Geological and Geochemical Studies of Flood Basalts (Aiba and Ashenge Basalts) in Maychew Area, Tigray, Northern Ethiopia: Implication for their Petrogenesis.
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Date
2018-06-27
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Addis Ababa University
Abstract
The study area, Maychew, is located in the northern eastern corner of the northwestern Ethiopian plateau. Petrological, petrographic, and geochemical (trace and major elements) data are presented and integrated for Maychew flood basalts to characterize the geochemistry and constrain petrogenetic processes involved in the evolution of these flood basalts. According to these data two types of basaltic groups have been identified; tilted basaltic units (Ashenge basalts) and horizontal to sub horizontal basaltic units (Aiba basalts).The studied samples have low Mg numbers (Mg#:37.99-58.92, excluding sample T2S8 and T4O18) and low compatible element contents such as Ni and Cr suggesting that these lavas have undergone fractionation en route to the surface. Moreover Ce/Pb, La/Nb, and La/Ta ratios, which are sensitive to crustal contamination, of the studied samples indicates that crustal contamination is more pronounced on samples of Aiba basalt than Ashenge basalts. Further, the investigated samples from study area exhibit trace element ratios such as Zr/Nb (3.77-10.88), Ba/La (5.75-13.55), La/Nb (0.76-1.89) and Ba/Nb (4.65-18.07), which overlap substantially the field of OIB. This suggests that these lavas are derived from mantle sources that are similar to OIB-type sources or contain high proportion of OIB-type sources at least on the basis of trace element ratios. Moreover, a plot of two highly incompatible trace elements, thought to be insensitive to fractional crystallization and partial melting, indicates that both of the basaltic groups were originated from a common mantle source. However, the observed distinct trends of these basaltic groups on such plots, and on some major and trace element variation diagrams could be related to variation in degree of crustal contamination and/ or fractional crystallization, rather than source heterogeneity. Hence, the two basaltic groups are originated from a common OIB like mantle source following different paths of petrogenetic processes; crustal contamination and/or fractional crystallization.
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Ethiopian Plateau, Fractional Crystallization, Crustal Contamination, Mantle Source, Petrogenetic Processes, OIB