SUB-OPHIOLITIC METAMORPHIC ROCKS OF THE LAC-BROMPTON AREA: A NEWLY-RECOGNIZED METAMORPHIC SOLE IN SOUTHERN QUÉBEC

The Lac-Brompton ophiolite (LBO) lies in tectonic contact with amphibolites and greenschist-grade metasedimentary rocks that represent the vestige of a metamorphic sole, similar to amphibolites of the Belmina Ridge in the Thetford-Mines area. In the Lac-Brompton region, these metamorphic rocks are overlain by, and in depositional contact with ophiolitic clasts-bearing polymictic conglomerates belonging to the St-Daniel Mélange. In order to better understand these rocks and investigate their origin, we have studied their metamorphic, structural and geochemical characteristics. Our results show that (1) the LBO metamorphic sole preserve structural evidence for three deformational phases. The first phase and associated amphibolite- to greenschist-grade metamorphism are Lower Ordovician and attributed to the emplacement (obduction) of the ophiolitic nappe onto the Laurentian margin. The second phase consists of reclined ENE-trending tight to isoclinal folds, presumably related to backthrusting deformation. The third phase consists of superposed Acadian folds. (2) the protolithe of amphibolites from the LBO metamorphic sole consists of a typical alkaline basalt, the composition of this basalt being very similar to rift-related mafic volcanics of the Tibbit Hill Formation. (3) garnets from the LBO metamorphic sole are microscopically zoned and show an average composition of Xalm=50% and Xgro=15%, whereas spessartine and grossulaire components are variable. Amphibole compositions vary from magnesio-hornblende to pargasite, and are indicative of albite-epidote to amphibolite-grade metamorphic facies. Epidote and chlorite are found as a retrograde metamorphic assemblage. Compared to Belmina Ridge, the LBO metamorphic sole shows a lower metamorphic grade and appears to originate from alkaline basalts rather than MORBs. Although that the LBO belongs to the same oceanic tract as the Thetford-Mines ophiolite, it seems that its obduction occurred on a different type of continental basement. However, as for the Thetford-Mines ophiolite, the emplacement history of the LBO involved the syn-obduction exhumation of its metamorphic sole, which is clearly expressed by retrograde metamorphism and the occurrence of an erosional unconformity at the contact with the overlying St-Daniel Mélange.