Petrogenesis of Plutonic Rocks from Southwest of Gimbi (Homa Area), Western Ethiopia

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Addis Ababa Universty


Southwest of Gimbi is located within the Western Ethiopian Precambrian shield that emplaced in between the low-grade volcanic and volcano-sedimentary rock assemblages and the high-grade gneiss and migmatites assemblages. The area is situated approximately about 508 km far from Addis Ababa. The main objective of the thesis is to characterize the petrology and geochemistry of plutonic rocks from Southwest of Gimbi area. To achieve the general and specific objectives of the research and come up with the expected result, starting from the beginning to the end different methods have been applied. Southwest of Gimbi Homa area is characterized by the plutonic rocks which have emplaced pre-, syn/late-to post-tectonic with associated to the major deformational events. Modally, plutonic rocks of Homa area range from rare diorite/gabbro through dominant granodiorite and monzogranite to alkali feldspar granite. Petrographic study shows that, Southwest of Gimbi Homa area granite rocks has medium to coarse grains and dominantly consist of K- feldspar phenocrysts, plagioclase, quartz and less biotite. All analyzed granite rocks from Southwest of Gimbi provide geochemical features of sub-alkaline series, high-K calc alkaline series, high silica concentration and high total alkali concentration. These granite rocks show enrichment in large ion lithophile elements and highly depleted in heavy rare earth elements. Southwest of Gimbi (Homa) granite rocks show enrichment in most incompatible elements (Rb and Pb) and depleted in compatible elements (Ba, Sr and Ti) reflecting the crustal source involvement in the rock genesis. Besides, the REE patterns of these granite rocks have subparallel and show pronounced negative Eu anomalies with increasing total REEs, typical of a feldspar fractionation trend. The considerable depletion in Nb, Ta and Ti and enrichment in large ion lithophile elements (Rb and Pb) on multi-element diagram indicates there is a subduction/volcanic arc related origin of granite rocks from Southwest of Gimbi. Southwest of Gimbi Homa granite rocks were genetically formed by small degree partial melting of crustal materials and by fractional crystallization of mantle derived mafic magma. Key words: Anomalies, Fractional crystallization, partial melting and ANS and MB.



Anomalies, Fractional crystallization, Partial melting and ANS and MB