論文ID: 240730
This study presents petrological and whole-rock geochemical data on basaltic lavas from the Tewa Zobar region of the northwestern Ethiopian plateau to investigate their petrogenesis, with petrographic analysis indicating the lavas consist of plagioclase, pyroxene, olivine, and Fe-Ti oxide minerals, exhibiting aphanitic, porphyritic, and glomeroporphyritic textures. Most of the basaltic samples (MgO = 7.4 wt. % - 7.9 wt. %) show alkaline composition, with the exception of one sample with lower MgO content of 6.4 wt.% showing a sub-alkaline nature. The alkaline basalts show steep primitive mantle-normalized trace element patterns with elevated high-field strength elements (HFSE) and heavy rare earth element (REE) profiles, including higher (Sm/Yb)N, (La/Yb)N, and (La/Sm)N values, whereas the sub-alkaline rock sample #Z56 has flatter heavy-REE profiles with lower (Sm/Yb)N, (La/Yb)N, and (La/Sm)N values. Based on the REE model, the sub-alkaline basalt suggests a melt fraction of 3-4%, while the alkaline basalt indicates a lower melt fraction of 1-2%, accompanied by approximately 1-2% residual garnet of enriched mantle source. These basalts also exhibit enriched trace element patterns, with the (Nb/Th)PM ratios typically showing a higher range, while the sub-alkaline basalt displays a notably lower Nb/Th ratio. Additionally, the alkali basalts generally have a higher Nb content relative to La, with (Nb/La)PM ratios showing a narrow range, while the sub-alkaline basalt presents a lower Nb/La ratio. The alkaline basalts analyzed, with element ratios such as La/Nb, Ba/Nb, and Ba/La —excluding Sample Z56, which shows distinct values—fall within the range of mantle-derived basalts, indicating the absence of crustal contamination. The middle rare earth element (MREE)/heavy rare earth element (HREE) and light rare earth element (LREE)/HREE ratios indicate that the basaltic magma was derived from a mantle source in the garnet-spinel transition zone at ∼80 km depth, with small degree of melting and significant contributions from both subcontinental lithospheric mantle (SCLM) and asthenospheric sources. Elevated Zr/Hf values and high incompatible trace element enrichment suggest that small-volume metasomatic fluids may have enhanced the source region. The elevated Nb/Ta ratio in the mafic lavas suggests the magma likely originated from a SCLM, potentially modified by metasomatic processes involving carbonatite-like component, while the geochemical similarities between the alkaline basalts and high-Ti 1 (HT1) magmatism point to a connection with mantle plume activity, which may have influenced the recent eruptions on the northwestern Ethiopian plateau.
