A REVISED TECTONIC HISTORY FOR PROTEROZOIC ROCKS OF THE SOUTHERN ROCKY MOUNTAINS IN NORTHERN NEW MEXICO

Newly obtained Lu-Hf garnet ages show that garnet growth, and therefore amphibolite facies metamorphism occurred entirely after ~1460 within the Tusas and Picuris Mountains of northern New Mexico. These mountains are part of a laterally continuous north directed fold and thrust belt that stretches across the Rocky Mountains of New Mexico. Aluminosilicate isograds that define metamorphic field gradients across the fold and thrust belt cross-cut the contractional structures. Furthermore, garnet microstructures are consistent with diachronous growth across the fold and thrust belt. Lu/Hf ages of garnet spread over ~50 Ma and vary systematically across the fold and thrust belt. Taken together the garnet ages are most consistent with a model where north directed thrusting drives metamorphism, with the thermal peak post-dating deformation. The abnormally high geotherms indicated by aluminosilicate mineral assemblages are likely produced by heat advected by 1.4 Ga plutons.