摘要
以同构造期代表古洋壳残片的蛇绿岩及产于古岛弧的玄武岩为基础，通过Ｎｄ，Ｐｂ同位素与微量元素示踪及岩浆源区分析，揭示出北秦岭元古宙上地幔以强亏损（εＮｄ（ｔ）＋６．３～＋７．３）和高的Ｙｂ／Ｈｆ，Ｎｂ／Ｌａ和Ｔｈ／Ｌａ比值为特征，北秦岭地壳和上地幔明显具有Ｐｂ同位素比值高的特征．北秦岭丹凤群岛弧火山岩、二郎坪群弧后玄武岩以及松树沟蛇绿岩中变拉斑玄武岩εＮｄ（ｔ）、放射成因Ｐｂ同位素、Ｙ／Ｔｂ和Ｔｉ－ＭｇＯ研究表明，本区玄武岩存在两类性质不同的岩浆源．一类与亏损的北秦岭岩石圈上地幔源区有关；另一类与携带海洋沉积物的洋壳板块俯冲参与有关．由此。
A Neoproterozoic convergence-subduction zone has been identified in the North Qinling belt from geological and geochemical studies.Nd isotopic composition and highly incompatible elements ratios(Nb/La,Th/La,Yb/Hf,Y/Tb) of Proterozoic mantle-derived rocks,and the Pb isotopic composition of Proterozoic and Early Paleozoic mantle-and crust-derived rocks,reveal that the North Qinling is characterized by high depletion( ε Nd (t) )=+6.3～+7.3)and higher Yb/Hf,Nb/La and Th/La ra￣tios in the upper mantle,and by distinctively high Pb isotopic ratios in both the crust and upper mantle.The Neoproterozoic island-arc basalts of the Danfeng Group are similar in the initial ε Nd (t) value (up to+7.3) and high Pb isotopic ratios to both the North Qinling upper mantle and the Neoproterozoic ophiolitic metatholeiite with geochemical features of MORB.The Sm-Nd model age and Yb/Hf ratio of Danfeng basalts are closer to those of the ophiolitic metatholeiite compared with the Paleo-and Mesoproterozoic mantle-deived rocks from North Qinling.The Danfeng basalts are suggested to be generated by magmas from two main sources based on the Y/Tb vs.Y and Ti vs.MgO plots.The source with Y/Tb ratio of about 36.5 is deduced mainly to be the North Qinling depleted lithospheric mantle with a similar Y/Tb ratio which constituted the mantle wedge above the subduction zone.The source with Y/Tb ratio of about 29 is reasonably considered as the subducted oceanic slab together with some pelagic sediments generally characterized by significantly lower Y/Tb ratio and ε Nd (t) value and higher radiogenic Pb compared with typical MORB.The increasing trend in Pb isotopic ratios with decreasing in ε Nd (t) value of Danfeng basalts in combination with lower Y/Tb ratio in many samples of the basalts implies the involvement of sedimentary material in the genesis of Danfeng island-arc volcanics.Thereby,the result demonstrates the recycling of crustal materials into the upper mantle at the convergence-subduction zone.
出处
《地球科学（中国地质大学学报）》
EI
CAS
CSCD
北大核心
1996年第5期469-475,共7页
Earth Science-Journal of China University of Geosciences
基金
国家自然科学基金
地质矿产部壳幔体系组成
物质交换及动力学开放研究实验室基金
关键词
古聚会带
壳幔再循环
秦岭造山带
岛弧岩浆作用
ancient convergence zone,crust-mantle recycling,Qinling orogenic belt,magmatism in island-arc setting.
