Generation of early Archaean grey gneisses through melting of older crust in the eastern Kaapvaal craton, southern Africa
Generation of early Archaean grey gneisses through melting of older crust in the eastern Kaapvaal craton, southern Africa
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DOI：
10.1016/j.precamres.2014.07.017
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发表时间：
2014-12-01
影响因子：
3.8
通讯作者：
Yang, Jinhui
中科院分区：
文献类型：
--
作者：
Kroener, Alfred;Hoffmann, J. Elis;Yang, Jinhui
本文报道了非洲南部卡普瓦尔克拉通最古老的组成部分--斯威士兰古片麻岩杂岩中古太古代灰色片麻岩的锆石年龄、锆石中Hf同位素、全岩地球化学和Hf-Nd同位素系统学。Hf在这些成分不均匀，多组分，混合片麻岩中的锆石同位素组成是高度可变的，即使在最古老的锆石可以追溯到3.66 Ga，这表明从熔体中的锆石生长代表了混合的少年和重熔材料来自分化的大陆地壳的始新世晚冥古宙年龄。与此相反，初始Nd和Hf全岩同位素组成往往是不一致的Hf在锆石的数据，主要是显示近似的Nd和强烈的放射成因的Hf的初始值的初始值。我们认为，这可能是两个Lu-Hf和Sm-Nd全岩同位素体系的干扰和部分重置在后期的部分熔融和地壳改造，最有可能在一个普遍的3.2 Ga的构造变质事件。原始地幔标准化微量元素模式显示变量的影响，残留斜长石和石榴石的来源，以及高含量的强不相容元素。结合Hf在锆石同位素数据的微量元素含量最好的解释是纳入老大陆地壳物质的灰色片麻岩的来源。我们的数据支持来自其他古太古代岩石的证据，即即使在最古老的地壳成分中也可以看到地壳循环，在早期大陆演化中发挥了重要作用。以前被归类为俯冲相关的英云闪长岩-奥长花岗岩-花岗闪长岩套的岩石是复杂的，它们的化学性质不能单独用于重建构造环境。(C)2014爱思唯尔有限公司版权所有。
We report zircon ages, Hf-in-zircon isotopes as well as whole-rock geochemistry and Hf-Nd isotopic systematics for Palaeoarchaean grey gneisses of the Ancient Gneiss Complex of Swaziland, the oldest components of the Kaapvaal craton, southern Africa. The Hf-in-zircon isotopic compositions in these compositionally heterogeneous, multicomponent, migmatitic gneisses are highly variable, even in the oldest zircons dating back to 3.66 Ga, suggesting growth of zircon from melts representing a mix of juvenile and anatectic material derived from differentiated continental crust of Eoarchean to late Hadean age. In contrast, the initial Nd and Hf whole-rock isotopic compositions are frequently not in agreement with the Hf-in-zircon data that mostly show approximately chondritic initial values for Nd and strongly radiogenic initial values for Hf. We consider it likely that both the Lu-Hf and Sm-Nd whole-rock isotopic systems were disturbed and partly reset during later episodes of partial melting and crustal reworking, most likely during a pervasive 3.2 Ga tectono-metamorphic event.Primitive mantle-normalized trace element patterns show the variable influence of residual plagioclase and garnet in the sources as well as high contents of strongly incompatible elements. In conjunction with the Hf-in-zircon isotopic data the trace element contents are best explained by the incorporation of older continental crustal material into the sources of the grey gneisses. Our data support evidence from other Palaeoarchean terranes that crustal recycling, as seen in even the oldest crustal components, played an important role in early continental evolution. Rocks previously classified as a subduction-related tonalite-trondhjemite-granodiorite suite are complex, and their chemistry alone cannot be used to reconstruct tectonic settings. (C) 2014 Elsevier B.V. All rights reserved.