The processes occurring during the Paleoproterozoic (2.5-1.4
Ga) along boundaries of different crustal blocks in North America are not
completely understood, a problem that Nathan Anderson is tackling with his
master’s thesis. The occurrence of maybe
the most interesting-named gneiss, the Bloody Dick gneiss, and the Maiden Peak
gneiss in SW Montana allow for the study of the evolution of the western margin
of North America, as they are near the Wyoming craton, the Great Falls tectonic
zone, and the Farmington zone.
Creating the first detailed complete map of the Bloody Dick
gneiss, Nathan collected samples to analyze in the lab. Analyses of zircons in the gneiss, along with
field observations showed that the Bloody Dick gneiss consists of multiple
gneiss units that were originally igneous rocks emplaced at ~2.47 Ga, and then
metamorphosed at ~1.76 Ga.
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| Broad scale paleomap of the region. Nathan's map is located at the pink dot... go read his paper or thesis to see the final result. |
This would be consistent with previous models suggesting northward-dipping subduction, and the Medicine Hat block being thrust over the Wyoming craton; however, juvenile hafnium values in the zircons suggests the rocks were derived from the mantle shortly before they crystalized. This would be consistent with a northward dipping subduction model, although if the direction of subduction was instead to the south, that model would fit the new results.
Nathan uses the results of his thesis to expand previous
models, and suggests a new model having a flip in subduction direction in the
Paleoproterozoic, similar to modern day New Zealand.
Recap by Ryan Goldsby
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