Chemical geodynamics of continental subduction-zone metamorphism: Insights from studies of the Chinese Continental Scientific Drilling (CCSD) core samples
Chemical geodynamics of continental subduction-zone metamorphism: Insights from studies of the Chinese Continental Scientific Drilling (CCSD) core samples
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DOI：
10.1016/j.tecto.2008.09.014
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发表时间：
2009-09
期刊：
影响因子：
2.9
通讯作者：
Yong‐Fei Zheng;Ren‐Xu Chen;Zi‐Fu Zhao
中科院分区：
文献类型：
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作者：
Yong‐Fei Zheng;Ren‐Xu Chen;Zi‐Fu Zhao
中国中东部大别-苏鲁造山带一直是研究超高压构造相关地球动力学过程的典型位置。我们对大陆深俯冲带中世界上最神秘的过程的大部分理解都是从这个带的各种记录中推断出来的。利用中国大陆科学钻探（CCSD）项目在苏鲁造山带岩心样品的深度剖面，对主孔（MH）的超高压变质岩在100 ~ 5000 m的连续深度上进行了一系列联合研究。研究结果为大陆俯冲带变质作用的化学地球动力学，特别是地表露头不能解决的问题提供了新的认识。我们对CCSD-MH岩心样品进行地球化学研究的现有结果概述如下。(1)建立了UHP变质矿物100 ~ 5000 m的O同位素剖面，在3300 m深处发现18o亏缺。除了沿大别-苏鲁造山带的面积18o衰竭超过3万km2外，华南地块北缘的三维18o衰竭超过10万km2。(2)矿物O同位素、H同位素和含水量的变化发生在榴辉岩-片麻岩转变过程中，与岩相变化一致。因此，不同岩性之间的接触是流体最有利的作用部位；榴辉岩退离榴辉岩-片麻岩边界的流体来源于减压脱溶。而与片麻岩相邻的榴辉岩则主要由片麻岩含水流体引起的逆行变质作用。对相邻样品的距离、岩石学和δ18O值关系的考察表明，不同岩性和相同岩性在20 ~ 50 cm尺度上的O同位素非均质性与大陆碰撞过程中流体流动的最大尺度相对应。(3)两个连续核心段的主要元素和微量元素研究表明，LILE和LREE迁移率高，而HFSE和HREE不迁移。有些榴辉岩的安山岩成分具有高SiO2、高碱、高LREE和LILE，而低CaO、MgO和FeO含量。这些特征可能是由于与片麻岩的化学交换，可能是由于在挖掘过程中伴生片麻岩的部分熔融产生的长硅熔体的交代作用。另一方面，部分榴辉岩与熔体萃取形成的难熔岩具有地球化学亲和性，表现为较强的LREE和LILE亏缺和无含水矿物。这些结果为UHP变质岩中熔体引起的元素迁移提供了证据，因此在挖掘过程中可能存在超临界流体。其中，SiO2、LREE、LILE等元素的丰度变化较大，主要发生在榴辉岩与片麻岩接触处。这表明它们在不同板构件之间的流动性，尽管它只发生在小尺度上，因此在局部开放系统中受到限制。(4)尽管存在广泛的退变，但退变流体在稳定同位素组成中具有内部缓冲作用，退变流体为氘源流体，来源于名义上无水矿物中结构羟基和分子水的减压析出。(5)岩石学和年代学研究表明，HP榴辉岩相重结晶发生在216±3 Ma，时间尺度在1.9 ~ 9.3 Myr以下。总的来说，大别-苏鲁UHP地区…
The Dabie–Sulu orogenic belt of east-central China has long been a type location for the study of geodynamic processes associated with ultrahigh-pressure (UHP) tectonics. Much of our understanding of the world's most enigmatic processes in continental deep-subduction zones has been deduced from various records in this belt. By taking advantage of having depth profiles from core samples of the Chinese Continental Scientific Drilling (CCSD) project in the Sulu orogen, a series of combined studies were carried out for UHP metamorphic rocks from the main hole (MH) at continuous depths of 100 to 5000 m. The results provide new insights into the chemical geodynamics of continental subduction-zone metamorphism, especially on the issues that are not able to be resolved from the surface outcrops. Available results from our geochemical studies of CCSD-MH core samples can be outlined as follows. (1) An O isotope profile of 100 to 5000 m is established for the UHP metamorphic minerals, with finding of18O depletion as deep as 3300 m. Along with areal18O depletion of over 30,000 km2along the Dabie–Sulu orogenic belt, three-dimensional18O depletion of over 100,000 km3occurs along the northern margin of the South China Block. (2) Changes in mineral O isotope, H isotope and water content occur in eclogite-gneiss transitions, concordant with petrographic changes. The contact between different lithologies is thus the most favorable place for fluid action; fluid for retrogression of the eclogites away from the eclogite-gneiss boundary was derived from the decompression exsolution. For the eclogites adjacent to gneiss, in contrast, the retrograde metamorphism was principally caused by aqueous fluid from the gneiss that is relatively rich in water. Inspection of the relationship between the distance, petrography and δ18O values of adjacent samples shows O isotope heterogeneities between the different and same lithologies on scales of 20 to 50 cm, corresponding to the maximum scales of fluid mobility during the continental collision. (3) Studies of major and trace elements in the two continuous core segments indicate high mobility of LILE and LREE but immobility of HFSE and HREE. Some eclogites have andesitic compositions with high SiO2, alkalis, LREE and LILE but low CaO, MgO and FeO contents. These features likely result from chemical exchange with gneisses, possibly due to the metasomatism of felsic melt produced by partial melting of the associated gneisses during the exhumation. On the other hand, some eclogites appear to have geochemical affinity to refractory rocks formed by melt extraction as evidence by strong LREE and LILE depletion and the absence of hydrous minerals. These results provide evidence for melt-induced element mobility in the UHP metamorphic rocks, and thus the possible presence of supercritical fluid during exhumation. In particular, large variations in the abundance of such elements as SiO2, LREE and LILE occur at the contact between eclogite and gneiss. This indicates their mobility between different slab components, although it only occurs on small scales and is thus limited in local open-systems. (4) Despite the widespread retrogression, retrograde fluid was internally buffered in stable isotope compositions, and the retrograde fluid was of deuteric origin and thus was derived from the decompression exsolution of structural hydroxyl and molecular water in nominally anhydrous minerals. (5) A combined study of petrography and geochronology reveals the episode of HP eclogite-facies recrystallization at 216±3 Ma, with timescale of 1.9 to 9.3 Myr or less. Collectively, the Dabie–Sulu UHP terrenes …