ORIGIN OF POST-PENOKEAN (1.74-1.8 GA) GRANITES IN THE UPPER GREAT LAKES REGION: HF ISOTOPE AND MAJOR, TRACE, AND REE EVIDENCE

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. Geon 18 Penokean accretion in the upper Great Lakes region was followed by protracted, predominantly crustal-melt derived granitic plutonism from 1800 to 1740 Ma, variously interpreted as ‘anorogenic’, orogenic-collapse related, or resulting from north-directed Yavapai age subduction. These bodies intrude Archean gneissic basement, Proterozoic meta-sedimentary sequences, and accreted juvenile arc terrane rocks. In Minnesota, uniform Nd-isotope model ages of 2.1 Ga from these rocks (Holm et al., 2005) are the same as Nd model ages obtained from many Penokean arc rocks of the Wisconsin magmatic terrane in northern Wisconsin (Barovich et al., 1989; Van Wyck & Johnson, 1997) suggesting that they were derived primarily or entirely by melting of pre-existing Penokean crust.

To test their origin, we measured the Hf isotope composition of zircons from ten post-Penokean granitic rocks from SD, MN, and WI all previously dated at the Univ. of Kansas, Lawrence. Isotopic compositions of the ten units are similar; with average initial 176/177 Hf ratios ranging from 0.28140 to 0.28180. Initial eps Hf value averages (of 4-6 measurements/sample) vary significantly from –9.7 (nwWI) to 3.7 (eSD), suggesting involvement of a heterogeneous host crust in the genesis of geon 17 magmas across the Penokean orogen. Hf-isotope depleted-mantle model ages vary significantly from 2.1 Ga to as high as 3.0 Ga (2.3 to 2.7 Ga in Minnesota). Major-element classification indicates a calc-alkaline to shoshonite trend of these rocks, likely indicating subduction-related genesis. Trace element tectonic discrimination indicate dominantly volcanic-arc (VAG) magmatism. Taken together, these data suggest that the post-Penokean granites are products of subduction-induced melting across a variable source terrane.