The Status of Sillimanite as an Index of Metamorphic Grade in the Kenya Basement System

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Abstract
Regional studies using certain minerals as indices of metamprphic grade indicate a coincidence between areas of granitic injection or migmatitization and the distribution of sillimanite. Occurrences of that mineral in knotted bodies or faserkiesel and in pegmatites and aplites within migmatites have, however, sometimes been attributed to late metasomatism rather than to metamorphic conditions. In view of the doubtful value of the mineral as an index of metamorphic grade in these circumstances its distribution in the Kenya Basement System is reviewed and the field relationships between sillimanite-bearing metamorphosed pelitic sediments and faserkiesel migmatites occurring in central Kenya are described.It is concluded that sillimanite in the migmatites is derived from partly assimilated sillimanite gneisses, which are of extensive but inconspicuous distribution in the non-migmatitized Basement System. The regional stability of sillimanite rather than kyanite over a great area suggests that high-grade metamorphic conditions prevalent in Kenya during Basement System times were accompanied by granitization processes culminating in the local formation of replacive granites in which sillimanite relics are preserved.