ALBERT

Description: 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 〈 15 mod.%). Almost all eruption centers are located on four NNE - SSW striking fissures, two of them passing the top regions of the volcanoes and the other two their eastern base areas. Nine facies areas, which developed in different environments (subaqueous to a maximum water depth of ca. 350m, transition subaqueous - subaerial, subaerial, subglacial), are distinguished by structure, texture and composition of the deposits in the area of both volcanoes. The strong alteration of one hyaloclastite type ("H3-type") was probably mainly caused by secondary heating by lava flows or intrusions. Some of these rocks are interpreted as warm subaqueous pyroclastic flow deposits. A new model for the evolution of Herdubreid and Herdubreidartogl comprises four eruption periods and 11 eruption epochs and eruptions: 1) The shield volcano of Herdubreidartögl was built up by subaerial effusive eruptions during the last interglacial. 2) During the waning period of the last glaciation hyaloclastites and lavas were produced initially at the base of both volcanoes (below 800-900m a.s.1.) in a sublacustrine environment. Subsequent hydroclastic and effusive eruptions took place close to and above water level. 3) Pillow lavas and hyaloclastites were erupted about 900 a.s.l. in a subglacial environment caused by a thickening of the ice sheet ea. 12-1 Ska ago as a result of climatic fluctuations. Plateau basalts were produced by pyroclastic and effusive activity after volcano build-up above the ice sheet. 4) During a postglacial period subaerial lava flows, agglutinates and fallout deposits were produced at Herdubreidartögl. Subglacial deposits can be clearly distinguished from sublacustrine deposits by their structure, texture and alteration. Apart from hydroclastic processes, pyroclastic processes play an important role in eruptions at less than 200m of water depth in both environments. Besides explosive fragmentation of the magma a considerable "secondary" fragmentation of the hyaloclastites take place during transport processes. The four eruption periods differ also in chemical composition of the rocks and magmatic evolution. During the first period mainly primitive olivine tholeiites (Mg# 64-75) were erupted directly from a magma reservoir. The olivine tholeiites (Mg# 56-67) dominating in the 2nd period probably result from magma mixing, whereas the tholeiites of the 3rd period (Mg# 46-56) are explained by fractional crystallization. The rocks of the 4th period (Mg# 61-66) are similar to those of the 1st and 2nd period. The eruptions of Herdubreid and Herdubreidartogl were fed from at least 2, but probably 3 magma reservoirs. The main and trace element compositions of the rocks of these volcanoes exclude a continuous, comagmatic evolution of the magmas from one magma source. The hydrostatic load pressure at time of eruption can be roughly estimated on the basis of the sulfur and H2O content of matrix glasses. The sulfur degassing of matrix glasses is not more than 40% in lavas and hyaloclastites erupted at 〉 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.

Description: The Herdubreidartögl are a hyaloclastite ridge siruated in the northern branch of the Icelandic rift zone and extending for about ten kilometres from NNE to SSW south of the Herdubreid table mountain. In contrast to most of Icelandic tuff ridges, the Herdubreidartögl have an extremely complex structure and in parts they show a rock sequence typical of table mountains. Parallel to the longitudinal axis of the ridge in its top region a number of craters are aligned over a distance of about five kilometres. These craters most probably formed the main centre of eruption. With respect to the petrographic features. 12 lithologic series were distinguished in the region of the Herdubreidartögl. They can roughly be classified as "basalt lavas" (among others flood basalts, olivine-dolerites, pillow lavas), "dikes/subvolcanic intrusiva" and "pyroclastites" (especially hyaloclastites ). These rocks are, with rare exceptions, fresh or only very little altered. Their chemical composition is rather uniform and similar to that of quite primitive olivine-tholeiites. In general, the series cannot be seperated petrochemically, only a few samples show a slight tendency towards a further evolution of the petrochemistry. A large part of the hyaloclastites in the area of the Herdubreidartögl was redeposited by epiclastic processes, but there are also sediments which indicate primary pyroclastic processes such as base surges or pyroclastic flows. Basing on the petrologic, petrochemical and sedimentologic investigations a pattern is postulated subdividing the evolution of the Herdubreidartögl into four cycles of eruption. During the first and earliest cycle, the lower southern part of the ridge was developed in a subaerial environment. The second cycle occurred under subaquatic conditions and formed the basis of the northern part of HerdubreidartögL The major portion of the ridge resulted from the third and the fourth cycle the eruptions of which alternatingly took place in a subaquatic environment (mostly shallow water) and under subaerial conditions.