THE LAC-BROMPTON OPHIOLITIC COMPLEX: STRATIGRAPHY AND STRUCTURE OF A NEWLY-RECOGNIZED OPHIOLITE IN THE SOUTHERN QUÉBEC APPALACHIANS, CANADA

The Southern Québec ophiolites consist of different segments of Lower Paleozoic oceanic crust and mantle obducted onto the Laurentian during the Taconian orogeny. Three main ophiolitic bodies are presently recognized, the Thetford-Mines, Asbestos and Mont-Orford ophiolitic complexes. NE of the Mont-Orford Complex, there is a large massif of undivided peridotite and serpentinite currently interpreted as a chaotic ophiolitic unit, the Lac-Monjoie ophiolitic mélange. Detailed mapping of ultramafic rocks and adjacent rock units of the area rather indicates that it represents a dismembered but well-preserved ophiolite that we introduce here as the Lac-Brompton ophiolitic Complex (LBOC). From base to top, the LBOC is made up mantle rocks (mainly harzburgites) overlain by a heterogeneous, and laterally discontinuous crustal sequence consisting of ultramafic cumulates(?), gabbros and basaltic volcanic rocks. Towards the base of the mantle sequence, there is a discontinuous horizon of greenschist- to amphibolite(?)-grade metamorphic rocks that may represent remnants of the ophiolite metamorphic sole. The LBOC is stratigraphically overlain by debris flows, conglomerates and medium- to fine-grained siliclastic rocks (included in the St-Daniel Mélange) that mark a major erosional unconformity. These sedimentary rocks are found in depositional contact with various parts of the LBOC, i.e. overlying basalts, gabbros, altered dunites, and even the metamorphic unit underlying the mantle sequence. The debris flows and conglomeratic rocks unit varies highly in thickness and in lithology. It is very similar to the Coleraine Breccia, a rock unit occupying the same stratigraphic position in the Thetford-Mines ophiolitic Complex. The presence of such an erosional unconformity and the fact that the LBOC is characterized by, at least, two generations of regional folding and faulting account for the high level of stratigraphic complexities and explain why it has been interpreted as a diapiric ophiolitic mélange. Our preliminary analysis and ongoing work in other ophiolites of Southern Québec suggest that the LBOC posses more lithological affinities with the Asbestos and Thetford-Mines complexes than with the adjacent Mont-Orford ophiolite.