DIVERSE VOLCANIC PRODUCTS OF INJECTION OF PLAGIOCLASE-ULTRAPHYRIC BASALT MAGMA INTO A ZONED SILICIC MAGMA CHAMBER IN THE NEOGENE OF ICELAND

A 7.8 Ma >3 km³ andesite to dacite lava, featuring an array of mixing and mingling textures, occurs in the Neogene plateau lava sequence in the mountains Laxardalsfjoll and Langadalsfjall in north­-central Iceland. The mixed lava has a unique marker of the degree of mixing: coarse (up to 3 cm), relatively equant plagioclase megacrysts. Domains within the lava vary from <1 to 20% plagioclase megacrysts, correlating with a range in bulk composition from andesite to dacite (58-69% wt.% SiO₂). This lava is locally overlain by a tuff that includes the same plagioclase as well as dacite to low­-silica rhyolite fiamme, and the tuff is locally overlain by a high-­silica rhyolite lava flow.

The megacrysts in the mixed lava are relatively homogeneous and calcic (An_81­-89), with narrow (<20 μm) low­-An rims. The megacrysts include distinctive concentric bands of melt inclusions. A basaltic lava with texturally and chemically identical plagioclase megacrysts (up to 35%) is exposed 9+ km south in Svinadalsfjall and Vatnsdalsfjall. The groundmass of the basaltic lava is evolved (~6.7 wt.% MgO), matching an aphyric lava that immediately overlies it. In both the mixed lava and the basalt, the low-­An rims are consistent with groundmass plagioclase compositions. The high-­An megacrysts are too calcic to have originated in either the mixed lava or the evolved basalt. As has been interpreted elsewhere, the megacrysts were likely picked up from an anorthositic crystal mush during transit of the evolved basaltic magma.

Variation diagrams show that the mixed lava occupies a triangular array with the ultraphyric basalt as one end member and a range of dacite to low­-silica rhyolite (including the fiamme in the tuff) as the other end member. When plotted with other silicic lavas in the local stratigraphy the trend of silicic compositions is continuous up to the high-­silica rhyolite that overlies the mixed lava.

The interpretation of the origin of this suite is that the dike system that fed the eruption of the ultraphyric basalt propagated laterally into a zoned silicic magma chamber, similar to the 1477 CE propagation of the Veidivotn fissure into the Torfajokull caldera. This triggered an eruption from the zone of mingling/mixing to form the mixed lava, followed by an eruption from the evolved upper portion of the magma chamber to form the upper high-­silica rhyolite.