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The Archean (2.8 Ga) Banded Iron Formation (BIF) of the Bell Lake region of Yellowknife greenstone belt, Canada is recrystallized to metamorphic assemblages of the amphibolite facies. This BIF is characterized by centimetre-scale Fe-rich and Si-rich mesobands. In the Si-rich mesobands, thin layers of magnetite microbands are developed in a quartz matrix. The Fe-rich mesobands are composed mainly of Ca-amphibole (hornblende), Fe-Mg amphibole (grunerite), and magnetite. The metamorphic foliation locally cuts across the mesoband boundaries, indicating the mesobanding was formed prior to peak metamorphism. Variations in mineral modal proportions between Fe-rich mesobands and microbands are diagnostic of depositional compositional differences between beds. Micro-X-ray fluorescence imaging reveals metamorphic differentiation within Fe-rich mesobands, with segregation of Fe-Mg amphibole, and the incompatible element Mn is concentrated at the margins of the Fe-rich mesobands during the amphibole-forming reactions. Ti was relatively immobile during metamorphic segregation and its distribution provides a record of the original structures in the Fe-rich mesobands. [ABSTRACT FROM AUTHOR]

Abstract: The longevity of the continental lithosphere mantle is explained by its unusual composition. This part of the mantle is made up mainly of forsterite-rich olivine (Fo92–94), with or without orthopyroxene, and it is essentially anhydrous. The former characteristic makes it buoyant, the latter makes it viscous, and the combination of these features that allow it to remain isolated from the convecting mantle. Highly forsteritic olivine is not normally produced during mantle melting. Possible explanations for its abundance in old Archean subcontinental lithospheric mantle include: (a) high-degree mantle melting in a plume or at an Archean ocean ridge; (b) accretion of this material to older lithosphere and its reworking in a subduction zone; (c) redistribution of material to eliminate high-density, low-viscosity lithologies. Following an evaluation of these models based on petrological and numerical modeling, we conclude that the most likely explanation is the accumulation of the residues of melting of one or more mantle plumes following by gravity-driven ejection of denser, Fe-rich components. [Copyright &y& Elsevier]