STRONG COMPOSITIONAL ZONATION IN PERALKALINE MAGMA - MENENGAI, KENYA RIFT-VALLEY

The second caldera-forming ash-flow tuff erupted from the Menengai volcano is up to 80 m thick and represents ca. 30 km3 of peralkaline trachytic magma. Activity started with the emplacement of air-fall tuffs, possibly from a central vent, followed by welded ash-flow tuffs and then by unwelded flow material, the proximal facies of which is lag breccia. At least part of the ash-flow deposits was emplaced through the ring fracture. The tuff was erupted from a compositionally zoned magma chamber which showed strong roofward enrichment in Fe, Mn, Na, Nb, Rb, Th, Y, Zr and the REE, and depletion in Al, Mg, Ca, K, Ti, P and Ba, corresponding to pantelleritic trachyte at the top, grading down through comenditic trachyte to more primitive (Ba-rich) trachyte. The maximum observed enrichment factor is X4 for Zr. The chamber may also have been zoned isotopically, Sr-87/Sr-86 becoming less radiogenic (from 0.7062 to 0.7048) and Nd more radiogenic (epsilon from 0 to - 2), upwards. Alkali feldspar phenocrysts generally became more sodic, and olivine and clinopyroxene phenocrysts more Fe-rich, towards the roof. Variable matrix glass compositions and unusually sodic cores of feldspar phenocrysts in some rocks are thought to result from mixing of liquids from more than one compositional layer during magma withdrawal. The partitioning of Ba, Rb, Sr and the REE between alkali feldspar and liquid was apparently dependent on melt peralkalinity. As the eruption proceeded, a range of compositions was progressively tapped. Compositional zonation in the chamber is inferred to have been continuous. The zonation resulted from up to 75% fractional crystallization of an alkali feldspar-olivine-clinopyroxene-titanomagnetite-apatite assemblage from the least-evolved trachytes, accompanied by minor assimilation of wall rocks, especially in the deeper parts of the chamber.