DIRECT TIMING CONSTRAINTS ON PALEOPROTEROZOIC METAMORPHISM, SOUTHERN LAKE SUPERIOR REGION: RESULTS FROM SHRIMP AND EMP U-PB DATING OF METAMORPHIC MONAZITES

The age of the Paleoproterozoic Penokean Orogeny (1875-1835 Ma) of the US mid-continent has been firmly established for decades by U-Pb pluton age data. The timing and nature of post-orogenic unroofing, and the recognition of repeated tectonothermal overprinting, has been established more recently by thermochronologic investigations. No reliable information exists however on the timing of initial higher-grade metamorphism which affected the southern Lake Superior region during the construction of Laurentia. Such information represents a critical missing link in our understanding of the tectonothermal evolution of the crust during and after Penokean orogenesis. We utilized the SHRIMP and the EMP to obtain geochronologic information on distinct monazite mineral domains from amphibolite grade rocks along the deformed Penokean continental margin. An Archean quartzofeldspathic gneiss unit from northern MI (Peavy district) and an amphibolite grade Paleoproterozoic metapelitic unit in east-central MN (Kettle River locality) yielded remarkably consistent spot ages of 1834±6 Ma. These dates are the first reliable metamorphic ages from this Paleoproterozoic orogen and indicate a widespread thermal pulse at ~1834 Ma in response to accretion-induced crustal thickening. The Kettle River sample shows replacement textures with spot analyses yielding a mean age of 1793±4 Ma. These younger data are interpreted to reflect overprinting associated with a recently recognized pulse of late-orogenic magmatism at 1798±3 Ma. Preliminary EMP monazite results from metamorphic samples collected in the vicinity of the ~1775 Ma East-Central Minnesota Batholith (in both MN and northern WI) give predominantly geon 17 spot ages. One sample revealed distinctly chemically zoned grains which produced distinct geon 18 Penokean cores and geon 17 rims. We conclude that further monazite geochronology holds promise for better elucidating the timing of syn-orogenic metamorphism, as well as overprinting post-orogenic events throughout the southern Lake Superior region.