The Precambrian rocks from Southern Ethiopia: petrology, geochemistry and their interaction with the Recent volcanism from the Ethiopian Rift Valley
Peccerillo, A.; Mandefro, B.; Solomon, G.; Bedru, H.; Tesfaye, K.
published: Jul 22, 1998
ArtNo. ESP154017303002, Price: 29.00 €
Major, trace element and Sr isotopic data are reported for rock samples from the Precambrian basement terranes of Sidamo, southern Ethiopia. The aims of this study are to contribute to the knowledge of the Ethiopian basement rocks, and to explore the implications for the role of the continental crust in the genesis and evolution of the Recent magmatism occurring along the Ethiopian Rift Valley. The investigated samples consist of various types of metamorphic and intrusive rocks such as gneiss, granofels, amphibolites, mafic metavolcanics, and syn- to post-metamorphic granitoids. The metamorphic rocks derive from both igneous and sedimentary protoliths. Their major and trace element abundances show large compositional variations, even though almost all the samples display low Ti, Nb and Ta, and high LILE/HFSE ratios. Metavolcanic rocks show major and trace element characteristics that resemble island-arc tholeiites and intermediate to acid calcalkaline rocks. Granitoid rocks range from granodiorite to leucogranite and are variably enriched in alkalies and incompatible elements. Texture and mineralogical composition suggest a syn-, late and post-metamorphic emplacement. Abundances and ratios of discriminant trace elements show that the syn- and late-metamorphic granitoids have compositions typical of volcanic arc granites, whereas the post-metamorphic intrusive rocks straddle the boundary between volcanic arc and within-plate granitoids. Trace element 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 synand late-metamorphic granites or to the bulk of metamorphic rocks. Measured 87Sr/88 Sr ratios of metamorphic rocks range from 0.703120 to 0.767678, whereas granitoids show values of 0.705287 to 0.778827. Geochemical data are also discussed to place constraints on the genesis and evolution of Recent rift-related magmatism. Acid volcanic rocks display geochemical characteristics that rule out a genesis by anatexis of the Precambrian crust and support the hypothesis of a derivation from mafic-intermediate parents by combined fractional crystallization and assimilation 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 several ratios of incompatible elements especially LILE/HFSE (e.g. Rb/Nb) and Ba/Rb ratios.