RE-EVALUATION OF PALEOPROTEROZOIC TECTONOTHERMAL EVENTS IN THE EASTERN PENOKEAN OROGEN, NORTHERN ONTARIO

In the Lake Superior region, timing of Penokean orogenesis sensu stricto (1875-1835 Ma) is well constrained by geochronology of variously deformed and crosscutting plutonic bodies and volcanic sequences. Significant geon 17 and 16 metamorphism and deformation is now known to strongly overprint the orogen in the region; for instance, 1775-1750 Ma amphibolite facies metamorphism is widespread, and 1660-1600 Ma Mazatzal deformation is well documented in Paleoproterozoic terranes of the central orogen. However, in the Lake Huron portion of the orogen to the east, Penokean-aged plutonism and volcanism are absent. Regional structures in the eastern Penokean orogen (EPO) include first and second order fold trains, the former assigned to the cryptic (older than 2000 Ma) Blezardian orogeny and the latter historically interpreted as Penokean deformation. Metamorphism has also typically been attributed to Penokean orogenesis, although rigorous timing constraints are lacking. We do note that 1750 Ma plutons in the EPO are spatially associated with metamorphic highs, suggesting that geon 17 metamorphism is likely. Paleoproterozoic K-Ar cooling-age data in the EPO are dispersed, with frequently reported ages between 1600-1500 Ma. In the central orogen (northern Michigan and Wisconsin), the locus of 1630-1580 Ma Ar-Ar biotite cooling ages delineates the region of Mazatzal deformation and related greenschist-facies metamorphism; furthermore, all ca. 1750 Ma plutons in central portion lie within the region overprinted by Matzatzal orogenesis. In the same way, 1750 Ma plutons in the EPO lie within the cores of large antiforms, which may suggest they have been subsequently deformed. Based upon the lack of reliable structural age constraints, the available geochronometric data, and the growing dataset that has arisen from the central and western (Minnesota) portions of the Penokean orogen, we postulate that the EPO may have experienced significant deformation and medium- to low-grade metamorphic overprinting associated with both Yavapai and Mazatzal convergence. Our on-going metamorphic monazite geochronology and Ar-Ar thermochronology should help to better elucidate the tectonothermal events in the EPO.