Petrology and Geochemistry of Oligocene-Miocene Flood and Shield basalts exposed between Abay River and Mt. Choke, North Western Ethiopian Plateau
Creators: name: Tilahun Tamirat Wondifraw
Publisher: name: National Academic Digital Repository of Ethiopia
Year: 2020

description: Petrographic description and geochemical (major and trace element) data are presented for<br> basaltic samples from Abay Valley (Dejen (DJ) – Gohatsion (GT) section) all the way through<br> Chemoga (CM), Lumame (LM) and RobeGebya (RG) to southern Mt. Choke, NW-Ethiopian<br> plateau flood basalts and Shield volcano. Basalts of Abay Valley (DJ – GT) show aphanitic texture<br> with plg-olv-px microcrystalline in groundmass; basalts of CM – LM show poorly porphyritic<br> with plg and px in px-olv-plg groundmass; basalts of RG – Mt. Choke on the other hand show<br> moderately porphyritic with olv and plg, the groundmass contains px-olv-plg and some opaque<br> minerals. Geochemical classification (TAS and AFM plot) shows the investigated area contains<br> three groups of basaltic rocks; (1) tholeiitic basalt, (2) alkali basalt and (3) transitional basalts.<br> Samples from Chemoga (CM) and Lumame (LM) fall in Alkali basalt field, whereas samples from<br> GohaTsion (GT) and Dejen (DJ) fall in the field of tholeiitic basalt, and samples from RobeGebyaChoke fall in transitional basalt. The variations in major and trace element compositions especially<br> between the tholeiitic basalts group and the other two groups cannot be explained by fractional<br> crystallization, crustal contamination or degree of partial melting of a homogeneous mantle source,<br> but rather reflect the involvement of variable mantle and/or crustal sources. In the plot of Ba/Nb against La/Nb, three trends are observed: (1) represented by the alkali and transitional basaltic groups, which fall within the field of OIB; (2) defined by the tholeiitic basalts group, which shows a trend from the field of OIB towards the MORB field; and (3) defined by the tholeiitic basalt group that trending away from the fields of OIB and MORB towards continental crust, possibly indicating the involvement of<br> crustal component in the tholeiitic basalts. In the primitive mantle normalized incompatible elements pattern the Alkali basalt and Transitional basalt groups have significant enrichment in HFSE (Nb and Ta) relative to the LILE and LREE (La and Ce), which is a notable feature in OIB-like patterns. On the other hand, the Tholeiitic group basalts show a significant positive Ba anomaly relative to the other LILE (Rb and K). HFSE (Nb and Ta) in the tholeiitic basalts are almost flat with LREE (La and Ce).Therefore, from the observed trace element patterns in the groups, the Alkali and Transitional group basalts are more likely mantle plume derived. The positive Ba anomaly, relative to the other LILE (Rb and K) together with low K2O/Na2O (0.26-0.42) and Rb/Sr ratio suggests the magma for tholeiitic basaltic group may formed from melting of amphibole bearing source-region. Since amphibole mineral is stable in the lithospheric mantle, the presence of anomalous Ba in the tholeiitic basalt indicating the source of those basalts were melting of an amphibole bearing sub-continental lithospheric mantle (SCLM) components. In the Nb/Zr versus Rb/Zr and Ba/Zr plot most of the samples plotted inside themantle array. Alkali basalt and transitional basalt show higher Nb/Zr, Ba/Zr and Rb/Zr ratios and falls in the mantle array more close to oceanic island basalt (OIB). The Tholeiitic basalts, on the other hand,<br> show lower Nb/Zr, Ba/Zr and Rb/Zr ratios with wider Ba/Zr ratio, falls inside mantle array more close<br> to E-type mid-oceanic ridge basalt (E-MORB), as well as outside the mantle array close to upper crust<br> (UC) field. Hence, unlike the alkali and transitional basalt, the tholeiitic basalts indicate crustal<br> contaminant involvement in their magma genesis. Therefore, the alkali and transitional basaltic groups<br> (shield volcanics), that show an OIB like composition, possibly originated from mantle plume source,<br> whereas the tholeiitic basaltic group (flood basalt) require source components from E-MORB and<br> SCLM mantles with minor crustal material. The geochemical characters of the different layers of the<br> tholeiitic basalts at Gohatsion section that are separated by paleosols show entirely similar feature,<br> implying that they belong to the same origin and mantle source.<br>