RHYOLITE VOLCANISM IN THE KRAFLA CENTRAL VOLCANO, NORTHEAST ICELAND

At the Krafla central volcano in north-east Iceland, two main phases o f rhyolite volcanism are identified. The earlier phase (last interglac ial) is related to the formation of a caldera, whereas the second phas e (last glacial) is related to the emplacement of a ring dike. Subsequ ently, only minor amounts of rhyolite have been erupted. The volcanic products of Krafla are volumetrically bimodal. Geochemically, there is a series of basaltic to basalto-andesitic rocks and a cluster of rhyo litic rocks. Rocks of intermediate to silicic composition (icelandites and dacites) show clear signs of mixing. The rhyolites are Fe-rich (t holeiitic), and aphyric to slightly porphyritic (plagioclase, augite, pigeonite, fayalitic olivine and magnetite). They are minimum melts on the quartz-plagioclase cotectic plane in the granite system (Qz-Or-Ab -An). The rhyolites at Krafla were produced by near-solidus, rather th an near-liquidus fractionation. They are interpreted as silicic minimu m melts of hydrothermally altered crust, mainly of basaltic compositio n. They were primarily generated on the peripheries of an active basal tic magma chamber or intrusive domain, where sufficient volumes of cru st were subjected to temperatures favorable for rhyolite genesis (850- 950 degrees C). The silicic melts were extracted crystal-free from the ir source in response to crustal deformation.