OBSERVATIONS ON THE ALLARD LAKE ANORTHOSITE, GRENVILLE PROVINCE, QUEBEC, AND IMPLICATIONS FOR PETROGENESIS OF THE CRUML BELT OF MASSIF ANORTHOSITES

The Allard Lake massif anorthosite (ALM, ~50º30'N,~64ºW), which hosts the giant Lac Tio hemoilmenite deposit, comprises the southeastern lobe of the Havre St. Pierre Complex, arguably the largest such complex in the Grenville Province of Quebec. The ALM consists mainly of anorthosite, noteworthy for its purity (typical rocks contain >95% plagioclase), and minor leuconorite; both lithologies contain antiperthitic andesine, hypersthene, hemoilmenite, and biotite in variable proportions. Whole-rock normative compositions are in the range An_37-48Or_4-7, with 1000-1200 ppm Sr, 200-500 ppm Ba, and <5 ppm Rb. Also found locally in the ALM are distinct, small-scale zones (tens of meters) of labradorite anorthosite-leuconorite (An_66-74Or_1-3, 350-550 ppm Sr, <50-100 ppm Ba), which are cut by dikes of the host andesine anorthosite. The ALM is also invaded by sheets of Fe-Ti-P-rich (~2-5 wt% P₂O₅, ~4-10 wt% TiO₂), mafic to ultramafic rock mapped previously as oxide-rich norite or ferrodiorite. However, in terms of texture, mineral assemblage and composition, and bulk-chemical composition, these FTP-rocks are absolutely identical to what we have termed oxide-apatite-gabbronorite (or "OAGN") elsewhere in Quebec anorthosites.

The overall characteristics of the ALM (high abundance of antiperthitic sodic plagioclase; high Sr and Ba in plagioclase; presence of hemoilmenite and biotite; abundance of orthopyroxene, dearth of clinopyroxene, and absence of olivine; presence of older labradorite anorthosite; occurrence of OAGNs) are the defining characteristics of the CRUML-belt massifs (Chateau-Richer, St. Urbain, Mattawa, Labrieville, etc.), a group of relatively young (~1010-1060 Ma) andesine anorthosite plutons that extend from near Quebec City to north of Chicoutimi. This finding expands the spatial extent of distinctive andesine-anorthosite magmatism over a much larger portion of the Grenville Province, and belies the claim that the anorthosites of the CRUML belt are volumetrically too small and compositionally too extreme to have any general bearing on the overall petrogenesis of massif anorthosites.