An isotopic and geochemical study of the northern Kaapvaal Craton and the Southern Marginal Zone of the Limpopo Belt: are they juxtaposed terranes?
An isotopic and geochemical study of the northern Kaapvaal Craton and the Southern Marginal Zone of the Limpopo Belt: are they juxtaposed terranes?
复制标题
卡普瓦尔克拉通北部和林波波带南缘带的同位素和地球化学研究：它们是并置的地体吗？
DOI：
10.1016/s0024-4937(99)00037-7
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发表时间：
2000
期刊：
影响因子：
3.5
通讯作者：
C. Smit
中科院分区：
文献类型：
--
作者：
K. Kreissig;T. Nägler;J. Kramers;D. D. Reenen;C. Smit
为了测试现有的侧加积模型太古代Kaapvaal在南非，变质岩和浅色花岗岩类从两个不同的省份，麻粒岩相的林波波带的南部边缘带和北方部分相邻的绿片岩角闪岩相Kaapvaal的加积，进行了研究。这两个省在同位素和微量元素特征方面彼此相似。它们表明不相容元素的普遍亏损与变质程度无关。这一观察结果与麻粒岩相变质作用期间的活动化理论相冲突。另一个共同的指纹是在变岩中异常高的Cr和Ni浓度。此外，他们定义了一个非常狭窄的范围内的Nd模式年龄（2.9-3.05 Ga），限制了平均地壳年龄，他们的地幔源区。花岗质片麻岩的Nd模式年龄变化较大，表明地壳内存在一定的年龄不均匀性。然而，这两个省的数据完全重叠。南部边缘带和Kaapvaal盆地也显示出一致的低207 Pb/204 Pb相对于他们的206 Pb/204 Pb比值，明显不同于其他南部非洲省份。这些结果支持了这样的观点，即南部边缘带代表的一部分Kaapvaal古陆暴露在更深的地壳水平，它排除了一个假设，这两个省是单独的合并古陆。此外，花岗岩类和变岩类之间的差异很小，这表明后者主要是由前者衍生而来的。南部边缘带麻粒岩相变岩与Kaapvaal岩带的低级变质岩暴露在同一水平上，表明沿着一个明显的构造边界隆起。
To test existing lateral accretion models for the Archean Kaapvaal Craton in South Africa, metapelites and leucocratic granitoids from two different provinces, the granulite facies Southern Marginal Zone of the Limpopo Belt and the northern part of the adjacent greenschist to amphibolite facies Kaapvaal Craton, have been studied. Both provinces resemble each other in terms of isotope and trace element characteristics. They show a general depletion of incompatible elements independent from the grade of metamorphism. This observation conflicts with theories involving mobilisation during granulite facies metamorphism. A further common fingerprint is the unusually high Cr and Ni concentrations in the metapelites. In addition, they define a very narrow range of Nd model ages (2.9–3.05 Ga) constraining the average crustal age of their cratonic source region. The granitoid gneisses show somewhat larger variations of Nd model ages indicating some age heterogenity within the crust. However, the data from both provinces completely overlap. The Southern Marginal Zone and the Kaapvaal Craton also show uniform low207Pb /204Pb relative to their206Pb /204Pb ratios that differ clearly from other Southern African provinces. These results support the view that the Southern Marginal Zone represents a part of the Kaapvaal Craton exposed at a deeper crustal level; it rules out a hypothesis that both provinces are separate amalgamated terranes. Furthermore, there is very little difference between granitoids and metapelites which indicates that the latter largely derived from the former. The exposure of the granulite facies metapelites of the Southern Marginal Zone at the same level as the lower grade metamorphic rocks of the Kaapvaal Craton indicates uplift along a sharp tectonic boundary.