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大陆俯冲微量元素和同位素的特征:中国苏鲁造山带CCSD中超高压变质岩的证据

发表时间:2014-11-15  浏览量:1650  下载量:703
全部作者: 郭海浩,肖益林
作者单位: 中国科学技术大学地球和空间科学学院,中国科学院壳幔物质与环境重点实验室
摘 要: 为更好地了解流体在俯冲和折返过程中的作用,这里对中国大陆深钻(Chinese continent scientific drilling,CCSD)中不同接触序列的样品进行全岩和矿物的元素分析,以及Li,B,O,Sr-Nd-Pb的同位素研究。仔细选择的4组样品地球化学变化显示深俯冲陆壳在脱水、峰期变质和退变质过程中的流体迁移性质。研究数据显示,O和Sr-Nd-Pb同位素在不同变质阶段的岩石之间基本保持不变,但Li和B同位素存在明显分馏。1组样品的脉体中流体活动性元素和主体榴辉岩呈现互补关系,代表变质脱水过程。2组样品是一个超高压(ultra high-pressure,UHP)的榴辉岩和一石榴橄榄岩的转换带。石榴橄榄岩和榴辉岩的MgO含量存在10%的差别,含水矿物和不同成分的单斜辉石成分证明,这2种不同来源岩石是在俯冲过程中聚合在一起的,并且富硅的含水流体从俯冲板块转移到了地幔。其他2组样品包含相对新鲜的榴辉岩和角闪岩,显示除了外部流体,内部流体导致角闪岩的形成。Li和B的含量与同位素的比值指示在脱水过程中存在Li,B的迁移和同位素的分馏。另外,退变质过程中重的Li和B同位素将加入流体中。在小尺度上,幔源的岩石可能经历了俯冲板片释放的流体交代作用。研究表明,在高级变质过程中,Li和B表现出不同形式的富集以及同位素的分馏作用。
关 键 词: 地质学;元素迁移;同位素分馏;俯冲和折返;苏鲁
Title: Trace elements and isotopes characters in continental subduction zones: constraints from ultra high pressure rocks from the CCSD, Sulu, China
Author: GUO Haihao, XIAO Yilin
Organization: School of Earth and Space Sciences, University of Science and Technology of China, CAS Key Laboratory of Crust-Mantle Materials and Environments
Abstract: In order to better understand the role of fluids during subduction and subsequent exhumation, we have investigated whole-rock and mineral chemistry and Li, B as well as O, Sr-Nd-Pb isotopes on selected continuous drill-core profiles through contrasting lithological boundaries from the Chinese continental scientific drilling (CCSD) program in Sulu, China. Four selected sample sets have been taken to investigate geochemical changes as a result of fluid mobilization during dehydration, peak metamorphism, and exhumation of subducted continental crust. Our data reveal that while O and Sr-Nd-Pb isotopic compositions remain unchanged, significant Li and B isotope fractionations occur between different lithologies that are in close contact during various metamorphic stages. On a sample set which is supposed to represent prograde dehydration, we observe that the element patterns of highly fluid-mobile elements in the veins are complementary to those of the host eclogite. A second sample set represents an example of ultra high-pressure (UHP) metamorphic crustal eclogite that is separated from a garnet peridotite by a thin transitional interface. Garnet-peridotite and eclogite are characterized by>10% difference in MgO, which, together with the presence of abundant hydroxyl-bearing minerals and compositionally different Cpx-II grains occurring as overgrowth parts at the rim of Cpx-I along the interface, demonstrate that both rocks must have originated from different magmas that have been tectonically juxtaposed during subduction and that hydrous fluids rich in silicate-component have been added from the subducting slab to the mantle. Two additional sample sets, comprising retrograde amphibolite and relatively fresh eclogite, demonstrate that besides external fluids, internal fluids can be responsible for the formation of amphibolite. Li and B concentrations and isotopic compositions point to losses and isotopic fractionation during progressive dehydration. On the other hand fluids with isotopically heavier Li and B are added during retrogression. On a small scale, mantle-derived rocks may be significantly metasomatized by fluids derived from the subducted slab. It is noteworthy that Li and B isotope fractionation may or may not behave coherently during slab subduction and subsequent exhumation.
Key words: geology; elemental transfer; isotopic fractionation; subduction and exhumation; Sulu
发表期数: 2014年11月第21期
引用格式: 郭海浩,肖益林. 大陆俯冲微量元素和同位素的特征:中国苏鲁造山带CCSD中超高压变质岩的证据[J]. 中国科技论文在线精品论文,2014,7(21):2098-2112.
 
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