THE RIVIÈRE-DES-PLANTE OPHIOLITIC MÉLANGE, SOUTHERN QUÉBEC APPALACHIANS – A TECTONIC MÉLANGE OR A DISMEMBERED AND ERODED OPHIOLITE ?

The Rivière-des-Plante ophiolitic mélange (RPOM) is part of the Dunnage zone and crops out along the Baie Verte-Brompton Line (BBL), 40 km to the NE of the Thetford-Mines ophiolite. It has been interpreted as a tectonic mélange formed and accreted to Laurentia during the Taconian orogeny. It mainly consists of serpentinized ultramafic rocks intruded by discontinuous masses of granitic rocks which have been described as metasediments correlated with those of the Chain Lakes Massif in western Maine. This interpretation is however revisited in this contribution. Our mapping indicates that the SE contact of the RPOM with both the Saint-Daniel Mélange and the Magog Group is an erosional unconformity. The NW limit rather corresponds to a SE-directed thrust fault marking the contact with the Caldwell Group, a volcano-sedimentary rock unit belonging to the Humber zone. Ultramafic rocks consist of dunite and harzburgite, whereas granitic rocks represent 50% of the RPOM. Gabbros and basalts are also locally present. The granites contain enclaves of metasedimentary and metabasic rocks in a medium-grained granular groundmass consisting of quartz, feldspar, biotite and muscovite. Preliminary step-heating 40Ar/39Ar data on biotite and white mica from metasedimentary enclaves yield high-temperature apparent steps of 450-460 Ma, giving minimum metamorphic ages, and plateau or low-temperature step ages suggesting a thermal overprint at 414-420 Ma. Similar 40Ar/39Ar muscovite ages have been measured in mylonitized, obduction-related granites crosscutting the mantle peridotite of the Thetford-Mines ophiolite. Two types of polymictic sedimentary breccias, both belonging to the Saint-Daniel Mélange, overly the RPOM; 1- a unit made up of clasts of metasedimentary rock, RPOM-type granite and minor serpentinite lying in a pelitic/sandy matrix, and 2- a pebbly to blocky olistostromal facies. The nature of clasts in the first type of breccia suggests that, as for the Thetford-Mines ophiolite, syn-obduction (?) erosion has reached the ophiolitic lower crust/upper mantle and the underlying metamorphic rocks of the Laurentian margin. These data suggest that the RPOM more likely represents a deeply-eroded ophiolitic massif rather than a tectonic mélange in which were incorporated «exotic» high-grade metamorphic rocks.