PROTEROZOIC RHYOLITE-QUARTZITE SEQUENCES OF THE SOUTHWEST: SYNTECTONIC “COVER” AND STRATRIGRAPHIC BREAKS (~1695 AND ~1660 MA) BETWEEN OROGENIC PULSES

Proterozoic rocks of the Southwest contain a distinctive association of high silica, alkalai rhyolite and ~ 1 km thick quartzarenite. They overlie a volcanogenic ?greenstone? basement that was previously deformed and metamorphosed. Pre-quartzite tectonism is documented in the Needle Mountains (high temperature deformational fabrics in the basement not present in the quartzite), Mazatzal Group (basal angular unconformity), Blue Ridge Quartzite (basal regolith on granite), and Gunnison areas (lower strain in quartzite than in basement). Quartzite successions fall into two age ranges: (1) 1.7-1.69 Ga (Hondo Group, NM, Mazatzal Group, AZ, Uncompahgre Group, Blue Ridge Quartzite, and Coal Creek Quartzite, CO) and (2) 1.66-1.65 Ga (Manzano Group, NM; White Ledges quartzite, AZ; Chino Valley quartzite, AZ). New dates and mapping of the Blue Ridge quartzite provide some of the best new constraints on the older group: depostion on a ~1706 Ma granite, crosscut by ~1686 Ma pegmatite. Rhyolite- quartzite successions are clearly syntectonic in a regional sense (both at 1.70-1.69 and 1.66-1.65 Ga) because volcanism and deposition were occurring at the same time as plutonism and deformation nearby. Thus, we do not interpret them to represent a shelf sequence or rifted margin, but rather as syntectonic basins, developed on stabilizing crust. They appear to mark unroofing of middle crustal rocks prior to and during their deposition. The angular unconformities are interpreted to record collisional exhumation of upthrust blocks and related deposition in small foreland basins. Their importance for understanding crustal evolution is underscored by their complex P-T-t-D paths, with multiple loops: 1) Volcanogenic basement was unroofed from middle crustal depths in the Yavapai orogeny. 2) Quartzites were deposited during continued thrust convergence in late stages of the Yavapai orogeny. 3) 1.70-1.69 Ga quartzites (and their basement) were buried (re-buried) to depths of 5-15 km via thrusting during the Mazatzal orogeny at 1.65 Ga. 4) similar rhyolite-quartzite successions were deposited syntectonically during the Mazatzal orogeny. 5) Both ages of rhyolite-quartzite were variably deformed and metamorphosed in the middle crust (5-15 km) at ~1.4 Ga before being exhumed to the surface by ~ 1.25 Ga.