U-PB AGES OF DETRITAL ZIRCONS IN THE HOLBROOK MEMBER OF THE MOENKOPI FORMATION NEAR WINSLOW, ARIZONA

Detrital zircon grains for this study were sampled from Middle Triassic (Anisian) fluvial sandstone forming the Holbrook Member of uppermost Moenkopi Formation as exposed on a prominent butte near Winslow, Arizona. U-Pb ages of 100 detrital zircon grains were analyzed by LA-ICP-MS using a beam diameter of 50 microns. Analytical data were then filtered to reject grains with >20% age discordance, leaving 88 grains that yielded reliable results. Varied detrital zircon age populations include prominent clusters of Permian-Triassic grains (16% in the range of 240-270 Ma) and Neoproterozoic grains (7% in the range of 560-620), with the largest tight cluster being Mesoproterozoic grains (25% in the range of 1400-1500 Ma). Subordinate and more subdued age clusters include both younger and older Mesoproterozoic grains (16% Grenville in the range of 1000-1300 Ma; 16% Yavapai-Mazatzal in the range of 1600-1800 Ma). The Permian-Triassic grains were probably derived from the East Mexico arc, Paleozoic and Neoproterozoic grains most likely from the Ouachita-Appalachian orogen, and the sharp Mesoproterozoic cluster (1400-1500 Ma) almost certainly from granitic rocks intruded into the nearby Yavapai-Mazatzal belt of southwestern Laurentia. Other Mesoproterozoic grains could also have been derived from southwestern Laurentia, although Grenville grains could have come from the Ouachita-Appalachian orogen or nearby eastern Mexico (Oaxaquia). In nonmarine Moenkopi strata, paleocurrents flowing to the west and northwest toward marine equivalents in Utah and Nevada are compatible with a fluvial system with headwaters embracing southwest Laurentia (Yavapai-Mazatzal belt), the Marathon suture of the Ouachita system, and the Permian-Triassic East Mexico arc built on Gondwanan crust in eastern Mexico