Shear zone-controlled magma emplacement or magma-assisted nucleation of shear zones? Insights from northeast Brazil

Abstract

In the Borborema province of northeast Brazil, neoproterozoic granitoids and large-scale transcurrent shear zones are spatially associated, suggesting a genetic link between magma bodies and shear zones. In some cases magma emplacement was clearly favored by shear zone activity, but for several plutons this model is not satisfactory. In these plutons, pre-full crystallization strike-slip deformation, evidenced by parallelism of magmatic foliations and lineations with the solid-state mylonitic fabric, and by a transition from magmatic to solid-state flow, is restricted to the vicinity of the shear zones. Evidence of shear zone activity prior to magma emplacement is lacking, and the magmatic foliation away from the shear zones is in most cases shallowly dipping and concordant with the slightly older, gently-dipping, regional gneissic foliation. Field and anisotropy of magnetic susceptibility mapping, together with petrographic and geochemical studies performed in one of the magmatic complexes of the Borborema province have revealed a structure and a magmatic fabric incompatible with the shear zone-controlled emplacement model. Away from the shear zones, this complex has retained a stratification inherited from the mixing of crystal-poor magmas of contrasting composition, and a magmatic fabric characterized by low-to moderate-dip magmatic foliations bearing a NW-trending lineation, which contrast with vertical foliations bearing NE- or E-W-trending stretching lineations in the shear zones and indicates that crystallization started prior to shear zone development. Based on evidence that magma emplacement predated strike-slip shearing and on information about the transition from magmatic to solid-state deformation observed in the studied plutons, we suggest that incompletely solidified plutons within the crust represent rheological heterogeneities that may induce strain localization and favor shear zone nucleation. We propose that in the studied cases deformation first concentrated within the plutons, where shear zones nucleated and grew. Subsequently, as the solid phase component increased, the crystal mush began to behave as a solid, and the shear zones propagated into the country rocks.