COMPLICATED GEODYNAMIC EVOLUTION IN THE NORTHERN WESTFJORDS, ICELAND ELUCIDATED BY ICELAND’S OLDEST SILICIC ROCKS

The petrogenesis of Iceland’s oldest silicic magmas may provide insight into early Iceland’s crustal evolution. As a result of Mid-Atlantic Ridge (MAR)–Iceland plume coupling and northwestern drift of the ridge axis, the MAR around Iceland—exposed on land as a series of rift zones—relocates progressively east of the main MAR axis to remain coupled with the plume. The oldest rocks in the Westfjords erupted from the Northwest Rift Zone (NWRZ) (active lifetime ~24-15 Ma), but magmatism appears to have shifted from the NWRZ to the Snæfellsnes-Skagi Rift Zone (SSRZ) around 15 Ma. Hrafnfjörður and Árnes central volcanoes are strongly suspected to have erupted from the SSRZ, which our U-Pb zircon SHRIMP-RG geochronology data (ages range from ~12.8 to 14.2 Ma) supports. However, reconstruction of the locations of the active Hrafnfjörður and Árnes central volcanoes based on these data—assuming a half-spreading rate of 1 cm/yr, the average over Iceland’s existence—elucidates a discrepancy. A separation distance of ~75–87 km should result between Hrafnfjörður and the now-extinct SSRZ axis, which matches well with measured distances, but Árnes lies between ~44–51 km from the SSRZ axis—not the anticipated ~64-76 km as modeled. In addition, given the age difference between Hrafnfjörður (~13.6 –14.2 Ma) and Árnes (~12.8–13.4 Ma), an approximate longitudinal difference of 10-15 km between the volcanoes is modeled; the measured difference is ~30 km. Zircon oxygen isotope (Árnes avg. δ¹⁸O~3.3‰; Hrafnfj. avg. δ¹⁸O~3.6‰) and hafnium isotope (Árnes avg. ε_Hf=+12.7; Hrafnfj. avg. ε_Hf~+15.6) compositions also suggest fundamental differences in silicic melt petrogenesis between these centers. Several mechanisms to account for the distance and geochemical disparities between Hrafnfjörður and Árnes may be viable, including trapped crust under the Westfjords (Martin and Sigmarsson, 2011), a dynamic spreading rate, mantle heterogeneity under Iceland (many authors), and changes in influence of the plume component to the bedrock over time (e.g. Banik et al., 2018). We conclude that none of these mechanisms alone can wholly account for the observations; instead, a combination of multiple factors is more likely.