U-PB AGES OF DETRITAL ZIRCONS FROM QUARZOSE EOLIANITE IN THE JURASSIC MOUNT WRIGHTSON FORMATION, SANTA RITA MOUNTAINS, ARIZONA

Quartzose eolianite is interbedded with Jurassic (185-170 Ma) volcanic rocks of the Mount Wrightson Formation in the Santa Rita Mountains of southern Arizona. An eolianite lens sampled from the upper member of the Mount Wrightson Formation on Cave Creek contains detrital zircon grains of both Mesozoic (27%) and older ages. U-Pb ages for individual zircon grains were determined by laser ablation ICPMS using a beam diameter of 50 microns. After analysis, 15 of 100 grains were removed from the study because of either >20% age discordance or poor analytical precision, leaving 85 reliable analyses. The Mesozoic grains (n=23) are dominantly Middle Jurassic to Late Triassic in age (176 and 221 age peaks on frequency curve), and were probably derived from the Cordilleran magmatic arc of which the Mount Wrightson volcanic rocks are a part. Pre-Mesozoic zircons include grain populations similar to those in Jurassic erg sands of the Colorado Plateau, and probably represent sediment blown into the Cordilleran arc from sand seas lying to the north. The most abundant pre-Mesozoic grains are Grenvillian (n=22), 840-1260 Ma; pre-Grenvillian Mesoproterozoic (n=12), 1320-1500 Ma; and Paleoproterozoic (n=18), 1600-2000 Ma. Minor Archean grains (n=6) are also present, as are Paleozoic and Pan-African grains (n=7). The Precambrian grains reflect derivation from various parts of the Laurentian craton lying to the east of the Cordilleran magmatic arc. The combination of the arc-derived Mesozoic zircon grains and the craton-derived pre-Mesozoic zircon grains is expected from the depositional setting of the Mount Wrightson eolianites.