THE PRECAMBRIAN ROCKS FROM SOUTHERN ETHIOPIA - PETROLOGY, GEOCHEMISTRY AND THEIR INTERACTION WITH THE RECENT VOLCANISM FROM THE ETHIOPIAN RIFT-VALLEY

Major, trace element and Sr isotopic data are reported for rock, sampl es from the Precambrian basement terranes of Sidamo, southern Ethiopia . The aims of this study are to contribute to the knowledge of the Eth iopian basement rocks, and to explore the implications for the role of the continental crust in the genesis and evolution of the Recent magm atism occurring along the Ethiopian Rift Valley. The investigated samp les consist of various types of metamorphic and intrusive rocks such a s gneiss, granofels, amphibolites, mafic metavolcanics, and syn- to po st-metamorphic granitoids. The metamorphic rocks derive from both igne ous and sedimentary protoliths. Their major and trace element abundanc es show large compositional variations, even though almost all the sam ples display low Ti, Nb and Ta, and high LILE/HFSE ratios. Metavolcani c rocks show major and trace element characteristics that resemble isl and-are tholeiites and intermediate to acid calcalkaline rocks. Granit oid rocks range from granodiorite to leucogranite and are variably enr iched in alkalies and incompatible elements. Texture and mineralogical composition suggest a syn-, late- and post-metamorphic emplacement. A bundances and ratios of discriminant trace elements show that the syn- and late-metamorphic granitoids have compositions typical of volcanic are granites, whereas the post-metamorphic intrusive rocks straddle t he boundary between volcanic are and within-plate granitoids. Trace el ement variation suggests an evolution by fractional crystallization or AFC for the first group, whereas post-metamorphic intrusives are the result of the interaction between an alkaline, Nb-Ta-Y-rich acid melt and an end-member with a composition similar to that of syn- and late- metamorphic granites or to the bulk of metamorphic rocks. Measured Sr- 87/Sr-88 ratios of metamorphic rocks range from 0.703120 to 0.767678, whereas granitoids show values of 0.705287 to 0.778827. Geochemical da ta are also discussed to place constraints on the genesis and evolutio n of Recent rift-related magmatism. Acid volcanic rocks display geoche mical characteristics that rule out a genesis by anatexis of the Preca mbrian crust and support the hypothesis of a derivation from mafic-int ermediate parents by combined fractional crystallization and assimilat ion processes. Trace element modelling suggests that the Rift basalts have significantly interacted with both the Precambrian crust and the associated acid magmas. This generated important modification of sever al ratios of incompatible elements especially LILE/HFSE (e.g. Rb/Nb) a nd Ba/Rb ratios.