MUSCOVITE AR/AR LASER ABLATION EVALUATION OF PROTEROZOIC THERMAL OVERPRINTING IN THE SOUTHERN LAKE SUPERIOR REGION

Application of modern geochronologic studies on Proterozoic rocks of the southern Lake Superior region has revealed the timing and extent of metamorphic and intrusive events superimposed on the 1850 Ma Penokean orogen. The Penokean orogenic belt has been intruded by two batholiths: the ca. 1775 Ma East-central Minnesota batholith (ECMB) and the ca. 1470 Ma Wolf River batholith (WRB) in Wisconsin. Conventional Ar/Ar mineral ages around the WRB show that it had a limited thermal influence on the Penokean crust as mica ages from country rock surrounding the WRB are primarily geon 16, interpreted as Mazatzal thermal resetting. In contrast, 1770 Ma U-Pb monazite ages around the ECMB illustrate that medium grade metamorphism accompanied plutonism; moreover the region yields geon 17 conventional plateau Ar/Ar mica ages. A sharp deformational front separates basement of geon 17 ages (Minnesota/ northern Michigan) from basement yielding reset geon 16 ages (n. Wisconsin).

Ar/Ar laser ablation analyses on four muscovite grains sampled north of the deformational front all reveal significant (100-300 m.y.) age gradients. Each grain reveals substantial regions of geon 17 ages, yet preserve smaller regions of older Penokean and younger Mazatzal ages. Because these rocks are overlain by 1700 Ma Baraboo Interval quartzites, we interpret the Mazatzal spot ages to indicate mild resetting – possibly associated with fluid flow outboard of the tectonic front. Significant age gradients are also preserved in three muscovite grains south of the Mazatzal tectonic front. One grain, which previously gave a 1614 Ma conventional plateau age, preserves a 1760 Ma core, a wide surrounding region of Mazatzal ages, and WRB rim ages. A second grain preserves a Mazatzal age core which progressively youngs to WRB rim ages. Nearer to the WRB, muscovite preserves a 1400 Ma core with ages as young as 1200 Ma at the rim. In most cases, we note that the conventional plateau age, where available, is close to the weighted mean of the spot fusion ages. Although step-heating experiments in this region do not effectively reveal the internal distribution of radiogenic argon within crystals, they do consistently reveal broad, first-order thermal overprinting effects. Ar/Ar laser ablation analyses are an effective tool for revealing more complex, second-order thermal effects in polymetamorphosed Precambrian terranes.