In situ Th/Pb dating of monazite in fibrous veins: Direct dating of veins and deformation in the shallow upper crust of the Mexican Orogen

Few methods exist for directly dating orogenic events in the upper crust. Here we report in-situ dates from (∼10 μm diameter) monazite grown in quartz-calcite fibrous veins, which precipitated at very low-grade metamorphic conditions (about 250 °C) in volcaniclastic and calcareous rocks of the Early Cretaceous Trancas Fm. in the Zimapán Basin, Central Mexico. Our integrated study includes regional and local structural and kinematic analysis of vein and folds, detailed textural and mineralogical characterization of vein-bearing minerals, and texturally-controlled Th/Pb geochronologic analysis of monazite by LA-MC-ICPMS. We find that monazite in the veins preferentially crystallized at the interface between vein-forming calcite and quartz fibers, as well as at the vein-host rock interface. Monazite ages range from 154 to 68 Ma, with the youngest sub-population yielding a weighted mean age of 76.8 ± 0.8 Ma, coincident with the age of basin-wide folding as earlier constrained by Ar/Ar illite dating. The oldest monazite ages coincide with the timing of deposition of the volcaniclastic host-rock. Given that veins are ubiquitous in shallow crustal rocks, this approach to dating of veins with enclosed monazite has great potential to improve the constraints on temporal resolution of fluid-rock interaction during of deformation.