R. Macdonald et al., STRONG COMPOSITIONAL ZONATION IN PERALKALINE MAGMA - MENENGAI, KENYA RIFT-VALLEY, Journal of volcanology and geothermal research, 60(3-4), 1994, pp. 301-325
The second caldera-forming ash-flow tuff erupted from the Menengai vol
cano is up to 80 m thick and represents ca. 30 km3 of peralkaline trac
hytic magma. Activity started with the emplacement of air-fall tuffs,
possibly from a central vent, followed by welded ash-flow tuffs and th
en by unwelded flow material, the proximal facies of which is lag brec
cia. At least part of the ash-flow deposits was emplaced through the r
ing 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 throu
gh comenditic trachyte to more primitive (Ba-rich) trachyte. The maxim
um 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), upward
s. Alkali feldspar phenocrysts generally became more sodic, and olivin
e and clinopyroxene phenocrysts more Fe-rich, towards the roof. Variab
le matrix glass compositions and unusually sodic cores of feldspar phe
nocrysts in some rocks are thought to result from mixing of liquids fr
om more than one compositional layer during magma withdrawal. The part
itioning of Ba, Rb, Sr and the REE between alkali feldspar and liquid
was apparently dependent on melt peralkalinity. As the eruption procee
ded, a range of compositions was progressively tapped. Compositional z
onation in the chamber is inferred to have been continuous. The zonati
on resulted from up to 75% fractional crystallization of an alkali spa
r-olivine-clinopyroxene-titanomagnetite-apatite assemblage from the le
ast-evolved trachytes, accompanied by minor assimilation of wall rocks
, especially in the deeper parts of the chamber.