The magnetic stratification of layered mafic intrusions: Natural examples and numerical models
The magnetic stratification of layered mafic intrusions: Natural examples and numerical models
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DOI：
10.1016/j.lithos.2009.03.042
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发表时间：
2009-07
期刊：
影响因子：
3.5
通讯作者：
E. Ferré;S. M. Maes;K. Butak
中科院分区：
文献类型：
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作者：
E. Ferré;S. M. Maes;K. Butak
镁铁质-超镁铁质侵入岩通常表现出明显的岩石学和地球化学分层，归因于岩浆室的分异。在一些大型侵入体中，这种分层的规模从几百米到几毫米。分层形成过程很多，既包括内部过程，如晶体沉淀、双扩散对流、流动分离和压实，也包括外部过程，如岩浆补给、提取或混合。镁铁质岩浆房可以作为封闭系统运行，也可以作为开放系统运行。最近通过Insizwa底板(南非)、大堤(津巴布韦)和布什维尔德复合体(南非)对钻孔岩心进行了几次调查。我们给出了关于这些侵入体的磁性结果的概述。镁铁质岩石的磁性主要由大多数层中磁铁矿的存在所决定。磁黄铁矿是唯一的铁磁性硫化物，通常局限于富硫化物的生物礁(Merensky，J-M，Böhmke)或侵入体底部的块状硫化物矿床。所有侵入体的磁特性(磁化率各向异性、剩磁、磁饱和、高场磁化率)在整个地层中都表现出明显的变化。这些磁带的重要性因不同区域变化的性质而不同。例如，磁化率的下降并不一定对应于一批新的岩浆，除非它还与磁各向异性参数的变化有关。在所研究的情况下，所有开放系统、磁化率和总铁含量(通过高场测量估计)都向这些侵入岩的顶部增加，表明铁的普遍富集型式。拉斑玄武岩岩浆的数值岩石学模型预测了静态差异的闭合系统侵入体(无对流)的磁化率变化。从磁性角度研究了连续岩浆补给和离散岩浆提取的影响。相反，这些模型可以用来解释自然例子中的磁层化模式，并检验其可能的原因。
Mafic–ultramafic intrusions commonly display a prominent petrographic and geochemical layering attributed to differentiation in a magma chamber. This layering occurs at scales from several hundreds of meters in some large intrusions, down to scales of a few millimeters. Layering-forming processes are numerous and include, both internal processes, such as crystal settling, double diffusive convection, flow segregation and compaction, and external processes such as magma-recharge, -extraction or -mixing. Mafic magma chambers may operate as a closed system or as an open system. Several recent investigations have been conducted on borehole cores through the Insizwa sill (South Africa), the Great Dyke (Zimbabwe), and the Bushveld Complex (South Africa). We present an overview of magnetic results on these intrusions. The magnetic properties of mafic rocks are dominated by the presence of magnetite in most layers. Pyrrhotite, the only ferrimagnetic sulfide, is generally restricted to sulfide-rich reefs (Merensky, J-M, Böhmke) or to massive sulfide deposits at the base of the intrusion. All intrusions display distinct variations of their magnetic properties (anisotropy of magnetic susceptibility, magnetic remanence, magnetic saturation, high field magnetic susceptibility) across the layering. The significance of these magnetic zones varies depending on the nature of variations from zone to zone. For example, a drop in magnetic susceptibility does not necessarily correspond to a new magma batch, unless it is also associated with a change in magnetic anisotropy parameters. In the studied cases, all open systems, the magnetic susceptibility and the total iron content (estimated from high field measurements) increase towards the top of these intrusions suggesting a general pattern of iron enrichment. A numerical petrologic model for a tholeiitic magma predicts magnetic susceptibility variations across a statically differentiated, closed system intrusion (no convection). The effects of sequential magma recharge and discrete magma extraction are investigated from the magnetic properties point of view. Conversely, these models can be used to interpret the magnetic stratification pattern in natural examples and test for its possible causes.