MONAZITE GEOCHRONOLOGY OF THE ORTEGA QUARTZITE: DOCUMENTING THE EXTENT OF 1.4 GA TECTONISM IN NORTHERN NEW MEXICO AND ACROSS THE OROGEN

Preliminary results of in-situ microprobe dating of monazite from the Ortega quartzite suggest an increasing influence of 1.4 Ga tectonism from north to south within the in the Tusas Mountains of northern New Mexico. Monazite from the Jawbone syncline within northernmost part of the range consistently yields 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 1.72-1.75 Ga detrital cores with 1.67-1.68 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 overprint 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 has also been found in several Proterozoic quartzites in Colorado, allowing the possibility to compare and correlate deformation and metamorphism across the region. 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 in deconvoluting the tectonic histories of other orogenic belts.