Magma emplacement and shear zone nucleation and development in northeast Brazil (Fazenda Nova and Pernambuco shear zones; State of Pernambuco)

Abstract

The Fazenda Nova and Serra da Japeganga batholiths and the Toritama pluton form a large granitoid massif located in the eastern part of the Borborema Province of Northeast Brazil. The sinistral, NE-SW trending, Fazenda Nova transcurrent shear zone (FNSZ) cuts across the contacts, and the deformation is localized within and in the immediate vicinity of the three magmatic bodies. The southern boundary of the Serra da Japeganga batholith is marked by the dextral, east-west trending, East Pernambuco shear zone (EPESZ). The internal structure in the Fazenda Nova/Serra da Japeganga complex, with increasingly more felsic units upwards, reflects the original stratigraphy in a vertically stratified magma chamber. Together with local strong discordant contacts, the common absence of shear zone-related deformation in the country rocks, and the lack of evidence for shear zone control on the internal magmatic fabric of the plutons away from the FNSZ and EPESZ, this suggests that magma emplacement predated shear zone development. Magma crystallization, however, was contemporaneous with shear zone evolution, as indicated by the rotation of the magmatic foliation approaching the shear zones, and by a continuous transition from magmatic to solid-state deformation fabric. During and after crystallization in the plutons, shear zones controlled the ascent and emplacement of subsequent magma batches which then formed dike swarms in both the FNSZ and EPESZ. Predominance of mafic dikes suggest that the shear zones extended downward to the upper mantle during their development. It is often suggested that fault zones control the ascent and styles of pluton emplacement. The results of this study, however, suggest an alternative point of view to explain the common association of granites and strike-slip faults. It is proposed that several shear zones in the intracontinental Borborema Province were nucleated due to thermal anomalies associated with preexisting magma chambers. Incompletely crystallized magmatic bodies represent significant rheological heterogeneities in the crust, that may have triggered strain localization and nucleation of these shear zones.