THE RIVIERE-DES-PLANTE OPHIOLITIC MELANGE, SOUTHERN QUEBEC : ORIGIN AND IMPLICATIONS FOR THE CHAIN LAKES MASSIF

The Rivière-des-Plante Ophiolitic Mélange (RPOM) is located along the Baie-Verte – Brompton line in southern Québec, 40 km NE of the Thetford-Mines ophiolite. It consists of harzburgite, serpentinite, ophicalcite and of a fragmental granofelsic rock and mylonitic gneiss that have been interpreted as belonging to the Chain Lakes Massif (CLM) of northern Maine. It is limited to the northwest by a SE-verging thrust fault. To the southeast, the contact with the overlying Saint-Daniel Mélange is a major erosional unconformity marked by debris flows and conglomerates containing fragments of the underlying peridotites and CLM-type rocks. In the Thetford-Mines area, the Bécancour antiform exposes greenschist-facies metasedimentary rocks formed during the Taconic orogeny (ca. 462 Ma), these rocks occur structurally below the Thetford-Mines ophiolite and show lithological facies similar to, but of lower metamorphic grade than those of the CLM. The granofels and gneisses of both the RPOM and CLM were previously interpreted as sediments mostly derived from the Laurentian margin, and metamorphosed during the Taconic orogeny. Our mapping and petrographic observations on the RPOM rather suggest that both the granofels and mylonitic gneisses were derived from a xenolith-rich granitoid crosscutting the mantle peridotites. This granitoid formed in response to crustal thickening and partial melting of Laurentian sedimentary rocks during ophiolite obduction. We believe that the RPOM therefore represents a deeply-eroded fragment of mantle peridotites belonging to the Thetford-Mines ophiolite, in which similar peridotite-hosted granitoids are also found. Existing monazite U/Pb data for the CLM suggest a 469 ± 4 Ma age for peak metamorphism and migmatization. We think that this event was synchronous with ophiolite obduction and formation of anatectic granitoids found in the Thetford-Mines ophiolite (ca. 470 Ma) and in the RPOM; partial melting and metamorphism during ophiolite obduction being related to the formation of granitic melts crosscutting the mantle peridotites.