THE NEOARCHEAN MENTOR ANORTHOSITE INTRUSIVE COMPLEX: THE LARGEST ANORTHOSITE INTRUSION OF THE SUPERIOR PROVINCE?

The ca. 2737 Ma Mentor anorthosite intrusive complex (MAIC) lies near the northern margin of the Wawa subprovince of the Archean Superior Province, in an area of northwestern Minnesota where the Wawa, Quetico, and Wabigoon subprovinces are juxtaposed. The rocks of interest are entirely concealed by 10s to >100 m of unconsolidated Quaternary sediments and localized Cretaceous strata and saprolite. The MAIC comprises a large volume of megacrystic anorthosite, with a lesser volume of oxide-rich gabbros. The gabbros are known, from a single borehole intersection at ~140m depth, to be enriched in vanadium; they may also have potential for chromium and titanium mineralization. New interpretations are based on aeromagnetic data and ground gravity data, constrained by approximately ten boreholes in the area. The anorthosite is weakly magnetized and dense, with a mean measured density of 2940 kg/m³, producing a 10-60 mGal gravity high. Pervasive epidote alteration is a suggested explanation for the high density of the anorthosite (the density of unaltered anorthite is 2730 kg/m³). The oxide-rich gabbros are strongly magnetized, producing aeromagnetic anomalies as large as 6000 nT and making them readily mappable across the complex. New geophysical interpretations suggest that the MAIC covers an area of about 640 km² along a strike length of about 85 km, and forward modeling suggests a depth extent as great as 7 km. Thus, the MAIC is significantly larger than previously interpreted and can be traced along strike for approximately double its originally interpreted length. The MAIC is here interpreted to be the largest known anorthosite complex in the Superior Province, as measured by preserved extent in map view. The MAIC is observed in drill core to intrude a package of basalt flows at its northwest boundary and is itself intruded by multiple low-density felsic plutons that produce 10-20 mGal, 4-20 km wide gravity lows. The origin of Archean anorthosites is controversial, and the tectonomagmatic significance of the MAIC is not well understood. However, the setting of the MAIC is consistent with other anorthosite complexes of the Superior Province, that commonly intrude packages of mafic volcanic flows and are themselves commonly intruded by felsic plutons.