PETROLOGIC RECONNAISSANCE OF THE LAC VALLANT ANORTHOSITIC SUITE [LVAS], GRENVILLE PROVINCE, QUEBEC

As part of an ongoing effort to characterize and understand the petrogenesis of Proterozoic massif anorthosites, we have initiated a study of the LVAS, a hook-shaped intrusion, ~20 km wide and ~150 km long, situated ~50 km NW of Baie Comeau.

All rocks studied are plagioclase-rich, medium- to very coarse-grained, massive to foliated to compositionally layered. Pale- to dark-gray to black varieties dominate, especially along the N and S margins, with mauve anorthosite-leucotroctolite being a widespread lithology in the interior. Hornblende is the dominant mafic mineral, with minor pyroxene, olivine, biotite, and magnetite, and spinel-bearing coronas around olivine. Such mineralogic and petrographic features reflect high-grade metamorphism. Chemical analyses by XRF methods of 28 samples, collected on a ~20 km segment of route 389, reveal four groups. Group A, found at the N margin of the LVAS, contains highly calcic normative plagioclase (~An₇₀Or₄). Group B, found throughout the main body of the LVAS, includes anorthosites and leucotroctolites (~An₅₀Or₆ – An₅₇Or₄); the mauve-colored rocks belong to this group. Group C, also common in the interior but also found along the S margin, comprises pale- to dark-gray anorthosites and leuconorites with feldspar compositions similar to group B. Group D, gray, coarse-grained anorthosites (An₅₁Or₄), has been recognized only in the interior of the LVAS.

Our findings, which are consistent with results reported by Gobeil et al. (Min. des Res. natur. Québec, RP 2006-01), invite comparison to other Grenville anorthosites. In terms of color (mauve/black) and composition, the LVAS resembles anorthosites of the Morin Complex and Lac Ste. Jean suite, respectively. The LVAS is also similar in character to anorthositic rocks of the ~1354 Ma Rivière Pentecôte Anorthosite (Martignole et al., 1993, J. Geol., 101). At the same time, plagioclase compositions and evidence for metamorphic recrystallization distinguish the LVAS from anorthosites of the 1010-1060 Ma CRUML belt, which are more sodic and potassic (An₃₀Or₁₀ – An₄₅Or₅). We find it intriguing that older, metamorphosed anorthosites contain labradorite whereas the younger, unmetamorphosed anorthosites contain andesine. The implications of such an apparent secular-compositional trend remain to be deciphered.