ABSTRACT

Crystallization of the 2.06 Ga Bushveld magma formed a 9 km (maximum) sequence of ultramafic and mafic rocks that generated a large volume of country fluid as it thermally metamorphosed a 3+ km section of previously unaltered underlying sedimentary rocks of the Transvaal sequence – a geometry similar to that seen as subducting lithospheric slabs are heated by overlying mantle rocks. The presence of a diatreme (breccia pipe) and other large, pipe-like features in the Bushveld Complex located proximal to diapiric upwelling of the basement rocks suggest that overpressured fluids generated during dehydration of the footwall sediments are focused by the diapiric structures such that the country fluids rapidly penetrate the Bushveld rock. A re-examination of existing stable and radiogenic isotopic evidence is consistent with contamination of Main Zone magmas by 1–2% country fluid. Numeric modelling of the footwall dehydration similarly shows that most of the country fluids will be confined to pipe-like channels as it percolates into the Bushveld sill. Modelling also suggests that the maximum extent of the metamorphic aureole was reached at about the same time that the Main Zone began to crystallize. It is proposed that rapid inflation of the Bushveld sill induced the sudden and catastrophic expulsion of overpressured country fluids to both generate the diatreme and contaminate the Main Zone magma, resulting in the Main Zone enrichment in crustal stable and radiogenic isotopic signatures (Sr, Nd, O and others). By analogy, it is also suggested that hydration melting in the mantle wedge is episodically driven by similar sudden influxes of slab fluids that are able to retain their geochemical and isotopic character by rapid channelled influx. This can be aided by flow focusing at diapirs structures at the upper slab-mantle contact.

摘要

2.06 Ga Bushveld 岩浆的结晶形成了 9 公里（最大）的超基性和基性岩序列，当它热变质了 3+ 公里的德兰士瓦序列的先前未改变的下伏沉积岩部分时，产生了大量的流体类似于俯冲的岩石圈板块被上覆的地幔岩石加热。Bushveld 杂岩体中存在一个锥状（角砾岩管）和其他大型管状特征，位于基底岩石的底辟上涌附近，这表明在下盘沉积物脱水过程中产生的超压流体被底辟结构集中，使得乡村流体迅速渗透布什维尔德岩石。对现有稳定和放射性同位素证据的重新检查与主带岩浆被 1-2% 的国家流体污染一致。下盘脱水的数值模拟类似地表明，大多数乡村流体在渗入 Bushveld 地台时将被限制在管状通道中。建模还表明，变质光环的最大范围大约在主带开始结晶的同时达到。有人提出，布什维尔德地台的快速膨胀导致超压流体突然和灾难性地排出，既产生了火山口，又污染了主带岩浆，导致主带富集地壳稳定和放射性同位素特征（Sr、Nd、 O 等）。类推，也有人认为，地幔楔中的水合熔化是由类似的板块流体突然涌入间歇性驱动的，板块流体能够通过快速通道涌入保持其地球化学和同位素特征。这可以通过在上部板片-地幔接触处的底辟结构处聚焦的流动来帮助。