GENERATION OF COEVAL THOLEIITIC AND CALC-ALKALINE MAGMAS IN ICELAND: INSIGHTS FROM IN SITU ZIRCON ANALYSES

Króksfjörður central volcano has long been regarded as one of Iceland’s most enigmatic magmatic centers due to the presence of calc-alkaline (CA) silicic magmas—the only instance of such compositions known in Iceland, which is dominantly tholeiitic (TH). Previous workers suggested petrogenesis of these CA magmas resulted from partial melting of amphibolites in the mid- to deep crust late in the volcano’s lifetime, while silicic THs at Króksfjörður resulted from shallow-crustal processes. We present in situ zircon U-Pb ages, O and Hf isotopes, and trace elements and whole rock Pb, Nd, and Hf isotope analyses from TH and CA silicic rocks at Króksfjörður. Our data reveal coeval CA and TH magmatism lasting >1 Myr (~12.2 Ma to 10.5 Ma), indicating a long-lived and complex system of magma reservoirs. Trace element concentrations in Króksfjörður zircons define two distinct fields corresponding to zircons from TH and CA samples, with ~1/4 of analyses overlapping both fields, indicating potential mixing of CA and TH zircon-bearing magmas and/or crystallization of zircons from hybrid TH/CA magmas. All zircons have δ¹⁸O from +0.6±0.4 (2 SE) to +3.7±0.3 resulting from incorporation of hydrothermally altered material into their source magmas, with no systematic difference between TH and CA source. In situ Hf isotope compositions in TH and CA-sourced zircons range from ε_Hf = +10.0 ± 1.5 (2SE) to +15.1 ± 1.1, again with overlap between the two compositions. Whole rock Hf, Nd, and Pb, isotope compositions illustrate differences in TH and CA magmas: CA units have more MORB-like ε_Hf (~+14.3 ± 0.2 (2SE)) and ε_Nd (7.8 ± 0.2 (2SE) to 8.3), while plume-like THs have ε_Hf = +~13.2 ± 0.3 and ε_Nd = +~7.3 ± 0.2. ²⁰⁶Pb/²⁰⁴Pb between CA and TH samples differs by ~0.4 (~18.5 vs. ~18.9, respectively)—a variation almost twice as large as the Pb isotopic variation in other Icelandic volcanoes. Our data suggest a magmatic system in which CA magmas are sourced from partial melting of a MORB-like source, in agreement with previous studies, but their evolution is spatially and temporally intertwined with TH magmas sourced from early-stage rifting. Zircon trace element concentrations indicate magma mixing likely occurred, but mixing volumes were likely limited since whole rock isotopic variation between CA and TH magmas is so pronounced.