Generation of Icelandic rhyolites: silicic lavas from the Torfajökull central volcano

Abstract

The Torfajökull central volcano in south-central Iceland contains the largest volume of exposed silicic extrusives in Iceland (∼225 km^3). Within SW-Torfajökull, postglacial mildly alkalic to peralkalic silicic lavas and lava domes (67–74 wt.% SiO_2) have erupted from a family of fissures 1–2.5 km apart within or just outside a large caldera (12×18 km). The silicic lavas show a fissure-dependent variation in composition, and form five chemically distinct units. The lavas are of low crystallinity (0–7 vol.%) and contain phenocrysts in the following order of decreasing abundance: plagioclase (An_(10-40)), Na-rich anorthoclase (<Or_(23)), clinopyroxene (Fs_(37-20)), FeTi oxides (Usp_(32-60); Ilm_(93-88)), hornblende (edenitic–ferroedenitic) and olivine (Fo_(22-37)), with apatite, pyrrhotite and zircon as accessory phases. The phenocryst assemblage (0.2–4.0 mm) consistently exhibits pervasive disequilibrium with the host melt (glass). Xenoliths include sparse, disaggregated, and partially fused leucocratic fragments as well as amphibole-bearing rocks of broadly intermediate composition. The δ^(18)O values of the silicic lavas are in the range 3.6–4.4, and these are lower than the δ^(18)O values of comagmatic, contemporaneous basaltic extrusives within SW-Torfajökull, implying that the former can not be derived from the latter by simple fractional crystallization. FeTi-oxide geothermometry reveals temperatures as low as 750–800°C. To explain the fissure-dependent chemical variations, ^(18)O depletions, low FeTi-oxide temperatures and pervasive crystal-melt disequilibrium, we propose the extraction and collection of small parcels of silicic melt from originally heterogeneous basaltic crustal rock through heterogeneous melting and wall rock collapse (solidification front instability, SFI). The original compositional heterogeneity of the source rock is due to (1) silicic segregations, in the form of pods and lenses characteristically formed in the upper parts of gabbroic intrusives, and (2) extreme isostatic subsidence of the earlier, less differentiated lavas of the Torfajökull central volcano. Ridge migration into older crustal terranes, coupled with establishment of concentrated volcanism at central volcanoes like Torfajökull due to propagating regional fissure swarms, supplies the heat source for this overall process. Continued magmatism in these fissures promotes extensive prograde heating of older crust and the progressive vitality and rise of the central volcano magmatic system that leads to, respectively, SFI and subsidence melting. The ensuing silicic melts (with relict crystals) are extracted, collected and extruded before reaching complete internal equilibrium. Chemically, this appears as a two-stage process of crystal fractionation. In general, the accumulation of high-temperature basaltic magmas at shallow depths beneath the Icelandic rift zones and major central volcanoes, coupled with unique tectonic conditions, allows large-scale reprocessing and recycling of the low-δ^(18)O, hydrothermally altered Icelandic crust. The end result is a compositionally bimodal proto-continental crust.