̃2.7Ga crust growth in the North China craton
̃2.7Ga crust growth in the North China craton
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DOI：
10.1016/j.precamres.2010.02.010
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发表时间：
2010-05
影响因子：
3.8
通讯作者：
Neng Jiang;Jing-hui Guo;M. Zhai;Shuangquan Zhang
中科院分区：
文献类型：
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作者：
Neng Jiang;Jing-hui Guo;M. Zhai;Shuangquan Zhang
大量的锆石U-Pb年代学数据表明，中国克拉通北部前寒武纪基底岩石有两个年龄峰，分别为1.81.9Ga和∼2.5Ga。不确定∼2.5Ga最显著的峰值是否真的记录了NCC的一个主要地壳增长期，从而使其不同于∼2.7Ga的全球主要地壳增长期。本文对华北裂谷6个前寒武纪样品进行了现场的锆石U-Pb、Hf同位素和全岩化学、锶-钕同位素分析，以了解地壳的生长和改造。样品的锆石U-Pb年龄为1.8~1.9，∼为2.5，∼为2.7Ga。两个锆石U-Pb年龄为∼2.7Ga的样品，其锆石Hf模式年龄(THfDm)和(或)全岩Nd模式年龄(TndDm)接近其锆石U-Pb值。显然，它们代表了年轻地壳的产生，从而为∼2.7Ga是一个新的地壳加入期提供了直接证据。具有∼1.8和∼2.5Ga锆石U-Pb年龄的样品也具有与∼2.7Ga样品相似的THfDM和/或TndDM，表明这些样品的地幔提取时间也为2.7±0.1Ga，其∼1.8和∼2.5Ga年龄可能代表了2.7±0.1Ga地壳的改造阶段，而不是年轻地壳的产生。改造作用或表现为变质作用，或表现为下地壳源岩浆作用。具体地说，∼2.5Ga英云闪长质片麻岩样品被认为是∼2.7Ga下地壳熔融的产物。由于许多∼2.5Ga英云闪长岩-奥长花岗闪长岩(TTGs)和钙碱性花岗岩类中含有∼2.7Ga包裹体和继承性锆石，我们推测它们可能也主要来自2.7±0.1Ga地壳。我们的数据表明，核子壳体中的∼2.7Ga陆壳的数量可能比今天暴露的数量更多。
Numerous U–Pb zircon geochronological data show that rocks of the Precambrian basement from the North China craton (NCC) have two age peaks at 1.8–1.9 and ∼2.5Ga, respectively. Whether the most prominent peak at ∼2.5Ga really registered a period of major crustal growth in the NCC thus making it different from the global major crustal growth period of ∼2.7Ga is uncertain. In this paper, we carried out in situ U–Pb and Hf isotopes of zircons and whole rock chemical and Sr–Nd isotopic analyses for six Precambrian samples from the NCC in order to understand crustal growth and reworking. The samples have U–Pb zircon ages of 1.8–1.9, ∼2.5 and ∼2.7Ga, respectively. The two samples with U–Pb zircon ages of ∼2.7Ga have zircon Hf model ages (THfDM) and/or whole rock Nd model ages (TNdDM) close to their U–Pb zircon ages. It is clear that they represent juvenile crust generation, thus providing direct evidence that ∼2.7Ga was a period of new crustal addition. The samples with U–Pb zircon ages of ∼1.8 and ∼2.5Ga also have THfDMand/or TNdDMsimilar to those of the ∼2.7Ga samples, indicating that these samples also have a mantle extraction time of 2.7±0.1Ga and their ∼1.8 and ∼2.5Ga ages may represent periods of reworking of the 2.7±0.1-Ga crust rather than juvenile crust generation. The reworking is manifested either as metamorphism or as lower crustal-derived magmatism. Specifically, the ∼2.5-Ga tonalitic gneiss sample is thought to be derived by melting of the ∼2.7Ga lower crust. Since many ∼2.5-Ga tonalite–trondhjemite–granodiorites (TTGs) and calc–alkaline granitoids in the NCC contain ∼2.7-Ga xenocrystic and inherited zircons, we postulate that they are probably also derived mainly from the 2.7±0.1-Ga crust. Our data imply that the quantity of ∼2.7Ga continental crust in the NCC is probably greater in extent than that exposed today.