TRACE ELEMENT SYSTEMATICS AND CHEMOSTRATIGRAPHY OF MAFIC-DOMINATED ASSEMBLAGES: EXAMPLES FROM THE ARCHEAN SUPERIOR PROVINCE

The most abundant rock type in most Archean greenstone belts are extrusive mafic rocks. Mapping within mafic-dominated assemblages can be challenging because of relatively homogeneous rock compositions, similar textures and flow facies, a lack of continuous bedrock exposure, and limited stratigraphic marker horizons or reliable younging indicators. Despite these difficulties, these mafic-dominated greenstone belt assemblages represent the most complete geologic record of mantle-crust dynamics and can provide important constraints on our understanding of Archean tectonic processes and metallogeny. Since mafic rocks are formed in a variety of geodynamic settings without any diagnostic physical differences in their appearance or flow facies, a reliable method to study their genesis is via trace elemental analyses. This study highlights the effectiveness of linking trace element ratios of mafic rocks with their geospatial variation to “map” volcanic stratigraphy in various greenstone belts in the Lake Superior region. Combinations of elemental ratios involving rare earth and/or high field strength elements (such as Th/La, La/Yb, Th/Nb) are effective in discriminating different petrogenetic processes of mafic magmas. Examples from mafic-dominated strata in the Shebandowan, Ely, Winston Lake, and Sturgeon Lake greenstone belts illustrate that this technique can be utilized at varying scales to map regional-scale ‘terranes’ to individual magmatic events. Despite the effectiveness in this method high, it becomes difficult to interpret the geodynamic settings forming these mafic rocks since tectonic models are poorly constrained in the Archean. Several potential causes of geochemical heterogeneity in the mafic strata are discussed and ranges from plume heterogeneity, varying crustal contamination, and variable influence of flux versus decompression melting of the mantle in back-arcs.