Petrogenesis of A-type granitoids from the Wallagga area, western Ethiopia: constraints from mineralogy, bulk-rock chemistry, Nd and Sr isotopic compositions

Abstract

Abstract A-type granitoids, either intruded into greenschist facies volcano-sedimentary sequence or emplaced at the contact between low- and high-grade terranes, constitute a significant proportion of the granitoid rocks in the Precambrian of western Ethiopia. These granitoids are characterized by Fe-rich biotite, ferro-hornblende, alkali-amphiboles, and alkali pyroxenes. High total alkalis, high FeO T /MgO (particularly in the peralkaline varieties and syenite), enriched rare earth element (REE), Y, Nb, Ta and low CaO, MgO, and Sr abundance characterize these granitoids. Chondrite-normalized REE patterns show enriched light REE, moderate to strong negative Eu anomalies, and more or less flat heavy REE patterns. Wide ranges of major and trace element compositions among these different A-type granitoids are attributed to variations of source compositions, crystal fractionation, and assimilation. Aluminum-in-hornblende barometric and coexisting amphibole-plagioclase thermometric estimates show that the Ganjii monzogranite crystallized at 600–630 °C and 5–6 kbar. Compared to A-type magmas elsewhere, this temperature estimate is low, suggesting that it might represent conditions of crystallization close to the solidus. Major and trace element modelling of the Ganjii monzogranite suggests derivation by crystal fractionation dominated by plagioclase, hornblende, and biotite from monzodioritic magma. A binary mixing model involving monzodiorite marginal facies and microgranitic dyke end members could not explain the chemical variation in the Ganjii monzogranite, suggesting that the effect of magma mixing on the petrogenetic evolution of the granite was insignificant. The peralkaline to mildly peraluminous Homa gneissic granite and the Tuppii granite, characterized by variable mineralogy, major, and trace element compositions are formed as a result of combined effects of fractionation and assimilation. The initial 87 Sr / 86 Sr of 0.70281 and positive e Nd (625 Ma) values (>+4) for the Ganjii monzogranite are consistent with generation of parental monzodioritic magma from the mantle. The depleted mantle Nd model age ( T DM ) of 0.86 Ga for this granite is coeval with early magmatic stages in the Arabian Nubian shield elsewhere. Despite the occurrence of Mesoproterozoic xenocrystic zircons in the Ganjii monzogranite, the Sr and Nd isotopic compositions indicate insignificant contribution of older crustal materials. The Tullu Kapii quartz syenite yielded e Nd (625 Ma) of +2.4 and +3, consistent with the results obtained from the Ganjii monzogranite, suggesting generation from a juvenile source. However, the older T DM (1.1 Ga) estimated for the Tullu Kapii quartz syenite indicates derivation from a relatively older source or contribution of pre-Pan-African crustal material, whose existence is indirectly substantiated by Mesoproterozic zircon inheritance and T DM ≅1.5 Ga obtained from a sample of calc-alkaline Ujjukka granitoids. Overall, A-type magmas in the area may have derived as a result of orogenic collapse that caused decompression melting of subcontinental lithospheric mantle during late Neoproterozoic. The abundance of comparable mantle-derived granitoids in other parts of the Arabian Nubian shield indicates considerable new crustal addition in Neoproterozoic time.