TIMING AND COMPOSITION OF VOLCANISM AT HARRAT ITHNAYN, WESTERN SAUDI ARABIA

Western Saudi Arabia hosts a number of young volcanic fields, although the ultimate cause of volcanic activity remains unclear. Younger volcanism (<12 Ma) is clearly represented by the north-south-trending region knows as the Makkah-Madinah-Nafud (MMN) line, which consists of three moderate sized volcanic fields: Harrat Rahat, Harrat Khaybar, and Harrat Ithnayn. The volcanic field of Harrat Ithnayn is the northern-most and the least studied volcanic field in 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. We apply geochronological, geochemical, and petrological methods to understand the origin and tectonic controls on volcanism in this region.

At about 2 Ma, Harrat Ithnayn started the most recent episode of the northern progression of volcanism along the MMN line. We report new age determinations on 10 lava flows by the ⁴⁰Ar-³⁹Ar laser step heating method. All ages are younger than 2 Ma and most of these lavas range between ~500 and 120 Ka. They provide good constraints on the timing of the chemical variation of volcanic activity at Harrat Ithnayn. Unlike Harrats Rahat and Khaybar, the volcanism at Harrat Ithnayn has also undergone less magmatic differentiation in the latest volcanic activity of its lifespan. 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 little evidence of crustal contamination. Age constraints agree with the hypothesis of south-to-north volcanic progression on 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 ~ 2-12% partial melting of a shallow garnet peridotite mantle source for magmas.