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 ¡À 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).
