Petrogenesis of Oligocene volcanic rocks of the Lake Tana area,Ethiopian large Igneous Province  

作　　者：Ayenachew Alemayhou Desta Asfawossen Asrat Minyahl Teferi Desta 

机构地区：[1]Department of Geology,School of Earth Sciences,Bahir Dar University,P.O.Box 79,Bahir Dar,Ethiopia [2]Department of Mining and Geological Engineering,Botswana International University of Science and Technology,Private Bag 16,Palapye,Botswana [3]School of Earth Sciences,Addis Ababa University,P.O.Box 1176,Addis Ababa,Ethiopia

出　　处：《Acta Geochimica》2023年第6期1104-1123,共20页地球化学学报（英文）

摘　　要：The Lake Tana area is located within a complex volcano-tectonic basin on the northwestern Ethiopian plateau.The basin is underlain by a thick succession of Oligocene transitional basalts and sub-alkaline rhyolites overlain in places,particularly south of the lake,by Quaternary alkaline to mildly transitional basalts,and dotted with Oligo-Miocene trachyte domes and plugs.This paper presents the results of integrated field,petrographic,and major and trace element geochemical studies of the Lake Tana area volcanic rocks,with particular emphasis on the Oligocene basalts and rhyolites.The studies reveal a clear petrogenetic link between the Oligocene basalts and rhyolites.The Oligocene basalts are:(1)plagioclase,olivine,and/or pyroxene phyric;(2)show an overall decreasing trend in MgO,Fe_(2)O_(3),and CaO with silica;(3)have relatively low Mg#,Ni and Cr contents and high Nb/La and Nb/Yb ratios;and(4)show LREE enriched and generally flat HREE patterns.All these imply the origin of the Oligocene basalts by shallow-level fractional crystallization of an enriched magma sourced at the asthenospheric mantle.The Oligocene rhyolites:(1)are enriched in incompatible while depleted in compatible trace elements,P and Ti;(2)show a strong negative Eu anomaly;(3)contain appreciable amounts of plagioclase,apatite,and Fe-Ti oxides;and(4)show clear geochemical similarity with well-constrained rhyolites from the Large Igneous Province(LIP)of the northwestern Ethiopian plateau.Low-pressure fractional crystallization of mantle-derived basaltic magma in crustal magma chambers explains the origin of these rhyolites.Our study further shows that the Oligocene basalts and rhyolites are co-genetic and the felsic rocks of the Lake Tana area are related differentiates of the flood basalt volcanism in the northwestern Ethiopian plateau.

关 键 词：Large igneous province Oligocene basalt Oligocene rhyolite Fractional crystallization Lake Tana 

分 类 号：P588.14[天文地球—岩石学]