THE ORIGIN OF SILICIC VOLCANISM IN A BASALTIC VOLCANIC FIELD: HARRAT KHAYBAR, WESTERN SAUDI ARABIA

The Arabian Peninsula hosts some of the world’s most extensive and well-exposed distributed volcanic fields, locally known as harrats. The Arabian volcanism initiated in the late Oligocene (ca. 30 Ma) and remains potentially active as illustrated by the most recent volcanic eruption in 1256 AD at Harrat Rahat and a shallow dike emplacement in 2009 at Harrat Lunayyir.

Harrat Khaybar, Saudi Arabia (ca. 1.7 Ma to Present) is one of the largest and the most compositionally diverse Arabian lava field. Rock compositions at Harrat Khaybar range from dominant alkali basalt to trachyte and comendite, where the transition from basaltic to rhyolitic has been attributed to an elevated magmatic flux, fractional crystallization, crustal assimilation, and/or halogen complexing. We are in the midst of a new study that tests these hypotheses using petrography, whole-rock geochemical and Sr-Nd-Pb isotopic analyses, two age determination techniques (Zircon (U-Th)/(He) double-dating and cosmogenic ³He surface-exposure dating), and petrogenetic modeling.

Interpretation of our new geochemical results to date confirms open-system magmatic processes. Whereas intermediate and silicic rocks show enrichment in light and heavy rare earth elements compared to the basaltic rocks, they mimic the pattern of basalts, except that evolved rocks display a strong Eu negative anomaly. Indices of closed-system fractional crystallization (e.g., Zr/Nb) reveal two linear trends: basalt to trachyte and trachyte to comendite. Sr isotopic compositions of the evolved rocks vary widely (⁸⁷Sr/⁸⁶Sr = 0.704 to 0.709) and correlate negatively with Mg#, whereas Pb isotopic compositions exhibit a narrow range (²⁰⁶Pb/²⁰⁴Pb = 18.5 to 18.7) and correlate positively with Mg#. Conversely, Nd isotopes remain relatively constant from basalt to the most evolved rocks (ℇ_Nd= 5.2 to 6.6). These data connote that silicic magmas are the product of assimilation and fractional crystallization processes on basalts at Harrat Khaybar. The assimilant is likely the Pan-African granitic basement.