Contrasting types of Grenvillian granulite facies aluminous gneisses: Insights on protoliths and metamorphic events from zircon morphologies and ages

Zircon morphology, internal texture, U/Pb age distributions and Th/U ratios are used to characterize aluminous gneisses in the Canyon domain (Manicouagan area, central Grenville Province) from two contrasting supracrustal sequences, metamorphosed at granulite-facies: one, a package of paragneiss and the other, a dominantly bimodal felsic–mafic volcanic sequence. The aluminous paragneisses yielded zircon with fragmental igneous cores, overgrown by metamorphic rims. These cores show a spread in ages between 1500 Ma and 2700 Ma, consistent with a supply of detritus from the adjacent Canadian Shield. In contrast, a felsic gneiss with aluminous nodules inferred to represent a hydrothermally-altered felsic volcanic rock of the bimodal sequence yielded well-preserved igneous zircon dated at 1238 ± 13 Ma, an age that integrated with previously published data links this volcanic sequence to a major episode of crustal extension in the central Grenville Province. Metamorphic zircon grains and rims from the paragneisses gave two groups of pre-Grenvillian ages that demonstrate the effect of major magmatic events on the country rocks in the Canyon domain: (a) 1391–1408 Ma, coeval with the emplacement of an older layered mafic to intermediate volcanic suite and (b) 1217–1260 Ma, coeval with the development of the 1238 Ma bimodal felsic–mafic volcanic sequence. In addition, metamorphic zircon from all the investigated aluminous gneisses records the high-grade Grenvillian metamorphism (∼1080–1040 Ma).