Abstract

Upper Ordovician Garin conglomerates (base of the Gaspé belt, Québec Appalachians) contain three igneous clast populations. (1) Calc-alkaline intermediat-felsic rocks resemble Exploits (New Brunswick, Maine) and Notre Dame (Québec, 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 Gaspé well with the Popelogan arc implies that the Red Indian Line passes through southern Gaspé. (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 (Québec, Newfoundland). They are interpreted as products of spreading and off-axis magmatism of a peri-Laurentian back-arc basin. We correlate McCrea mélange 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 Newfoundland's geology into the Gaspé Appalachians of Québec. A mid-ocean-ridge basalt–like intrusion in the McCrea mélange (Ar-Ar age: 471.2 ± 11.2 Ma) is coeval with peri-Laurentian ophiolites in Newfoundland and may record a back-arc opening event.

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