DISCOVERY OF THE L'ENFER NORITE: POSSIBLE EVIDENCE FOR NORITIC MAGMAS IN THE PETROGENESIS OF ANDESINE ANORTHOSITES, SE GRENVILLE PROVINCE, QUEBEC

Olivine-bearing, high-Al gabbroic magma has been suggested as the parent for several Proterozoic (~1125-1430 Ma) massif anorthosite complexes (e.g., Laramie, Nain, Marcy), in part based on dikes and chilled margins of troctolitic to high-Al basaltic composition in association with these massifs. However, such rocks have never been found within or near to any of the late- to post-tectonic (~1010-1060 Ma) andesine anorthosites in south-central Quebec (Chateau-Richer, St. Urbain, Lac Chaudiere, Lac à Jack, Mattawa, Labrieville). In fact, ol is lacking in these rocks and cpx is rare, whereas opx is common, such that leuconorite is a widespread rock type. New field evidence for the possible involvement of noritic magmas in the origin of these anorthosites comes from the recently discovered L’Enfer norite pluton, a small body (~6 km²) found ~50 km NW of St. Urbain, adjacent to the Lac à Jack massif. This pluton does not appear on any existing geologic map, but shows up as a prominent aeromagnetic high. This magnetic high corresponds well with the dimensions of the pluton, which crops out in two lobes separated by a screen of country rock. The interior of the pluton is coarse- to very-coarse-grained, with spectacular “mega-diabasic” textures in which tabular plag grains (~1-4 cm long) are intergrown mainly with opx. Cumulate layering occurs locally, and rare ultramafic rocks with abundant ol and Fe-Ti oxides are also present. Most samples contain accessory mag, ilm, cpx, ap, and bt, with locally abundant red-brown amph. Medium-grained, homogeneous rocks within a few meters of the contact may represent a chilled margin. Marginal and interior rocks (5 samples each) have broadly overlapping compositions [in wt%: 46-55 SiO₂, 12-19 Al₂O₃, 1.2-5.5 TiO₂, 0.6-1.5 K₂O,  normative An_35-44, mg=0.40-0.54, 686-1116 ppm Sr, 191-473 ppm Ba]. Although the age of the L’Enfer norite is unknown, it constitutes pristine igneous rock. Thus, we infer that it is coeval with nearby post-tectonic anorthosites, and that it might represent a parental magma-type, from which plag could separate to form anorthosite. Support for this interpretation includes: (1) plag An-contents that match those of spatially associated anorthosites; (2) opx as the dominant mafic mineral; (3) ubiquitous hydrous phases; (4) relatively Ti-rich compositions; and (5) high Sr.