Deep subduction of continental crust in accretionary orogen: Evidence from U–Pb dating on diamond-bearing zircons from the Qinling orogen, central China
Deep subduction of continental crust in accretionary orogen: Evidence from U–Pb dating on diamond-bearing zircons from the Qinling orogen, central China
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增生造山带中大陆地壳深俯冲：来自中国中部秦岭造山带含金刚石锆石U-Pb定年的证据
DOI：
10.1016/j.lithos.2013.12.021
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发表时间：
2014-03
期刊：
影响因子：
3.5
通讯作者：
Hu-Jun Gong
中科院分区：
文献类型：
--
作者：
Xiao-Chi Liu;Zheng-Wei Qin;Sai-Hong Yang;Hu-Jun Gong
增生造山带被认为是大陆地壳形成的最重要场所。然而，目前尚不清楚在这种背景下大陆地壳是否已显着地迁移回地幔并伴随着其横向生长。大陆超高压岩石是大陆地壳再循环的最佳指标，但在增生造山带中却很少发现，这阻碍了对这一重要过程的探索。秦岭造山带可能发生早古生代超高压变质作用，为解决这一问题提供了绝佳的机会。本文报道了对秦岭造山带角闪岩样品中锆石 U-Pb 年龄、微量元素和矿物包裹体的综合研究。锆石晶体表现出典型的变质生长带、低Th/U比、平坦的HREE图案和不显着的Eu异常。他们给出的加权平均 U-Pb 年龄为 490.4 ± 5.8 Ma。最重要的是，从其中一颗锆石晶体中发现了原位金刚石内含物。因此，将获得的 490.4 ± 5.8 Ma 年龄作为记录超高压变质作用的峰值。原位金刚石包裹体的发现为秦岭造山带超高压变质作用提供了重要证据，表明北秦岭微大陆在与华北地块南缘附着的二郎坪岛弧碰撞时俯冲至地幔深度>120km。结合之前的结果，我们认为北秦岭超高压地体经历了从 > 120 km 到约 100 km 的快速折返。 30 公里，间隔 10 马里。由于在一些增生造山带中已经发现了一些超高压变质作用的实例，因此需要更加关注增生造山过程中大陆超高压变质作用的发生。此外，早古生代期间，北秦岭造山带有明显的地壳生长记录。推测增生造山带除了形成大陆地壳外，还可能在大陆地壳再循环到地幔中发挥重要作用。
Accretionary orogenic belts are considered the most important sites for the generation of continental crust. However, it is still not very clear whether in such settings the continental crust has been significantly transported back to the mantle accompanied with its lateral growth. Continental UHP rocks, the best indicators of the recycling of the continental crust, have rarely been discovered in accretionary orogenic belts, which hinder the exploration of this essential process. The possible occurrence of early Paleozoic UHP metamorphism in the Qinling orogen provides an excellent opportunity to address this question. This article reports an integrated study of U–Pb age, trace element and mineral inclusion of zircon from an amphibolite sample in the Qinling orogen. The zircon crystals show typical metamorphic growth zoning, low Th/U ratios, flat HREE patterns, and insignificant Eu anomalies. They give a weighted mean U–Pb age of 490.4 ± 5.8 Ma. Most importantly, an in situ diamond inclusion was identified from one of the zircon crystals. Therefore, the obtained 490.4 ± 5.8 Ma age was taken as registering the peak UHP metamorphism. The discovery of in situ diamond inclusion provides important evidence for the UHP metamorphism of the Qinling orogen and indicates that the North Qinling microcontinent was subducted to mantle depths of > 120 km when it collided with the Erlangping arc attached to the southern margin of the North China Block. Combining with previous results, we suggested that the North Qinling UHP terrane underwent fast exhumation from > 120 km to ca. 30 km within 10 Myr intervals. As a few examples of UHP metamorphism have been discovered in some accretionary orogens, more attention needs to be paid to the occurrence of continental UHP metamorphism during accretionary orogenic process. Furthermore, evident crustal growth has been documented in the North Qinling orogen during the early Paleozoic. It is inferred that the accretionary orogenic belts may play important roles in recycling of continental crust into the mantle besides generation of continental crust.
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