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首页> 外文期刊>Journal of Volcanology and Geothermal Research >Magnetic Fabric Of The Ricany Granite, Bohemian Massif: A Record Of Helical Magma Flow?
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Magnetic Fabric Of The Ricany Granite, Bohemian Massif: A Record Of Helical Magma Flow?

机译：Ricany花岗岩的磁性织物，波希米亚地块：螺旋岩浆流动的记录？

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摘要

A highly unusual fabric pattern and inferred flow mechanism was discovered in the shallow-level Ricany granite pluton, Bohemian Massif, using an integrated structural and anisotropy of magnetic susceptibility (AMS) study. The pluton consists of an outer strongly porphyritic and an inner weakly porphyritic biotite granite separated by a wide gradational contact. Both varieties share a steep margin-parallel magmatic foliation (defined by K-feldspar phenocrysts and biotite) and magnetic (AMS) foliation carried by biotite. The steep foliation bears a shallowly-plunging magnetic lineation arranged parallel to the circumference of the pluton margin and a steep magnetic lineation in the weakly porphyritic pluton center. We propose a new mechanism of magma flow, in which the bulk ascent along a steep-sided, cylindrical conduit was partitioned into a high-viscosity, phenocryst-rich outer margin flowing helically and a low-viscosity, phenocryst-poor center flowing vertically. This interpretation is supported quantitatively by a simple model of magma flow within a cylindrical pipe, in which the linear decrease in phenocryst content from the pluton margin inwards causes a power-law decrease in the effective viscosity of the magma. According to the Poiseuille equation, such a power-law viscosity distribution across the conduit produces a central zone of high magma ascent velocity. The magnetic (AMS) fabric pattern in the Ricany pluton may thus preserve a record of helical magma ascent driven by viscosity partitioning in a steep-sided conduit, which presumably linked an underlying magma chamber with a volcanic feeder at the surface. Except for one deep-seated granite-migmatite complex, no such helical fabric pattern has been documented in a granite pluton as of yet. We propose that the viscosity-partitioned helical flow may be a more common magma ascent mechanism in shallow-level volcano-plutonic systems than previously envisaged.
机译：利用磁化率（AMS）的综合结构和各向异性研究，在波西米亚地块的浅层Ricany花岗岩岩体中发现了一种非常不寻常的织物图案和推测的流动机制。岩体由一个外部的强斑岩质和一个内部的弱斑岩状黑云母花岗岩组成，该花岗岩由广泛的梯度接触所分隔。两种变种都具有陡峭的边缘平行岩浆页岩（由钾长石斑岩和黑云母定义）和由黑云母携带的磁性（AMS）页岩。陡峭的叶状体带有与铁心边缘的圆周平行排列的浅俯冲磁性线，而在弱斑状的岩心中则带有陡峭的磁线。我们提出了一种新的岩浆流动机制，其中沿陡峭的圆柱状管道的整体上升被划分为一个高粘度，富含表晶的外缘螺旋状流动，以及一个低粘度，缺乏表晶的中心垂直流动。圆柱管内岩浆流动的简单模型在定量上支持了这种解释，在该模型中，从the体边缘向内的隐晶石含量线性下降，导致岩浆有效粘度的幂律下降。根据Poiseuille方程，导管上的这种幂律粘度分布会产生高岩浆上升速度的中央区域。因此，Ricany岩体中的磁性（AMS）织物图案可以保存由陡壁管道中的粘度分配驱动的螺旋岩浆上升的记录，该管道可能将下面的岩浆室与地面的火山岩馈线相连。除了一种深层的花岗岩-辉锰矿复合物外，到目前为止，在花岗岩岩体中还没有记录到这种螺旋状的织物图案。我们提出，与以前设想的相比，在浅层火山-火山成岩系统中，黏度分配的螺旋流可能是更常见的岩浆上升机制。

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