Geochemical constraints on the origin of mafic and ultrapotassic dykes from the southern Manicouagan area, Grenville Province.

摘要

The Grenville Province has a long record of magmatic activity, including arc magmatism related to the pre-Grenvillian evolution of eastern Laurentia and magmatism during the final collision that shaped the orogen. This study focuses on recently identified mafic dykes exposed in the Gagnon Terrane and the Canyon Domain as well as late tectonic, ultrapotassic dykes exposed in the Canyon Domain, in the central part of the Province.In the Canyon Domain, mafic dykes intrude a &sim1.4 Ga volcanoclastic sequence, inferred to represent remnants of an island arc, in the vicinity of a &sim1.2 Ga bimodal sequence inferred to have formed in a back arc setting. These dykes are variably deformed and recrystallized under medium-P granulite facies conditions. These dykes have a geochemical signature of subalkaline tholeiitic basalts to andesites, similar REE patterns and epsilonNd 1300 values from +4.6 10 +6.2. However, two groups have been recognized based on the presence (Group 2) or absence (Group 1) of pronounced negative Nb anomalies. The dykes of Group 2 have an arc geochemical signature and may have been emplaced during late stages of the development of the &sim1.4 Ga arc. In contrast, the dykes of Group 1 have a within-plate geochemical signature and may be correlative with the &sim1.2 Ga (Elzevirian) bimodal meta-volcanic sequence exposed east of the Canyon Domain. It is therefore suggested that the &sim1.2 Ga volcanic sequence developed in proximity to the Canyon Domain and that their juxtaposition is not the result of Grenvillian age tectonic transport.A swarm of &sim0.98 Ga late tectonic, ultrapotassic dykes occur in the central and southern Canyon Domain. These dykes crosscut the foliation, but have an internal fabric in the north. Farther south they are massive and display rims of granitic pegmatite. The ultrapotassie dykes consist of biotite-quartz-apatite +/- K-feldspar +/- plagioclase +/- clinopyroxene +/- orthopyroxene +/- amphibole with highly variable proportions. Those lacking internal fabric preserve an igneous texture with large biotite and apatite in interstitial pools dominated by K-feldspar and quartz.Some ultrapotassic dykes are characterized by primitive MgIn the Gagnon Terrane, mafic dykes inferred to be &sim1.74 Ga old, intrude Archean orthogneisses east of a major shear zone. These dykes are foliated and recrystallized under upper amphibolite to granulite facies conditions. These rocks have a geochemical signature of subalkaline tholeiitic basalts to andesites and, some, show primitive Mg