Improved Constraints on the Eruptive History of Northern Harrat Rahat Volcanic Field, Kingdom of Saudi Arabia

Abstract:

Harrat Rahat is a large (~20,000 km²) alkalic volcanic field located in central western Saudi Arabia. A variety of eruptive products ranging from alkali basalt to trachyte have erupted at Harrat Rahat over the past ~10 m.y., with the most recent eruptions occurring at 641 CE (uncertain) and 1256 CE in the northern part of the volcanic field. Despite the field’s young age and its close proximity to two major city centers, the eruptive history of Harrat Rahat remains poorly constrained. Previous researchers grouped the volcanic strata of northern Harrat Rahat into seven subunits based on limited K-Ar and ⁴⁰Ar/³⁹Ar dating, and on the degree of erosion displayed by the eruptive products. The youngest eruptive products (subunits Qm7 – Qm4) are thought to be ≤600 ka, whereas the older lavas (Qm3 – Qm1) are thought to be >600 ka. However, due to the sparse geochronologic control on the ages of the eruptive units, it remains unclear if the currently defined subunits accurately reflect the age distribution of lavas in northern Harrat Rahat. Additionally, the temporal relation between basaltic magmatism and the more evolved eruptive products has yet to be examined. To better constrain the eruptive history of Harrat Rahat we measured >50 new ⁴⁰Ar/³⁹Ar eruption ages for Qm1 through Qm5 lavas in northern Harrat Rahat. These new ⁴⁰Ar/³⁹Ar ages suggest that the majority of volcanism in the region occurred ≤400 ka and is significantly younger than previously thought, indicating that the magmatic system at Harrat Rahat has been more active over the past 400 kyr then previously recognized. Additionally, these new age data suggest that nearly all trachytic magmatism occurred <125 ka and was preceded by a pulse of more mafic magmatism. It is likely the magmatic system at Harrat Rahat reached an evolved state late in the history of the volcanic field due to increased and/or prolonged input of basaltic magmas into the crust.