U-Pb and 40Ar/39Ar geochronology and tectonic setting of the peralkaline magmatism in the Borborema Province (NE Brazil)

Aegirine-augite granite and syenite plutons and dikes form a distinct magmatic pulse aligned in the NE-SW di-rection in the central and northern Borborema Province. We use SHRIMP zircon U-Pb and laser step-heating 40Ar/39Ar determinations to constrain the crystallization and cooling ages of syenites and alkali-feldspar gran-ites and gabbros deformed by shear zones. Concordant zircon ages of the Triunfo and Cacimbas syenites vary between 596 & PLUSMN; 4 and 594 & PLUSMN; 4 Ma while sodic amphiboles from a granitic dike of Manaira swarm yield 40Ar/39Ar plateau ages of 606 & PLUSMN; 5, 605 & PLUSMN; 6 and 599 & PLUSMN; 4 Ma. Muscovite from host mylonitic gneisses, in turn, yields plateau ages of 600 & PLUSMN; 3 Ma and 586 & PLUSMN; 2 Ma. Close to the Patos shear zone which limits the Central and Northern Borborema domains, foliated felsic dike of the Serra do Algoda & SIM;o suite yields a concordia age of 592 & PLUSMN; 2 Ma whereas the Catingueira granitic pluton records numerous inherited zircons. The Tapero & PRIME;a gabbro stock, situated next to the connection between E-and NE-trending shear zones, yields concordant ages of 592 & PLUSMN; 2 Ma. These results constrain the alkaline magmatism of the Central and Northern domains between 600 and 590 Ma, i.e., shortly after the continental collisions that amalgamated the province in the Ediacaran. Cooling was faster in the Triunfo-Manaira suite and apparently slower close to the Patos shear zone. Host rock structures indicate that the Triunfo-Manaira suite emplaced in the brittle upper crust, whereas the alkaline rocks of the Serra do Algoda & SIM;o suite emplaced in HT/LP ductile transtensional setting. A possible tectonic scenario includes slab break-off in a continental collision setting, partial melting of the sub-continental mantle and lower crust and magma transfer to the middle-upper crust. The distribution of these rocks next to the margins of volcanosedimentary belts suggests that their ascent and emplacement explored deep fault segments reactivated by crustal extension.

Automated summary

A study on the Aegirine-augite granite and syenite plutons and dikes in the Borborema Province provides chronological constraints using SHRIMP zircon U-Pb and laser step-heating 40Ar/39Ar determinations. The results suggest that the alkaline magmatism in the central and northern parts of the province occurred between 600 and 590 million years ago, shortly after the continental collisions that formed the province in the Ediacaran. The emplacement of these rocks was influenced by crustal extension and reactivation of deep fault segments.