Geology, geochemistry and petrogenesis of intrusive rocks of the Wallagga area, western Ethiopia

Crystalline rocks from the western Ethiopian Precambrian terrain comprise t wo major rock groups: (1) the often migmatised eastern and western high-gra de gneisses; and (2) the central low-grade metavolcanosedimentary rocks. Gr anitoid bodies of different ages and compositions intrude these rocks. Fiel d observations, petrographic investigations, and geochemical features suppo rt a two-fold classification of the granitoid rocks. The volcanic are grani toids (VAG) are most common and include the Ujjukka granite and granodiorit e, and the Dhagaa Booqa and Guttin K-feldspar megacrystic granites. The sec ond and geographically more restricted group represents within-plate or ano rogenic (A-type) granitoids, typified by the Tullu Kapii syenite. Geochemic ally, the Tullu Kapii syenite is characterised by moderate to high contents of SiO2, total alkalis, Y, Nb, Ta, Hf, Zn, Zr, Ga, Sigma REE, higher ratio s of Fe2O3(total)/MgO, Rb/Sr, Rb/Ba, and lower contents of CaO, MgO, Sr, an d Ba, compared to the other granitoids. The VAG group shows chondrite-norma lised REE patterns with slightly enriched LREE and flat HREE patterns witho ut significant Eu anomalies. In contrast, the Tullu Kapii A-type granitoid is characterised by a nearly horizontal REE pattern with variable negative Eu anomalies. The Ujjukka granite and granodiorite; and the Dhagaa Booqa an d Guttin K-feldspar megacrystic granites originated in a two-step process, which involves batch equilibrium melting of basaltic or andesitic material producing tonalitic magma, followed by fractional crystallisation. The Tull u Kapii syenite was the product of partial melting of source rocks enriched in high field strength elements. (C) 2000 Elsevier Science Limited. All ri ghts reserved.