Lassen, et al., reported in 2001 that field observations and major- and trace-element geochemical data are presented for Neoarchean alkalic intrusions of the Quetico metasedimentary belt in northwestern Ontario and northern Minnesota. The intrusions are characterized by high total alkalis (Na2O+K2O) (up to 13 weight per cent) for a given SiO2 content and show enrichment in large-ion lithophile elements and light rare-earth elements, and varied depletion in high field-strength elements. They include silica-undersaturated units. The differentiation history of the intrusions involves fractionation of clinopyroxene, hornblende, feldspar, Fe-Ti oxides, apatite, and titanite. The Blalock intrusion can also be classified as part of the sanukitoid suite. Other intrusions studied are more evolved, with distinct alkalic affinities.
The term ‘sanukitoid’ has been defined by Stern et al.(1989) as referring to rocks containing between 55 and 60 weight per cent SiO2, with Mg >0.6, Ni >100 ppm, Cr >200 ppm, K2O >1 weight per cent, Rb/Sr <0.1, Ba >500 ppm, Sr >500 ppm, enrichment in LREEs, and no or minor Eu anomalies. The term ‘sanukitoid suite’ includes more evolved rocks derived from sanukitoid through fractional crystallization (Stern et al., 1990). Rocks formed by processes similar to those of sanukite may have chemical compositions outside the sanukitoid field (see Tatsumi, 1981). Alkalic rocks do not have a strict definition, but the term generally refers to rocks containing high alkali contents at a given SiO2 content as indicated in the total alkali versus silica plot of Irvine and Baragar (1971). Following the Irvine and Baragar classification diagram, sanukitoid rocks are alkalic if they contain more than 5.0 to 5.5 weight per cent total alkalis, since SiO2 levels are between 55 and 60 weight per cent. Most sanukitoid rocks in the western Superior Province are alkalic according to this definition.
The Blalock intrusion is not a sanukitoid sensu stricto, because some units have lower Ni and Cr and higher SiO2 and Rb/Sr. However, the most mafic units fall within the sanukitoid field as defined by Stern et al.(1989) and the intrusion is of sanukitoid affinity as plotted for Ni+Cr versus Ba+Sr and K2O versus Rb/Sr, as well as the sanukitoid field. For the Blalock intrusion, Ni content ranges from 35 ppm to 142 ppm and Cr content, from 53 ppm to 226 ppm; Rb/Sr ranges from 0.05 to 0.2, K2O is between 3.2 and 6.1 weight per cent, Ba content is between 1221 ppm and 2282 ppm and Sr content, between 902 ppm and 1968 ppm. The LREE content ranges from approximately 200 to 500 times chondritic values. The Blalock intrusion is approximately 8 Ma older than the dated alkalic intrusions of this study. It differs from most sanukitoid bodies of the western Superior Province, which occur in metavolcanic subprovinces, in being hosted by metasedimentary rocks of the Quetico Subprovince.
Most units of the Gheen intrusion are too low in silica (45–55 weight per cent), Ni (29–84 ppm), and Cr (40–289 ppm) to be classified as sanukitoid, but fit all other criteria of the definition. Other intrusions within this study do not fit the sanukitoid classification. The studied intrusions show a large range in composition, from ultramafic, such as the North Elbow Lake intrusion, to highly evolved syenitic intrusions, such as the Harnett Lake intrusion. The Blalock intrusion is intermediate and follows the general trend as defined by all intrusions in this study. This may imply that sanukitoid bodies need not represent primitive, unfractionated magmas.
A progressive change in magma types has been noted by many researchers in the Wabigoon and Wawa subprovinces of the western Superior Province (e.g. Card, 1990). The tholeiitic and calc-alkalic volcanism and tonalite-trondhjemite-granodiorite plutonism characteristic of greenstone subprovinces terminated by ca. 2700 Ma. This was followed the by intrusion of calc-alkalic to slightly alkalic sanukitoid suites at ca. 2690 Ma, formed by relatively large degrees of melting of a LILE-enriched upper mantle (Shirey and Hanson, 1984). Alkalic suites intruded at ca. 2680 Ma range from syenite to nepheline syenite and rare carbonatitic compositions, and their parental magmas formed by smaller degrees of partial melting of a LILE-enriched upper mantle.
The involvement of a deeper asthenospheric source in the petrogenesis of these late mantle-derived magmas has been inferred by many workers (e.g. Stevenson et al., 1999). The involvement of a sublithospheric source is indicated by the Beaverhouse Lake intrusion, which is distinct in its lack of negative Nb-Ta anomalies in primitive-mantle-normalized diagrams. Instead, its chemical characteristics resemble those of present-day ocean-island basalts and imply that it is related to hot spots (Hattori and Percival, 1999). The upwelling of asthenospheric mantle could have provided heat for extensive magmatism in the southwestern Superior Province by melting of metasomatized lithospheric mantle.
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