Responsible sourcing of minerals: putting values into the value chain
Responsible sourcing of minerals: putting values into the value chain
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负责任的矿产采购：将价值纳入价值链
DOI：
10.1080/25726838.2019.1599195
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发表时间：
2019
期刊：
影响因子：
--
通讯作者：
Bilham N
中科院分区：
文献类型：
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作者：
Bilham N
超高压（UHP）变质作用长期以来一直与深俯冲有关。超高压矿物，如柯石英和微金刚石，往往发现在离散的镁铁质单位托管在较大规模的大陆地形。人们普遍认为，这些大陆地形的整体也必须被俯冲到超高压条件。然而，证据表明，长英质材料在榴辉岩相条件下转化是缺乏的。超高压榴辉岩香肠产于花岗岩和花岗片麻岩中。花岗岩被角闪岩相、巴罗变质作用所覆盖，因此它们的早期历史很难解释。我们提出的证据复杂的岩浆，热液和变质的历史保存在2云母堇青石花岗岩。第一阶段：火成岩域与它们的结构关系和原生火成岩矿物一起被沿着保存下来。第二阶段：侵位过程中花岗岩的后期热液蚀变在矿物化学上是明显的，特别是在火成岩域中保存的白色云母。第三阶段：大陆物质俯冲过程中高压变质作用的证据被保存为火成岩黑云母周围的石榴石薄冠和堇青石之后的假像。第四阶段：在较低压力下的后期变质作用在矿物组合、矿物化学和石英显微结构中是明显的，这些矿物组合、矿物化学和石英显微结构显示出温度的降低。结合岩相学、地质年代学和热力学模型，我们可以破译这些花岗岩的历史，确定它们的早期岩浆和岩浆热液历史，以及它们是否在榴辉岩相条件下转化。花岗岩的变质作用取决于中下地壳水的供应。有证据表明，在花岗岩和镁铁质榴辉岩的退变质过程中，存在多次水化事件。然而，水在中下地壳中的作用，特别是在俯冲的早期阶段，以及水如何影响大陆俯冲的地球动力学，目前还不清楚，这是我们研究的一个关键方面。
Ultra-high pressure (UHP) metamorphism has long been associated with deep subduction. UHP minerals, such as coesite and microdiamond, are often found in discrete mafic units hosted in larger scale continental terrains. It is widely accepted that the entirety of these continental terrains must also have been subducted to UHP conditions. However, evidence that the felsic material transformed at eclogite facies conditions is lacking. The UHP eclogite boudins are hosted in granite and granite gneiss. The granites have been overprinted by amphibolite facies, Barrovian metamorphism, so their early history is difficult to decipher. We present evidence of a complex magmatic, hydrothermal and metamorphic history preserved within a 2-mica cordierite granite. Stage 1: Igneous domains are preserved along with their textural relationships and primary igneous minerals. Stage 2: Late stage hydrothermal alteration of the granite during emplacement is evident in the mineral chemistry, particularly in the white micas preserved in the igneous domains. Stage 3: Evidence of high-pressure metamorphism during subduction of the continental material is preserved as thin coronas of garnet around igneous biotite and in pseudomorphs after cordierite. Stage 4: Later metamorphism at lower pressures is evident in the mineral assemblages, mineral chemistry and quartz microstructures which show decreasing temperatures.Using a combination of petrography, geochronology and thermodynamic modelling, we can decipher the history of these granites and determine their early magmatic and magmatic-hydrothermal history and whether they transformed at eclogite facies conditions. The transformation of granite depends on the availability of water in the mid to lower crust. There is evidence for multiple hydration events in the granite and mafic eclogites on their retrograde path. However, the role of water in the mid to lower crust, particularly in the early stages of subduction, and how this influences the geodynamics of continental subduction is not understood and forms a key facet of our research.