GEOCHEMISTRY OF THE CARY MOUND GRANITE, CENTRAL WISCONSIN: TECTONIC IMPLICATIONS

The Cary Mound granite is an approximately 6.5 km by 3 km oblong pluton located approximately 15 km south of Marshfield, Wisconsin. The pluton exhibits both a red and a dark granite phase. Rhyolite lava flows are spatially associated with the intrusive phases, but no contact is visible. Both phases of the granite display granophyre texture and miarolitic cavities indicating epizonal emplacement. Previous studies have not established petrogenetic kinship between the dark and red granite phases, nor between the granites and the rhyolite. An existing age date of 1.835 Ga for the Carey Mound granite suggests intrusion during the cessation of Penokean deformation. Crystallization ages on granite emplaced near the end of an orogenic event offers an excellent way to constrain the cessation age of orogenic activity. Several approaches can be adopted to determine if granite emplacement was syn- or post- orogenic. Combined geochemical, geochronologic, and magnetic susceptibility studies would help resolve this problem. This study presents field and petrographic observations, mineral chemical, and XRF whole rock major and trace element analyses on the Cary Mound granite to assess magma evolution and the tectonic implications for emplacement. The chemical data will establish relationships between the red and dark granite phases, as well clarify if the geographically associated rhyolite represents co-magmatic volcanic ejecta.