Major rock units of the Morin Complex, southwestern Quebec

The Morin Complex contains the largest mass of anorthositic rocks in southwestern Quebec. The major meta-igneous units of the complex from older to younger comprise anorthosite-leucogabbro, pyroxene monzodiorite and pyroxene quartz monzonite compositions. Anorthosite-leucogabbro and pyroxene quartz monzonite underlie by far the largest areas. Relict igneous textures and structures are preserved in much of the anorthositeleucogabbro except in the southern part of the east lobe where recrystallization to polygonal textures is far advanced or complete. Most measured plagioclase compositions in anorthositeleucogabbro lie in the range An42 to An54. The Or content of plagioclase averages about 5 mol per cent but drops to about 1. 5 mol per cent in recrystallized plagioclases. Orthopyroxenes typically contain 60 to 70 mol per cent MgSi03 in anorthosite-leucogabbro and range down to less than 40 mol per cent MgSi03 in pyroxene quartz monzonite. Anorthosite-leucogabbro is interpreted as essentially a plagioclase cumulate with interstital pyroxenes. Rocks corresponding to the liquid from which this cumulate crystallized have not been identified and may not be present at the existing level of exposure. The pyroxene and oxide-rich pyroxene monzodiorite probably also formed in part by crystal accumulation processes. The pyroxene quartz monzonites, on the other hand, show little or no evidence of having formed by crystal accumulation. Fe-Ti oxide concentrations are common in parts of the complex. Many of these are associated with contacts between anorthositeleucogabbro and pyroxene monzodiorite. Direct dating of rocks of the complex has not been carried out but recent Rb-Sr whole rock ages indicate that the age of last regional metamorphism (Grenvillian Orogeny) was 1100-1125 m. y. ago. It has also been suggested that this is also a close approximation to the age of emplacement of the smaller Lac Croche meta- anorthositic complex nearby.