西藏乃东县扎嘎当O型埃达克岩成因——来自地球化学和Sr-Nd-Pb同位素的制约

Petrogenesis of the Zhagadang O-type adakite in Naidong County of Tibet:Constraints from its geochemistry and Sr-Nd-Pb isotopic compositions

埃达克岩是具有特定地球化学性质的一套中酸性火成岩组合,具有多种成因模式和特殊的构造意义。位于西藏泽当一带的泽当蛇绿岩对研究新特提斯洋的演化具有重要的意义,选取泽当蛇绿岩内出露的奥长花岗岩为研究对象,开展了岩石学、岩石地球化学、Sr-Nd-Pb同位素等研究。研究显示,扎嘎当奥长花岗岩地球化学特征表现为高硅、高铝、富钠(Na 2 O/K 2 O>2)、低镁,准铝质钙碱性,同时具高Sr和Sr/Y值,低Y、Yb,属O型埃达克岩。岩石稀土元素总量偏低,富集轻稀土元素和大离子亲石元素Rb、K、Ba,亏损高场强元素Nb、Ti、P,Eu具微弱的正异常。岩石具有较高的Sr、Sm、Nd、Pb含量,I Sr=0.704251~0.704329,I Nd=0.512776~0.512807,εNd(t)=6.61~7.22。岩石铅同位素组成较均一,206 Pb/204 Pb=18.378~18.495,207 Pb/204 Pb=15.542~15.554和208 Pb/204 Pb=38.336~38.406。研究表明,扎嘎当奥长花岗岩源区来源于具有地幔特征的洋壳,由消减大洋板片在一定深度部分熔融形成。综合泽当地区的地质特征和前人研究成果,认为泽当洋内弧是存在的。

Adakite,as a set of intermediate-acid igneous rock assemblages with special geochemical characteristics,has a variety of genetic models and special tectonic significance.The Zedang ophiolite is of great significance to the study of the evolution of the Neo-Tethys Ocean.The trondhjemite outcropped in the Zedang ophiolite was selected to study its petrology,petrochemistry and Sr-Nd-Pb isotopes.The results show that the Zhagadang trondhjemite is a high SiO 2 and Al 2 O 3,rich Na 2 O(Na 2 O/K 2 O>2),low MgO,and para-aluminous calc-alkaline series.It is characterized by high Sr and Sr/Y,low Y and Yb.These geochemical characteristics are comparable with those of O-type adakite.The Zhagadang trondhjemite is rich in large ion lithophile elements(e.g.,Rb,K and Ba),poor in high field-strength elements(e.g.,Nb,Ti and P)and rare earth elements,and exhibits a weak positive Eu anomaly.The Sr-Nd-Pb isotopic compositions indicate that the Zhagadang trondhjemite is characterized by high Sr,Sm,Nd and Pb contents,with I Sr of 0.704251~0.704329,I Nd of 0.512776~0.512807,positiveεNd(t)values(6.61~7.22)and relatively uniform lead isotopic composition(206 Pb/204 Pb=18.378~18.495,207 Pb/204 Pb=15.542~15.554 and 208 Pb/204 Pb=38.336~38.406).The study indicates that the Zhagadang trondhjemite bearing mantle characteristics was formed by partial melting of the subducting oceanic slab at a certain depth.The result confirms the existence of the Zedang intra-oceanic.