GENERATION OF ICELANDIC RHYOLITES - SILICIC LAVAS FROM THE TORFAJOKULL CENTRAL VOLCANO

The Torfajokull central volcano in south-central Iceland contains the largest volume of exposed silicic extrusives in Iceland (similar to 22 5 km(3)). Within SW-Torfajokull, postglacial mildly alkalic to peralka lic silicic lavas and lava domes (67-74 wt.% SiO2) have erupted from a family of fissures 1-2.5 km apart within or just outside a large cald era (12 x 18 km). The silicic lavas show a fissure-dependent variation in composition, and form five chemically distinct units. The lavas ar e of low crystallinity (0-7 vol.%) and contain phenocrysts in the foll owing 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 di sequilibrium with the host melt (glass). Xenoliths include sparse, dis aggregated, and partially fused leucocratic fragments as well as amphi bole-bearing rocks of broadly intermediate composition. The delta(18)O values of the silicic lavas are in the range 3.6-4.4, and these are l ower than the delta(18)O values of comagmatic, contemporaneous basalti c extrusives within SW-Torfajokull, implying that the former can not b e derived from the latter by simple fractional crystallization. FeTi-o xide geothermometry reveals temperatures as low as 750-800 degrees C. To explain the is fissure-dependent chemical variations, O-18 depletio ns, low FeTi-oxide temperatures and pervasive crystal-melt disequilibr ium, we propose the extraction and collection of small parcels of sili cic melt from originally heterogeneous basaltic crustal rock through h eterogeneous melting and wall rock collapse (solidification front inst ability, SFI). The original compositional heterogeneity of the source rock is due to (1) silicic segregations, in the form of pods and lense s characteristically formed in the upper parts of gabbroic intrusives, and (2) extreme isostatic subsidence of the earlier, less differentia ted lava of the Torfajokull central volcano. Ridge migration into olde r crustal terranes, coupled with establishment of concentrated volcani sm at central volcanoes like Torfajokull due to propagating regional f issure swarms, supplies the heat source for this overall process. Cont inued magmatism in these fissures promotes extensive prograde heating of older crust and the progressive vitality and rise of the central vo lcano magmatic system that leads to, respectively, SFI and subsidence melting. The ensuing silicic melts (with relict crystals) are extracte d, collected and extruded before reaching complete internal equilibriu m. Chemically, this appears as a two-stage process cf crystal fraction ation. In general, the accumulation of high-temperature basaltic magma s at shallow depths beneath the Icelandic rift zones and major central volcanoes, coupled with unique tectonic conditions, allows large-scal e reprocessing and recycling of the low-delta(18)O, hydrothermally alt ered Icelandic crust. The end result is a compositionally bimodal prot o-continental crust. (C) 1998 Elsevier Science B.V. All rights reserve d.