U-PB AGES OF DETRITAL ZIRCONS FROM SANDSTONES OF THE UPPER JURASSIC MORRISON FORMATION ON THE COLORADO PLATEAU

Detrital zircons were separated from 10 fluvial sandstone samples of the Upper Jurassic Morrison Formation on the Colorado Plateau. U-Pb ages were obtained for approximately 100 individual grains per sample using LA-ICP-MS with a beam diameter of 35 microns, but grain ages with >20% discordance or >10% uncertainty were rejected, leaving ~920 reliable grain ages. Zircon grains in 5 samples from Salt Wash Member (Bullfrog Creek UT, Fremont River UT, Montezuma Canyon UT, Beclabito Dome NM, Slick Rock CO) and one sample each from Tidwell Member (near Notom UT), Recapture Member (near Gallup NM), and Fiftymile Member (at Fiftymile Bench UT) are markedly heterogeneous in age, and were apparently reworked in large part from Jurassic eolianites (Dickinson et al., this meeting) exposed either within the Sevier thrust belt of Utah or as sedimentary cover along the Mesozoic Mogollon Highlands of central Arizona and New Mexico, or both, although 150-250 Ma Mesozoic grains derived from the Cordilleran magmatic arc form 5%-15% of the grain populations. Prominent age groups are 400-700 Ma Paleozoic-Neoproterozoic grains (20%-35%), with twin peaks within the intervals 375-450 Ma and 550-625 Ma, and 950-1780 Ma Mesoproterozoic grains (45%-60%), with Grenville peaks (1075-1175 Ma) dominant over older peaks (1425-1475 Ma; 1525-1700 Ma) that reflect derivation of detritus from the Yavapai-Mazatzal crustal belt intruded by anorogenic plutons of younger age. A consistent paucity of 1850-2100 Ma grains (<5%) precludes significant derivation of detrital zircons from the Paleozoic allochthons of central Nevada and the Sierra Nevada. Zircon populations in the more feldspathic and less quartzose Westwater Canyon Member at Recapture Creek UT and Todilto Park NM are more restricted in age, containing more arc-derived grains (~35%) and a predominance of Yavapai-Mazatzal detritus over Grenville detritus among Proterozoic grains (55%-60%). The more restricted age ranges suggest derivation mainly from basement and arc cover lying to the southwest of the Colorado Plateau. Ages of detrital zircons thus support the longstanding inference that Salt Wash and Westwater Canyon Members had different provenance.