Mica deformation signatures: geochemical and strain investigation of phlogopite and biotite from the Drybones Bay and Mud Lake kimberlites (Northwest Territories, Canada)

Phlogopite in kimberlites commonly displays evidence of complex deformation history. Textural evidence includes kink-banding, undulatory extinction and bent cleavage; this is conventionally ascribed to strain in the lithospheric mantle. However, the extent and intensity of deformation in phlogopite from kimberlites have not been systematically explored. In this work, the degree of strain-related mosaicity, resulting from plastic deformation in micas from the Drybones Bay and Mud Lake kimberlites (Canada), was quantified by measuring ‘streaking’ along chi (χ) direction in 2D X-ray diffraction images, as the sum of full-width-at half-maximum Σ(FWHMχ) where Σ(FWHMχ) > 2.5^o indicates deformed phlogopite grains. The examined phlogopite was categorized into the low-Ti-Cr and low-Cr, Ti-rich varieties. The low Σ(FWHMχ) signature of the low-Cr, Ti-rich phlogopite grains matches their low-strain environment during or immediately after kimberlite emplacement. On the other hand, appreciable variations in Σ(FWHMχ) with respect to both deformed and undeformed low-Ti-Cr phlogopite cannot be feasibly explained by transport in a turbulent kimberlite magma during rapid ascent from the mantle. It seems most likely that the low-strained low-Ti-Cr phlogopite grains are genetically unrelated to their high-strained counterparts, which are interpreted as xenocrysts derived from metasomatized mantle xenoliths. We conclude that optically observable deformation microtextures in phlogopite are not a reliable indicator of their xenocrystic origin for the following reasons: (1) the low-Ti-Cr phlogopite population comprises both deformed and undeformed crystals; (2) some deformed low-Ti-Cr phlogopite grains are cognate phenocrysts; and (3) strain features in phlogopite xenocrysts can also be developed under crustal conditions in response to post-emplacement tectonics.