New Mexico Geological Society Annual Spring Meeting — Abstracts

Preliminary results of in-situ microbe dating of monazite from the Ortega quartzite suggest an increasing influence of 1.4 Ga tectonism from north to south within the Tusas Mountains of northern New Mexico. Monazite from the Jawbone syncline within northernmost part of the range consistently yield ages of 1.75 to 1.72 Ga. These monazite grains are interpreted to be mostly detrital in origin, with REE and age zoning reflecting the history of the source terranes. Monazite from an anticline immediately to the south has 172-1.75 Ge detrital cores with 1.67-1.65 Ga rims, implying that initial fold formation occurred during the ~1.67-1.65 Ga Mazatzal orogeny. Monazite from the middle and southern Tusas Mountains is predominantly 1.4 Ga in age. This suggests that a previously documented gradient in deformation and metamorphism from north to south may reflect a multistage tectonic history for the range, with an increasingly intense over print of 1.4 Ga tectonism to the south.

The discovery of abundant monazite in regionally extensive, 1-2 km thick quartzites found throughout the Proterozoic orogenic belt of the southwestern United States may provide important new constraints on the region’s tectonic history, specifically, the extent and influence of 1.4 Ga tectonism on the formation and modification of fundamental large-scale structures. These quartzites define the present regional geometry of exposed rocks within the Proterozoic Mazatzal Province, and are believed to strongly influence local structure. In addition to northern New Mexico, monazite dating in thick quartzites represents a powerful tool by which we can better understand the evolution and stabilization of Proterozoic crust in the southwestern United States, and may be an important new technique to deconvoluting the tectonic histories of other orogenic belts.