Unraveling the relationship between a tonalitic pluton and shear zones: Insights from magnetic fabrics and microstructures of the Alto Maranhão batholith, Mineiro belt, southern São Francisco craton

Abstract

The Alto Maranhão batholith (SE Brazil) comprises granitic juvenile arc-related rocks within the Paleoproterozoic Mineiro Belt. The pluton is bounded by the NE-SW sinistral Jeceaba-Bom Sucesso (JBSZ) and NW-SE dextral Congonhas-Itaverava (CISZ) shear zones. Field, microstructural and magnetic fabric studies were employed in order to constrain its emplacement model and the spatial and temporal relationships between plutons and shear zones during Paleoproterozoic orogenic events. Combined meso- and microscale fabric analyses allowed us to individualize two different domains or lobes across the intrusion. The magmatic foliation, defined by biotite and hornblende, strikes NE-SW in the western lobe and rotates to NW-SE in the eastern lobe, moderately to steeply dipping, trending accordingly with the solid-state foliation exhibited by the country rocks. Field observations show that magmatic structures are preserved, such as elongated to rounded syn-magmatic enclaves and magmatic layering. The microstructures described for both domains are a direct record that deformation occurs locally and under magmatic conditions, with limited features of solid-state deformation such as undulose extinction in quartz and plagioclase on both tonalite and dioritic enclaves. AMS data exhibits dominantly steep magnetic lineations in the eastern lobe of the intrusion, except for sampling sites close to the CISZ, whose lineations are shallowly plunging. In both cases, the magnetic lineation trends NW-SE. Towards the western lobe, the magnetic lineation becomes horizontal and randomly oriented close to its westernmost tip. The magnetic and magmatic foliations follow the same orientation, therefore striking NW-SE in the eastern lobe and NE-SW in the west. Considering the batholith's heterogeneities highlighted by the AMS measurements, we suggest that the eastern lobe of the intrusion is characterized as a feeder zone in which magma was injected along the CISZ, being subsequently spread into the pluton's western lobe, until being stopped by a rheological barrier defined by JBSZ and country rocks, controlling its boomerang-like form. The proposed emplacement model suggests that the Alto Maranhão batholith was emplaced in a contractional site, by arc-normal dextral simple shear revealing that transcurrent deformation was taking place concurrently with the growth of the São Francisco paleocontinent, and addition of mantle-derived magmas to the continental crust.