Monogenetic volcanism in the western Arabian Peninsula: Insights from Late Quaternary eruptions in northern Harrat Rahat, Kingdom of Saudi Arabia

Abstract

Harrat Rahat is an intra-plate volcanic field in western Saudi Arabia. It covers ~20,000 km2 in area, hosts around 1000 vents, and has emitted a wide range of compositions from 10 Ma to historical times. The research presented here focuses on petrological, geochronological and physical volcanological study of some of the youngest (≤0.3 Ma) and one older (1.2 - 0.9 Ma) volcanic centre(s) in northern Harrat Rahat, where the city of Al-Madinah (~1.5 million inhabitants) is located. Rocks from these centres range from alkali-basalt to hawaiite (SiO2 44.5 - 50.3 wt.% and MgO 3.5 - 9.3 wt.%), and display intra-plate (PREMA) characteristics. Each eruption produced a distinct composition, either within a very narrow compositional range or in trends over a wider range. Mantle-normalised trace element abundance patterns display parallel curvilinear trends with signatures that lie between OIB and MORB. Rocks with the highest MgO content display less enrichment in rare earth elements (REE) than those with the lowest. Magmatic 3He/4He isotope ratios vary between 9.3 and 11.8 Ra and define the source as plume-like. Trace element modelling and geothermobarometry reveal that partial melting occurred in the asthenospheric mantle (>100 km depth, ~1500 °C, >3 GPa). Fractionation of olivine and clinopyroxene en route to the surface can account for variations in composition. The elevated plume-like He ratios and PREMA signature are interpreted to reflect melting of a fossil plume head located in the upper asthenosphere. New 3He surface exposure ages were obtained for some of the volcanic centres, allowing a revision of existing stratigraphic information. The volcanic centres are dominated by lava flows, but explosive eruptions leading to pyroclastic cones and off-cone pyroclastic fall deposits also occurred. Eruptions occurred through point-like, complex, and multiple vents (i.e., along fissures). Lava flows are characterised by a down-flow variation in surface morphologies (morphotypes), which together define a new classification scheme. Pyroclastic cones are best-characterised by features such as bedding, agglutination, and size of the fragments; together these dominantly reflect Hawaiian and/or Strombolian eruptive styles. Rare evidence of phreatomagmatism was found in one volcanic centre. Detailed geochemical study of individual centres, along with stratigraphic and physical volcanological descriptions, revealed that magma reaching the surface first was the least evolved and produced the least explosive phases. It is hypothesised that variations in composition, resulting from fractionation, are related to magma ascent time, with greater degrees of fractionation representing longer transport times. As a whole, ‘harrats’ in the Arabian Peninsula can be considered transitional between monogenetic volcanic fields and continental flood basalt provinces sensu stricto.