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Unraveling The Evolution of Silicic Eruptions in a Basaltic Province: Insights From Harrat Khaybar, Western Saudi Arabia

Abdullah Alohali , Shanaka de Silva, Chris Russo

Abstract

Some of the major and well-exposed distributed volcanic fields on Earth are on the Arabian Peninsula, locally known as harrats. Harrat Khaybar, Saudi Arabia (ca. 1.7 Ma to Present) is one of the largest and most compositionally diverse Arabian lava field. Its rock compositions range from predominant alkali basalt to trachyte and comendite, where the progression from basalt to rhyolite is attributed to increased magmatic flux, fractional crystallization, crustal assimilation, and/or halogen complexing. We are currently conducting a comprehensive study to evaluate these hypotheses by integrating petrographic observations, whole-rock geochemistry, Sr-Nd-Pb isotopic analyses, 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 (ℇNd= 5.2 to 6.6). These data connote that silicic magmas are the product of assimilation and fractional crystallization processes on basalts. The assimilant is likely the Pan-African granitic basement.