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Collaborative Research: Influence of a Proterozoic Southern Laurentia Long-Lived Convergent Orogen In the Lake Superior region, USA

The PI's propose to employ recent advancements in geochronometric techniques to refine their understanding and models of the Proterozoic geology of the southern Lake Superior region. Their objectives are two-fold: 1) to use U-Pb monazite geochronometry (both ion probe and EMP) to document and directly date Penokean and subsequent metamorphic events (i.e., Yavapai, Mazatzal and Wolf River metamorphism) in an attempt to link these episodes to continued subduction after the Penokean orogeny; and 2) to use Ar-Ar laser microprobe grain-scale age mapping to evaluate the influence of multiple crustal events on the regional thermal record. Their results will be used ultimately to establish the Proterozoic geology of this historical region in the context of a long-lived convergent orogen model (Karlstrom et al., 2001).<br/><br/>Metamorphic geochronometry. Results of recent NSF-supported research (Holm and Van<br/>Schmus) on the origin of post-Penokean granites show that magmatism long thought to be solely<br/>1760 Ma, actually occurred in three separate, generally southeastward-migrating pulses at 1800,<br/>1775, and 1750 Ma. Van Schmus, Holm and their co-authors have postulated that these<br/>magmatic pulses could correlate with northward-directed subduction associated with southward<br/>growth of the continent. In one possible scenario, geon 17 Yavapai-age slab rollback of a<br/>northward subducting oceanic plate caused continental arc magmatism to step generally<br/>southeastward between 1800 and 1750 Ma. As the slab steepened, it would have disengaged<br/>from the overlying, overthickened crust. The reduced compressional stresses and increased<br/>thermal input allowed for extensional collapse of the orogen and subsequent but short-lived<br/>crustal stabilization. The PI's propose to carry out a detailed metamorphic study to test the hypothesis that widespread metamorphism (and not simply widespread cooling) occurred with the migrating arc magmatism and might be linked spatially to tectonic extrusion (i.e., collapse) structures. Their preliminary SHRIMP and EMP dating of metamorphic monazites appear to support this. These data will provide the first direct timing constraints on both Penokean and post-Penokean metamorphism in the southern Lake Superior region.<br/><br/>Metamorphic thermochronometry. In the 1990's, NSF-funded (Holm) Ar-Ar mineral age data<br/>were used to document the timing and extent of orogenic cooling and later thermal reheating<br/>across the Penokean orogen. For instance, abundant 1760-1720 Ma plateau cooling ages are<br/>interpreted to be associated with a rapid crustal collapse/stabilization period and 1630-1600 Ma<br/>plateau ages are interpreted to indicate reheating/deformation associated with the Matzatal<br/>orogeny to the south. However, preliminary Ar-ion laser microprobe age data documents the<br/>presence of significant age gradients across muscovite crystals (500 m.y. in one case). These<br/>results merit further scrutiny of the existing thermochronologic data. Specifically, do the 1630-<br/>1600 Ma Ar-Ar plateau ages i) reflect the time of widespread deformation and mild reheating; ii)<br/>represent partial resetting associated with geon 14 magmatism; iii) or perhaps represent slow<br/>cooling? Ar-Ar muscovite laser probe age mapping will allow the PI's to better constrain the medium-temperature history of the region and, especially to relate it to younger overprinting events in the thermally reheated areas. The data collected in this study are critical to properly interpret the new igneous ages (obtained by Van Schmus, Holm, and others) and existing Ar-Ar ages and to evaluate the Proterozoic geology of southern Lake Superior region in the context of a long-lived convergent orogen model.

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