NEW INSIGHT INTO 1.7-1.65 YAVAPAI-MAZATZAL AGED TECTONISM IN THE TUSAS MOUNTAINS OF NORTHERN NEW MEXICO

Metasedimentary and metaigneous rocks in northern New Mexico, provide constraints on the tectonic evolution of the southern Laurentian margin from 1.8-1.7 Ga. Tectonic overprinting in this region occurred at ~1.4 Ga., obscuring the deformational and metamorphic history prior to this pervasive tectono-metamorphic event. Two conflicting end-member models suggest that multiple generations of folds, structural fabric formation, and metamorphism occurred either at 1.6 Ga or at 1.4 Ga. Recent observations in the northern Tusas Mountains of New Mexico suggest that the 1.4 Ga. overprint is mostly absent, providing a window into Yavapai-Mazatazal aged (~1.7 Ga) tectonism. In the central-northern portion of this range, north of the Spring Creek shear zone, early-formed structures such as cross-cutting fabrics with the Tres Piedras and Tusas Mountain metagranitic plutons (respectively dated by U/Pb on zircon at 1.70-1.693 Ga and 1.693 +/-3 Ga) and preservation of 1.67-1.66 Ga Ar/Ar hornblende ages contrast overprinted fabrics and 1.40-1.38 Ga Ar/Ar ages in the south. New analysis of porpyroblastic growth in thin Al-rich pelitic layers of slightly variable bulk composition display a range of index minerals from Ctd +Chl to St+Bt+/-Ky to Gt+Bt+/-St over a distance of 3-5km, varying spatially with respect to the metaplutonic units. These assemblages generally post date tectonic fabrics. Pseudosection analysis using Perple_X yield P-T estimates of 425-575˚C at ~4 kbars, showing a 50-150˚C temperature gradient across this distance. The temperature gradient of inclined isotherms suggests the preservation of a contact areole that developed during the 1.69 Ga. emplacement of the Tusas and Tres Piedras metagranites. This isotherm pattern contrasts a horizontal thermal gradient associated with the 1.4 Ga event preserved south of the Spring Creek shear zone. 1.4 Ga overprinting is represented by syntectonic Ky+Bt growth on Gt+St assemblages that yield temperature conditions similar to those near the plutons, but slightly higher pressure (~1 kbar). The preservation of a contact areole and its relationship to structural fabric provides a clear window into Yavapai-Mazatzal aged tectonism. Our data suggest that syntectonic emplacement of the granitic plutons occurred during regional tectonism at 1.70-1.65 Ga.