Hawkesworth Chris, Cawood Peter A, Dhuime Bruno
School of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, UK.
School of Earth, Atmosphere and Environment, Monash University, Melbourne, VIC 3800, Australia.
Front Earth Sci (Lausanne). 2020 Aug 6;8. doi: 10.3389/feart.2020.00326.
The Earth is the only known planet where plate tectonics is active, and different studies have concluded that plate tectonics commenced at times from the early Hadean to 700 Ma. Many arguments rely on proxies established on recent examples, such as paired metamorphic belts and magma geochemistry, and it can be difficult to establish the significance of such proxies in a hotter, older Earth. There is the question of scale, and how the results of different case studies are put in a wider global context. We explore approaches that indicate when plate tectonics became the dominant global regime, in part by evaluating when the effects of plate tectonics were established globally, rather than the first sign of its existence regionally. The geological record reflects when the continental crust became rigid enough to facilitate plate tectonics, through the onset of dyke swarms and large sedimentary basins, from relatively high-pressure metamorphism and evidence for crustal thickening. Paired metamorphic belts are a feature of destructive plate margins over the last 700 Myr, but it is difficult to establish whether metamorphic events are associated spatially as well as temporally in older terrains. From 3.8-2.7 Ga, suites of high Th/Nb (subduction-related on the modern Earth) and low Th/Nb (non-subduction-related) magmas were generated at similar times in different locations, and there is a striking link between the geochemistry and the regional tectonic style. Archaean cratons stabilised at different times in different areas from 3.1-2.5 Ga, and the composition of juvenile continental crust changed from mafic to more intermediate compositions. Xenon isotope data indicate that there was little recycling of volatiles before 3 Ga. Evidence for the juxtaposition of continental fragments back to ~2.8 Ga, each with disparate histories highlights that fragments of crust were moving around laterally on the Earth. The reduction in crustal growth at ~ 3 Ga is attributed to an increase in the rates at which differentiated continental crust was destroyed, and that coupled with the other changes at the end of the Archaean are taken to reflect the onset of plate tectonics as the dominant global regime.
地球是唯一已知的板块构造活跃的行星，不同研究得出结论，板块构造开始的时间从太古代早期到7亿年前不等。许多观点依赖于基于近期实例建立的代理指标，如成对变质带和岩浆地球化学，而在更热、更古老的地球上确定这些代理指标的意义可能很困难。存在尺度问题，以及不同案例研究的结果如何置于更广泛的全球背景中。我们探索一些方法，以表明板块构造何时成为全球主导机制，部分是通过评估板块构造的影响何时在全球范围内确立，而不是其在局部地区存在的最初迹象。地质记录反映了大陆地壳何时变得足够坚硬以促进板块构造，这通过岩墙群和大型沉积盆地的出现、相对高压变质作用以及地壳增厚的证据得以体现。成对变质带是过去7亿年中消减型板块边缘的一个特征，但很难确定在更古老的地形中变质事件在空间和时间上是否相关联。在38亿年至27亿年前，不同地点在相似时间产生了高钍/铌（在现代地球上与俯冲有关）和低钍/铌（与非俯冲有关）的岩浆组合，并且地球化学与区域构造样式之间存在显著联系。太古宙克拉通在不同地区于31亿年至25亿年前的不同时间稳定下来，新生大陆地壳的成分从镁铁质变为更中性的成分。氙同位素数据表明，在30亿年前之前挥发性物质的再循环很少。追溯到约28亿年前大陆碎片并列的证据，每个碎片都有不同的历史，这突出表明地壳碎片在地球上横向移动。约30亿年前地壳生长的减少归因于分异的大陆地壳被破坏速率的增加，而这与太古宙末期的其他变化一起被认为反映了板块构造作为全球主导机制的开始。
