Late Paleoproterozoic (Geon 18 And 17) Reactivation of the Neoarchean Great Lakes Tectonic Zone, Northern Michigan, USA: Evidence from Kinematic Analysis, Thermobarometry and 40ar/39ar Geochronology08/01/2007
The Neoarchean Great Lakes Tectonic Zone in the midcontinent of North America exerted a strong control during the Proterozoic stabilization of the Superior craton. In northern Michigan, this zone underlies the Paleoproterozoic Marquette synclinorium, a Penokean compressional feature that was reactivated during post-Penokean orogenic collapse and gneiss dome formation. Early north-side up motion along steep reverse faults under lower greenschist facies conditions was related to basin inversion of the Marquette trough sediments during Penokean orogenesis. In the western portion of the Marquette synclinorium, these early Penokean fabrics are overprinted by a high-grade event recorded by synkinematic mineral growth in high-grade shear zones. These late shear zones accommodate S-side up motion with Archean basement uplifted relative to Proterozoic supracrustal sequences, activity related to the emplacement of the ca.1800Ma Humboldt Granite. Metamorphic amphiboles from the center of a circular pattern of metamorphic isograds (the Republic node) record plateau ages of ∼1730Ma with progressively older ages across the Marquette synclinorium away from the central metamorphic high. Model Archean ages are preserved in the most distal amphibole samples north of the Marquette syncline, reflecting the preservation of amphibole crystallization in the biotite zone. Mica ages record complete resetting and a similar age progression with the youngest ages (∼1715Ma) in the central metamorphic high and older ages (∼1806Ma) towards the margin. Thermobarometry indicates peak temperatures of ∼620°C for the central sillimanite zone decreasing to ∼500°C for the garnet zone, and maximum pressures of 300–400MPa (GASP and GRIPS assemblages). Collectively, these characteristics are interpreted to reflect two separate reactivation episodes of the GLTZ during Penokean accretion (Marquette syncline formation) and post-Penokean formation of granite-cored gneiss domes (collapse).