NEW P-T-TIME PATHS FOR THE PICURIS OROGENY IN NORTHERN NEW MEXICO

Recent garnet and monazite ages confirm a single regional thermal event spanning 1450 to 1350 Ma in the Picuris and Tusas Mountains, northern New Mexico. However, published P-T conditions during this event vary considerably depending on location, spanning 3-5.5 kbar and 550-600°C. We present new P-T-time paths utilizing isochemical P-T phase diagram sections, mineral compositions, and garnet ages (Sm-Nd & Lu-Hf) to better understand the tectonic and thermal history of the Picuris orogeny.

Seven samples across the prominent Plomo-Pecos shear zone record metamorphism at 490 to 615°C and 4.5 to 7.5 kbar. Garnet major and trace element zoning in each sample is compatible with prograde growth, with little evidence for new growth following a hiatus. Maximum P-T conditions range up to c. 1.5 kbar and 100°C higher than previously estimated. Individual samples show temperature increases of 55-105°C and pressure increases of 0.2-1.6 kbar, with near isobaric to loading paths. Temperatures of initial garnet equilibrium show a strong correlation with age and proximity to the Plomo-Pecos shear zone. The lowest garnet-in temperatures and oldest garnet ages occur south of the shear zone, and the highest peak temperatures and youngest garnet ages occur north of the shear zone. Peak pressures systematically increase from south to north across the shear zone. Together these data are compatible with garnet growth during a single thermal event, which initiated at low pressure pre- or syn-initial faulting at 1450-1430 Ma and then continued to peak conditions by 1405-1350 Ma. Bulk rock compositions south of the fault facilitated early garnet growth while compositions north of the fault resulted in later growth at higher P-T conditions. In this scenario, prolonged regional heating affected both sides of the fault with a heating rate of c. 0.7°C/m.y. and individual P-T paths are compatible with thermal relaxation following juxtaposition of the Vadito Group over the Trampas and Hondo Groups along the Plomo-Pecos shear zone.