Melt Generation and Fluid Flow in the Thermal Aureole of the Bushveld Complex
Melt Generation and Fluid Flow in the Thermal Aureole of the Bushveld Complex
复制标题
Bushveld 复合体热光环中的熔体生成和流体流动
DOI：
10.1093/petrology/44.6.1031
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发表时间：
2003
影响因子：
3.9
通讯作者：
A. Fallick
中科院分区：
文献类型：
--
作者：
N. Harris;Andy McMillan;M. Holness;R. Uken;M. Watkeys;N. Rogers;A. Fallick
花岗岩片侵位到混合岩带的东部接触晕的布什维尔德复杂的流体增强，不一致的黑云母熔融的下伏西尔弗顿形成页岩在俯冲变质。钡的浓度在岩席（>1000 ppm）和角页岩（>800 ppm）中都是极端的。我们的结论是，富钡，热液流体诱导熔融的光环，和流体运输的Ba 2+，并在较小程度上，Sr 2+和Eu 2+，坚持在熔体区亚固相线条件下。高Ba地区的Sr同位素系统学确定了2161 ± 106 Ma的误差，初始（87 Sr/86 Sr）比值为0.705 ± 0.001。变质沉积岩不受流体渗透均匀化的同时，但增加了初始比例，这表明，而同位素均匀化之间的露头渗透流体，没有证据表明区域均匀化。氧同位素组成的砂质沉积物中的光环是不相关的距离接触，这表明渗透流体平衡同位素与沉积物。它们的18 O值（11.3 - 12.1）的升高与沉积岩性挥发分的流体来源一致。在露头和薄切片规模的纹理分析的岩石质和沙岩的光环（Lakenvalei和Magaliesberg组）表明，熔融的程度是高度不均匀的，即使在颗粒尺度上，并导致不均匀分布的渗透的含水流体的渗透的渗透剂裂缝和晶界。石英的阴极发光成像显示出显着的差异，在熔融的岩石中具有均匀的发光，在岩石中含有很少或没有熔体，保留纹理继承的区域变质原岩的量的精细结构。简单的有限差分热模型的光环表明，熔化区的宽度（>500米）是不符合传导热传递，因此，热结构已被修改的流体平流。
Granite sheets emplaced into the migmatite zone of the eastern contact aureole of the Bushveld Complex resulted from fluid-enhanced, incongruent biotite melting of the underlying Silverton Formation shales during prograde metamorphism. Ba concentrations are extreme in both the sheets (>1000 ppm) and the hornfels (>800 ppm) into which they have been emplaced. We conclude that a Ba-rich, hydrothermal fluid induced melting in the aureole, and that fluid transport of Ba2+, and to a lesser extent, Sr2+ and Eu2+, persisted in the melt zones under subsolidus conditions. Sr-isotope systematics from high-Ba localities define an errorchron of 2161 ± 106 Ma with an initial (87Sr/86Sr) ratio of 0·705 ± 0·001. Metasedimentary rocks unaffected by fluid infiltration were homogenized at the same time but with an increased initial ratio, suggesting that whereas isotope homogenization was achieved between outcrops permeated by fluids, there is no evidence of regional homogenization. Oxygen-isotope compositions of psammitic metasediments in the aureole are uncorrelated with distance from the contact, suggesting the infiltrating fluid equilibrated isotopically with the metasediments. Their elevated 18O values (11·3–12·1) are consistent with a fluid source from devolatilization of the sedimentary lithologies. Textural analysis at both outcrop and thin-section scale of arkose and psammites in the aureole (Lakenvalei and Magaliesberg Formations) shows that the extent of melting was highly heterogeneous, even on the grain scale, and resulted from heterogeneously distributed infiltration of aqueous fluid on dilatant cracks and grain boundaries. Cathodoluminescence imaging of quartz shows a marked difference in the amount of fine structure, with that in the melted rocks having uniform luminescence in contrast to that in rocks containing little or no melt, which preserve textures inherited from the regionally metamorphosed protolith. Simple finite-difference thermal modelling of the aureole suggests that the width of the melt zone (>500 m) is inconsistent with conductive heat transfer, and hence that the thermal structure has been modified by fluid advection.