ḨARRAT RAHAT REVISITED – NEW RESULTS ON AN ARABIA PLATE VOLCANIC FIELD PREVIOUSLY STUDIED BY VIC CAMP AND JOHN ROOBOL

Cenozoic mafic alkalic volcanic fields, each known in Arabic as a harrat, scatter 3000 km across the Arabia plate from southern Yemen to southeast Turkey. In 1987–1992 Vic Camp and John Roobol published geologic maps of major Arabian harrats and reports on eruptive histories, tectonics, and petrogenesis. Ḩarrat Rahat, one of the largest, erupted close to Al Madīnah in 1256 C.E., the only well-documented historical eruption of the province. Numerous other youthful looking but pre-historic lava flows near Al Madīnah lack capping Neolithic stone structures that are widespread regionally. Vic and John (& Coleman et al., 1983) interpreted such lavas as having erupted after the Arabian Neolithic pluvial period but before written records. If so, eruptive frequency near Al Madīnah would be high with an accordingly severe volcanic threat. Consequently, two recent efforts reexamined northern Ḩarrat Rahat: The King Abdullaziz Univ.–Univ. of Auckland Volcanic Risks in Saudi Arabia project and a collaboration between the Saudi Geological Survey and the USGS. Major results of the latter (https://pubs.usgs.gov/publication/pp1862) include that the young-appearing lava flows near Al Madīnah erupted at 20–60 ka, not in the last 6 kyr. Possibly, Neolithic people avoided the rugged young lavas when older flows, smoothed by erosion, were nearby. A cluster of uneroded scoria cones in Al Madīnah’s western suburb erupted at 13 ka, not in 641 C.E. as previously conjectured. That historically noted 641 C.E. eruption was probably in Ḩarrat Khaybar 160 km away. Ḩarrat Rahat’s late Pleistocene eruptive activity was pulsed with inter-eruption repose averaging 4 kyr over the last 180 kyr. Seismic and magnetotelluric surveys detect no crustal magma reservoirs, but a gravity survey shows the harrat overlies a shallow graben. Petrologic results include that the mantle source is depleted asthenosphere admixed with up to 30 wt.% Afar plume (like) component; melting extent is low (~3 wt.%), hence melts are alkalic, and is in the garnet to spinel facies transition. This is close to the geophysically imaged 60–80 km deep lithosphere–asthenosphere boundary that is anomalously shallow for the lithosphere’s age beneath the major harrat belt, indicating that magmatism is associated with a basal erosion process, rather than driven by passive extension.