EMPLACEMENT CONDITIONS AND TH/PB DATING OF MONAZITE BEARING SYNTECTONIC VEINS: A CASE STUDY FROM THE MEXICAN FOLD-THRUST BELT (MFTB), CENTRAL MEXICO

Veins are common structures in the upper crust, as they result from multiple and diverse episodes of fluid-rock interaction and deformation. They preserve information of their growth and also of fluids active during deformation as fluid inclusions. When veins grow at high confinement, they experiment similar pressure (P) and temperature (T) to the host rock during deformation. Such conditions and the nature of the fluids active during deformation can be constrained through careful petrography, mineralogy, microthermometry of fluid inclusions and geochemical modeling. Moreover, if veins contain radioactive isotope bearing minerals (e.g. monazite), it is possible to obtain their age of emplacement and thus of deformation.

We study the deformation history of the Zimapan Basin (ZB), in the MFTB, in central Mexico. Such basin was shortened with calcite and quartz veins emplaced early, during and late to folding. Such veins are hosted in a volcaniclastic sandstone and shale succession of the Early Cretaceous Trancas Fm., and they contain micron-size monazite crystals. The aims of this study are: A better understanding of processes and conditions of monazite growth and test its potential as a geochronometer of deformation and explore the use of veins as PTt markers of deformation.

Fluid inclusion microthermometry and other geothermometers, indicate that the peak of shortening of the ZB occurred at 200°C-300°C. The P estimated with the analysis of isochores of primary fluid inclusion reached at least 250 MPa. Fluids active during deformation were dominantly aqueous with a low to medium salinity (7-23 wt% NaCl eq.). Geochemical analyses suggest deposit reducing conditions during deformation. Stability diagrams for phosphate species were constructed for temperature and pressure conditions obtained from deformation. Finally, ²⁰⁸Pb/²³²Th in situ monazite ages obtained with LA-ICPMS resulted in three main age ranges: 62±7Ma to 99±9Ma, 108±9Ma and >120Ma. The youngest age group shows a weighted mean age of 80±3.6 Ma which is consistent with age of folding of the ZB; while the other ages are intermediate between folding and deposition of the Trancas Fm. These results suggest that under the conditions previously described, monazite grows and can be a geochronometer, and veins have the potential to be used as PTt markers.