RESULTS AND IMPLICATIONS OF MONAZITE GEOCHRONOLOGY FROM THE WESTERN PENOKEAN OROGEN, MINNESOTA

We are using monazite geochronology to better constrain the timing of Paleoproterozoic medium-grade metamorphism preserved across the 1870-1830 Ma Penokean orogeny in the southern Lake Superior region. In east-central Minnesota, late and post-tectonic plutons intruded the internal zone of the Penokean orogen at ca. 1800 Ma and ca. 1775 Ma. In the northern internal zone, the south-dipping Malmo fault juxtaposes post-Penokean plutons in its hanging wall against slightly lower grade metamorphic rocks of the medial fold-thrust belt in its footwall. Just north of the Malmo fault, a medial zone garnet-schist sample records a strong ca. 1830 Ma metamorphic episode and a secondary ca. 1800 Ma thermal pulse. In the internal zone, south of the Malmo fault, we obtained in situ metamorphic EMP ages from three garnet-staurolite Paleoproterozoic metasedimentary schists and one Archean garnet-cordierite gneiss. In contrast to the medial zone, in situ monazite ages from the internal zone reveal a profound ca. 1770 Ma thermal imprint associated with intrusion of the 1775 Ma East-central Minnesota batholith and a secondary ca. 1800 thermal imprint. The considerable distance of some of these samples from the western edge of the exposed batholith and the absence of Penokean metamorphic ages suggests that the 1770 Ma thermal pulse must have been dramatic. Our new monazite age data reveal that the Malmo fault juxtaposes rocks of different metamorphic age (1830 Ma to the north from 1770 Ma age to the south). We propose that the Malmo fault is a post-Penokean geon 17 structure that exhumed the plutonic terrane of east-central Minnesota. The Malmo fault and the correlative Flambeau-Flowage fault in northern Wisconsin bound the north side of an extensive >500 km long corridor of Paleoproterozoic gneiss domes. Our data presented here support the conclusion that the gneiss dome corridor formed after the Penokean orogeny during a period of orogenic collapse leading to rapid crustal stabilization of the southern Lake Superior region.