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首页> 外文期刊>Contributions to Mineralogy and Petrology >Fluid/rock interaction and mass transfer in continental subduction zones: constraints from trace elements and isotopes (Li, B, O, Sr, Nd, Pb) in UHP rocks from the Chinese Continental Scientific Drilling Program, Sulu, East China
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Fluid/rock interaction and mass transfer in continental subduction zones: constraints from trace elements and isotopes (Li, B, O, Sr, Nd, Pb) in UHP rocks from the Chinese Continental Scientific Drilling Program, Sulu, East China

机译：大陆俯冲带的流体/岩石相互作用和质量传递：来自中国大陆科学钻井计划（中国东部苏鲁）的UHP岩石中微量元素和同位素（Li，B，O，Sr，Nd，Pb）的约束

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In order to better understand the role of fluids during subduction and subsequent exhumation, we have investigated whole-rock and mineral chemistry (major and trace elements) 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 carefully selected sample sets have been chosen to investigate geochemical changes as a result of fluid mobilization during dehydration, peak metamorphism, and exhumation of deeply subducted continental crust. Our data reveal that while O and Sr-Nd-Pb isotopic compositions remain more or less unchanged, significant Li and/or B isotope fractionations occur between different lithologies that are in close contact during various metamorphic stages. Samples that are supposed to represent prograde dehydration as indicated by veins formed at high pressures (HP) are characterized by element patterns of highly fluid-mobile elements in the veins that are complementary to those of the host eclogite. A second sample set represents a UHP metamorphic crustal eclogite that is separated from a garnet peridotite by a thin transitional interface. Garnet peridotite and eclogite are characterized by a >10% difference in MgO, which, together with the presence of abundant hydroxyl-bearing minerals and compositionally different clinopyroxene grains demonstrate that both rocks have been derived from different sources that have been tectonically juxtaposed during subduction, and that hydrous silicate-rich fluids 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. Our study indicates that during high-grade metamorphism, Li and B may show different patterns of enrichment and of isotopic fractionation.
机译：为了更好地了解流体在俯冲和随后的掘出过程中的作用，我们在选定的连续钻探下研究了全岩石和矿物化学（主要元素和微量元素），Li，B以及O，Sr，Nd，Pb同位素通过对比来自中国苏禄的中国大陆科学钻探计划（CCSD）的岩性边界来获得岩心剖面。选择了四个精心选择的样本集来研究地球化学变化，这些变化是由于流体在脱水，峰变质和深俯冲的陆壳发掘过程中的动员所致。我们的数据表明，尽管O和Sr-Nd-Pb同位素组成或多或少保持不变，但在不同变质阶段紧密接触的不同岩性之间会发生显着的Li和/或B同位素分馏。如在高压（HP）下形成的静脉所指示的那样，该样品被认为代表前期脱水，其特征在于与主榴辉岩互补的静脉中高流动性元素的元素模式。第二个样品组代表UHP变质的榴辉岩，其通过薄的过渡界面与石榴石橄榄岩隔离。石榴石橄榄岩和榴辉岩的特征是MgO差异> 10％，再加上大量的含羟基矿物和成分不同的斜辉石晶粒，表明这两种岩石均来自俯冲过程中构造并列的不同来源，并且从俯冲板向地幔中添加了富含硅酸盐的含水流体。包括逆向闪石和相对新鲜的榴辉岩在内的另外两个样品组证明，除了外部流体外，内部流体还可能导致闪石的形成。 Li和B的浓度和同位素组成表明在进行性脱水过程中损失和同位素分馏。另一方面，在倒退过程中会添加Li和B同位素更重的流体。在小范围内，地幔衍生的岩石可能会被俯冲板块衍生的流体显着地交代。我们的研究表明，在高级变质过程中，Li和B可能表现出不同的富集和同位素分馏模式。

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