ORIGIN OF 1700-1800 MA ROCKS IN THE CENTRAL YAVAPAI AND PENOKEAN PROVINCES: INSIGHTS FROM ZIRCON HF ISOTOPE DATA

Zircons from granitic rocks are ideally suited to constrain the petrogenetic history of intrusive suites as their isotopic compositions reflect the relative contributions of mantle and crustal sources in their genesis. Six new U-Pb zircon ages from basement drill core samples in the central Yavapai Province (primarily Nebraska) yielded ages ranging from 1694 Ma to 1825 Ma. In conjunction with the U-Pb analyses, the same zircon separates were analyzed for their Lu-Hf isotopic compositions. These central Yavapai Province samples have positive mean initial εHf values ranging from +1.65 to +8.45. Overall, new Hf results from the central Yavapai Province demonstrate crust across a broad region of the Midcontinent was derived from a juvenile source with some contributions of older Paleoproterozoic crust. Similar age samples throughout Nebraska and from east-central Minnesota (Penokean Province) yield Nd-isotope model ages (TDM) ranging from 1.9 Ga to 2.2 Ga, possibly suggesting they were derived primarily or entirely by melting of pre-existing Penokean crust (Van Schmus et al., 1993). However, initial εHf average values from the 1.75-1.80 Ga east-central Minnesota granites vary from -5.3 to +1.3 and give Hf depleted-mantle model ages ranging from 2.3 to 2.7 Ga. The east-central Minnesota Hf isotope data therefore suggest melting of a heterogeneous host crust. This conclusion is consistent with geologic and geophysical relations, which indicate these granites intruded both Archean gneissic basement, Proterozoic metasedimentary sequences, and Penokean accreted juvenile arc rocks.