Abstract

Detailed field work conducted in the Dunnage zone of the Quebec Appalachians, is herein combined with 40Ar/39Ar dating on a series of ophiolitic massifs, crosscutting granites, and associated metamorphic rocks occurring along the Baie Verte–Brompton line, the Taconian suture between Laurentia and Lower Paleozoic peri-Laurentian oceanic terranes. Studied massifs are the Lac-Brompton ophiolite and the Rivière-des-Plante Ultramafic Complex in southern Quebec, and the Nadeau Ophiolitic Mélange in the Gaspé Peninsula. Our work suggests that these massifs form remnants of eroded ophiolitic nappes, which are unconformably overlain by the Saint-Daniel and Rivière-Port-Daniel mélanges, and correlate with the Thetford-Mines and Mont-Albert ophiolitic complexes. Our 40Ar/39Ar data and compiled regional age constraints indicate that ophiolite obduction was diachronous along the strike of the orogen. The timing of obduction and mélange formation varies according to the irregular geometry of the Early Paleozoic Laurentian margin, with earlier collision occurring along, or at the margins of promontories. Obduction was initiated with the formation of infraophiolitic metamorphic soles between ca. 479 and 472 Ma in southern Quebec and the Nadeau Ophiolitic Mélange, and possibly as late as ca. 470–466 Ma for the Mont-Albert Complex. These sole rocks were later exhumed and translated onto the Laurentian margin with the overlying ophiolites between 475 and 460–457 Ma. The uplifting and erosion of the orogenic wedge during the waning stages of obduction, has resulted in the sedimentation of olistostromal mélanges and onlapping flysch units above the ophiolitic nappes, as well as foredeep flysch successions during the latest Arenig(?) to earliest Caradoc.

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