Crust-Enriched, Mantle-Derived Tonalites in the Early Proterozoic Penokean Orogen of Wisconsin

The early Proterozoic (1.82 to 1.89 Ga) Penokean orogenic province represents the initial phase of a major period of crustal growth of the North American Proterozoic craton. In central Wisconsin, the orogeny is dominated by the intrusion of variably deformed and metamorphosed tonalite-trondhjemite suites, in part hosted by Archean gneisses and migmatites. The magmatism is calc-alkaline and similar in composition to modern high-K orogenic andesites. Moderate abundances of Sr (600-750 ppm) and REE in the most primitive tonalites are consistent with derivation from an eclogitic, altered tholeiite rather than a crustal (Archean), amphibolitic tholeiite, or enriched mantle source. Existing Nd-Sr isotopic data for other tonalites of the Wisconsin Penokean orogen also indicate a mantle origin but with some admixture of Archean crustal component. This study shows that an Archean component most likely was incorporated during emplacement. The tonalite to trondhjemite differentiation of these magmatic suites is not compatible with any form of simple fractionation. The most probable model is one of coupled fractionation-contamination involving changing proportions and composition of removed solid phases plus incorporation (to 7 wt %) of Archean crust such as that represented by some of the host gneissic terrane.