THE LAC PICHÉ ANORTHOSITE, GRENVILLE PROVINCE, QUEBEC: THE NEWEST MEMBER OF THE CRUML BELT

A distinctive feature of Grenville massif anorthosites is their tendency to appear as negative anomalies on aeromagnetic maps. And so it was that our attention was drawn to a small (~10 km wide) aeromagnetic low located ~80 km north of Quebec City in an area where no anorthosite was shown on any geologic map. A reconnaissance (2003) revealed that anorthosite does indeed underlie the area covered by the magnetic low, and we named this body the "Lac Piché Anorthosite." Analyses of a few samples showed it to be an andesine anorthosite (~An₃₈Or₈), with high Sr (~1000 ppm) and modest Ba (~250 ppm). Noteworthy features include the ubiquitous presence of hemoilmenite as the sole Fe-Ti oxide phase, the presence of orthopyroxene and minor biotite as the mafic silicates, and scattered inclusions of labradorite anorthosite (~An₅₅Or₃). Detailed mapping (2004) indicated that the anorthosite has the shape of a dome, with outward, shallow-dipping (<20º) foliations. The massif consists of coarse-grained pink to brown seriate porphyritic anorthosite and leuconorite in the center, and finer-grained, green aphyric anorthosite along its margin. The anorthosite is bordered by a narrow (~100-500 m), continuous rim of jotunite, which in turn is surrounded by a large plutonic mass of mangerite. No deposits of oxide ores were identified, but conformable layers of oxide-apatite gabbronorite (OAGN) containing local zones of oxide-apatite rock (nelsonite) occur within the mangerite. Follow-up laboratory work confirmed initial findings about rock compositions and revealed, in addition, that the pluton is reversely zoned (An_34-37 in the center An_37-40 along the border).

The characteristics of the Lac Piché Anorthosite are thus indistinguishable from those of the main CRUML-belt massifs (Château Richer, St. Urbain, Mattawa, Labrieville), which form a chain of plutons extending some 300 km from near Quebec City to north of Chicoutimi. The recognition of the Lac Piché body as a CRUML-belt massif thus expands the spatial extent of this distinctive late- to post-tectonic (~1000-1050 Ma) event of anorthosite-jotunite-mangerite (AJM) magmatism. Examination of other aeromagnetic lows may lead to the discovery of additional, previously unrecognized bodies of massif anorthosite in the Grenville Province.