INSIGHTS INTO THE LATE PALEOZOIC GEOLOGY OF AN ANCESTRAL ROCKY MOUNTAIN RANGE FROM DETRITAL ZIRCONS IN THE FOUNTAIN FORMATION, MANITOU SPRINGS, COLORADO

The Ancestral Rocky Mountains (ARM) are frequently cited as a significant source of clastic sediment in Laurentian Pangea during the Late Paleozoic. Many studies assume the geology of the ARM was similar to the modern day Yavapai-Mazatzal basement exposed in the Rocky Mountains, but this assumption can be evaluated by examining proximal sedimentary sequences containing rocks actually eroded from the ARM during the Late Paleozoic. The FFM, exposed near Manitou Springs, Colorado, is an ideal unit for such a study because it preserves sediment eroded from the Pikes Peak Batholith (PPB), a granitoid complex exposed in the adjacent Ute Pass uplift that is distinctive for both its age (ca. 1.1 Ga) and its alkaline composition. The goal of this study is to characterize, using LA-ICPMS, the U-Pb age distribution and trace element composition of zircon crystals from fluvial sandstones, cobbles and boulders in the FFM. Three samples of coarse-grained sandstone from the upper FFM yield simple age distributions; although measured ages (n>260) range from 1015 Ma to 1965 Ma, 70% are between 1000 Ma and 1200 Ma (peak at 1085 Ma) and 25% are between 1350 Ma and 1550 Ma (peak at 1450 Ma). Three cobbles of medium-grained granite from the middle FFM all yield measured age distributions (n>65 total) dominated by prominent peaks at about 1440 Ma and about 1470 Ma, which we interpret as the approximate crystallization ages of the granite bodies. Two cobbles of medium-grained quartzite yield age distributions spanning 1425 Ma to 2700 Ma, but with 93% of the measured ages (n>190) between 1650 Ma and 1850 Ma (peak at 1740 Ma). Taken together, our results indicate that the geology of the Ute Pass range involved Yavapai-Mazatzal basement intruded by ca. 1.4 Ga granite bodies, and plutons, dikes and/or volcanic rocks (Sanders and Hawkins, 1999) associated with the PPB. In addition, zircon trace element compositions provide a basis for discriminating different source rocks. For example, ca. 1.1 Ga zircons from the upper FFM have higher Nb/U (>0.04), steeper REE patterns [(Lu/Nd)cn > 100], and prominent negative Eu anomalies [(Eu/Eu*)<0.2] compared to 1.4 Ga zircon grains. The distinctive age and composition of 1.1 Ga zircon grains from the FFM suggest that the PPB is a potential point source for clastic sediment derived from the southern ARM during the Late Paleozoic.