Isotopic Signatures of Neoproterozoic to Cambrian Ultrapotassic Syenitic Magmas,Northeastern Brazil: Evidence for an Enriched Mantle Source

Neoproterozoic to Cambrian ultrapotassic (K₂O up to 13 wt%) peralkalic alkali-feldspar-rich syenitic plutons were emplaced along the boundary between the Cachoeirinha-Salgueiro and Alto Pajeu tectonostratigraphic terranes of the Borborema structural province, northeastern Brazil. Syenite and alkalic pyroxenitic magmas coexisted in these plutons, which locally carry mica pyroxenite xenoliths. In the Triunfo batholith, the largest peralkalic pluton in the region, syenites and alkalic pyroxenites have high pyroxene-corrected δ¹⁸O values (+8.1 to + 8.5‰_SMOW in the syenite and +7.6 to + 7.7‰ in the alkalic pyroxenite), high δ³⁴S (+12.3‰_CDT in syenite and + 11.2‰_CDT in alkalic pyroxenite), high initial ⁸⁷Sr/⁸⁶Sr ratios (0.7098, syenite and alkalic pyroxenite data lying on the same Rb-Sr isochron), and low εNd (?15.3 to ?17.2 in syenite and ?16.1 in pyroxenite). Whole-rock δ¹⁸O_SMOW for mica pyroxenite xenoliths varies from +7.5 to +8.0‰_SMOW. Syenite, alkalic pyroxenite, and xenoliths all are enriched in large-ion lithophile elements (LILE). These geochemical and isotopic signatures suggest that the magmas were derived from an incompatible-element-enriched mantle source; this protolith probably resulted from hybridization by addition of crustal material via subduction at ～2.4 Ga, as estimated from Nd model ages. Partial melting of metasomatized material and magma emplacement at a late stage of the Brasiliano Orogeny (566 Ma) were controlled by mantle-deep shear zones during the amalgamation of the Cachoeirinha-Salgueiro and Alto Pajeu terranes.