Petrological and geochemical characteristics related to early rifting of Iapetus Ocean, Quebec Appalachians

Abstract

A detailed petrological and geochemical study was conducted on Lower Cambrian volcanic rocks associated with the Chaudiere River Nappe (C.N.R) and Drummondville Olistostrome (D.O.) in the Quebec Southern Appalachians. Three major volcanic rock assemblages were identified that correspond to very low-grade metamorphosed basalts, andesites, and rhyolites. The basaltic flows can be subdivided into four geochemical groups: subalkaline, transitional-subalkaline, transitional-alkaline, and alkali basalts. The geodynamic model proposed for the generation of these volcanic rocks involves, for groups 2, 3, and 4 basalts, partial melting of a previously metasomatized spinel lherzolite source and diapiric upwelling of an enriched melt. Only group 1 basalts could have been generated by an unmetasomatized depleted lherzolite source. Andesites may be generated by 85% fractional crystallization of basalts from group 3. Rhyolite could be produced by melting of continental crust. Both sedimentary and volcanic events are compatible with progressive opening of a continental rift leading to formation of a mature oceanic basin. -from Authors