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 and207Pb/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 and207Pb/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 and207Pb/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.
Article PDF
Similar content being viewed by others
Avoid common mistakes on your manuscript.
References
Abbate E, Bonazzi A, Dal Piaz GV, Del Moro A, Gosso G, Ibrahim HA, Savioli Mariani E (1985) Il settore occidentale dell'Unita di Inda Ad ed il granito di Arar (Somalia settentrionale). Quaderni Geol Somalia 8: 7–25.
Abbate E, Bruni P, Fazzuoli M, Sagri M (1987) The continental margin of northern Somalia and the evolution of the Gulf of Aden. Proceedings of the Geosom 87 Meeting, Mogadishu, p 3.
Almond DC (1983) The concepts of “Pan-African Episode” and the “Mozambique Belt” in relation to the geology of east and northeast Africa. Bull Fac Earth Sci, King Abdulaziz Univ 6: 71–87.
Berhe SM (1990) Ophiolites in Northeast and East Africa: implications for Proterozoic crustal growth. J Geol Soc London 147: 41–57.
Black R, Liégeois JP (1993) Cratons, mobile belts, alkaline rocks and continental lithospheric mantle: the Pan-African testimony. J. Geol Soc London 150: 89–98.
Burke K, Sengör AMC (1986) Tectonic escape in the evolution of the continental crust. Am Geophys Union, Geodyn Ser 14: 41–53.
Claesson S, Pallister JS, Tatsumoto M (1984) Samarium-neodymium data on two late Proterozoic ophiolites of Saudi Arabia and implications for crustal and mantle evolution. Contrib Mineral Petrol 85: 244–252.
Dalziel IWD (1992) Late Precambrian rifted margins, the origin of the Iapetus and Pacific ocean basins, and Gondwana amalgamation: time and space considerations. EOS 73: 364.
D'Amico C, Ibrahim HA, Sassi FP (1981) Outline of the Somalian Basement. Geol Rundsch 70: 882–896.
Daniels JL (1965) Geology of the Borama and Hargeisa Districts, Somalia. Hargeisa. Geol Surv Som Rep: 118 pp.
Debon F, Le Fort P (1983) A chemical-mineralogical classification of common plutonic rocks and associations. Trans R Soc Edinb Earth Sci 73: 135–149.
DePaolo DJ (1981) Neodymium isotopes in the Colorado front range and crust-mantle evolution in the Proterozoic. Nature 291: 193–196.
Didier J (1973) Granites and their Enclaves. Elsevier Scientific, Amsterdam, 393 pp.
Doe BR, Zartman RE (1979) Plumbotectonics, the Phanerozoic. In: Barns HL (ed) Geochemistry of Hydrothermal Ore Deposits. Wiley Interscience, New York-Brisbane-Toronto, pp 22–70.
Fabre J (1988) Les series Paléozoïque d'Afrique: une approace. J Afr Earth Sci 7/1: 1–40.
Gass IG (1977) The evolution of the Pan-African crystalline basement in NE Africa and Arabia. J Geol Soc London 134: 129–138.
Gass IG, Ries AC, Shackleton RM, Smewing JD (1990) Tectonics, geochronology and geochemistry of the precambrian rocks of Oman. In: Robertson AHF, Searle MP, Ries AC (eds) The Geology and Tectonics of the Oman Region. Spec Publ Geol Soc London 49: 585–599.
Govindaraju K (1984) Compilation of working values for 170 international reference samples of mainly silicate rocks and minerals. Geostandards Newsl Spec Issue, App I, Nancy, 8: 1–28.
Gray CM, Oversby VM (1972) The behaviour of lead isotopes during granulite facies metamorphism. Geochim Cosmochim Acta 36: 939–952.
Greenwood JE (1960) Report on the geology of the Las Koreh-Elayu area, Erigavo district, quarter degree sheets Nos. 5 and 6. Somaliland Protectorate Geol Surv 3: 36 pp.
Greenwood JE (1961) The Inda Ad series of the former Somaliland Protectorate. Bull Overseas Geol Min Res, Oxford 8: 288–296.
Greenwood WR, Anderson RE, Fleck RJ, Roberts RJ (1980) Precambrian geologic history and plate tectonics evolution of the Arabian shield. Bull Saudi Arabian Dir Gen Min Res 24.
Haider A (1989) Géologie de la formation ferrifère Précambrienne et du complexe granulitique encaissant de Buur (Sud de la Somatic) — implications sur l'evolution crustale du socle de Buur. Unpubl PhD Thesis, Inst Nat Polytech de Lorraine, Nancy, 160 pp.
Harms U, Schandelmeier H, Darbyshire DPF (1990) Pan-African reworked early/middle Proterozoic crust in NE Africa west of the Nile: Sr and Nd isotope evidence, J Geol Soc London 147; 859–872.
Harris NBW, Duyverman HJ, Almond DC (1983) The trace element and isotope geochemistry of the Sabaloka igneous complex, Sudan. J Geol Soc London 140: 245–256.
Harris NBW, Hawkesworth CJ, Ries AC (1984) Crustal evolution in northeast and east Africa from model Nd ages. Nature 309: 773–776.
Harris NBW, Gass IG, Hawkesworth CJ (1990) A geochemical approach to allochthonous terranes: a Pan-African case study. Phil Trans R Soc London A 331: 533–548.
Kennedy WQ (1964) The structural differentiation of Africa in the Pan-African (± 500 My) tectonic episode. Univ of Leeds, Res Inst African Geol, Dept Earth Sci, Ann Rep 8: 48–49.
Key RH, Charsley TJ, Hackman BD, Wilkinson AF, Rundle CC (1989) Superimposed Upper Proterozoic collision-controlled orogenies in the Mozambique Belt of Kenya. Precambrian Res 44: 197–225.
Klerkx J, Deutsch S (1977) Résultats préliminaires obtenus par la méthode Rb-Sr sur l'âge des formations précambriennes de la region d'Uweinat (Lybie). Mus Roy Afr Centr, Dept Geol Min, Ann Rep (1976): 83–94.
Krogh TR (1973) A low contamination method for hydrothermal decomposition of zircon and extraction of U and Pb for isotopic age determination. Geochim Cosmochim Acta 37: 485–494.
Kröner A (1977) Precambrian mobile belts of southern and eastern Africa — ancient sutures or sites of ensialic mobility? A case for crustal evolution towards plate tectonics. Tectonophysics 40: 101–135.
Kröner A, Eyal M, Eyal Y, Sassi F, Teklay M (1989) Extensions of the Arabian—Nubian shield into southern Israel, Ethiopia and northern Somalia. Terra Abstr. EUG, Strasbourg, 1: 363.
Küster D (1990) Rare-metal pegmatites of Wamba, central Nigeria — their formation in relationship to late Pan-African granites. Miner Depos 25: 25–33.
Küster D. Rb-Sr isotope systematics of muscovite from Pan-African pegmatites of western and northeastern Africa. Mineral Petrol, in press Küster D, Harms U (1991) Late Proterozoic/Early Phanerozoic Rapakivi Granitoids in NE-Africa: evidence for Pan-African crustal consolidation. Abstr Vol, Int Symp Rapakivi Granites and Related Rocks, Helsinki, Finland, p 31.
Küster D, Utke A, Leupolt L, Lenoir JL, Haider A (1990a) Pan-African granitoid magmatism in northeastern and southern Somalia. Berl Geowiss Abh, Berlin 120: 519–536.
Küster D, Matheis G, Jacob KH, Mohamed FH, Caen-Vachette M (1990b) Geochemical indications of Pan-African rare-metal potentials in NE-Africa — variations in time and style. Berl Geowiss Abh, Berlin 120: 551–570.
Lameyre J, Bowden P (1982) Plutonic rock types series — discrimination of various granitoid series and related rocks. J Vole Geotherm Res 14: 169–186.
Lancelot JR (1975) Les systémes U-Pb chronomètres et traceurs de l'évolution des roches terrestres. Unpubl PhD Thesis, Univ Paris VII, 280 pp.
Lenoir JL, Haider A (1990) Contribution to the knowledge of the petrogenesis and geochronology of the Basement Complex of southern Somalia (Buur region). CIFEG Occasional Publ, 15th Coll African Geology, p 269.
Ludwig KR (1980) Calculation of uncertainties of U-Pb isotope date. Earth Planet Sci Lett 46: 212–220.
Mason JE, Warden AJ (1956) The Heis-Mait-Wagderia area. Somaliland Protectorate Geol Surv Rep 1: 23 pp.
Merla G, Abbate E, Canuti P, Sagri M, Tacconi P (1973) Geological map of Ethiopia and Somalia, scale 1:2.000.000. Consiglio Nazionale delle Ricerche, Italy.
Moorbath S, Welke HJ, Gale NH (1969) The significance of lead isotope studies in ancient, high-grade metamorphic basement complexes, as exemplified by the Lewisian rocks of northwest Scotland. Earth Planet Sci Lett 6: 245–256.
Mosley PN (1993) Geological evolution of the late Proterozoic “Mozambique belt” of Kenya. Tectonophysics 221: 223–250.
Muhongo S, Lenoir JL (1994) Pan-African granulite-facies metamorphism in the Mozambique belt of Tanzania: evidence from U-Pb on zircon geochronology. J Geol Soc London 151: 343–347.
Pearce JA, Harris NBW, Tindle AG (1984) Trace element discrimination diagrams for the tectonic interpretation of granitic rocks. J Petrol 25/4: 956–983.
Pinna P, Jourde G, Calvez JY, Mroz JP, Marques, JM (1993) The Mozambique Belt in northern Mozambique: Neoproterozoic (1100–850 Ma) crustal growth and tectogenesis, and superimposed Pan-African (800–550 Ma) tectonism. Precambrian Res 62:1–59.
Sacchi R, Zanferrari A (1987) Notes on some shear zones of northern Somalia. J Afr Earth Sci 6/3: 323–326.
Shackleton RM (1986) Precambrian collision tectonics in Africa. In Coward MP, Ries AC (eds) Collision Tectonics. Spec Publ Geol Soc London 19: 329–349.
Snelling NJ (1963) Age determination unit. Ann Rep Overseas Geol Surv 1962, London, pp 30–39.
Stacey JS, Kramers JD (1975) Approximation of terrestrial lead isotope evolution by a two-stage model. Earth Planet Sci Lett 26: 207–221.
Stacey JS, Stoeser DB (1983) Distribution of oceanic and continental leads in the Arabian-Nubian Shield. Contrib Mineral Petrol 84: 91–105.
Stacey JS, Doe BR, Roberts RJ, Delevaux MH, Gramlich JW (1980) A lead isotope study of mineralization in the Saudi Arabian Shield. Contrib Mineral Petrol 74: 175–188.
Steiger RH, Jäger E (1977) Subcommission on geochronology: convention on the use of decay constants in geo- and cosmochronology. Earth Planet Sci Lett 36: 359–362.
Stern RJ (1981) Petrogenesis and tectonic setting of Late Precambrian ensimatic volcanic rocks, Central Eastern Desert of Egypt. Precambrian Res 16: 195–230.
Stoeser DB, Camp VE (1985) Pan-African microplate accretion of the Arabian Shield. Geol Soc Am Bull 96: 817–826.
Taylor SR, McLennan SM (1981) The composition and evolution of the continental crust: rare earth element evidence from sedimentary rocks. Phil Trans R Soc London 301: 381–399.
Utke A, Huth A, Matheis G, Hawa HH (1990) Geological and structural setting of the Maydh and Inda Ad basement units in northeast Somalia. Berl Geowiss Abh, Berlin 120: 537–550.
Vidal P, Cocherie A, Le Fort P (1982) Geochemical investigations of the origin of the Manaslu leucogranite (Himalaya, Nepal). Geochim Cosmochim Acta 46: 2279–2292.
Warden AJ (1981) Correlation and evolution of the Precambrian of the Horn of Africa and southwestern Arabia. Unpubl PhD Thesis, Mont-Univ, Leoben, 333 pp.
Warden AJ, Horkel AD (1984) The geological evolution of the NE-branch of the Mozambique Belt (Kenya, Somalia, Ethiopia). Mitt Österr Geol Ges 77: 161–184.
Whalen JB, Currie KL, Chappell BW (1987) A-type granites: geochemical characteristics, discrimination and petrogenesis. Contrib Mineral Petrol 95: 407–419.
Williamson JH (1968) Least squares fitting of a straight line. Can J Phys 46: 1845–1847.
Worku H, Yifa K (1992) The tectonic evolution of the Precambrian metamorphic rocks of the Adola Belt (southern Ethiopia). J Afr Earth Sci 14: 37–55.
Zartman RE, Doe BR (1981) Plumbotectonics — the model. Tectonophysics 75: 135–162.
Author information
Authors and Affiliations
Rights and permissions
About this article
Cite this article
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/BF01083233
Received:
Accepted:
Issue Date:
DOI: https://doi.org/10.1007/BF01083233