THE IMPORTANCE OF ANDESITES FOR INTERPRETING TECTONIC PROCESSES DURING THE ARCHEAN

The role of modern-style subduction in Archean plate tectonic processes is controversial. Uniformitarianism suggests that lithological and geochemical similarities of some Archean rocks with modern volcanic and continental arcs indicate the presence of similar petrogenetic processes and similar geodynamic settings. However, many thermodynamic models suggest that the hotter mantle and higher crustal buoyancy during the Archean makes the process of subduction more difficult to achieve. Andesites and other intermediate volcanic assemblages are characteristic magma compositions found at modern arcs, but are generally a minor component of most Archean greenstone belts. Additionally, intermediate volcanic rock assemblages have been demonstrated to be formed in both subduction- and plume-dominated petrogenesis. Therefore, understanding the petrogenesis of Archean intermediate assemblages, especially when they are voluminous, is a very important component in determining the presence or absence of modern plate tectonic processes at the time of formation.

This study describes the lithogeochemistry and Nd-isotopic composition of andesitic volcanic assemblages in the Shebandowan greenstone belt of the Wawa-Abitibi terrane that were deposited prior to deformation and tectonic assembly of the Superior Province. The intermediate rocks of the Shebandowan greenstone belt are voluminous, are weakly deformed, and are low metamorphic grade. The major and trace element geochemistry of the andesitic assemblages are like modern arcs and also contain significant amounts of magnesian andesites and adakites. Neodymium isotopic analyses indicate local interaction with older crust and evolved Nd-isotopic values correlate with other crustal contamination proxies such as Th/Ce and Ti/Sc ratios. Modelling mixing and assimilation-fractional crystallization interactions between plume-derived and crust-derived melts reveal that plume-crust interactions cannot explain the compositional array obtained from the intermediate rocks in the Shebandowan greenstone belt. Microplate tectonic settings, such as those in the western Pacific Ocean, are an excellent modern analog for the interpreted geodynamic settings that formed the intermediate rocks in the Shebandowan greenstone belt.