Contrasting Si-saturation in troctolite-anorthosite intrusions along the Manicouagan corridor of the Abitibi-Grenville transect

Abstract

A survey of troctolite-anorthosite complexes along the Manicouagan corridor of the Abitibi-Grenville Lithoprobe transect reveals the existence of two distinct crystallization sequences in their cumulate rocks. In two intrusions (Brien and Raudot), olivine crystallized first, followed by plagioclase, Fe-oxides, and then clinopyroxene; and magmatic orthopyroxene is absent. In five other intrusions (Lucie, Tétépisca, Blache, Baie-Comeau, and Pentecôte), plagioclase and olivine are followed by orthopyroxene, and then clinopyroxene. These two distinct crystallization sequences correlate with compositional differences in gabbroic marginal rocks and dykes associated with the intrusions. The Raudot intrusion exhibits a fine-grained gabbroic margin that is Fe- and Al-rich, but Si-poor and nepheline normative. In contrast, the marginal rocks of the Blache, Baie-Comeau, and Pentecôte intrusions are typically gabbronorites that are poorer in Fe and richer in Si. Dyke rocks cutting this latter group of intrusions share these compositional characteristics and are typically orthopyroxene normative. The differences between the two types of intrusions appear to reflect the degree of Si-saturation of their parental magmas with respect to the troctolite divide. Together, the array of estimated parental magma compositions of the troctolite-anorthosite intrusions mimics that of Tertiary picritic magmas. The low forsterite (Fo) contents in olivine and the late appearance of clinopyroxene in the cumulates of the intrusions may reflect the interaction of mantle-derived parental magmas with relatively juvenile feldspathic crust.