Petrogenesis of Flood Basalts From Hirna Area, South Eastern Ethiopian Plateau
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Date
2013-07-07
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Addis Ababa University
Abstract
This study presents new petrography, major and trace element data from Hirna basalts, southeastern Ethiopian plateau which directly overlies the Cretaceous Upper sandstone for a maximum thickness of 540m. The study exhibit the presence of variation in geochemical distribution over the area. Major and trace element variation mostly controlled by source region heterogeneity and fractionation history of the primary magma. Generally silica saturated and undersaturated basalts have been recognized. Silica undersaturated basalts characterized by high Fe2O3 (17.34-17.54 wt%), and low SiO2 (34.89-35.02 wt %) and show enrichment in highly incompatible trace elements Sr (806-1198ppm), Zr (323-422), Nb (111-142). Silica saturated basalts on the other hand exhibit SiO2 (45.45-49.04 wt %) MgO varies between 4.55-6.16wt% and comparatively the concentration of highly incompatible trace element in silica saturated basalts are low Sr (594-72 ppm), Zr (145-171ppm), Nb (17-19ppm). The incompatible trace element ratios between the two basalts form two distinctive groups. Silica saturated basalts exhibit La/Y <1, La/Lu 65-70, La/Yb 9.5-10.61, La/Nb >1 and silica undersaturated basalts show La/Y 2.96-3.12, La/Lu 322-323, La/Yb 44-45, La/Nb <1. This incompatible trace element ratio variation and the crossing pattern on the Chondritic normalized REE (rare earth element) plot between the two groups shows source region heterogeneity. Enrichment of LREE (light rare earth elements) and depletion of HREE (heavy rare earth element) have been observed for both basalts. This signature is indicative of melting in the presence of residual garnet in the source region. The highly compatible trace element concentration of the area is very limited (Ni<45ppm and Cr<60ppm) and have low MgO/ (MgO+Fe2O3) ratio (0.26-0.33) it is a characteristics of high involved magma spectrum. The basaltic successions of the area‟s major and trace element data petrogenetic model suggests fractionation of Olivine, pyroxene, Fe-Ti oxide and ±plagioclase are the major factors responsible for the modification of the primary magma.
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Petrogenesis, Flood Basalts, Hirna Area, South Eastern Ethiopian Plateau