Yirgu, GezahegnAyalew, Dereje2018-07-032023-11-092018-07-032023-11-091994-01http://10.90.10.223:4000/handle/123456789/5828The Gedemsa volcano lies on the floor of the northern sector of the Main Ethiopian Rift. It is characterized by a polygenic caldera resulting from large pyroclastic eruptions. K/Ar dating performed by previous studies indicates an age of 0.8 to 0.1 Ma for the exposed volcanic products. Volcanologic and stratigraphic studies allowed recognition of several phases of activity during the evolution of Gedemsa. The lowest exposed products are represented by acidic lavas, which are covered by thick plinian fall pumice deposits. This are followed by an ignimbrite deposit and by intra-caldera lava flows and interbedded pyroclastic products. The caldera, is clearly a composite structure resulting from several collapses which occurred after plinian and ignimbritic eruptions. A separate stage of volcanic activity connected to the Wonji Fa ult System (basaltic volcanism) formed surge deposits and numerous basaltic cinder cones and lavas, both within and outside the caldera depression. The volcanic products from Gedemsa volcano are petrologically and geochemically diverse. Alkaline and peralkaline silicic lavas and pyroclastics (trachytes and rhyolites) are by far the most abundant products. The mafic rocks are only represented by the mafic inclusions occurring within some of the post-caldera products. The younger riftrelated activity is, instead, represented by Na-transitional basalt. On the whole, the rocks occurring in the area have a very marked bimodal distribution, a situation which is typical of almost all the young volcanism of the Ethiopian Rift Valley. Major and trace element variations of peralkaline silicic volcanic rocks from Gedemsa volcano support an origin by crystal/liquid fractionation starting from mafic parental liquids, with separation of olivine, plagioclase, clinopyroxene and opaques during the early to intermediate stages and of alkalifeldspar and a few mafic phases (alkali amphiboles and pyroxenes) during the late stages of evolution. These processes generated strong enrichments in incompatible trace elements and depletion in compatible elements in the acidic magmas. Consequently, rhyolites display extremely high values of Zr, Y, Rb and F and low to very low abundances of ferromagnesian trace elements, Sr and Ba. Some rhyolites, however, have low Na and fluorine, most probably due to interaction with groundwaters. Such a process may represent an explanattion of the high F contents in the groundwaters of the Wonji area and of other zones inside the rift. Although, crystal fractionation best fits the geochemical variations, field and petrographic observations indicate that mixing processes were also active during the magma evolution.enPetrology and GeochemiThesis