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The Rivière-des-Plante ultramafic Complex lies along the Baie Verte–Brompton line in southern Québec and has previously been interpreted as an ophiolitic mélange. It is bounded on the northwest by a northwest-dipping thrust fault and unconformably overlain by conglomerates belonging to the Saint-Daniel Mélange to the southeast. It consists of harzburgite, serpentinite, ophicalcite, gabbro, granite, and granofelsic to mylonitic fragmental rocks. The latter have been interpreted as “exotic” metasedimentary rocks correlative with those of the Chain Lakes massif of western Maine. Our mapping suggests that the Rivière-des-Plante ultramafic Complex is not a mélange, but rather a deeply eroded ophiolitic remnant mostly represented by mantle peridotites that correlate with those of the Thetford-Mines ophiolite. The granofelsic to mylonitic rocks represent xenolith-bearing granitoids crosscutting the peridotites rather than “exotic” blocks derived from the Chain Lakes massif. These granites are similar to ca. 470 Ma peridotite-hosted granitoids of the Thetford-Mines ophiolite, which were generated by anatexis of the Laurentian margin during ophiolite obduction. A comparison of metasedimentary rocks of the Chain Lakes massif with those of the southern Québec Laurentian margin, as well as stratigraphic and geochronological data for both the southern Québec and western Maine Appalachians, suggests that the Chain Lakes likely represents more or less in situ Laurentian margin, and that metamorphism and anatexis dated at 469 Ma may have been caused by the obduction of the southern Québec ophiolites.

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