The roles of extreme fractional crystallization and magma mixing in the petrogenesis of large-volume silicic magmas from the Oligocene Afro-Arabian Large Igneous Province

Understanding the temporal and petrogenic processes by which large volumes of silicic magma are generated has been a long-standing but elusive task, especially when considering the huge volumes of magma erupted in quick succession in large igneous provinces (potentially >>1000 km³ per eruptive event). The Oligocene Afro-Arabian large igneous province is distributed between the Yemen and Ethiopian conjugate rifted margins and in Yemen, silicic units locally represent up to 50% of the volcanic stratigraphy. Eruptive volume estimates for individual units, based on volcanic stratigraphy and correlated Indian Ocean deep-sea tephras, exceed 2000 km³ dense rock equivalent_. The Green Tuff (29.59 ± 0.12 Ma) is a pyroclastic airfall deposit that erupted immediately preceding column collapse and emplacement of the SAM pyroclastic density current. The Green Tuff has 65 to 76 wt% SiO₂, Or_45-80 potassium feldspar, and En₄₁Fs₁₈Wo₄₁ to En₄₅Fs₁₇Wo₃₈ pyroxene. The SAM Ignimbrite is distinctive in the field due to its pink color and clear textural division into a lower massive base and upper platy jointed section. It has 69 to 76 wt% SiO₂, Or_18-100 potassium feldspar, Ab_92-100 plagioclase, En₃₄Fs₂₃Wo₄₂ to En₆₂Fs₂₀Wo₁₇ pyroxene, and amphibole with Mg# 0.2 to 0.7. High density sampling through the Green Tuff and the SAM Ignimbrite (ca. 0.5 to 1 m) and new ICP-MS major and trace element and whole rock radiogenic Pb isotope MC-ICP-MS analyses reveals mixing between two geochemically and isotopically distinct low and high ²⁰⁸Pb/²⁰⁶Pb magmas within each eruption sequence. The base of the Green Tuff has a ²⁰⁸Pb/²⁰⁶Pb composition of 2.0596 and increases to 2.0632 at 7 m upsection before shifting to 2.0587 and decreasing to 2.0613 by the top of the section. ²⁰⁸Pb/²⁰⁶Pb throughout most of the SAM Ignimbrite is 2.0596 -2.0632 but shifts to 2.0140-2.0515 at 13 m upsection. Trace element patterns show similar shifts at the same heights as changes in the Pb isotope composition within each unit. Trace element patterns suggest the SAM Ignimbrite and Green Tuff are generated by extreme fractional crystallization of basalt while changes in the isotopic composition during the eruption sequence is consistent with magma mixing playing a significant role in the formation of these large-volume silicic magmas.