Geochemical signatures of komatiites and origin of the Stoughton-Roquemaure Group, Abitibi Greenstone belt, Canada.

摘要

The genesis of sub-greenschist-facies-metamorphosed komatiitic rocks of the Stoughton-Roquemaure Group (SRG) in the Abitibi Greenstone belt, Canada, is evaluated based on the geochemistry of preserved primary clinopyroxenes. This study tries to distinguish between two main models of origin by the application of major and trace elements. In general, clinopyroxenes in the SRG ultramafic komatiites, komatiites and basaltic komatiites are similar in composition. They contain low atomic proportions of nonquadrilateral components (e.g., Ti, Al and Na). The tectonic discrimination diagrams of Nisbet and Pearce (1977) and Leterrier et al. (1982) suggest both a plume and subduction origin for these rocks, whereas, those of Beccaluva et al. (1989) are compatible with a subduction origin. The SRG clinopyroxenes are seen to have similar compositions to those from Barberton (South Africa) and Belingwe komatiites (Zimbabwe), but they differ from Gorgona komatiites (Colombia). In addition, they can be compared with very low- and low-Ti ophiolitic basalts, island arc tholeiites, boninites, and basaltic andesites and andesites thought to evolve in subduction zones.; The SRG trace element data indicate that the rare earth elements (REE) have been immobile during metamorphism. REE patterns show depletion in the light REE ([La/SM]N 0.3) and unfractionated heavy REE ([Gd/Y] N = 0.8-1.18). The clinopyroxenes are depleted in high field strength elements (Nb, Zr, Ti and Y) relative to the more incompatible REE but show enrichment in large ion lithophile elements (Rb, Sr). These characteristics are attributed to a subduction zone origin. The overlap between plume and subduction affinity in the discrimination diagrams, and the subduction amity from the trace element data make it difficult to assign a particular palaeotectonic setting. A parallel line of descent is proposed in which a rising mantle plume intercepted a subducting slab at shallow depth.