The Petrology of Thingmuli, a Tertiary Volcano in Eastern Iceland

Abstract

The Tertiary flood-basalt sequence of eastern Iceland is intermittently disturbed by central volcanic activity with the voluminous eruption of acid magma. Associated with one of these central volcanoes, described in this paper, is an intense swarm of acid and basic dykes, a set of acid cone-sheets, and extensive superimposed hydrothermal alteration. The lavas and intrusions which make up the volcano grade in composition from olivine-tholeiites, through olivine-free tholeiites, basaltic-andesites, and andesites (icelandites), to rhyolites. This series is unusually rich in iron, titanium, and manganese, and poor in magnesium; alumina and total alkalis also tend to be low in the basic members. Magnetite (sensu laid) plays a varied role in the order of crystallization, and it is only in the intermediate stages of this fractionated series that magnetite is available for crystal fractionation. The otherwise progressive enrich-ment of iron relative to magnesium throughout the successive liquids of the series is halted during an intermediate stage, as magnetite becomes an early-crystallizing phase.