Imaging the roots of a post-collisional pluton: Implications for the voluminous Cambrian magmatism in the Aracuai orogen (Brazil)

The Cambrian Santa Angelica intrusive complex (SAIC - SE, Brazil), is composed of two lobes with concentric fabric and bull's eyes shapes, each lobe containing a mafic nuclei and granitic borders. It intruded during the post-collisional phase of the Aracuai orogen (AO). Although SAIC'S internal structure is described as exclusively magmatic, prominent solid-state deformation occurs at its borders, as well as an internal shear zone (ISZ). We studied the mechanisms that caused the different architectures displayed by the post-collisional bodies when emplaced in distinct crustal levels across the AO. We forward-modelled gravity data from the SAIC and its country rocks and performed structural analysis through anisotropy of magnetic susceptibility (AMS) and microstructural data of oriented thin sections in the SAIC border and the intrusion's country rocks. Magnetic measurements indicates that multidomain magnetite and pyrrhotite control the magnetic fabric of SAIC's country rocks. 2D gravity modelling shows that the northeast lobe outcrops its roots and its thickness is less than half of the southwest one. Three zones have been identified based on the occurrence of the solid-state microstructures: the country rocks and the border of the pluton display higher temperatures solid-state microstructures, overlapped by lower temperature ones, while the ISZ only shows high temperature structures. We suggest the SAIC experienced a reverse diapiric mechanism caused by the negative buoyancy of its mafic nuclei during crystallization. As the mafic cores sink through a ductile and hot halo in midcrust conditions, they deform the granitic rocks at the borders, as well as the country rocks, also generating the internal shear zone by the relative motion observed between the lobes. Furthermore, we infer that the buoyancy of the mafic and felsic magmas, together with crustal rheology, were the main constraints responsible to restrain the emplacement depth of the postcollisional plutons in the AO.

Automated summary

The Cambrian Santa Angelica intrusive complex in SE Brazil consists of two lobes with concentric fabric and bull's eyes shapes, intruding during the post-collisional phase of the Aracuai orogen. While described as magmatic internally, the complex shows solid-state deformation at its borders and an internal shear zone. This study suggests a reverse diapiric mechanism due to the negative buoyancy of mafic nuclei during crystallization.