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GSA Bulletin; January 2009; v. 121; no. 1-2; p. 17-38; DOI: 10.1130/B26251.1
© 2009 Geological Society of America
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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

Céline Dupuis1,{dagger}, Michel Malo1,*, Jean Bédard2,§, Bill Davis3 and Mike Villeneuve3

1 Institut National de la Recherche Scientifique, Eau, Terre et Environnement, 490 de la Couronne, Québec, Québec G1K 9A9, Canada
2 Geological Survey of Canada–Québec, Earth Science Sector, 490 de la Couronne, Québec, Québec G1K 9A9, Canada
3 Geological Survey of Canada–Ottawa, Earth Science Sector, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada

Correspondence: {dagger}E-mail: celine.dupuis.1{at}ulaval.ca.

Correspondence: *E-mail: michel.malo{at}ete.inrs.ca.

Correspondence: §E-mail: Jean.Bedard{at}rncan.gc.ca.

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.

Key Words: igneous clasts • calc-alkaline • tholeiite • volcanism • back-arc basin • Dunnage zone • Gaspé Appalachians






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