New Mexico Geological Society Annual Spring Meeting — Abstracts

Paleoproterozoic quartzite-rhyolite assemblages remain an enigmatic and poorly dated tectonic feature of the 1.70-1.65 Ga crustal assembly of southern Laurentia. An understanding of these sequences will provide insight into the processes involved in the formation of continental lithosphere. 1-2 km think, quartz arenites with greater than 95% quartz are found throughout the southwest and are likely associated with the Yavapai and Mazatzal orogenies. Due to the lack of modern analogs, the depositional and tectonic settings of these first-cycle quartzites remain controversial.

Two distinctive suite of these sequences have been identified based on U-Pb geochronology; ~1.70 Ga and ~1.66 Ga. This study focuses on one if the younger of the quartzite-rhyolite assemblages, located in the Monzano Mountains of central New Mexico. In this locality, the rhyolites are stratigraphically above and below the Monzano Group Quartzite, allowing a unique opportunity to precisely date these quartzites. Previously, a wide range of dates have been found for the Monzano area rhyolites, 1662 ± 1 to 1680 ± 20 Ma (Shastri, 1993), 1680 ± 20 Ma (Bowring et. Al, 1983), and 1700 ± 20 Ma (Unruh, unpubl). Using detrital zircons and monazites from the quartzite, and zircons from the rhyolite, new dates will tightly constrain the timing of deposition, timing of deformation, and the provenance of the quartzite. This data, along with field mapping and comparative studies between correlative quartzite-rhyolite sequences in Arizona and elsewhere in the Southwest, will provide essential information for understanding the tectonic evolution of continental lithosphere.