CONSTRAINING PALEOPROTEROZOIC TIMING AND SETTINGS OF DEPOSITION AND TECTONISM IN THE BIG THOMPSON AND RIST CANYON AREAS, NORTHERN COLORADO FRONT RANGE

The northern Colorado Front Range offers excellent exposures of both arc and basin rocks formed through Paleoproterozoic accretionary tectonics. To clarify the chronology of tectonic events and associations around the Big Thompson and Rist Canyons, metamorphosed clastic sedimentary rocks and granodioritic and tonalitic intrusive rocks were collectively studied by bulk rock geochemistry, U-Pb zircon geochronology, and U-Th-total Pb monazite geochronology.

Most zircon found in a quartzite has zoning indicative of igneous sources and predominately range in age from 1750 Ma to 1950 Ma, with a few zircons scattered in age back to the Archean. This detritus likely originated from the Green Mt. Arc, Trans-Hudson/Penokean Orogen, and Wyoming Province, all found generally to the north. The youngest concordant analysis is c. 1750 Ma and is very close to the youngest ages found in three other reported detrital zircon analyses from rocks presumably deposited in the same basin. Tonalitic rocks (c. 1742 Ma) are geochemically similar to tonalite-trondhjemite-granodiorite suites found in the Archean and more recent adakitic type rocks, suggesting the tonalitic magma formed from the melting of eclogitized shallowly subducted ocean crust. Granodioritic rocks (c. 1731 Ma) possess geochemistry typical of mantle derived subduction zone melts. Monazite in an andalusite-sillimanite-mica schist is aligned with, and has asymmetric rim growth parallel to the first deformational foliation found in the region. All monazite appears to be part of the same c. 1723 Ma age population formed during the first deformation event.

These results provide a picture describing a relatively rapid transition from deposition in an arc marginal basin to magmatism and deformation during accretion. More data is required to determine the exact geometry and relationship of subduction zone(s) and arc(s) in the area, but some models suggest that rocks west of Denver (100 km to the south) and Gunnison-Salida Arc rocks (250 km to the south) were rifted from the Green Mt. Arc during slab rollback after c. 1770 Ma. Alternatively, these areas could be independent arcs that initially formed at about the same time as the Green Mt. Arc. In either case, the Big Thompson-Rist Canyon rocks likely record accretion of these areas to the Green Mt. Arc.