摘要
伦圭新生代火山岩位于东非裂谷西支最南端,已有的大量地质与地球化学证据表明伦圭火山岩是典型的碱性双峰式火山岩,其SiO_(2)含量在46%~56%之间出现明显的间断。基性火山岩和酸性火山岩均表现出富集轻稀土元素,轻、重稀土元素强烈分馏,无明显Eu异常的特征。基性岩大离子亲石元素Rb、Ba、Th、U、Sr富集,高场强元素Zr、Hf、Ti亏损,Nb-Ta相对于La弱亏损,具有低Th/Ce(<0.05)和Th/La(0.07~0.09)值。酸性岩不相容元素整体上呈现不同程度的富集,Sr、P、Ti亏损,Nb-Ta相对于La弱富集,显示较强的岩浆分异作用。基性岩和酸性岩均表现出一定的Pb富集,说明有微量的地壳混染。基性岩εNd(t)为+1.50~+3.70,(^(87)Sr/^(86)Sr)i为0.7048~0.7057;除T19样品具有负的εNd(t)值外,其他所有酸性岩εNd(t)值介于+0.21~+5.13,(^(87)Sr/^(86)Sr)i为0.7043~0.7061,可见伦圭基性火山岩和酸性火山岩Sr、Nd同位素特征具有一致性。根据同位素地球化学和元素地球化学特征,认为其基性岩来源于一个整体亏损的地幔,并受到了大陆岩石圈地幔的混染,而酸性火山岩则是基性岩浆分离结晶作用的产物。
The Rungwe Cenozoic volcanic rocks,which are located at the southern end of the western branch of the East African Rift,have been provento be typical alkaline bimodal volcanic rocks by many lines of geological and geochemical evidence.The SiO_(2)content of Rungwe volcanic rocks exhibits a gap between 46%and 56%.Both basic and acidic volcanic rocks are enriched in light rare earth elements with highly fractionated LREEs and HREEs.In chondrite-normalized spider grams,all rocks have no Eu anomalies.The basic rocks are characterized by high abundance of Rb,Ba,Th,U,Sr,and depletion in HFSE(Zr,Hf,Ti),Nb-Ta are weakly depleted compared to La,with low Th/Ce(<0.5)and Th/La(0.07~0.09)value.The incompatible elements of acidic rocks are enriched todifferent degrees,and depleted in Sr,P and Ti.Compared to La,Nb-Ta is weakly enriched suggesting stronger crystallization differentiation of magma.Pb is enriched in all the samples,indicating that there is a trace of crustal contamination.The basic rocks are characterized byεNd(t)=+1.50~+3.70,(^(87)Sr/^(86)Sr)i=0.7048~0.7057,while acidic rocks are characterized byεNd(t)=+0.21~+5.13,(^(87)Sr/^(86)Sr)i=0.7043~0.7061 except the T19 sample which has negativeεNd(t)value.These data indicate that the acidic rocks are similar in Sr-Nd isotopes to basic rocks and that the basic rocks were derived from a depleted mantle with contaminationfromthe continental lithospheric mantle,while the associated acidic rocks were formed by fractional crystallization of the basic magma.
作者
孙凯
张琳琳
吴兴源
刘晓阳
何胜飞
许康康
龚鹏辉
任军平
SUN Kai;ZHANG Linlin;WU Xingyuan;LIU Xiaoyang;HE Shengfei;XU Kangkang;GONG Penghui;REN Junping(School of Earth Science and Resources,China University of Geosciences(Beijing),Beijing.100083;Tianjin Center.China Geological Survey,Tianjin,300170;Eastern and Southern A frica Geosciences Coo peration Research Center,CGS,Tianjin,300170;Sinomine Rock and Mineral Analysis(Tianjin)Co.,Ltd,,Tianjin,300270)
出处
《地质学报》
EI
CAS
CSCD
北大核心
2021年第4期962-975,共14页
Acta Geologica Sinica
基金
中国地质调查项目(编号DD20160108、DD20190439和1212011220910)
商务部技术援外项目(编号[2015]352)资助的成果。
作者简介
孙凯,男,1986年生。工程师,博士研究生,主要从事南部非洲地质矿产勘查与研究工作。通讯地址:300170,天津市大直沽八号路4号,中国地质调查局天津地质调查中心境外地质室,Email:tjcentersk@foxmail.com。
