Abstract
Magmatic activity circumscribed to a restricted area lasting for tenths of millions of years allows to link changes in magma chemistry to deformation, and thus to determine the tectonic evolution of a region. Here we describe an area of about 180 × 40 km in eastern Borborema Province (northeastern Brazil) where several pulses of magmatism occurred in the time interval 656–562 Ma. The oldest pulses are represented by orthogneisses bearing a flat-lying foliation resulting from deformation and metamorphism of granitoids emplaced in three stages: 656–645 Ma, 632–618 Ma, and 606–597 Ma. The oldest and most important unit (656–652 Ma-old) has large length/width ratio (c. 10), alkali-calcic signature, lacks pronounced negative Ta anomalies whereas showing positive Hf and Zr anomalies in normalized trace element diagrams, and yields crystallization age similar to the deposition age of surrounding, dominantly clastic metasedimentary units. These characteristics point to emplacement in an extensional setting and most probably fit in a rift/sag basin model, with thermal thinning of the lithosphere generating regional subsidence and magmatism. Rocks related to the 632–618 Ma-old stage are represented by orthogneisses with petrographic and geochemical characteristics akin to I-type granites, indicating a shift to an orogenic setting at around 640–630 Ma. Increased radioactive heating during continued thickening promoted anatexis of metasedimentary rocks at mid-crustal levels and segregation of small volumes of peraluminous melts at 606–597 Ma. The subsequent period of magmatic activity at 587–581 Ma witnesses the transition to a transcurrent regime, with intrusion of large granitic and syenitic batholiths with high-K calc-alkalic to shoshonitic affinity. Intrusion of these plutons added to heat resulting from strain heating to produce peraluminous granites at 573–566 Ma through partial melting of metasedimentary sources. The final magmatic episode at c. 562 Ma suggests involvement of the lithospheric mantle and crustal sources, perhaps related to localized delamination. Generally, younger orthogneisses and plutons contain zircons inherited from previous magmatic events, pointing to important crustal reworking. Together with the geochemistry of the rocks, these results imply only modest addition of juvenile material and thus no net crustal growth in eastern Borborema Province during the 660–560 Ma time interval.