PALEOPROTEROZOIC CRUSTAL GROWTH IN THE CENTRAL US: HF AND OXYGEN ISOTOPE EVIDENCE FROM MAGMATIC ZIRCON IN THE YAVAPAI PROVINCE OF NORTH-CENTRAL COLORADO

The nature of continental crust growth in deep time is strongly debated. Different approaches, many using radiogenic isotopes and commonly on whole rock samples (Sr, Nd, Hf), have been used to characterize crustal growth through time. In the last decade, combined in situ measurements of d¹⁸O, Hf, U-Pb on zircon evolved as the most robust and reliable way to assess juvenile vs. recycled nature of the crust.

These methods were applied to Paleoproterozoic orthogneisses to assess two competing models, both based on Nd and U-Pb dating, for crustal evolution of the Yavapai terrane in north-central Colorado (CO). One involves continuous southward accretion of arc crust, the other rifting in central CO with juvenile input from the mantle.

Nearly 300 δ¹⁸O analyses were carried out on zircons from 22 samples from northern, central and southern regions in the Yavapai terrane, sampling the oldest magmatic rocks in each domain as previously determined by ion microprobe U-Pb zircon dating. d¹⁸O averages were calculated for each sample after post-analysis vetting by CL imaging and Raman spectroscopy. Three felsic samples contain several distinct groups, but most samples showed limited scatter in δ¹⁸O (4.8-6.6) and mainly within uncertainty for zircon in equilibrium with the mantle, and typical for older, juvenile crustal provinces. Overall, felsic samples show a wider range of δ¹⁸O (4.3-6.6) than intermediate-felsic ones (5.2-6,2), but averages for both groups are within the mantle array. Samples with unusually low δ¹⁸O (3.1-4.4) may be a result of magma interaction with hydrothermal fluids.

Some U-Pb magmatic ages in the southern region are 20 to 30 Myrs younger than the typical 1760-1785 Ma range to the north, but there is no age vs. δ¹⁸O correlation, and thus no trend towards more crustal input into the parent magma source over time. There are no distinct differences between the N and S regions or a particular mantle input in the central domain, postulated to be produced by rift magmatism in one of the models. In whole rock Nd vs. δ¹⁸O space most samples plot near mantle values for both systems, except for 4 samples from the central and 2 from the southern region with a weak trend towards an older crustal component. This will be explored using Hf isotope analyses obtained on zircon domains matched with the oxygen results.