Silicic volcanism at Ljósufjöll, Iceland: insights into evolution and eruptive history from Ar-Ar dating

Ljósufjöll volcano is the largest outcrop of silicic volcanic material in the volcanic Snaefellsnes flank (non-rifting) zone of Iceland. The silicic eruptives range from trachytes to alkaline and peralkaline (comenditic) rhyolites and show evidence for eruption in both subaerial and subglacial environments. Thirteen silicic eruptive units have been identified and mapped by a combination of field observations and geochemical correlation. The trachytes probably formed by fractional crystallisation of a basaltic magma to form an alkali feldspar-rich trachytic mush, with continued fractionation of the interstitial melt producing low Ba and Sr rhyolite magma. Ar-Ar dating of feldspars and matrix (glass or holocrystalline groundmass) has been carried out on twelve of the units. Many units have a complex Ar-isotopic system, with many samples showing evidence for inherited ⁴⁰Ar in the form of feldspar xenocrysts. The deduced eruption ages span an age range from < 129ka to > 650ka. The eruptive history of Ljósufjöll probably extends further back than this but the products of previous eruptions have since been removed by erosion or buried. A syenite xenolith was dated at 1.1Ma, indicating that silicic, alkaline magmatism has been occurring at Ljósufjöll for over one million years. Eruptions of silicic material at Ljósufjöll appear to be more common during times of rapid climate change and fluctuating ice volume.