Refined chronology of late Quaternary eruptions at Harrat Khaybar, Saudi Arabia, with implications for magma dynamics and regional volcanic history

Abstract

Determining accurate and precise ages for Quaternary volcanic centers is essential for reconstructing volcanic field histories, understanding magmatic processes, and assessing potential hazards or risk. Harrat Khaybar, western Saudi Arabia, is one of the youngest and potentially most active volcanic fields on the Arabian plate, has been active since ca. 1.7 Ma, and is characterized by a spectrum of rock compositions ranging from predominantly alkalic basalt to trachyte and comendite. Previous work in Harrat Khaybar utilizing 40 Ar/ 39 Ar incremental heating geochronology to constrain morphological preservation and superpositional relationships bracketed the volcanic activity into broad age groups in intervals of ∼150 k.y., and the youngest and most compositionally evolved volcanoes, including Jabal Abyad, Jabal Bayda, and Jabal Qidr, were assigned to the age groups between ca. 300 ka and present. Herein, we establish a detailed chronology of prehistoric silicic and historical basaltic eruptions at central Harrat Khaybar using four independent eruption age determination techniques: zircon double-dating (ZDD), which combines 238 U- 230 Th disequilibrium or U-Pb with (U-Th)/He dating; zircon U-Pb dating; cosmogenic 3 He dating; and cosmogenic 36 Cl geochronology. These were employed to accurately date six volcanic centers, including the comenditic Jabal Abyad, Jabal Bayda, Jabal Ibayl, and Jabal Alhayyirah, the trachytic Jabal Aluthmor, and the basaltic Jabal Qidr. Additionally, our previously published 40 Ar/ 39 Ar ages have been recalculated using isochron intercept (nonatmospheric) 40 Ar/ 36 Ar for the trapped Ar component. Our new results reveal that zircon rims from Jabal Abyad and Jabal Bayda define isochron 238 U- 230 Th crystallization ages of 125 ± 4 ka and 144 ± 6 ka, concordant with ZDD eruption ages of 132 ± 4 ka and 149 ± 5 ka, respectively. Zircon U-Pb crystallization and (U-Th)/He eruption ages from Jabal Alhayyirah are concordant at 471 ± 14 ka and 458 ± 18 ka, respectively. Finally, zircons from the nearby Jabal Ibayl yield a U-Pb weighted mean crystallization age of 566 ± 16 ka concordant with the corresponding (U-Th)/He eruption age of 554 ± 12 ka, both of which are notably older than the previously proposed eruption ages of 300−150 ka. Recalculations of published 40 Ar/ 39 Ar ages for the youngest volcanoes at central Harrat Khaybar are now in excellent agreement with new geochronological data. Our new age data reveal several new insights into the development of Harrat Khaybar. It is now clear that the comenditic eruptions do not belong to the same eruptive phase and indicate an extended history during which comendites have episodically punctuated the basaltic volcanism since at least 600 ka. The data indicate that Jabal Ibayl and Jabal Alhayyirah represent separate older volcanic events, whereas the younger Jabal Abyad and Jabal Bayda volcanoes appear to be coeval, and their spatial proximity implies that they share a magmatic lineage and maybe a common plumbing system. Zircon age spectra of the comendites reveal obvious xenocrysts and antecrysts indicating assimilation of basement and plutonic progenitors, but otherwise they define broad unimodal populations of crystallization ages that overlap within error with the respective eruption ages. We interpret this to indicate that zircon crystallization continued up to the time of eruption. The cosmogenic ages of Jabal Aluthmor and Jabal Qidr reveal that these centers erupted as recently as ca. 2000 and 760 years ago, respectively. Broadly coeval young silicic and basaltic eruptions at northern Harrat Rahat, the harrat immediately south of Harrat Khaybar, may imply a shared geodynamic forcing between the two adjacent volcanic fields.