Processes of crust formation in the early Earth imaged through Hf isotopes from the East Pilbara Terrane
Processes of crust formation in the early Earth imaged through Hf isotopes from the East Pilbara Terrane
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DOI：
10.1016/j.precamres.2017.05.004
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发表时间：
2017-08-01
影响因子：
3.8
通讯作者：
Zhao, Jian-Xin
中科院分区：
文献类型：
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作者：
Gardiner, Nicholas J.;Hickman, Arthur H.;Zhao, Jian-Xin
西澳大利亚的皮尔巴拉火山是地球上保存最完好的古到中太古代地形之一。东皮尔巴拉地体是典型的花岗岩-绿岩带，其穹隆状杂岩是由三次主要岩浆事件形成的。这些花岗岩穹隆是由变质的花岗质火成岩组成的，在3.53 - 2.83 Ga期间，表现出从早期英云闪长岩-奥长花岗岩-花岗闪长岩（TTG）岩石向富钾花岗岩的岩浆演化。因此，东皮尔巴拉一直是寻求限制早期地球动力学过程的工作者的焦点。了解这一争论的一种方法是使用对岩浆源敏感的工具来询问连续的火成岩超套。我们提出了新的锆石Hf和全岩Nd同位素数据从四个主要的超套房的埃德加山圆顶，东皮尔巴拉地体。早期CA 3.45 Ga TTG表现出同位素特征，这意味着它们部分来自现有的地壳，加上一些少年的材料。随后的古太古代岩浆事件显示出一个长期的趋势，更进化的同位素签名，解释为现有地壳的改造增加占主导地位，只有少量的新的少年地壳。这意味着这些后来的古太古代超套房很大程度上来自于更古老的花岗岩地壳的熔融，质量平衡模型表明有约1000的输入。20%幼年（耗尽地幔）材料。幼年物质的有限增加和现有地壳随时间的增加的改造，不支持现代式俯冲的模型，但与> 3.2 Ga的东皮尔巴拉地体的火山高原型环境中的垂直构造过程一致。所有的古太古代埃德加山样品解决了两个阶段的Hf模式年龄约。3.7 Ga和Nd模式年龄。3.6佐治亚州这些始太古代模型年龄支持存在一个神秘的前3.5 Ga原地壳，虽然未知的程度。中太古代晚期花岗岩的分析产生了富钾二长花岗岩的高度演化的Hf同位素特征。这是一致的趋势，在东皮尔巴拉地体从钠TTG型岩浆作用在古太古代早期的富钾岩浆在中太古代晚期。(C)2017 Elsevier B.V.版权所有。
The Pilbara Craton, Western Australia, is one of the best preserved Palaeo- to Mesoarchaean terrains on Earth. The East Pilbara Terrane is the archetypical granite-greenstone belt, the dome-like complexes of which were formed through three major magmatic events. These granite domes are comprised of metamorphosed granitic igneous rocks that exhibit a magmatic evolution from early tonalite-trondhjemite-granodiorite (TTG) rocks towards K-rich granites over the period 3.53-2.83 Ga. Accordingly, East Pilbara has been a focus for workers seeking to constrain early Archaean geodynamic processes. One way to inform on this debate is to interrogate successive igneous supersuites using tools sensitive to magmatic source. We present new zircon Hf and whole-rock Nd isotope data from four major supersuites of the Mount Edgar Dome, of the East Pilbara Terrane. Early ca. 3.45 Ga TTGs exhibit isotopic signatures that imply their partial derivation from existing crust, with addition of some juvenile material. Subsequent Palaeoarchaean magmatic events show a secular trend towards more evolved isotopic signatures, interpreted as a dominance of increasing reworking of existing crust, with only minor addition of new juvenile crust. The implication of this is that these later Palaeoarchaean supersuites were largely derived from the melting of older granitic crust, with mass balance modelling suggesting an input of ca. 20% juvenile (depleted mantle) material. The limited addition of juvenile material and increased reworking of existing crust with time, does not support a model of modern-style subduction, but is consistent with vertical tectonic processes in a volcanic plateau-type setting for the East Pilbara Terrane >3.2 Ga. All Palaeoarchaean Mount Edgar samples resolve to two-stage Hf model ages of ca. 3.7 Ga and Nd model ages of ca. 3.6 Ga. These Eoarchaean model ages support the existence of a cryptic pre 3.5 Ga protocrust, albeit of unknown extent. Analysis of late Mesoarchaean granites yields highly evolved Hf isotope signatures of K-rich monzogranites. This is consistent with a trend in the East Pilbara Terrane from sodic TTG type magmatism in the early Palaeoarchaean to K-rich magmas in the late Mesoarchaean. (C) 2017 Elsevier B.V. All rights reserved.