THE LAC BROMPTON OPHIOLITE COMPLEX: STRATIGRAPHY, GEOCHEMISTRY, AND IMPLICATIONS FOR THE SOUTHERN QUÉBEC OPHIOLITE BELT, CANADA

The southern Québec ophiolites record volcanic arc build-up and forearc to back-arc spreading. The northern part, represented by the Thetford-Mines and Asbestos ophiolites, is predominantly of forearc affinity marked by the extensive occurrence of boninitic volcanic rocks. Southward, the Mont-Orford ophiolite and the Bolton Group volcanics have been attributed to arc construction and back-arc spreading. The Lac-Brompton ophiolite (LBO), previously interpreted as a serpentinite diapir, is located between the Asbestos and Mont-Orford complexes. Detailed mapping and geochemistry of volcanic and plutonic rocks of the LBO shows that it shares stratigraphical similarities with the Thetford-Mines and Asbestos ophiolites.

The LBO crops out in the core of a NNE-plunging Acadian antiform. The mantle sequence is conformably overlain by a discontinuous unit of boninitic volcanics and by conglomeratic to fine-grained sedimentary rocks of to the Saint-Daniel Mélange. Ultramafic rocks of the LBO consist mainly of mantle harzburgites and dunites. Minor occurences of pyroxenites, gabbros and diorites crosscut the mantle sequence. Two types of granitic intrusions have been identified within the LBO, plagiogranites and two-micas K-feldspar-rich granites. The plagiogranite is characterized by low contents of Al2O3, CaO, REE, Y, and by Th enrichment relative to Nb. The differentiation of a basic magma originating from partial melting of a depleted forearc mantle wedge is favoured for this granitic rock. The two-micas granite is typical of similar intrusions occurring in the mantle sequence of the Thetford Mines ophiolite, which have been attributed to the partial melting of continental sedimentary rocks during obduction. Basalts occurring south of the LBO are part of the Mont-Orford ophiolite. They exhibit REE and Nb enrichment typical of T-MORBs, a back-arc rifting environment best accounts for such geochemical signatures. The stratigraphic and geochemical characteristics of the LBO suggest that it was formed on a slow-spreading ridge environment in a forearc setting. The LBO, together with the Thetford-Mines and Asbestos massifs, indicate that a Lower Ordovician forearc crust is preserved for hundreds of km in the southern Québec Appalachians.