Struktur und Entstehung subglazialer/sublakustrischer Vulkane am Beispiel des Vulkankomplexes Herðubreid/Herðubreiðartögl in Island

The volcanoes Herdubreid (ea. 20km2, 1682m a.s.l.) and Herdubreidartögl (ea. 30km2, 1070m a.s.l.) in the northern branch of the Icelandic rift zone consist mainly of sheet and pillow lavas and 8 types of hyaloclastite, which were distinguished on the basis of structural and textural criteria. The rocks are picritic olivine tholeiites, olivine tholeiites and tholeiites (Mg# 46-75) which have between 16% of normative olivine and 2% of normative quartz. Plagioclase (An79 - An90) and olivine (Fo79 - Fo89 ) are the dominant phenocrysts. Oinopyroxene, spinell and Fe/Ti-oxides occur in minor amounts. Herdubreid and the largest part of Herdubreidartögl are tablemountains. The southern base area of Herdubreidartögl is built of relics of a shield volcano and is separated from the northern part by an E-W striking fault. The tablemountain succession of pillow lavas, hyaloclastites and sheet lavas ("plateau basalts") is developed twice on top of each other at Herdubreid. The upper one (above ca. 900m a.s.1.) differs in texture and structure of the hyaloclastites ("H2-type") and the high plagioclase content (as a rule > 20 mod.%) from the lower succession (plagioclase as a rule 200m water depth, and at least 50% in glasses from < 200m water depth. The H2O contents in matrix glasses follow approximately the pressure dependent saturation line of H2O in basaltic melts