ISOTOPIC INSIGHTS ON GENERATION OF SILICIC MAGMAS IN ICELAND: CONSTRAINTS ON MAGMATIC-TECTONIC CONTROL AT HAFNARFJALL-SKARÐSHEIÐI VOLCANO

On Iceland, an active mid-ocean ridge and a major mantle plume coincide to produce an island plateau where silicic (SiO₂ ≥ 65 wt%) rocks are abundant (~10-15% of exposed rocks) compared to typical oceanic crust. Current models for silicic magma petrogenesis in Iceland invoke partial melting of hydrothermally altered basalts, or fractional crystallization of primary basaltic magma, or a combination of both processes, acting either independently or in unison. Studies directed at understanding petrogenesis in Iceland provide constraints on magmatic processes in Iceland, but they often do not consider the specific tectonic history and environment in which each volcano’s silicic magmas are produced. The 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 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 e_Hf (+11.9 to 13.3) and e_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 magmas from a propagating rift rather than an established one. This observation is consistent with silicic magma generation dominantly resulting from fractional crystallization of mantle melts, with lesser contribution from partially melted altered crust to account for 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 source 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.