Geochemical and Petrographic Characteristics of Crystal Rich Volcanic Rocks From Lalibela Area, Northwestern Ethiopia.
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Date
2018-06-03
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Addis Ababa University
Abstract
The Bilbala magmatic complex is situated in northwestern Ethiopian plateau near to northern main Ethiopia rift. The aim of this work was to investigate and identify the sources involved in the petrogenesis of the pre –rift basalts of the Bilbala area around Lalibela wich located with approximate distance of 730 Km from Addis Ababa. Field observation, ptrographic analysis and geochemical data analysis are the main method used to meet the aim or objective of this study. Based on field observation and description the study area is covered by basaltic rocks. The paper presents the results of a comprehensive major element, trace element, and petrographic study of Oligocene volcanic sequences from the northwestern Ethiopian flood basalt province around Lalibela. A petrographic analysis result with field observation indicates the presence of clinopyroxene, olivine and plagioclase as phenocrysts and groundmass with Fe-Ti oxides, opaque minerals and volcanic glass. The major and trace element compositions with mineral assemblage of crystal rich basaltic rocks in the area show the HT basalts characteristics of the Ethiopian continental flood basalt. The analyzed samples result displays high contents of TiO2 (3.36-6.87%), FeO* (11.59-15.71%), CaO/Al2O3 (0.89-1.989%) and enrichment of incompatible trace elements and LREE. On primitive mantle normalized diagram of the studied samples there is positive Ta and Nb anomalies, but Sr with Pb shows negative anomalies.Enrichment of LREE and a relative depletion of HREE in the studied samplesis observed and this suggest the presence of residual garnet in the source, and from trace elements concentration and ratios there is little evidence for contamination during the magmatic ascent.The Ce/Pb, Nb/Th and Nb /U ratios of the study area basaltic rocks display 21.3-51.7, 9.5-13.8 and 33.1-45.2 respectively. These elemental ratios have nearly OIB type character. The studied samples have low Zr/Nb ratios (< 10). The low Zr/Nb value of this mafic lava might imply a dominant mantle plum contribution during the genesis of the study area lavas.
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Keywords
Bilbala, Fractional Crystallization, Crustal Contamination, Partial Melting, Mantle Plume, Depletion, Enrichment.