FINGERPRINTING THE K T IMPACT SITE AND DETERMINING THE TIME OF IMPACTBY U-PB DATING OF SINGLE SHOCKED ZIRCONS FROM DISTAL EJECTA/

U-Pb isotopic dating of single 1-3 mug zircons from K/T distal ejecta from a site in the Raton Basin, Colorado provides a powerful new tool with which to determine both the time of the impact event and the age of the basement at the impact site. Data for the least shocked zircons are slightly displaced from the 544 +/- 5 Ma primary age for a compon ent of the target site, while those for highly shocked and granular gr ains are strongly displaced towards the time of impact at 65.5 +/- 3.0 Ma. Such shocked and granular zircons have never been reported from a ny source, including explosive volcanic rocks. Zircon is refractory an d has one of the highest thermal blocking temperatures; hence, it can record both shock features and primary and secondary ages without modi fication by post-crystallization processes. Unlike shocked quartz, whi ch can come from almost anywhere on the Earth's crust, shocked zircons can be shown to come from a specific site because basement ages vary on the scale of meters to kilometers. With U-Pb zircon dating, it is n ow possible to correlate ejecta layers derived from the same target si te, test the single versus multiple impact hypothesis, and identify th e target source of impact ejecta. The ages obtained in this study indi cate that the Manson impact site, Iowa, which has basement rocks that are mid-Proterozoic in age, cannot be the source of K/T distal ejecta. The K/T distal ejecta probably originated from a single impact site b ecause most grains have the same primary age.