwestern border, shows sinistral kinematics. These granitoids share a similar mineralogical composition, characterized by potassic feldspar, plagioclase, quartz, and amphibole in varying modal proportions. The BZP is a inequigranular very coarse-grained unit with syenogranitic to monzogranitic composition, whereas the SNP is porphyritic and ranges from quartz-monzonitic to monzogranitic composition. Both plutons host inequigranular enclaves characterized by their finer textures than the host rocks, with biotite-dioritic to biotite-monzonitic compositions. Electron microprobe analyses reveal a edenitic composition of amphibole in both the granitoids and associated enclaves, indicating magma mingling processes, an interpretation further supported by Harker-type variation diagrams. Additionally, biotite compositions reflect a progressive evolution in oxidation state and volatile content (H₂O). The SNP and BZP host granitoids and their enclaves are metaluminous, and show high K₂O contents (4.6–6.4 wt%) and K₂O/Na₂O ratios (1.2–1.8), which confirms their shoshonitic affinity. These rocks are enriched in Ba, Sr, and light rare earth elements and exhibit negative anomalies in Ti, Nb, and P; no negative Eu anomalies were registered. The U–Pb zircon ages of BZP (589 ± 1.4 Ma) and SNP (574 ± 1.4 Ma) are interpreted as crystallization ages. Both plutons exhibit high initial ⁸⁷Sr/⁸⁶Sr ratios (0.709–0.726) and negative εNd(t) values (−16.2 to −14.7), indicating significant contributions from Paleoproterozoic crustal components (Tdm ∼ 2.0 Ga). We propose that the granitic magmas were formed by crustal anatexis induced by K-rich mafic melts derived from a metasomatized lithospheric mantle. The large volume of mafic magma, as evidenced by the numerous enclaves and the various degrees of mixing observed, supports this interpretation. It is possible that some of the chemical signatures observed in the plutons and enclaves were inherited from the mantle source, which was metasomatized during Paleoproterozoic subduction.