Redefining the Metamorphic History of the Oldest Rocks in the Southern Rocky Mountains

Proterozoic rocks exposed in the Rocky Mountains of New Mexico record the history of the assembly of North America, providing insight into crustal growth, accretion, and recycling. Here, we present six wellconstrained Lu-Hf garnet ages showing that regional amphibolite-facies metamorphism and deformation occurred between 1.46 and 1.40 Ga. The samples come from the kyanite, sillimanite, and andalusite zones of the "Al2SiO5 triple-point terrane" of northern New Mexico. Porphyroblast-matrix textures in dated samples provide definitive evidence for garnet growth during three phases of deformation associated with crustal thickening during the development of a regional-scale north-vergent contractional orogenic belt. The development of first-generation deformation fabrics is constrained to a period between 1.46 and ca. 1.45 Ga; second-generation deformation fabrics are constrained to the period 1.42-1.40 Ga; and third-generation fabrics were formed after ca. 1.40 Ga. Our results demonstrate that crustal-scale structures and regional metamorphism in northern New Mexico are both the result of a Mesoproterozoic orogenic event, the Picuris orogeny. We found no evidence for amphibolite-facies metamorphism during the Paleoproterozoic Mazatzal or Yavapai orogenies in northern New Mexico.