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
首发时间:2013-07-05
摘要:为了更好了解流体在俯冲和折返过程中的作用,我们对中国大陆深钻(CCSD)中不同接触序列的样品进行了全岩和矿物的元素分析,Li, B, O, Sr, Nd, Pb的同位素研究。仔细选择的4组样品的地球化学变化显示了深俯冲陆壳在脱水过程,峰期变质和退变质过程的流体迁移性质。我们的数据显示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。一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。二组样品是一个超高压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10%的差别,含水矿物和不同成分的单斜辉石成分证明这两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。其它两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。在小的尺度上,幔源的岩石可能经历了俯冲板片释放的流体的交代作用。本研究表明在高级变质过程中,Li和B表现不同形式的富集以及同位素的分馏作用。
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Trace elements and isotopes characters in continental subduction zones: constraints from UHP rocks from the CCSD, Sulu, East China
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 Program (CCSD) 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 datas 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 as indicated by veins formed at high-pressures (HP), 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 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.
Keywords: Fluid/rock interaction elemental transfer isotopic fractionation subduction and exhumation Sulu
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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
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