Petrogenetic Evolution of Silicic Magmas in a Basaltic Volcanic Field: Harrat Khaybar, Saudi Arabia

Abstract

Distributed volcanic fields are the most common type of volcanism on Earth. While typically characterized by mafic eruptions, some fields such as Harrat Khaybar, in western Saudi Arabia also host explosive silicic eruptions. Determining the timing and origin of silicic volcanism within basaltic provinces is crucial for accurate volcanic hazard assessments. This dissertation investigates the petrogenetic evolution of silicic magmas at Harrat Khaybar, which is one of the youngest and most compositionally diverse volcanic fields in the Arabian Peninsula (1.7 Ma to present). Specifically, it tests two key hypotheses: (1) the youngest two eruptive phases are younger than previously estimated by the 40Ar/39Ar incremental heating technique (≤300 ka) and (2) the silicic magmas are primarily derived by assimilation-fractional crystallization (AFC) of basaltic melts. The first study tests the first hypothesis using zircon-double dating (238U–230Th disequilibrium/U-Pb and (U–Th)/He dating), cosmogenic 3He, and cosmogenic 36Cl. Recently published 40Ar/39Ar incremental heating ages of groundmass were also recalculated using isochron intercept (non-atmospheric) 40Ar/36Ar for the trapped Ar component. Our zircon age data reveal that Harrat Khaybar’s bimodality began ca. 968 ka, significantly earlier than previously recognized. Some evolved centers, such as Jabal Abyad and Jabal Bayda, have remained synchronously active for over 200 kyr, suggesting a shared magmatic plumbing system. Cosmogenic surface-exposure ages of the youngest basaltic lavas show that some of the recent basaltic centers are long-lived and last erupted ca. 760 years ago at Jabal Qidr. Recalculations of published 40Ar/39Ar ages show excellent agreement with our geochronological data. Collectively, the new ages show a strong temporal correlation of volcanic activity with the neighboring volcanic field of northern Harrat Rahat, suggesting a possible regional connection or shared geodynamic mechanism influencing both volcanic fields. The second study uses petrography, geochemistry, and Sr–Nd–Pb isotopes to model crustal processes and silicic magma evolution beneath Harrat Khaybar. Results support a model in which intermediate-silicic magmas formed by >90% fractional crystallization of primitive basalt, with minimal crustal assimilation (r < 0.05), primarily involving biotite from Pan-African basement rocks. Isotopic variation among evolved volcanoes suggests a multi-chamber system rather than a single zoned reservoir. Notably, younger silicic magmas exhibit stronger crustal assimilation signatures than older ones, indicating progressive thermal maturation of the upper crust beneath Harrat Khaybar. Together, these studies provide robust constraints on the timing and magmatic evolution of evolved volcanism at Harrat Khaybar and shed some light on the mechanisms by which evolved magmas might form within basalt-dominated volcanic systems. By linking magma origin to long-lived crustal processes and regional volcanic timing, this work advances our understanding of the timescales of magmatic differentiation in distributed volcanic fields. These findings not only refine hazard models for western Arabia but also offer a transferable framework for assessing silicic eruption potential in basalt-dominated provinces worldwide.