Contemporaneously erupted tholeiitic and calc-alkaline magmas in the Archean Colomb-Chaboullie greenstone belt, James Bay, Quebec: Petrologic implications

摘要

Archean greenstone terranes are key to understanding the geological processes active during early Earth history. These areas are also of economic interest due to their association with volcanogenic massive sulphide (VMS) deposits, and several other types of mineral deposits. The Archean Colomb-Chaboullie greenstone belt is located between the Nemiscau and Opatica subprovinces of the Superior Province, Quebec, Canada. This belt comprises three submarine volcanic units: pillowed to massive basaltic lavas, variably porphyritic massive to pillowed intermediate lavas (+/- intrusions), and intermediate to felsic volcaniclastic rocks. The calc-alkaline intermediate lavas and volcaniclastic assemblages are intercalated with the tholeiitic to transitional basaltic lavas. This implies that these volcanic rocks were contemporaneously formed within a limited geographical area. The major element trends of the basalts are explained by crystal fractionation while the trace elements are influenced by independently occurring minor degrees of crustal contamination. The major and trace element geochemistry of the calc-alkaline rocks share many geochemical characteristics with pre-existing crustal contaminants. We therefore propose that the intermediate and felsic assemblages were produced via crustal contamination and mixing processes within a pre-existing magma chamber. The basalts spent less time in the crust and had a more direct passage to the surface.