AFRO-ARABIAN OLIGOCENE FLOOD VOLCANIC STRATIGRAPHY AND EXTREME GEOCHEMICAL HETEROGENEITY: INSIGHTS INTO EVOLUTION OF VOLUMINOUS SILICIC PYROCLASTIC ERUPTIONS

Oligocene Afro-Arabian flood volcanism comprises a bimodal suite of basaltic and rhyolitic volcanic rocks erupted from 31-26 Ma. The volcanic stratigraphy can be divided into three parts characterized by differences in dominant erupted compositions and extrusive volumes: (i) main flood basalts: voluminous basaltic lavas and minor silicic tuffs; (ii) main silicic volcanics: large-volume silicic ignimbrites and tuffs; and (iii) upper bimodal volcanics: small-volume, laterally restricted ignimbrites, tuffs and basaltic lavas. Major and trace element compositions and Pb-Nd isotope ratios are consistent with >90% of the silicic volcanism (εNd= +3.8 to +4.7) originating from extreme fractional crystallization of the basalts (εNd= +2.1 to +6.0), with an additional minor component of silicic volcanism generated by Pan-African crustal anatexis (εNd= +0.4 to +0.9). Individual voluminous silicic eruptions have been unequivocally chemically and isotopically correlated from Yemen across the Red Sea conjugate rifted margin to Ethiopia and to tephra layers in Indian Ocean ODP drill cores ~2700 km away using in situ trace element and Pb isotope analysis. The distal tephras preserve extreme geochemical heterogeneity that is not extractable in the equivalent welded, on-land pyroclastic deposits. Individual eruptions record large major element variations, e.g. SiO₂: 43.1 to 76.1 wt.%. Major and trace elements trends indicate that fractional crystallization was the major process in generating compositional variations, and silicic units have incompatible trace elements that uniquely link them to the upper bimodal volcanics rather than the underlying, volumetrically dominant main flood basalts. The detailed compositional variations of ash shards from these distal tephra layers preserves unique snapshots of zoned magma chambers at the instant of eruption, and reveal the formation and petrogenetic history of these large-volume, chemically zoned silicic magmatic systems.