TECTONIC IMPLICATIONS OF METAMORPHIC AGE PATTERN REVEALED BY EMP MONAZITE DATING ALONG THE PENOKEAN CONTINENTAL MARGIN IN NORTHERN WISCONSIN, USA

We are using EMP monazite geochronology to determine the timing and age pattern of metamorphism across the Paleoproterozoic Penokean orogen. Reinterpretation of the Penokean continental margin in northern Wisconsin defines fault-bounded subterranes which preserve contrasting metamorphic conditions (sillimanite versus kyanite). The kyanite-bearing Watersmeet subterrane lies directly north of the sillimanite-bearing Park Falls subterrane which, in turn, lies north of the Niagara suture zone. In total, 85 spot analyses were performed on three metapelitic schist samples, yielding two age populations. The Watersmeet subterrane yielded bimodal monazite ages (with peaks at 1839 and 1765 Ma). The Park Falls subterrane yielded only geon 17 ages (mean age of 1743 Ma).

Although further work is needed, it appears that the kyanite-bearing Watersmeet structural panel records both geon 18 and geon 17 metamorphism, whereas the sillimanite -bearing Park Falls panel preserves only geon 17 ages. These metamorphic age constraints suggest that the fault-bounded panels were probably juxtaposed after the Penokean orogeny during a period of widespread metamorphism followed by rapid unroofing (as suggested by abundant geon 17 cooling ages). We present a tectonic model in which the Watersmeet subterrane rocks are tectonically buried during geon 18 Penokean orogenesis and then tectonically extruded northward during orogenic collapse (and remetamorphosed) at ca. 1775-1750 Ma. Intrusion of syn-collapse plutons was associated with the younger geon 17 metamorphism. The subterranes are locally depositionally overlain by the ca. 1700 Ma red quartzites (the Barron quartzite in northwest Wisconsin), and therefore must have been juxtaposed prior to 1700 Ma. More age and thermobarometric data should allow us to test and refine models for orogenic evolution such as this one.