Petrologic and Volcanologic Constraints on Depths of Evolved Magma Generation and Storage at Northern Harrat Rahat and Harrat Khaybar, Kingdom of Saudi Arabia

Abstract:

We present preliminary results of a petrologic study of northern Harrat Rahat that is part of an interdisciplinary hazard assessment of this rift-related volcanic field in Saudi Arabia. Alkali basalts and hawaiites occur with mugearites and trachyphonolites. This silica-undersaturated suite is consistent with fractionation at crustal depths greater than 25 km, i.e., at pressures for which experimental studies find that sodic clinopyroxene dominates the liquidus assemblage, resulting in a differentiation trend in which SiO₂ rises only moderately. Trachyphonolites lack positive Eu anomalies despite abundant anorthoclase phenocrysts, and exhibit linear crystal-size distributions indicative of a simple, one-stage crystallization history. We suggest that trachyphonolite melts are extracted from extensively crystallized alkali basalts in the middle to lower crust. Any crystals entrained from depth (e.g., sodic clinopyroxene) are resorbed during ascent of these water-poor magmas to shallow levels, where they cool and crystallize. Abundant phenocrysts and low H₂O contents resulted in viscous magmas that produced numerous cryptodomes along with domes and spines that pierced the uplifts. Eruption of trachyphonolite domes was accompanied by poorly fluidized scoria flows that traveled up to 4 km from their vents. Explosive disruption of domes produced block flows that ring craters. Vulcanian deposits with accretionary lapilli and lithics of rounded metamorphic rocks point to the involvement of groundwater sourced in basinal sediments eroded from Precambrian basement. Although phreatomagmatic eruptions produced tuff rings up to 1.5 km in diameter, there is no evidence for collapse calderas, suggesting there have not been sizable reservoirs of evolving magma in the shallow crust, consistent with petrologic evidence for a deep origin of the trachyphonolite magmas.

In contrast, Harrat Khaybar, which lies 140 km NNE of Harrat Rahat, erupted silica-saturated evolved magmas: trachytes and alkali rhyolites. Experimental studies demonstrate they can be produced by alkali basalt fractionation at low pressures equivalent to those of the upper crust. The Jabal Bayda tuff ring is the product of a phreatomagmatic eruption of phenocryst-poor alkali rhyolite that produced a plinian fall deposit.