DATING DEFORMATION, EXHUMATION, AND FLUID-ROCK INTERACTIONS IN FAULT SYSTEMS USING APATITE U-PB AND TRACE ELEMENT LA-ICP-MS ANALYSIS

Apatite U-Pb depth-profiling and trace and rare earth element (TREE) LA-ICP-MS analyses in combination with detailed imaging have the power to recover the spatial age and chemical information that reveal the thermal history and fluid-rock interactions along crustal-scale faults in the middle and lower crust, and to distinguish between different thermal scenarios and isotopic/elemental resetting mechanisms. Apatite U-Pb thermochronometry, geochemistry, and microtextural analysis was performed on samples collected from the footwall, hanging wall, and along a mylonitic shear zone (known as the MMB) within a basement massif in the North Pyrenean Zone, France. Results show the middle-lower crust in the footwall was exhumed along the MMB in the Early Cretaceous during the period extreme crustal thinning and mantle exhumation along the Iberia-European margin. Apatite grains record partial recrystallization associated with shearing and circulation of deeply-sourced fluids along the fault evident by the mixed ages (300-100 Ma), geochemical signatures, and patchy microtextures. Samples from the hanging wall show partial recrystallization via crack-seal deformation mechanisms near the MMB (0-80m) and are unaffected further from the MMB (~100m) with preserved magmatic apatite U-Pb crystallization ages of ~300 Ma and magmatic growth zoning. These results provide important constraints for linking middle to lower crustal processes with the coeval upper crustal brittle faulting, exhumation, metasomatism and talc-chlorite mineralization within the basement massif. Results demonstrate the power of this integrated approach of U-Pb geochronology and TREE analysis by LA-ICP-MS coupled with microtextural analysis in interpreting apatite U-Pb ages, directly dating deformation near the brittle-ductile transition, and providing ages and constraints on the petrologic and geochemical conditions of (re)crystallization and fluid-rock interactions along fault zones within middle crustal depths.