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Compositional Differences between Felsic Volcanic rocks from the Margin and Center of the Northern Main Ethiopian Rift
Abstract
Most of the felsic rift margin volcanic rocks have greater Al2O3, K2O, Nb, Zr, Rb, and Sr, and lesser CaO, Zr/Nb, and CaO/Al2O3 than rift center volcanic rocks. These contrasts may have been inherited from differences in the compositions of their parental basic magmas, which were produced by variable degrees of partial melting.
In both series, the felsic volcanic rocks generally have higher initial Sr- isotopic (0.7038-0.7073) ratios than their basic equivalents (0.7035-0.7046). Nd- isotopic ratios of most felsic rift center samples (0.5129-0.5126) are similar to their associated basic volcanic rocks. In contrast, the Nd-isotopic ratios (0.5128-0.5124) of felsic rift margin volcanic rocks are commonly lower than their companion basic volcanic rocks (0.512806-0.512893), and are relatively lower than rift center equivalents. The elemental and Sr-Nd isotopic compositions of the volcanic rocks suggest that fractional crystallization from differing basic parents accompanied by a limited assimilation (AFC) was the dominant process controlling the genesis of the MER felsic volcanic rocks.
Keywords: Ethiopia; Northern Main Ethiopian Rift; Bimodal Volcanism; parental difference; Sr-Nd Isotopes, Fractional Crystallisation
