PROTEROZOIC QUARTZITES AT HAMILTON MOUNDS, CENTRAL WISCONSIN: IMPLICATIONS FOR GEON 17 MAGMATISM, SEDIMENTATION, AND TECTONICS IN THE SOUTHERN LAKE SUPERIOR REGION

Quartzite inliers in central Wisconsin have long been enigmatic with regard to the Proterozoic evolution of the southern Lake Superior region. At Hamilton Mounds the intrusion of quartzite by 1.76 Ga granite has led some to suggest that Baraboo Interval sedimentation (1.75-1.63 Ga) was spatially and temporally associated with Geon 17 magmatism. Re-examination of the Hamilton Mounds area reveals the presence of two different quartzites, which we designate the Hamilton Mounds (HMQ) and Seven Sisters (SSQ) quartzites. The HMQ consists of gray, laminated micaceous and feldspathic quartzite, contains calcareous layers and concretions, and is intruded by 1.76 Ga granite. In contrast, the SSQ consists of pink, massive to cross-bedded supermature quartzite and resembles Baraboo Interval quartzites elsewhere. Local brecciation and introduction of K in the SSQ may be the result of Geon 14 fluid migration, as documented in the Baraboo and Sioux quartzites. Although the contact between the HMQ and SSQ is not exposed, we interpret the HMQ and intrusive granite to represent basement, upon which the SSQ was deposited unconformably. Thus, like the Baraboo Quartzite, the SSQ appears to be younger than, and rests upon, Geon 17 basement. The results of this investigation are consistent with the Baraboo Interval (1.75-1.63 Ga) representing an episode of extensive weathering and widespread deposition of supermature quartz arenite on a stabilized craton in the interval following cessation of Geon 17 magmatism and prior to Geon 16 Mazatzal deformation.