GEOCHRONOLOGIC CONSTRAINTS ON PROTEROZOIC BASEMENT EVOLUTION AND QUARTZITE DEPOSITION, WEST NEEDLE MOUNTAINS, COLORADO

U-Pb geochronology from basement assemblages and Pb-Pb detrital zircon geochronology from quartzites of the overlying Uncompaghre Formation provide precise new timing constraints on the Proterozoic tectonic evolution of the West Needle Mountains, Colorado.  Basement samples collected from a single outcrop ~0.5 km southwest of Coal Bank Pass include polydeformed Twilight Gneiss and cross-cutting, foliated mafic and granitic dikes.   The tonalitic protolith of the gneiss crystallized at 1774±3 Ma (zircon) and underwent at least one episode of penetrative deformation (D₁) prior to intrusion of the dikes.  Metamorphic zircon and titanite from a foliated mafic dike suggest that a second phase of deformation (D₂) involving upright tight-to-isoclinal folding and foliation development at amphibolite facies occurred at 1706±4 Ma.  Granitic dikes cutting D₂ fabrics were emplaced at 1706±2 Ma and were subsequently deformed during a younger (Mazatzal?) orogenic pulse (D₃).  Fabrics developed during D₂ are consistent with the NE-striking regional tectonic grain developed during the ca. 1.7 Ga Yavapai Orogeny.  Furthermore, the confluence of deformation, magmatism, and metamorphism at ca. 1706 Ma represents a robust local maximum age for the deposition of quartzite and schist of the Uncompaghre Fm. 

Laser ablation ICP-MS analysis of detrital zircon from the Uncompaghre Fm. basal conglomerate sampled near Snowden Peak yielded a range of Paleoproterozoic ²⁰⁷Pb/²⁰⁶Pb ages with a minor component of Archean-aged detritus.  The peak detrital age of the population was 1728±13 Ma, and the youngest reliable ages consistently clustered at ca. 1700 Ma.  These ages are consistent with data from similar Proterozoic quartzite sequences across the region and suggest that quartzite was deposited on exhumed Yavapai-aged basement shortly following the orogenic peak.