The volcanic history and geochemical evolution of the Hveragerði Region, S. W. Iceland.

Abstract

The Hveragerði Region is situated at the Hengill Triple junction, SW Iceland, where there are three volcanic systems. The crust in the area is constructed from both fissure (elongate) and lava shield (conical) eruptive units. Hengill is the presently active spreading zone with the Hengill Central Volcano, whereas the Hveragerði region is inactive with the extinct Grensdalur Central Volcano. Recent geophysical research indicates the presence of high and low density volumes within the upper 5 km of the crust in this area. The location of the density anomalies coincides with surface geological features, such as Recent lava shields, and the extinct and active central volcanoes. A geological map of the Hveragerði Volcanic System has been constructed, and approximately 450 basaltic samples have been examined petrographically and analysed for whole- rock, volcanic glass and mineral chemical data from this region. Observations from these data, coupled with the geological and geophysical observations, suggest that the lava shields are fed straight from the base of the crust, whereas fissure eruptions originate from shallow crustal reservoirs The character of the crustal reservoir has been highly variable in the past 1 Ma, and has varied from a melt-dominated reservoir, to a crystal mush-dominated one. Each lava shield is compositionally distinct and is thought to preserve the mantle-melting signature. The compositional variation amongst the lava shields suggests that instantaneous melts are able to segregate from the mantle without complete mixing with accumulated melts from the entire length of the melting column. The depleted instantaneous melts from the crest and top of the melting column will either form picritic lava shields or they may interact with more fractionated crustal reservoirs and undergo quench crystallisation of megacrystic plagioclase (An 80-90). These crystals with associated pyroxene and olivine become flototion cumulates. There are episodes within the stratigraphy where off-axis lava shield and highly megacrystic fissure eruptions dominate, and such periods may represent periods of low magma supply. On the submerged mid-ocean ridges, linear and conical features are also observed, and these may be analogous to the fissure and lava shields, respectively. The basalt types reported here from the Hveragerdi region have also been reported off-shore, and they may therefore represent basalts derived from similar magmatic processes in a similar magmatic plumbing system. However, an initial observation of the relationship of 12 dredged basalts from 63˚ 10'N on the Reykjanes Ridge suggests that this is not the case at this locality.