RESULTS OF EMPA MONAZITE GEOCHRONOLOGY FROM PENOKEAN SHEAR ZONES IN THE SOUTHERN LAKE SUPERIOR REGION

The Niagara fault zone and the Eau Pleine shear zone are two Penokean age sutures along which the Wisconsin magmatic terrane (juvenile island arc rocks) and the Marshfield terrane (Archean microcontinent) were accreted onto southern Laurentia. In order to date the deformational history of these shear zones, monazite U-Th-Pb geochronometry was conducted via in-situ analysis using the electron microprobe on monazite grains from each of the shear zones. A medium-grained, biotite quartzofeldspathic (Archean?) gneiss from the Eau Pleine shear zone contains small, somewhat elliptical monazite grains aligned with the foliation plane. Thirty-five spots from six monazite grains yield an EMPA total-Pb monazite age of 1846±12Ma. Compositional domains within these grains do not correlate with age domains. This 1846 Ma age is consistent with the fact that the Eau Pleine shear zone truncates 1860-1890 Ma island arc rocks and is cut by an 1833 Ma alkali-feldspar granite.

We dated monazite from two finer-grained tectonite samples collected at different localities along the Niagara fault zone. Thirteen small (15-35 micron diameter) monazite grains located in a fine-grained, quartz sericite mylonite appear to be compositionally homogeneous, although some have narrow higher-Th rims. Only two small monazite grains were found in a fine-grained, biotite-garnet rich tectonite. Eighteen total spots from all 15 grains analyzed reveal two age domains of 1628±16 Ma (MSWD=0.91) and 1496±16 Ma (MSWD=0.47). These post-Penokean ages correlate closely with known tectonothermal events: the 1630 Ma Mazatzal deformation and the 1470 Ma Wolf River batholith intrusion. Although preliminary, we suggest the ~1630 Ma age domain provides further evidence for Mazatzal age deformation reaching as far north as the Niagara fault zone. The younger ~1500 Ma age domain likely represents monazite precipitation related to fluid flow in response to the emplacement of the Wolf River batholith. Large-scale channeled fluid flow of this age has recently been documented in Wisconsin and Minnesota along the Baraboo Interval quartzite/basement nonconformity by Medaris et al. (2003). Application of higher resolution techniques on finer-grained tectonites should help to further elucidate the timing and nature of post-Penokean overprinting events in this region.