A lost arc-back-arc terrane of the Dunnage oceanic tract recorded in clasts from the Garin Formation and McCrea mélange in the Gaspé Appalachians of Québec

Abstract

Upper Ordovician Garin conglomerates (base of the Gaspe belt, Quebec Appalachians) contain three igneous clast populations. (1) Calc-alkaline intermediat-felsic rocks resemble Exploits (New Brunswick, Maine) and Notre Dame (Quebec, Newfoundland) subzone lavas. Clasts (monzonite-rhyodacite) give U-Pb zircon ages between 465 and 466 Ma, precluding correlation with Exploits rocks. We suggest that the suite represents peri-Laurentian continental arc magmas coeval with the Red Indian Lake Group of Newfoundland. Correlation of lavas from a Gaspe well with the Popelogan arc implies that the Red Indian Line passes through southern Gaspe. (2) Mafic tholeiitic-to-alkaline clasts resemble New Brunswick and Maine (Exploits subzone) alkaline lavas. We interpret them as magmas associated with initiation of a back-arc basin, possibly on the Laurentian margin. (3) Mafic-intermediate tholeiitic to calc-alkaline clasts have an oceanic subduction component and correlate with Notre Dame subzone lavas (Quebec, Newfoundland). They are interpreted as products of spreading and off-axis magmatism of a peri-Laurentian back-arc basin. We correlate McCrea melange lavas to the calc-alkaline intermediat-felsic and mafic-intermediate suites and interpret them as samples of a Notre Dame subzone terrane coeval with the Red Indian Lake Group of Newfoundland during initiation of a successor arc–back-arc, a terrane which is no longer preserved in New Brunswick and Maine but that provides some evidence for possible extensions of Newfoundland9s geology into the Gaspe Appalachians of Quebec. A mid-ocean-ridge basalt–like intrusion in the McCrea melange (Ar-Ar age: 471.2 ± 11.2 Ma) is coeval with peri-Laurentian ophiolites in Newfoundland and may record a back-arc opening event.