Geological constraints on the origin of the mantle root beneath the Canadian shield
Geological constraints on the origin of the mantle root beneath the Canadian shield
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加拿大地盾下地幔根起源的地质限制
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发表时间：
1990
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通讯作者：
P. Hoffman
中科院分区：
文献类型：
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作者：
P. Hoffman
北美大陆板块由一组以加拿大地盾为中心的西太平洋微大陆和主要埋藏在西部和南部内陆台地沉积盖层之下的幼年元古代地壳组成。地盾的下面是异常的低温地幔根，在地台下面是不存在的。由于地盾和地台之间的地壳厚度或密度似乎没有系统性的差异，地盾的长寿命膨胀意味着地幔根赋予的内在浮力超过了其较冷的温度。同位素和地震各向异性数据表明地幔根的平均年龄，接近上覆地壳的形成时间。优先发展的地幔根下的地壳是一致的起源部分俯冲板高度亏损的海洋岩石圈叠瓦，假设浮力俯冲是更常见的，在大洋中。地幔根的形成并不依赖于碰撞造山作用，正如已经提出的那样，但西太平洋板块的地幔有足够的浮力和耐火性，能够在后来的构造增厚中幸存下来。地幔根持续存在于被基性岩脉群横切并经历了造山后地壳熔融的短暂事件的大洋地壳下，但在地幔柱起始部位，地幔根减少了。在地盾和地台之间分别划分前寒武纪和元古宙地壳，导致地盾不能作为一个整体代表前寒武纪地壳。对地盾的研究错误地得出结论，认为前寒武纪地壳的高比例形成于侏罗纪，元古代的特征是陆表火山作用和沉积作用，以及地壳的“改造”。此外，克拉通侵蚀碎屑的同位素比值可能会高估大陆地壳的平均年龄。
Cratonic North America is composed of a cluster of Archaean microcontinents centred on the Canadian shield, and juvenile Proterozoic crust that lies mainly buried beneath the sedimentary cover of the western and southern interior platforms. The shield is underlain by an anomalous low-temperature mantle root that is absent beneath the platform. As there appears to be no systematic difference in crustal thickness or density between the shield and the platform, the long-lived arching of the shield implies an intrinsic buoyancy imparted by the mantle root that more than offsets its colder temperature. Isotopic and seismic anisotropy data indicate an Archaean age for the mantle root, close to the time of formation of the overlying crust. The preferential development of the mantle root beneath Archaean crust is consistent with an origin by imbrication of partly subducted slabs of highly depleted oceanic lithosphere, assuming that buoyant subduction was more common in the Archaean. Formation of the mantle root was not dependent on collisional orogenesis, as has been suggested, but the Archaean cratonic mantle was sufficiently buoyant and refractory to survive later tectonic thickening. The mantle root persists beneath Archaean crust that was transected by mafic dyke swarms and subjected to short-lived episodes of post-orogenic crustal melting, but the root is reduced at mantle plume initiation sites. The partitioning of Archaean and Proterozoic crust between the shield and the platform, respectively, causes the shield to misrepresent Precambrian crust as a whole. Studies of the shield falsely conclude that a high percentage of Precambrian crust formed in the Archaean, and that the Proterozoic was characterized by epicontinental volcanism and sedimentation, and crustal ‘reworking’. Furthermore, the isotopic ratios of detritus eroded from the craton may tend to overestimate the mean age of continental crust.