Geology and petrography of the Dyrfjöll central volcano, Eastern Iceland

The Tertiary Dyrfjöll Central Volcano had before erosion a volume of at least 150 km3 and was composed of approximately 75 - 80 % of basaltic rocks, up to 20 % of silicic rocks, and minor amounts of intermediate rocks. In shape it resembled an elliptical shield volcano with the long axis striking N 20° E. No dyke swarm, as typically is found associated with volcanic centres in Iceland, is seen at the Dyrfjöll volcano. This may be due to a lack of erosion to deeper levels of the volcano, where the dykes are typically most numerous. A relatively low degree of hydrothermal alteration also most likely indicates that the present level of erosion has not reached the core of the volcano. The history of the Dyrfjöll volcano can be divided into five stages. In the first stage the foundation of the volcano was built from basaltic lava flows. The second stage was silicic, forming extrusives and shallow intrusives in the basaltic foundation. During the third stage the growth of the volcano continued, mostly from basaltic lava flows, but a few intermediate and silicic lava flows also occur. A Plinian type eruption and the subsequent formation of a caldera on top of the volcano marked the fourth stage. The history of the volcano closed with its burial by basaltic lava flows from neighbouring volcanic systems. In the vicinity of the Dyrfjöll Central Volcano four other central volcanoes occur, which together may have comprised a large volcanic centre. Explosive volcanic activity of a degree unusual for Iceland is indicated by large amounts of ignimbrite. Petrochemical ly, the rocks of the volcano belong to the tholeiitic series, ranging in composition from basalts to rhyolites. The basaltic rocks, however, show a slight transitional trend, indicated by an alkali content just above the Hawaiian Division Line and higher contents of titanium and iron compared to typical rocks of the tholeiitic series. It is therefore concluded, with reference to presently active volcanoes in Iceland, that the Dyrfjöll Central Volcano was situated close to the margin of the riftzone of its day, or may even have been situated just outside it.