Nd and Pb isotopes indicate significant direct involvement of older continental crust during the generation of the Penokean synorogenic intrusive suite in Wisconsin. Initial values for plutons show a clear trend of increasing ϵNd, from −6 to +4 southward from the Superior Province to the suture between the Pembine-Wausau and Marshfield terranes, indicating decreasing amounts of contamination by older crust. Crossing into the Archean Marshfield terrane, there is a rapid shift back down to lower ϵNd values (−1 to −7). Feldspar common Pb from synorogenic plutons generally defines a mixing array between Archean crust and primitive arc crust, which has Pb isotope compositions that are similar to those of the Proterozoic mantle.
A mixing model based on Nd and Pb concentrations and isotopic ratios indicates that synorogenic intrusive rocks acquired their isotopic ratios through interaction with Archean crust, rather than through contamination of a mantle source region by sediment subduction. In addition, direct dating of exposed basement in the Pembine-Wausau terrane indicates, for the first time, that Archean crust is present here (U/Pb zircon age of 2607 ± 22 Ma). The combined Nd and Pb isotope data demonstrate the importance of distinguishing between contamination of the mantle and remelting crust basement for the origin of old Nd model ages in Precambrian orogens. Such distinctions are critical for evaluating crustal growth estimates in Precambrian orogenic terranes.
The preferred tectonic model for the Penokean orogeny involves back-arc rifting of the southern Superior Province during a major period of Penokean orogenesis, followed by collision of the Marshfield terrane to the south, which was also intruded by Penokean-age magmas. In addition to the isotopic and geochronology data presented here, this model is supported by gravity data which indicate the presence of low-density material (probably Archean crust) south of the Niagara fault; this feature is interpreted to be a buried extension of the southern margin of the Superior Province beneath the Pembine-Wausau terrane. During the Penokean orogeny, synorogenic intrusive rocks acquired their isotope compositions through direct interaction with Archean crust.