Western Saudi Arabia hosts a number of young volcanic fields, known as “Harrats”. Harrats cover a significant proportion of western Saudi Arabia and are associated with significant volcanic hazards. However, the ultimate cause of volcanic activity remains unclear. Younger volcanism (<12 Ma) is clearly represented by the north-south-trending region known as the Makkah-Madinah-Nafud (MMN) line, which consists of three moderate sized volcanic fields: Harrat Rahat, Harrat Khaybar, and Harrat Ithnayn. Harrat Ithnayn is the northern-most and the least studied volcanic field of the MMN line, and it has been suggested that Ithnayn represents the youngest field produced by age progressive volcanism along the MMN line. Harrat Ithnayn is thus a critical piece in the puzzle in determining the causes of the volcanic activity in the MMN line region. This research focuses primarily on investigating the age and composition of the volcanic activity at Harrat Ithnayn and how it changes through time. I apply geochronological, geochemical, and petrological methods to understand the origin and tectonic controls on volcanism in this region.
I report new age determinations on 10 lava flows, one sample from the northern part of Harrat Khaybar and 9 samples from Harrat Ithnayn, by the ⁴⁰Ar-³⁹Ar laser step heating method. All ages are younger than 2 Ma and most of these lavas range in age between ~500 and 120 Ka. These ages constrain the timing and chemical variations of volcanic activity at Harrat Ithnayn. Unlike older Harrats Rahat and Khaybar, the volcanism at Harrat Ithnayn has also undergone less magmatic evolution, suggesting a lack of shallow crustal magma bodies. Similar to Harrat Hutaymah and other younger volcanic fields peripheral to the MMN line, olivine ± clinopyroxene dominated crystal fractionation at a range of upper mantle and crustal pressures, with some evidence of crustal assimilation. The new age constraints agree with the hypothesis of south-to-north volcanic progression of volcanism along the MMN line. In addition, active mantle upwelling, decompression melting and possible asthenospheric flow from the Afar mantle plume, appears to have been the source of this volcanic activity, produced from ~ 2-14% partial melting of a shallow garnet peridotite mantle source for magmas.