ZIRCON GEOCHRONOLOGY OF THE BASAL LAYER OF THE MARKAGUNT GRAVITY SLIDE, MARYSVALE VOLCANIC FIELD, UTAH, USA

The southwestern Marysvale volcanic field is in the High Plateaus of Utah between the Colorado Plateau on the east and the Basin and Range Province on the west on the eastern end of the Pioche-Marysvale volcanic belt. Within the Tertiary Marysvale volcanic field is the Markagunt Plateau, which is capped by the remains of a volcanic field sector collapse termed the Markagunt gravity slide (MGS), estimated to have displaced 3000 cubic kilometers of mass. The MGS is one of the largest subareal landslides ever recognized on Earth and is interpreted to have formed from the collapse of the southern sector of the Marysvale volcanic field. The base of the slide includes a layer of brecciated volcanic material up to several meters in thickness that was generated during catastrophic slide emplacement. Basal layer material also occurs as injectites into overlying allochthonous volcanic rocks. We collected two samples of the basal layer (Panguich Lake and Haycock Mountain). Zircons were separated from these rocks using traditional gravitational and magnetic techniques. The zircons were analyzed by at the University of Arizona Lasercrhon Center. The basal layer at Haycock Mountain (n=105) has zircons that range in age from 23-3507 Ma, and has a maximum depositional age of 23.02 +/- 0.2 Ma. The basal layer at Panguich Lake (n=69) has zircons that range in age from 23-1864 Ma, and has a maximum depositional age of 23.02 +/- 0.15 Ma. The maximum depositional ages of these two localities are statistically identical and indicates that the MGS must have been emplaces after 23.02 Ma. These data also indicate that the basal layer includes zircons that are older than slide emplacement. These zircons may have been derived from either upper or lower plate rocks during transport and disaggregation. Whether or not the MSG emplacement was eruption-related is not evident in these data.