ARCHITECTURE OF VOLCANIC ASSEMBLAGES IN AN ARCHEAN GREENSTONE BELT: CLUES FROM MAPPING, GEOCHRONOLOGY, LITHOGEOCHEMISTRY AND ND ISOTOPES, AND IMPLICATIONS FOR TECTONIC MODELS IN THE SUPERIOR PROVINCE

Models for the generation and amalgamation of volcanic assemblages in Archean greenstone belts range from thrust stack/subduction based models to models that are driven by “in situ” catalytic crustal delamination. The resulting primary architecture of volcanic assemblages produced by each of these processes should be different (e.g. allochthonous vs. autochthonous). As such, deciphering the original contact relationships between volcanic assemblages in the Archean is critical in understanding the tectonic processes that were responsible for forming them. Often the original contacts between assemblages are overprinted by late deformation events, and a multi-disciplinary approach must be used in order to reconstruct them.

The Island Lake greenstone belt is located in the Superior Province, Manitoba, Canada. An integrated mapping, lithogeochemical, geochronological and Nd isotope study was undertaken to better understand the volcanic assemblages in the belt and tectonic processes in this part of the Superior Province. Three volcanic assemblages are observed in the belt; two episodes of Mesoarchean volcanism at 2.897 Ga and 2.852 Ga, and a Neoarchean episode of volcanism at 2.744 Ga. Belt-scale shear zones were thought to bound the volcanic assemblages, however mapping, geochronological and lithogeochemical data indicates that more than one of the volcanic assemblages outcrop on either side of these structures, and as such the shear zones do not represent the primary contacts between the assemblages. Lithogeochemical data from the mafic volcanic rocks indicates that the oldest volcanic assemblages are enriched in Th and LREE and have negative Nb, Ta, Ti anomalies, while the youngest volcanic assemblages lack any significant anomalies and enrichment. Nd isotope data from extrusive felsic rocks in the assemblages and contemporaneous plutons show a trend of increasingly negative εNd values with decreasing age, and all have Nd model ages over 3.0 Ga. This data is consistent with a model in which the volcanic assemblages have autochthonous relationships with each other and are part of a relatively intact primary volcanic pile. These assemblages were erupted onto a stable 3.0 Ga cratonic block as a result of periods of southward dipping Meso – and Neoarchean subduction.