ORIGIN AND REGIONAL SIGNIFICANCE OF LATE PRECAMBRIAN AND EARLY PALEOZOIC GRANITOIDS IN THE PAN-AFRICAN BELT OF SOMALIA

Granitoids within the Precambrian basement of north-eastern and southe rn Somalia are subdivided on the basis of geology, geochronology and p etrology into three different assemblages. The post-kinematic assembla ge in north-eastern Somalia (almost-equal-to 630 Ma) comprises granodi orites and granites which belong to a medium-K calc-alkaline suite. Av erage initial Sr, Nd and Pb isotopic ratios [Sr(i) = 0.7048, epsilon(N d) = -1.8, Pb-206/Pb-204(i) = 17.704 and Pb-207/Pb-204(i) = 15.611] in dicate that these melts were derived from a mantle or juvenile crustal source with only slight involvement of pre-existing crust as a contam inant. Two different assemblages are found in southern Somalia. The ol der assemblage is composed of crustal anatectic, synkinematic, parauto chthonous granites (almost-equal-to 600 Ma) related to amphibolite fac ies retrogression of an intensively reworked pre-Pan-African crust [Sr (i) = 0.7100, epsilon(Nd) = -8.4, Pb-206/Pb-204(i) = 15.403 and Pb-207 /Pb-204(i) = 15.259]. These monzo-and syenogranites are moderately pot assic and peraluminous. The younger assemblage (almost-equal-to 470 Ma ) consists of post-kinematic monzonites to syenogranites with A-type a ffinities. Initial Sr, Nd and Pb isotopic data for this metaluminous a ssemblage [Sr(i) = 0.7114, epsilon(Nd) = -13.1, Pb-206/Pb-204(i) = 16. 913 and Pb-207/Pb-204(i) = 15.512] indicate a significant lower crusta l component but, however, also a mantle signature. The late Proterozoi c to early Palaeozoic granitoids in Somalia thus express contrasting r egimes, characterized by strong juvenile input in the north, close to the Arabian - Nubian Shield, whereas intense crustal reworking with li ttle addition of juvenile material prevailed in the south. Somalia was definitively not a cratonic area during the Pan-African, but a zone o f high crustal mobility.