Petrogenesis of isolated high-silica rhyolites in Iceland

High-silica rhyolites (HSRs) are rhyolites containing > 75 wt% SiO 2 on an anhydrous basis. In this contribution we have studied four Icelandic isolated occurrences away from central volcanoes (i.e., Icelandic volcanoes with a well-developed volcanic plumbing system): three in active rift zones (Mælifell, Ketilhyrnur and Prestahnúkur) and one in a flank/off-rift zone (Sultarfell). With no established central volcano, in which magmas can evolve by fractional crystallisation in its plumbing system, we propose that these high-silica volcanic rocks are the product of partial melting of tonalitic Icelandic crust (i.e., plagiogranites). We have modelled their petrogenesis using alphaMELTS (a command line frontend of rhyolite-MELTS), first involving fractional crystallisation from a mantle-derived basaltic melt to generate the plagiogranite, and then partial melting of plagiogranites at shallow depths (~ 4 km), compatible with a pressure range constrained by the haplogranite system minima. Rayleigh and batch melting models are consistent with this method, fairly reproducing some of the incompatible trace elements compositions found in the HSR occurrences (e.g., Y and Rb ~ 100 ppm), but failing to reproduce others (e.g., Zr < 400 ppm and Nb < 60). This inconsistency however supports the proposed partial melting model, in which zircon and ilmenite, found in plagiogranites, remain in a refractory restite. A partial melting model also supports unusual assemblages found in Prestahnúkur samples, comprising allanite, zircon, fayalite, hedenbergite, and ilmenite, along with an intermediate plagioclase and quartz. The discrepancy between the mineral chemistry of these phases and the expected compositions according to the modelling suggest that some of this mineralogy is likely to be xenocrystic, with mineral phases such as zircon being derived directly from underlying plagiogranites.