Geochronologic constraints on deformation and metasomatism along an exhumed mylonitic shear zone using apatite U-Pb, geochemistry, and microtextural analysis

Directly dating and characterizing deformation and metasomatism in fault zones is critical to understanding fault zone processes and lithospheric evolution. Integrated apatite U-Pb geo-thermochronometry, geochemistry, and microtextural analysis has potential to directly date and characterize deformation and fluid-rock interactions in shear zones using a single mineral phase. Integrated apatite U-Pb, microtextural, and trace and rare earth element results from an exhumed mylonitic shear zone, the Main Mylonitic Band in the Northern Pyrenees, constrain timing of deformation and metasomatism along the shear zone as Early Cretaceous, synchronous with ductile deformation and exhumation of middle-lower crustal gneisses in the footwall, and significant mineralization and metasomatism in the brittle hanging wall. Samples from the hanging wall preserve magmatic apatite U-Pb crystallization ages that overlap with zircon and monazite U-Pb ages of similar to 300 Ma, granitic geochemical trends, and primary magmatic growth zoning. Footwall samples below the MMB record dynamic recrystallization of apatite associated with mylonitization and exhumation of the gneisses in the Early Cretaceous. Apatite along the MMB record partial recrystallization via dissolution-reprecipitation associated with shearing and circulation of deeply sourced fluids along the shear zone evident by mixed ages (300-100 Ma), geochemical signatures enriched in light rare earth elements, and patchy microtextures. The MMB likely acted as a conduit for deeply sourced fluids that affected the brittle-ductile transition, strain localization, and fluid pathways through the continental crust. This study demonstrates that the integration of apatite U-Pb geo-thermochronology, geochemistry, and microtextural analysis is a powerful approach to dating deformation and metasomatism, characterizing grain scale deformation and (re)crystallization processes, and geochemically characterizing fluid-rock interactions in exhumed middle crustal shear zones. (C) 2020 Elsevier B.V. All rights reserved.