GEOCHRONOLOGY OF THE GREAT LAKES TECTONIC ZONE AND SURROUNDING REGION, CENTRAL UPPER PENINSULA, MICHIGAN, USA

The Great Lakes Tectonic Zone (GLTZ) is a fundamental boundary between two contrasting Archean terranes of the Superior Province: the granite-greenstone terrane of the Wawa-Abitibi Subprovince on the north and the gneisses of the Minnesota River Valley Subprovince on the south. The boundary was first described by Morey and Sims (1976) in Minnesota and, although almost entirely concealed, has subsequently been defined by geophysical studies across the northcentral US. In the Marquette region, central Upper Peninsula of Michigan (USA), the GLTZ is well-exposed along a strike length of ~11 km and approximately 2.3 km wide with approximately 1.6 km width of mylonitized rocks of the Wawa-Abitibi Subprovince on the north and approximately 0.7 km-wide zone of Minnesota River Valley mylonites on the south (Sims, 1991,1993). The post-Archean reactivation of the GLTZ is assumed yet the influence of this structural feature on regional events is uncertain. Here we integrate field observations with LA-ICP-MS U-Pb zircon geochronology to establish a regional geochronology framework including constraining the emplacement of the New Swanzy batholith, truncated on the northeast by the GLTZ and we assume to be the protolith of the mylonites along the southwest margin of the GLTZ, and a ~2.9 Ga gneiss sampled directly to the south of the GLTZ. We also will present a LA-ICP-MS U-Pb zircon age for a recently sampled, undeformed granitic dike observed to be cross-cutting the mylonitic foliation of the GLTZ, constraining the minimum age for movement on the GLTZ.

Morey and Sims (1976) Geol. Soc. Am. Bull. 87, 141-152

Sims (1991) US Geol. Surv. Bull. 1904-E, 17

Sims (1993) US Geol. Surv. Miscellaneous Investigations Series Map I-2356.