The Archean komatiites of the Lake of the Woods greenstone belt, Kenora, Ontario formed on the western extension of the Superior Province southern margin and have not been studied using modern analytical methods. Although Archean plate tectonic processes have been the subject of decades of research, the nature of these processes remains the subject of considerable debate. Recent work has investigated the link between komatiites and Archean subduction zones. Komatiites are widespread in Archean terranes and together with spatially associated tholeiitic basalts form an important part of many Late Archean greenstone belts, therefore a better understanding of Archean geodynamic processes and comparison to modern day processes is required.
Detailed mapping in the Upper Keewatin Assemblage identified komatiites on the southern margin of the Long Bay Group. The komatiites are typically metamorphosed to upper greenschist facies and include a variety of schists that do not show any preserved primary textures or mineralogy. Polyhedrally jointed flow tops were observed in rare locations. Mineral assemblages include dominantly anthophyllite-tremolite-chlorite and serpentine-tremolite-chlorite schists, as well as lesser talc-tremolite-chlorite schists. These units are moderately to intensely foliated with chlorite and lesser amphibole defining the foliation and also include randomly oriented bladed amphibole grains that typically have tremolite cores and anthophyllite rims. The amphiboles show a chemical transition from core to rim with a loss in Ca as anthophyllite appears. Accessory phases include chromite, magnetite, ilmenite and apatite. Ultramafic rocks are very fine-grained and mineralogy has been described using a compilation of petrography, x-ray diffraction and scanning electron microscope analysis.
Whole-rock geochemical analyses were conducted on 110 samples. The Upper Keewatin Assemblage is composed of dominantly mafic to intermediate volcanic rocks that are typically of tholeiitic affinity with rare calc-alkalic units. A total of 41 samples were determined to be ultramafic in which the komatiite units are Al-undepleted komatiites that display major and trace element concentrations consistent with melts derived from above the garnet stability zone. They can be subdivided into three suites with primitive mantle patterns that display strong Th and Nb depletions with flat HREEs (heavy rare earth elements), weak Th and Nb depletions with flat HREEs and enriched Th with moderate Nb depletions and flat HREEs. Neodymium isotope analyses, in conjunction with trace element geochemistry, suggests that some units have been weakly to moderately contaminated. Mafic tholeiitic units have low- and high-Ti varieties, in which most units are dark grey to black amphibolites and rare chlorite-tremolite schists. The mafic units shows similar contamination trends to the ultramafic units.
The tectonic setting of the Upper Keewatin Assemblage requires voluminous mafic magmatism with relatively sporadic ultramafic magmatism in a subaqueous setting giventhe widespread pillow basalts. The belt evolution is interpreted to represent an initial primitive oceanic arc with subaqueous tholeiitic magmatism followed by calc-alkalic dominant magmatism following arc evolution. Tholeiites originating from the primitive arc display weak Nb depletions, and through geochemical modelling it is possible that they are the contaminant that produced Th-enriched komatiites. Although a hydrous source could not be confirmed for the Nb-depleted komatiites, contamination cannot account for the variation in trace elements. It is likely that plume-arc interactions have occurred, similar to scenarios documented in the Superior Province; however, the mechanisms cannot be determined. Deep-penetrating fractures can accommodate along-strike variations in the dip of subduction zones, which typically segment island arcs into various belts. Large-scale crustal weaknesses would act as potential magma conduits for ascending plumes, resulting in a chemical and isotopic difference between the upwelling asthenosphere and arc magmatism. This would result in likely interaction between the rising plume and subduction-derived fluids, producing the Nb-depletions present in komatiite samples within the assemblage.