Abstract

Sheared and unsheared xenoliths of garnet-harzburgite from the K1 Venetia kimberlite pipe (South Africa) are composed primarily of olivine and orthopyroxene (partially replaced by serpentine, magnetite and chlorite) and abundant porphyroblasts of garnet with kelyphitic rims of orthopyroxene, clinopyroxene and spinel. Mylonitized, orthopyroxene defines a mineral elongation lineation whereas bent orthopyroxene and sigmoidal garnet with wings of orthopyroxene define consistent senses of motion within individual xenoliths. The kelyphite surrounding both deformed and undeformed garnet grains is not deformed and appears to have resulted from the inversion of garnet and olivine to orthopyroxene, clinopyroxene and spinel, corresponding to the isograd separating garnet peridotite from spinel peridotite. Subsequent hydration around rims and along fractures within xenoliths resulted in the formation of serpentine, magnetite and chlorite from olivine and orthopyroxene. This hydration apparently resulted from reaction with the entraining kimberlite magma. The preservation of mylonite rather than its annealing and recrystallization to coarse-grained rocks at high temperature strain free mantle conditions requires quenching shortly after mylonite formation. This preservation taken with the presence of undeformed kelyphite around garnet grains implies that mylonitization and entrainment of garnet-harzburgite into kimberlite magma and kelyphite formation occurred during very rapid magma ascent.

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