CRYSTAL ACCUMULATION AND MELTING IN FELSIC ALKALINE MAGMAS, AND THE SYENITE-TRACHYTE PROBLEM (Invited Presentation)

I review and discuss potentially fruitful petrological predictions of the "mush model" [1-3] when applied to felsic alkaline magmas (phonolites, trachytes and their plutonic equivalents). The essential feature of the model is that cumulate formation is favored when a body of cooling, convecting magma with entrained crystals reaches a critical solids content at which crystals start touching each other, impeding convection. A supernatant crystal-poor evolved melt lens then segregates above a thick underlying cumulate mush by hindering settling and compaction of crystals. Subsequent melting of fusible cumulate due to heating by recharging mafic magma generates crystal-poor magmas of accumulative composition. Effects peculiar to alkaline systems include density separation of feldspars from sodalite-group minerals and leucite, finding expression in the flotation cumulate naujaites of Ilimaussaq and in leucite-phyric potassic phonolites. The syenite-trachyte problem [4] arises because liquids of this composition are perched on the thermal barrier between granitic and phonolitic minima in the system Q - ne - ks , and may be stated as “Why are trachytes and syenites common among alkaline suites if there is no impediment to liquids of these compositions 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. Although experiments indicate that small amounts (<10%) of mafic phases are the first to crystallize from trachytic and phonolitic melts, high crystallinities are only achieved after alkali feldspar joins the assemblage. Hence, cumulates formed by hindered settling and compaction are syenitic, with a composition closer to the feldspar join in Q - ne - ks than the starting composition. Melting of syenitic cumulates produces trachytic magma. In felsic alkaline systems subject to mafic recharge, these processes act as a ‘trachyte attractor’, shepherding compositions close to the feldspar join. References: [1] Bachmann & Bergantz (2004) J. Petrol. 45, 1565-1582; [2] Hildreth (2004) JVGR 136, 169-198; [3] Wolff et al. (2015) Lithos 236–7, 275–286; [4] Morse (1969) GSA Mem 112.