U–Pb ages from the Nimish Formation and Montagnais glomeroporphyritic gabbro of the central New Québec Orogen, Canada

Three U–Pb zircon dates were obtained from two igneous suites associated with cycle 2 sedimentation in the central New Québec Orogen (Labrador Trough). In the Dyke Lake area of the western part of the orogen (Schefferville zone), the Nimish Formation includes a polymictic conglomerate containing quartz syenite cobbles that crystallized at 1877.8 ± 1.3 Ma. These cobbles are petrographically and geochemically linked to the Nimish volcanics, which are intercalated with the Sokoman iron formation. Consequently, the syenite date gives an approximate age for both Nimish magmatism and ironstone deposition in the Schefferville zone. A date of ca. 2.65 Ga obtained from a Nimish trachyte in the same area indicates that zircons in the sample are xenocrysts. Farther east, at Howse Lake (Howse zone), a Montagnais plagioclase-glomeroporphyritic gabbro sill crystallized at 1884.0 ± 1.6 Ma. The Howse Lake sill, which intrudes turbidites of the Menihek Formation, is considered comagmatic with the basalts that cap the formation, and with the Willbob basalts to the east (Doublet Terrane). Consequently, the Menihek turbidites, as well as the underlying Sokoman Formation, were deposited prior to 1884 Ma in the Howse zone. In contrast, the syenite date indicates that deposition of the Menihek Formation in the Schefferville zone did not commence until after 1878 Ma. The 1884 Ma date from the Howse Lake sill also provides an estimate for the timing of cycle 2 tholeiitic volcanism in the eastern part of the orogen. The correlation between the upper Menihek and Willbob basalts suggests that the Murdoch and Thompson Lake formations, which underlie the Willbob basalts in the Doublet Terrane, are the stratigraphie equivalents of the lower and middle portions of the Menihek Formation, respectively.