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

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 assem- blage 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, end = --1.8, 2°6pb/2°4pb(i) = 17.704 and 2°Tpb/2°4pb(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 grani- tes (~ 600 Ma) related to amphibolite facies retrogres- sion of an intensively reworked pre-Pan-African crust (Sr i = 0.7100, ~Na = --8.4, 2°6pb/2°4pb(i) = 15.403 and 2°7pb/2°4Pb(i) = 15.259). Thesemonzo-andsyenograni- tes are moderately potassic and peraluminous. The younger assemblage (~ 470 Ma) consists of post-kine- matic monzonites to syenogranites with A-type affinities. Initial Sr, Nd and Pb isotopic data for this metaluminous assemblage (Sri = 0.7114, end = --13.1, 2°6pb/2°4pb(i) = 16.913 and 2°TPb/2°4pb(i) = 15.512) indicate a signifi- cant lower crustal component but, however, also a mantle signature. The late Proterozoic to early Palaeozoic grani- toids in Somalia thus express contrasting regimes, charac- terized by strong juvenile input in the north, close to the Arabian- Nubian Shield, whereas intense crustal rewor- king with little addition of juvenile material prevailed in the south. Somalia was definitively not a cratonic area