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Origin and regional significance of late Precambrian and early Palaeozoic granitoids in the Pan-African belt of Somalia

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Abstract

Granitoids within the Precambrian basement of north-eastern and southern Somalia are subdivided on the basis of geology, geochronology and petrology into three different assemblages. The post-kinematic assemblage in north-eastern Somalia (≈ 630 Ma) comprises granodiorites and granites which belong to a medium-K calc-alkaline suite. Average initial Sr, Nd and Pb isotopic ratios [Sri = 0.7048, ɛNd = −1.8, 206Pb/204Pb(i) = 17.704 and 207Pb/204Pb(i) = 15.611] indicate that these melts were derived from a mantle or juvenile crustal source with only slight involvement of pre-existing crust as a contaminant. Two different assemblages are found in southern Somalia. The older assemblage is composed of crustal anatectic, synkinematic, parautochthonous granites (≈ 600 Ma) related to amphibolite facies retrogression of an intensively reworked pre-Pan-African crust [Sri = 0.7100, ɛNd = −8.4, 206Pb/204Pb(i) = 15.403 and 207Pb/204Pb(i) = 15.259]. These monzo- and syenogranites are moderately potassic and peraluminous. The younger assemblage (≈ 470 Ma) consists of post-kinematic monzonites to syenogranites with A-type affinities. Initial Sr, Nd and Pb isotopic data for this metaluminous assemblage [Sri = 0.7114, ɛNd = −13.1, 207Pb/204Pb(i) = 16.913 and 207Pb/204Pb(i) = 15.512] indicate a significant lower crustal component but, however, also a mantle signature. The late Proterozoic to early Palaeozoic granitoids in Somalia thus express contrasting regimes, characterized by strong juvenile input in the north, close to the Arabian-Nubian Shield, whereas intense crustal reworking with little addition of juvenile material prevailed in the south. Somalia was definitively not a cratonic area during the Pan-African, but a zone of high crustal mobility.

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Authors and Affiliations

  1. Département de Géologie, Musée royal de l'Afrique centrale, B-3080, Tervuren, Belgium

    J. -L. Lenoir & J. -P. Liégeois

  2. SFB 69, TU Berlin, Ackerstrasse 71, D-13355, Berlin, Germany

    D. Küster, A. Utke & G. Matheis

  3. Department of Earth Sciences, Open University, Milton Keynes, MK7 6AA, UK

    A. Haider

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  1. J. -L. Lenoir
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  2. J. -P. Liégeois
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  4. A. Utke
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  6. A. Haider
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Lenoir, J.L., Liégeois, J.P., Küster, D. et al. Origin and regional significance of late Precambrian and early Palaeozoic granitoids in the Pan-African belt of Somalia. Geol Rundsch 83, 624–641 (1994). https://doi.org/10.1007/BF00194167

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