MODELS FOR THE ORIGIN OF THE MESOPROTEROZOIC "GRANITE-RHYOLITE" PROVINCE

The Mesoproterozoic Granite-Rhyolite Province underlies much of the midcontinent of North America and ranges in age from 1.5 to 1.3 Ga old. In general, older parts of this province (1.5-1.4 Ga old) occur in the east and younger (1.4-1.3 Ga old) examples are more common in the southern and western parts of the province. This province is composed primarily of undeformed high silica rhyolites and related epizonal granitic plutons. Igneous rocks of intermediate and mafic compositions are volumetrically rare.

These rocks have been traditionally referred to as being “anorogenic”, based on several factors. There is a lack of widespread deformation in the rocks, the igneous rocks are not calc-alkaline , and the types of metamorphic and sedimentary rocks associated with convergent or divergent plate boundaries today are mostly missing. Also, the granites have been classified as having an A-type geochemical signature. Geochemical data collected on a suite of these rocks suggest a source region that was heterogeneous in composition and age, although there is little evidence of contribution of any Archean crust to the source region(s) of these granites and rhyolites.

Potential models for the origin of this province will be discussed. These will include the possibility of the Granite-Rhyolite Province being a Large Igneous Province and what role sub-crustal lithospheric delamination may have played in this widespread and relatively long-lived magmatic event.