Mineral stability in peralkaline silicic rocks: Information from trachytes of the Menengai volcano, Kenya

Abstract

Electron microprobe analyses are presented for phenocrysts and matrix glass in peralkaline, silica-oversaturated trachytes from the Menengai volcano, Kenya Rift Valley. The dominant phenocryst assemblage is alkali feldspar–hedenbergite–fayalite–titanomagnetite–apatite. Aenigmatite, amphibole and quartz occur rarely in more peralkaline rocks. QUILF calculations indicate that the trachytic magmas crystallised at temperatures of 854–870°C, at relatively low oxidation states (ΔFMQ −1.6 to −1.7) and silica activity (aSiO2 (Qtz) of 0.60–0.66). The new analyses are used, along with published data, to outline the distribution of the main phases in the compositional spectrum of peralkaline quartz trachytes and rhyolites. There is uncertainty about the nature, or even existence, of a low-temperature zone in the alkali feldspar primary phase region, the equivalent of the thermal valley in the haplogranite system. Quartz phenocrysts may appear early or late in the crystallisation sequence, even in rocks of similar bulk composition, its appearance perhaps being a function of the F content of the melts. Whereas hedenbergite and fayalite show fairly systematic compositional trends with increasing host rock peralkalinity, amphibole compositions are variable, for reasons not yet understood. Aenigmatite crystallisation is at least partly controlled by oxygen fugacity and silica activity. With rare exceptions, ilmenite and titanomagnetite are incompatible phases but the factors controlling their relative stabilities are not clear. It appears that peralkaline trachyte–rhyolite sequences evolve along many crystallisation paths, the paths perhaps being strongly influenced by pH2O, pF2, melt F/Cl ratios and perhaps total pressure.