Abstract

The syenite-trachyte problem arises because liquids of this composition are perched on the thermal barrier between granitic and phonolitic minima in petrogeny’s residua system, SiO2–NaAlSiO4–KAlSiO4 (quartz-nepheline-kalsilite, Q-Ne-Ks), and may be stated as: Why do trachytic liquids seemingly mark an endpoint in the evolution of many alkaline suites if there is no impediment to their fractionating to either a rhyolitic or a phonolitic derivative? The outstanding feature of the crystal assemblages precipitated by trachytic melts is that they are essentially monomineralic in alkali feldspar at the high crystallinities that favor formation of cumulate beds through hindered settling and compaction, and are therefore syenitic (sensu stricto). Hence, cumulates formed from a wide range of silica over- and undersaturated magmas have compositions closer to the feldspar join in Q-Ne-Ks than the starting composition. Subsequent melting of fusible cumulate due to heating by recharging mafic magma generates crystal-poor trachytic magmas of accumulative composition. In dynamic felsic alkaline systems subject to mafic recharge heating, these processes act as a “trachyte attractor,” shepherding compositions close to the feldspar join.

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