DETRITAL ZIRCON GEOCHRONOLOGY OF THE PENNSYLVANIAN-PERMIAN INGLESIDE FORMATION IN NORTHERN COLORADO: EVIDENCE FOR THE ARRIVAL OF APPALACHIAN ZIRCONS AT THE ANCESTRAL ROCKIES

The Pennsylvanian-Permian Ingleside Formation in Colorado was deposited along the flanks of the Ancestral Front Range as a result of late Paleozoic tectonism and eustatic sea level changes. We present new detrital zircon U-Pb ages from the type section of the Ingleside Formation at Owl Canyon to place age limits on deposition and to understand the sediment provenance. Samples were collected from a shoreface sandstone unit in the lower Ingleside Formation and an eolian sandstone unit in the upper Ingleside Formation, representing late Pennsylvanian and early Permian time, respectively. Paleocurrent indicators throughout the section indicate primarily S- to E-directed sediment transport. We determined U-Pb ages on 120 zircons from each sample using LA-ICPMS, and ages with excessive discordance (>20% discordant or >5% reverse discordant) were rejected. Both sandstone samples dominantly yield 1800-1600 Ma ages, reflecting zircon sources in the Yavapai-Mazatzal terranes that cored the Ancestral Rocky Mountain uplifts. The oldest cluster of zircon ages obtained is in the range of 2000-3100 Ma. These ages are rare in the United States and have been documented in the Mojave terrane and basement uplifts in the southern Canadian Cordillera. Small populations of detrital zircons yield age peaks at 1300-900 Ma, suggesting derivation directly from the Grenville basement terrane. Both samples also contain small populations of Paleozoic zircon ranging from 330 to 470 Ma that coincides with high magmatic flux during the Taconic and Acadian orogenies in the Appalachian orogen. In addition, some~305-320 Ma zircons with 20-30% discordance are also present, possibly indicating an Alleghanian-age source . We interpret these Paleozoic zircons to have been sourced from Appalachian orogen. Previous detrital zircon studies indicate that Paleozoic zircons are absent from the underlying Fountain Formation, which is dominated by the 1600-1800 Ma population of locally-derived zircons. Thus, the transition from fluvial deposition of the Fountain Formation and marine to eolian deposition of the Ingleside Formation marks the arrival of exotic zircons formed during the Appalachian orogeny.