TRACE- AND RARE-EARTH ELEMENT EVIDENCE FOR POST-PENOKEAN ANOROGENIC GRANITE AND RHYOLITE GENESIS IN CENTRAL WISCONSIN

Early geochemical work on the Proterozoic Waushara Granite and Berlin Rhyolite of central Wisconsin used major- and limited trace element data to conclude that these units are geochemical equivalents (i.e., Waushara Granite is the intrusive equivalent of Berlin Rhyolite). Furthermore, the limited data for these units were used to conclude that these petrogenetically similar rocks were generated in an anorogenic setting (e.g., Smith, 1983). In light of more recent advances in the analyses of trace- and rare-earth elements and the understanding of their relationship to tectonic setting, we draw on new geochemical data to test these hypotheses.

In this study, we use trace- and rare-earth element analyses on recently collected samples to recognize geochemical similarities between the Waushara Granite (exposed at Redgranite) and the Berlin Rhyolite. Furthermore, we compare these trace- and rare-earth element data to recently published data for two Proterozoic A-type granite suites (1.4 Ga, Pikes Peak, CO, Smith et al., 1999; 1.1 Ga, St. Francois Mtns, MO, Menuge et al., 2001) and a suite of Proterozoic continental arc granites (1.8 Ga, Hollings and Ansdell, 2002) in order to interpret the geologic setting in which these two rock bodies formed. The trace- and rare-earth element data for the Redgranite and Berlin rocks show tectonic affinities related to anorogenic (A-type) settings, confirming the conclusions of prior geochemical work. More specifically, our data indicate that central Wisconsin high-silica rocks formed as within-plate granites (WPG; Pearce et al., 1984), associated with a post-collisional extensional/rift setting and are distinct from the calc-alkaline activity associated with orogenesis. This conclusion is consistent with the interpretations of other granite “inliers” in central and south-central Wisconsin. Similar granites give zircon U-Pb ages that are Proterozoic (~1.76 Ga; VanSchmus, 1978), correlative with post-Penokean igneous activity.