Abstract
Pliocene felsic rift margin and Quaternary rift center volcanic rocks from the northern Main Ethiopian Rift (MER) exhibit contrasts in major and trace element contents and Sr-Nd isotopic ratios. Quaternary rift center felsic volcanic rocks are mainly peralkaline trachytes and rhyolites, whereas Pliocene felsic rift margin volcanic rocks are represented by benmoreites, weakly peralkaline trachytes and rare rhyolites. 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
Highlights
The Main Ethiopian Rift (MER), the Southwestern Ethiopian Rift Zone (SWERZ), the Tana Rift and the Afar region represent the northernmost part of the East African Rift System (Fig. 1)
Geochemical, and isotopic characteristics of the volcanic units, we describe the compositional differences between the volcanic rocks at the rift margin and rift center, and discuss the petrogenetic relationships between basic and felsic magmas in order to assess the influence of basic parents and continental crust in the genesis of the felsic melts
Studies of the northern Main Ethiopian Rift (MER) volcanic rocks offer insight into the genetic relations of basic and felsic volcanic rocks, and establish that compositional contrasts occur in equivalent volcanic rocks at rift margin and center magma series within a single intraplate continental setting
Summary
The Main Ethiopian Rift (MER), the Southwestern Ethiopian Rift Zone (SWERZ), the Tana Rift and the Afar region represent the northernmost part of the East African Rift System (Fig. 1). Recent geochemical and isotopic studies have focused on the Oligocene-Miocene to Quaternary basic-felsic volcanism that accompanied the formation of the MER. These studies have proposed the involvement of distinct mantle components in various proportions, and the importance of the Afar mantle plume and lithospheric mantle in the sources of the basic lavas (Fig. 1). Several geochemical and isotopic studies of felsic products have been carried out in the rift center at Debre Zeit, Gedemsa, and Asela-Ziway (Fig. 1) (Gasparon et al, 1993; Peccerillo et al, 1995; Trua et al, 1999; Abebe et al, 1998; Boccaletti et al, 1999), on volcanic rocks which are mostly younger than 2 Ma. The felsic volcanic products at the rift margin (Addis Ababa) and rift center (Nazreth) regions and their compositional variations are not yet well studied. Geochemical, and isotopic characteristics of the volcanic units, we describe the compositional differences between the volcanic rocks at the rift margin and rift center, and discuss the petrogenetic relationships between basic and felsic magmas in order to assess the influence of basic parents and continental crust in the genesis of the felsic melts
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