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The Axum–Adwa basalt–trachyte complex: a late magmatic activity at the periphery of the Afar plume

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Abstract

The Axum–Adwa igneous complex consists of a basalt–trachyte (syenite) suite emplaced at the northern periphery of the Ethiopian plateau, after the paroxysmal eruption of the Oligocene (ca 30 Ma) continental flood basalts (CFB), which is related to the Afar plume activity. 40Ar/39Ar and K–Ar ages, carried out for the first time on felsic and basaltic rocks, constrain the magmatic age of the greater part of the complex around Axum to 19–15 Ma, whereas trachytic lavas from volcanic centres NE of Adwa are dated ca 27 Ma. The felsic compositions straddle the critical SiO2-saturation boundary, ranging from normative quartz trachyte lavas east of Adwa to normative (and modal) nepheline syenite subvolcanic domes (the obelisks stones of ancient axumites) around Axum. Petrogenetic modelling based on rock chemical data and phase equilibria calculations by PELE (Boudreau 1999) shows that low-pressure fractional crystallization processes, starting from mildly alkaline- and alkaline basalts comparable to those present in the complex, could generate SiO2-saturated trachytes and SiO2-undersaturated syenites, respectively, which correspond to residual liquid fractions of 17 and 10 %. The observed differentiation processes are consistent with the development of rifting events and formation of shallow magma chambers plausibly located between displaced (tilted) crustal blocks that favoured trapping of basaltic parental magmas and their fractionation to felsic differentiates. In syenitic domes, late- to post-magmatic processes are sometimes evidenced by secondary mineral associations (e.g. Bete Giorgis dome) which overprint the magmatic parageneses, and mainly induce additional nepheline and sodic pyroxene neo-crystallization. These metasomatic reactions were promoted by the circulation of Na–Cl-rich deuteric fluids (600–400 °C), as indicated by mineral and bulk rock chemical budgets as well as by δ18O analyses on mineral separates. The occurrence of this magmatism post-dating the CFB event, characterized by comparatively lower volume of more alkaline products, conforms to the progressive vanishing of the Afar plume thermal effects and the parallel decrease of the partial melting degrees of the related mantle sources. This evolution is also concomitant with the variation of the tectono-magmatic regime from regional lithospheric extension (CFB eruption) to localized rifting processes that favoured magmatic differentiation.

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Acknowledgments

The authors gratefully acknowledge the three anonymous Reviewers for their constructive criticisms and the Editor J. Hoefs for his final comments. The authors gratefully acknowledge the analytical support provided by R. Tassinari (XRF and ICP-MS analyses), R. Carampin (electron microprobe analyses), M. Laurenzi (Ar–Ar datings) and L. Dallai (oxygen isotopic analyses), as well as B. Bonin for his useful comments on an early version of the manuscript.

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  1. Dipartimento di Fisica e Scienze della Terra, Università di Ferrara, Via Saragat 1, 44100, Ferrara, Italy

    C. Natali, L. Beccaluva, G. Bianchini & F. Siena

  2. Istituto di Geoscienze e Georisorse, C.N.R, Via Moruzzi 1, I-56124, Pisa, Italy

    C. Natali & G. Bianchini

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  1. C. Natali
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Correspondence to G. Bianchini.

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Communicated by J. Hoefs.

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Natali, C., Beccaluva, L., Bianchini, G. et al. The Axum–Adwa basalt–trachyte complex: a late magmatic activity at the periphery of the Afar plume. Contrib Mineral Petrol 166, 351–370 (2013). https://doi.org/10.1007/s00410-013-0879-0

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