THE DURATION, PRESSURE, AND TEMPERATURE OF METAMORPHISM, PICURIS AND TUSAS MOUNTAINS, NM

Proterozoic metamorphic rocks in the Picuris and Tusas mountains of northern New Mexico are important because they include pelitic schist containing multiple Al₂SiO₅ polymorphs which have been attributed to multiple thermal events or a single long duration metamorphic event. These metamorphic rocks are cut by the Plomo-Pecos Shear Zone (PPSZ), possibly a significant tectonic boundary. We present new garnet Sm-Nd ages and P-T data across the shear zone that constrain the duration and conditions of amphibolite facies metamorphism in these mid-crustal rocks.

Garnet Sm-Nd ages have a weighted mean of 1398±22 Ma (N=10/10), indistinguishable from all but 2 of 8 published garnet Lu-Hf ages. Most garnet have major element zoning dominated by decreasing Mn and increasing Fe and Mg from core to rim. This is interpreted as prograde with limited evidence for diffusional relaxation. Garnet trace element zoning is more complex with breaks that could indicate complex reaction history, multi stage growth, and/or resorption. In garnet dated with both chronometers, the Lu zoning may be sufficient to cause bias of Lu-Hf ages to older cores and major element zoning may be partly modified, compatible with some diffusion. The 2 oldest, c. 1450 Ma, garnet Lu-Hf ages could reflect the onset of prolonged regional metamorphism or an earlier thermal event. However, the oldest garnet Lu-Hf age (1451±26 Ma) in the Picuris Mountains overlaps with the garnet Sm-Nd result for the same sample when all garnet aliquots are included. The 3 youngest, c. 1370 Ma, garnet Sm-Nd ages may reflect minor diffusional re-equilibration; however, the data are inconclusive. Thermobarometry, and P-T estimates from garnet zoning and mineral assemblage diagrams are used to construct prograde clockwise paths for metamorphism. Some of these P-T paths start with stable andalusite, but all peak with pressures above the Al₂SiO₅ triple point. In the Picuris Mountains, peak temperatures reached up to ~610°C and peak pressures varied from ~5 kbar south to ~6.5 kbar north of the PPSZ.

We interpret that all of the Picuris garnet growth was during a single protracted regional event. In this interpretation, garnet grew within a maximum of c. 75 m.y., but more likely c. 50 m.y. The results indicate that garnet growth P-T paths reached pressures and temperatures well above the triple point, and that the PPSZ had significant vertical displacement.