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

现代板块构造以冷（<350°C/GPa）和深（100 ~ 100 km）俯冲为特征。它的发生标志着地球历史上的一个重大变化，但其发生的时间在中太古代（3 Ga）到新元古代（1.0 ~ 0.7 Ga）之间存在很大争议。现代板块构造的一个关键证据是超高压（UHP）岩石的出现，反映了地壳物质的深俯冲至100公里深处。由于地质记录的不完整，以及探测超高压岩石的费力和空间效率低下的技术，对超高压岩石的搜索存在偏见。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。这种方法将用于验证我们的中心假设，即深层俯冲过程导致超高压条件在新元古代之前就已经发生了。该项目分为两个阶段。第一阶段的目标是改进新方法，以便开发一种高效的技术来筛选存在超高压岩石的区域。将从两个区域分析排放中小型集水区的沉积物样品，这些样品显示了各种不同的超高压指标，特别是来自几种主矿物和岩性的粘土和/或微金刚石内含物。之所以选择德国的Erzgebirge和挪威的西部片麻岩地区，是因为之前有大量的工作，已经存在的矿物分离，以及已经证明的岩性多样性。第二阶段的目标是调查两个古元古代区域，即加拿大东北部的跨哈德逊造山带和西格陵兰岛的Nagssugtoqidian造山带，在那里发现了超高压条件，但尚未被不可辩驳的证据（如硅质岩和/或微金刚石包裹体）证明。将采用第一阶段改进的方法，用超高压指标筛选从这些地区流出的河流沉积物中的碎屑矿物。一个成功的项目将(i)证明深俯冲过程已经在古元古代起作用，（ii）允许扩展考虑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板块构造的发生时间。

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.