Þingmúli revisited: New zircon-based perspectives on rhyolite petrogenesis in Iceland

Abstract

Þingmúli (Thingmuli), an extinct volcano in eastern Iceland, has long been an archetype for fractional crystallization of tholeiitic basalt to produce rhyolite. However, a recent re-examination of Þingmúli suggests that some silicic magmas were produced under more oxidizing conditions from a combination of magma mixing and assimilation, thereby indicating two petrogenetic pathways. We present the first zircon-based analyses from Þingmúli to evaluate the extent to which the zircon record corroborates these recent findings. Zircon U-Pb age dates of ~ 8.5–10 Ma agree with published whole-rock and regional values. Zircon ε_Hf ranges from ~ + 13.6 to 16.3; zircon δ¹⁸O is largely restricted to ~ 3.1–3.9‰, with crystals from one sample averaging ~ 2‰. Trace elements reveal two distinct compositional groups: Group 1—common at Þingmúli but rare in the Iceland zircon record; and Group 2, which is representative of typical Icelandic zircon, but under-represented at Þingmúli. The rhyolite from which Group 1 zircon crystallize results from extreme fractional crystallization of incompatible-element enriched melt with variable, but probably small, amounts of unusually low-¹⁸O material. Group 2 zircon, with compositions distinct from Group 1 (notably, Eu/Eu*>0.1), reveal a different origin story—one counter to the traditionally held view of petrogenesis at Þingmúli. A combination of variable mantle input composition and differentiation dominated by AFC processes involving crustal input are required to generate the evolved melts from which Group 2 zircon crystallized. This dynamic petrogenesis, common in active Icelandic central volcanic systems, dominates rhyolite petrogenesis at Þingmúli.