Magmatic-tectonic control on the generation of silicic magmas in Iceland: Constraints from Hafnarfjall-Skarðsheiði volcano

Hafnarfjall-Skarðsheiði (H-S) central volcano, located at the edge of the Western rift zone in Iceland, provides a snapshot into silicic magma generation that occurred soon after establishment of a rift. We present in situ zircon U-Pb ages and oxygen and hafnium isotope compositions, complemented by whole-rock major and trace element and Pb, Nd, and Hf whole rock isotope data, from the dominant silicic units erupted throughout H-S's eruptive history. Zircon U-Pb ages (ca. 5.4 to 3.9 Ma) and field relationships indicate silicic magmatism was episodic. However, relatively low (for Iceland) whole rock ε_Hf (+11.9 to 13.3) and ε_Nd (+7.2 to 7.6), in addition to Pb isotope data from basalt and rhyolite units indicate that the same mantle-derived source is dominantly responsible for the geochemical characteristics observed in both magma types, which are more similar to those from magmas from a propagating rift than an established one. This observation is consistent with a role for fractional crystallization of mantle melts in addition to contributions of partially melted altered crust to explain the low δ¹⁸O_zrc values (1.5 to 4.6‰) observed. This study highlights the importance of the evolutionary state of the rift, crustal history, and mantle heterogeneity all as contributing factors to the isotopic composition of silicic Icelandic magmas. We invoke a petrogenetic model where the timescales of rift drift explain the long-lived, episodic silicic magmatism produced during rift propagation at Hafnarfjall-Skarðsheiði volcano.