Mesoscale pervasive felsic magma migration: alternatives to dyking
Mesoscale pervasive felsic magma migration: alternatives to dyking
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DOI：
10.1016/s0024-4937(98)00075-9
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发表时间：
1999-03
期刊：
影响因子：
3.5
通讯作者：
R. Weinberg
中科院分区：
文献类型：
--
作者：
R. Weinberg
本文回顾了岩墙作为一种长英质岩浆从源头提取并运移到地壳浅层的机制的文献，并回顾了最近的文献，这些文献提出了一系列可供选择的岩浆在热地壳带中迁移的机制，这些机制产生了中尺度渗透的花岗岩片状侵入体。最近的论文强烈支持将岩浆运移作为控制岩浆迁移的主要机制。然而，从长英质岩浆来源启动堤坝是充满困难的，即使岩浆立即可以运输，如在岩浆室中。在部分熔融的源区内，岩浆可能存在于各种形状、大小和连通性程度不同的结构中。虽然对局部熔融带内单个岩墙的生长以及大型岩墙自蔓延到固体底壳中都有较详细的研究，但很少有人注意到能够产生广泛地壳尺度岩墙的岩墙网络生长的关键中间步骤。花岗岩墙群的稀有表明，如果岩墙是岩浆运输的首选机制，长英质岩浆源在其一生中只产生很少的主要运移墙。或者，堤坝不是一种重要的机制。控制一个这样的堤坝排水的集水区的水量的参数，以及控制源头内堤坝网络结构的其他基本几何参数，都是未知的。堤坝排出部分熔融来源的能力主要取决于这些变量，特别是来源的水平渗透率。粘性长英质岩浆在岩石孔隙中的缓慢运动表明，在围堤过程中排出的岩浆大多是先前从孔隙中提取的岩浆，并存在于不规则的岩浆体或岩墙网络中。观察到混合岩带中通常存在大量浮力岩浆，而这些带中的岩墙起到次要作用，这表明岩墙效率低下，只能提取源区可用总熔体的一小部分。为支持这一结论，最近对出土的热地壳带的详细研究揭示了一系列以中尺度普遍岩浆流动为特征的替代迁移机制(露头规模，而不是多孔流动)。无处不在的迁移导致岩浆片优先平行于高渗透带，如叶理或层理平面。除了局部围岩外，还提出了三种解释岩浆普遍迁移的替代机制，即构造泵送；岩浆楔入低粘度岩石；以及挥发分驱动的侵入。由于其无聚焦、无处不在的特点，这些机制仅限于热的地壳，那里的岩浆不会暴露在快速冻结中。提出了一种模型，即与岩浆一起平流的热量将地壳地温向上推，从而使无处不在的岩浆迁移到较浅的深度。
This paper reviews the literature on dyking as a mechanism of felsic magma extraction from a source and transport to shallower crustal levels, and review the recent literature suggesting a range of alternative mechanisms of magma migration in hot crustal zones which produce mesoscale pervasive granite sheet intrusions. Recent papers have strongly favoured dyking as the main mechanism controlling magma migration. However, the initiation of dykes from a felsic magma source is fraught with difficulties, even when magma is immediately available for transportation, as in magma chambers. Within a partially molten source, magma may reside in a range of structures with a wide range of shapes, sizes and degrees of connectivity. Whereas the growth of individual dykes within a partially molten zone, and the self-propagation of large dykes into subsolidus crust, have both been studied in some detail, little attention has been given to the crucial intermediate step of the growth of a dyke network capable of producing wide crustal scale dykes. The rarity of granite dyke swarms suggests that, if dyking is the preferred mechanism of magma transport, felsic magma sources produce only few major transporting dykes during their lifetime. Alternatively, dyking is not an important mechanism. The parameters controlling the volume of the catchment drained by one such dyke, as well as other basic geometrical parameters controlling the structure of the dyke network within the source, are unknown. The ability of dyking to drain a partially molten source depends crucially on these variables and particularly on the horizontal permeability of the source. The slow velocity of viscous felsic magmas traveling in rock pores implies that magma drained during dyking is mostly that previously extracted from the pores, and resident in irregular magma bodies or dyke networks. The observation that large volumes of buoyant magma are commonly present in migmatite zones, and that dyking in these zones plays a secondary role, suggest that dyking is inefficient and is able to extract only a fraction of the total melt available in the source. In support of this conclusion, recent detailed studies of exhumed hot crustal zones have revealed a range of alternative migration mechanisms characterized by mesoscale pervasive magma flow (outcrop scale as opposed to porous flow). Pervasive migration gives rise to magma sheets preferentially emplaced parallel to high-permeability zones such as foliation or bedding planes. Apart from local dyking, three alternative mechanisms have been proposed to account for pervasive migration of magmas, namely tectonic pumping; magma wedging into low-viscosity rocks; and volatile-driven intrusion. Because of their unfocused, pervasive character, these mechanisms are restricted to hot crust where magmas are not exposed to rapid freezing. A model is proposed whereby heat advected with the magma pushes crustal geotherms upwards, allowing pervasive magma migration to shallower depths.