Magma mixing and hybridization processes at the alkalic, silicic, Torfajökull central volcano triggered by tholeiitic Veidivötn fissuring, south Iceland

Abstract

Abstract Around 19°W64°N, in the Eastern Volcanic Zone of south Iceland, the southwestern extremity of the tholeiitic Veidivotn fissure swarm abuts the northern periphery of the mildly alkaline silicic central volcano Torfajokull. Effusive mixed-magma eruptions in this area have been initiated by crustal rifting associated with lateral injection of tholeiitic magma into the rhyolitic Torfajokull chamber. Mixed rocks, in which cm- to dm-sized mafic inclusions constitute ∼ 10% of the rhyolite lava are distinguished from hybrid rocks which are thorough mixtures having an homogeneous fabric. Mapping of the Domadalshraun lava (0.05–0.1 km3) reveals the early eruption of hybrid spatter and lava followed by less thoroughly blended mixed lava. Geochemical study discloses the participation of three end-members. Plagioclase-phyric tholeiite containing ∼ 10–20% pl + cpx + o1 precipitated at ∼ 1140–1160°C is typical of the Veidivotn component. The Torfajokull rhyolite contains ∼ 10% crystals, dominantly anorthoclase and oligoclase, and is found to be compositionally zoned. Early-late trends include SiO2: 71.6−70.1%, A.I.: 1.07−0.86, Sr: 61–86 ppm and result from feldspar fractionation. The third end-member is a transitional alkali basaltic andesite ( ≡ hawaiite ) belonging to the mafic magma suite associated with Torfajokull. This intermediate magma resided at depth within the stratified Torfajokull chamber and contains xenocrystic feldspars which have settled out of the differentiating rhyolite. The calculated positions in tholeiite-rhyolite-basaltic andesite composition space of eight hybrid rocks show that hybridization was not a random process. No rhyolite/transitional alkali hybrids are present. The only two tholeiite/transitional alkali hybrids are both 0.45/0.55 blends of these two end-members. No rhyolite-bearing hybrid contains more than 20% transitional alkali magma. It is proposed that rhyolite/tholeiite hybrids are most likely to be generated where a shallow tholeiitic fissure has been laterally intruded into and above the roof of the rhyolitic chamber. Near the top of the fissure, tholeiite containing excess water may have been sufficiently vesicular for its bulk density to equal that of the rhyolitic magma. Calculations identify this critical depth as 0.75–1 km-coincident with calculated quenching pressures of small vesicular tholeiitic clots in the mixed lavas (assuming 1 wt.% water). Dense tholeiite at deeper levels in the dyke collapses into the chamber and is replaced by buoyantly rising rhyolite. Hybridization ensues where the rhyolite and vesicular tholeiite come together in the fissure. These and other relative-density controlled processes account for the selective nature of the hybridization process and the order in which hybrid and mixed magmas were erupted. The late 15th century Laugahraun and Sudurnamshraun flows at Landmannalaugar reveal participation of only tholeiitic and rhyolitic end-members. Hybrids are poorly represented and it is hypothesized that this is due to the deep ( ≳ 1 km ) intrusion of undersaturated tholeiite, which leads to the generation of mixed rather than hybrid magmas. These and other tholeiite/rhyolite mixed-magma eruptions in the area were triggered by lateral flow of tholeiite from the Veidivotn system, initiated by the overfilling of the Veidivotn magma chamber, 40 km to the NE, in the style recognized at Krafla and elsewhere in Iceland. Future concern over any renewed activity in Veidivotn should not, therefore, prohibit consideration of rhyolitic or mixed-magma eruptions being induced in the Torfajokull region.