PALEOPROTEROZOIC CRUSTAL EVOLUTION OF THE COLORADO FRONT RANGE: COMBINED U-PB, HF AND OXYGEN ISOTOPE EVIDENCE FROM MAGMATIC ZIRCON

We present combined in situ measurements of d¹⁸O and Hf, integrated with U-Pb data on magmatic zircon from Paleoproterozoic orthogneisses and granitoid rocks from a N-S traverse across the CO Front Range to assess the juvenile vs. recycled nature of the Yavapai terrane crust. The aim was to assess two models of crustal growth based on prior Nd whole rock and U-Pb zircon data. Simplified, one invokes continuous southward accretion of arc crust, while the second involves rifting in central CO with juvenile input from the mantle.

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.