The oldest part of the Barberton granitoid-greenstone terrain, South Africa: evidence for crust formation between 3.5 and 3.7 Ga
The oldest part of the Barberton granitoid-greenstone terrain, South Africa: evidence for crust formation between 3.5 and 3.7 Ga
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南非巴伯顿花岗岩-绿岩地形最古老的部分：3.5 至 3.7 Ga 之间地壳形成的证据
DOI：
10.1016/0301-9268(95)00072-0
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发表时间：
1996
影响因子：
3.8
通讯作者：
G. Byerly
中科院分区：
文献类型：
--
作者：
A. Kröner;E. Hegner;J. Wendt;G. Byerly
由于逆冲、推覆堆积和走滑断层等强烈变形，巴伯顿绿岩带（BGB）南部的许多地层和年龄关系仍未解决。通过使用各种技术进行单锆石测年，建立了一个相对未受干扰的序列，从下部 Onverwacht 群（∼ 3.48-3.45 Ga）开始，然后是上部 Onverwacht（∼ 3.42-3.3 Ga）、无花果树群（∼ 3.26-3.23 Ga）和 Moodies 群（> 3.22 Ga），但 Theespruit 组的位置仍然不确定。利用单锆石蒸发和蒸气消解技术，我们获得了 BGB 东南部 Steynsdorp 背斜中绘制为 Theespruit 的长英质岩石的非常均匀的 207Pb/206Pb 和 UPb 年龄，分别为 3544 ± 3 至 3547 ± 3 Ma，比所有其他测年绿岩单位年龄大约 100 Ma。这些岩石被 3502–3511 Ma 老 Steynsdorp TTG 岩体侵入，其中含有年龄为 3553 ± 4 Ma 的锆石异晶。侵入 Komati 组（下 Onverwacht 群）变火山岩的 3.5 Ga 花岗闪长岩塞含有两颗 3702 ± 2 Ma 锆石异晶，这是迄今为止在巴伯顿-斯威士兰地区测量到的最古老的锆石，证明该地区存在非常古老的地壳。 Theespruit 长英质变火山岩的 ϵNd(t) 值在 + 1.1 和 − 1.1 之间，Nd TDM 模型年龄在 3.5 到 3.7 Ga 之间，表明其原岩受到较古老大陆地壳的不同污染。上述锆石年龄将 BGB 的历史延伸至 ∼ 3.55 Ga。我们认为 Steynsdorp 背斜区域构成了 BGB 最古老的核，在其上先后通过构造和岩浆作用增生了较年轻的单元。我们还推测，BGB 南部的镁铁质-长英质火山单元可能代表了独特的海洋高原，而不是在 3.55 至 3.42 Ga 之前合并的海底物质。我们的数据支持这样的概念：BGB 由许多离散的、以断层为界的地体组成，因此大规模的岩性相关性是不合理的。
Many stratigraphic and age relationships in the southern part of the Barberton greenstone belt (BGB) remain unresolved due to strong deformation including thrusting, nappe stacking and strike-slip faulting. A relatively undisturbed sequence from the lower Onverwacht Group (∼ 3.48-3.45 Ga), followed by the upper Onverwacht (∼ 3.42-3.3 Ga), the Fig Tree Group (∼ 3.26-3.23 Ga) and the Moodies Group (> 3.22 Ga) was established by single zircon dating using various techniques, but the position of the Theespruit Formation is still uncertain. Using the single zircon evaporation and the vapour digestion techniques we obtained remarkably uniform207Pb/206Pb and UPb ages of 3544 ± 3 to 3547 ± 3 Ma for felsic rocks mapped as Theespruit in the Steynsdorp Anticline of the southeastern BGB, some 100 Ma older than all other dated greenstone units. These rocks were intruded by the 3502–3511 Ma old Steynsdorp TTG pluton containing zircon xenocrysts as old as 3553 ± 4 Ma. A 3.5 Ga granodiorite plug intrusive into metavolcanics of the Komati Formation (lower Onverwacht Group) contained two 3702 ± 2 Ma zircon xenocrysts, the oldest so far measured in the Barberton-Swaziland area and testifying to the presence of very ancient crust in the region. The Theespruit felsic metavolcanics have ϵNd(t)values between + 1.1 and − 1.1 and Nd TDMmodel ages between 3.5 and 3.7 Ga, suggesting variable contamination of their protoliths with older continental crust. The above zircon ages extend the history of the BGB back to ∼ 3.55 Ga. We suggest that the area of the Steynsdorp Anticline constitutes the oldest nucleus of the BGB onto which successively younger units were tectonically and magmatically accreted. We also speculate that the mafic-felsic volcanic units of the southern BGB may perhaps represent distinct oceanic plateaux, rather than ocean floor material, which amalgamated between 3.55 and 3.42 Ga ago. Our data support the concept that the BGB consists of a number of discrete, fault-bounded terranes, and that large-scale lithological correlations are therefore not justified.