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

A detailed petrological and geochemical study was conducted on Lower Cambrian volcanic rocks associated with the Chaudiere River Nappe (C.R.N.) Drummondville Olisotostrome (D.O.) in the Quebec Southern Appalachians. These volcanic rocks are interpreted as products of igneous events related to the break-up of continental crust and the opening of the Iapetus Ocean. 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 first three groups consist of olivine- and plagioclase-phyric basalts erupted as pillows, massive flows, flows breccias and blocky lavas. Alkali basalts represent massive flows characterized by kaersutite-Ti-augite-biotite-and apatite-phenocrysts. Groups 2 and 4 basalts show affinities with intraplate 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. Estimated degrees of partial melting range from 22 percent for subalkaline basalts to 5 percent for alkali basalts. Andesites may be generated by 85 percent fractional crystallizationmore » of basalts from group 3. Rhyolite could be produced by melting of continental crust. Sedimentary units in contact with or observed in proximity to the volcanic outcrops suggest that groups 1 and 2 basalts were erupted in a relatively deep and low-energy environment whereas groups 3 and 4 were erupted on or close to a continental margin. Both sedimentary and volcanic events are compatible with pregressive opening of a continental rift leading to formation of a mature oceanic basin. 44 refs., 9 figs., 3 tabs.« less