Deep subduction in Earth history: Seeking for traces in the sedimentary record

现代板块构造以冷俯冲(&lt；350℃/Gpa)和深俯冲(&gt；100公里)为特征。它的出现标志着地球历史上的一次重大变化，然而，从中太古代(3Ga)到新元古代(1.0Ga到0.7Ga)，地球的时间存在很大争议。现代板块构造的一个关键证据是超高压(UHP)岩石的出现，这些岩石反映了地壳物质向下深达100公里的深度俯冲。寻找超高压岩石受到出了名的不完整地质记录的影响，并受到检测超高压岩石的费力和空间效率低下的技术的阻碍。我们最近开发并测试了一种新的方法，能够利用碎屑矿物中的超高压包裹体在集水区范围内追踪超高压岩石。这一方法将被用来检验我们的中心假设，即深俯冲过程导致超高压条件在新元古代之前已经运行。该项目分为两个阶段。第一阶段的目的是改进新的方法，以便开发一种高效的技术，以筛选存在超高压岩石的地区。排出中小型集水区的沉积物样品将从两个区域进行分析，这两个区域显示了来自几种宿主矿物和岩性的各种不同的超高压指示物，特别是柯石英和/或微钻石包裹体。德国的ErzgeBirge和挪威的西片麻岩地区之所以被选中，是因为大量的前期工作，已经存在的矿物分离，以及已证明的岩性差异。第二阶段旨在调查两个古元古代地区，加拿大东北部的横贯哈德逊造山带和西格陵兰岛的Nagssugtoqdian造山带，那里的超高压条件得到了指示，但尚未被柯石英和/或微钻石包裹体等确凿证据所证实。排出这些地区的河流的沉积物将用超高压指示器筛选碎屑矿物，该方法在第一阶段得到改进。一个成功的项目将(I)证明深部俯冲过程已经在古元古代进行，以及(Ii)允许扩大所考虑的超高压变质作用的区域和时期，以便更好地限制地球上现代式板块构造的开始时间。

Modern-style plate tectonics are characterized by cold (<350°C/GPa) and deep (>100 km) subduction. Its onset marks a major change in the history of planet Earth, the timing of which, however, is highly controversial ranging from Mesoarchean (3 Ga) to Neoproterozoic (1.0 to 0.7 Ga). A key evidence for modern-style plate tectonics is the occurrence of ultrahigh-pressure (UHP) rocks reflecting deep subduction of crustal material down to depths >100 km. Searching for UHP rocks is biased by the notoriously incomplete geological record and hampered by laborious and spatially inefficient techniques to detect UHP rocks. We have recently developed and tested a new approach capable to trace UHP rocks at the catchment scale using UHP inclusions in detrital minerals. This approach will be used to test our central hypothesis that deep subduction processes leading to UHP conditions already operated prior to the Neoproterozoic.The project is subdivided into two phases. The first phase is aimed at improving the new approach in order to develop a highly efficient technique to screen a region regarding the presence of UHP rocks. Sediment samples draining small to medium sized catchments will be analyzed from two regions, which show a variety of distinct UHP indicators, in particular coesite and/or microdiamond inclusions, from several host minerals and lithologies. The Erzgebirge in Germany as well as the Western Gneiss Region in Norway has been chosen because of extensive previous work, already existing mineral separates, and the demonstrated lithological diversity.The second phase is aimed at investigating two Paleoproterozoic regions, the Trans-Hudson Orogen in North-East Canada and the Nagssugtoqidian Orogen in West Greenland, where UHP conditions are indicated but not yet proven by irrefutable evidence such as coesite and/or microdiamond inclusions. Sediments from streams draining these regions will be screened for detrital minerals with UHP indicators applying the approach refined during the first phase. A successful project will (i) demonstrate that deep subduction processes already operated in the Paleoproterozoic, and (ii) allow for expanding regions and periods considered for UHP metamorphism in order to better constrain the timing of onset of modern-style plate tectonics on Earth.