Petrogenesis of metaluminous A-type rhyolites from the St Francois Mountains, Missouri and the Mesoproterozoic evolution of the southern Laurentian margin

Predominantly acid volcanic and intrusive rocks of the approximately 1.48 Ga Eastern Granite-Rhyolite Province are exposed only in the St Francois Mountains inlier, Missouri. Combined Nd isotopic, major and trace element analyses are presented for the predominantly acid volcanic rocks of the St Francois Mountains. They are metaluminous to mildly peraluminous, A-type compositions with positive initial epsilon(Nd) values of 3.1-5.0. They are interpreted to be the result of crystal fractionation of partially melted calc-alkaline igneous rocks formed at a recently active subduction zone. The likelihood of a juvenile, calc-alkaline, subduction-generated protolith for the St Francois Mountains rhyolites leads to a hypothesis in which dacitic parental magmas resulted from partial melting in a continental 'back-arc' setting. Basaltic magmatism due to decompression melting of the mantle resulted from lithospheric extension and thinning and provided the heat source for melting in the middle or lower crust. Penecontemporaneous magmatism elsewhere in Laurentia included subduction related calc-alkaline rocks in the Pinware Terrane of Labrador; some felsic igneous rocks of uncertain plate tectonic setting within the Grenville Province also have 1.5-1.6 Ga T-DM ages. These are interpreted as magmatic products of approximately 1.5 Ga northward dipping subduction beneath much of the length of the southern margin of Laurentia. The Eastern Granite Rhyolite Province is consequently seen as an integral inboard expression of this subduction related igneous activity. Petrogenetic models involving a mantle plume or the development of a continental rift are not required to explain this granite-rhyolite terrane and evidence against their involvement is discussed. (C) 2002 Elsevier Science B.V. All rights reserved.