PALEOPROTEROZOIC CRUSTAL EVOLUTION OF THE COLORADO FRONT RANGE: COMBINED U-PB, HF AND OXYGEN ISOTOPE EVIDENCE FROM MAGMATIC ZIRCON
Oxygen and Hf isotope analyses on zircons from a limited set of 22 samples from the oldest magmatic rocks in northern, central and southern CO Front Range areas as previously determined by ion microprobe U-Pb zircon dating. d18O averages calculated for each sample after post-analysis vetting of CL images and assessment of crystallinity by Raman spectroscopy. There is a slight distinction between felsic samples with a wider range of d18O (4.3-6.6) and intermediate samples with a narrower range (5.2-6,2), but both groups have averages consistent with equilibrium with the mantle, thus consistent with juvenile magmas. Few samples with unusually low d18O (3.1-4.4) may result from magma interacting with hydrothermal fluids.
Within our data set, U-Pb magmatic ages in the southern region are 20-30 myrs younger than the typical 1760-1785 Ma range to the north. However, there is no overall evident trend between U-Pb age and oxygen or Hf isotopes. While some samples from the southern, Gunnison-Salida, area show Hf-oxygen isotope trends towards limited input from pre-existing crust, the overall characteristics are juvenile. A larger dataset, including metasedimentary rock types is likely needed to resolve the models for crustal growth of the Yavapai province.
We welcome discussion of this data with respect to the different proposed models, both at the poster and during the post conference field trip.