Igneous Layering in Basaltic Magma Chambers-猫眼课题宝

DOI：

10.1007/978-94-017-9652-1_2

发表时间：

2015

期刊：

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中科院分区：

其他

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作者：

O. Namur;B. Abily;A. Boudreau;F. Blanchette;J. M. Bush;G. Ceuleneer;B. Charlier;C. Donaldson;J. Duchesne;Michael Denis Higgins;D. Morata;T. Nielsen;B. O’Driscoll;B. O’Driscoll;K. Pang;T. Peacock;C. Spandler;A. Toramaru;I. Veksler

层状是镁铁质和超镁铁质层状侵入岩的一个共同特征，通常由一系列具有对比矿物模式和/或矿物结构特征的层组成，包括晶粒尺寸和方向以及局部变化的矿物成分。各层的形态通常是平面的，但在一些层状侵入体中观察到更复杂的形状。分层在层厚度、均匀性、横向连续性、地层循环性以及与周围层接触的清晰度方面表现出各种特征。它通常还与交错层理、槽状结构或层终止等沉积结构有相似之处。现在人们普遍认为，玄武岩岩浆室大多在室边缘形成的稍微过冷的边界层中就地结晶。因此，大多数已知的现有分层不能归因于简单的晶体沉降过程。基于详细的现场关系、地球化学分析以及理论和实验研究，文献中提出了其他潜在机制来解释层状火成岩的形成。在这项研究中，我们回顾了分层形成的重要机制，我们将分层形成过程分为动态和非动态分层过程。动态过程发生在岩浆室填充或结晶过程中。它们包括具有对比密度和/或晶粒尺寸的晶体的差异沉降或浮选、含晶体岩浆的流动偏析以及对流液体运动进入岩浆室期间的晶体偏析。双扩散对流，在岩浆室内产生分层液柱，也能产生分层。其他动态过程包括岩浆注入岩浆室，导致岩浆分层或岩浆混合，以及主岩浆室或凝固晶糊内的硅酸盐液体不混溶。非动态成层过程主要包括结晶强烈条件（例如压力、氧逸度）的快速变化，破坏和改变稳定的液相线组合，以及柯晶曲线的短暂偏移。分层也可能是由于成核速率的变化以及晶浆中的矿物通过颗粒旋转、溶解沉淀而重组造成的，这是由于颗粒尺寸分布、矿物模式或压差方面的初始不均匀性造成的。许多这些过程是由能量耗散驱动的，可以称为平衡或自组织过程。

Layering is a common feature in mafic and ultramafic layered intrusions and generally consists of a succession of layers characterized by contrasted mineral modes and/or mineral textures, including grain size and orientation and, locally, changing mineral compositions. The morphology of the layers is commonly planar, but more complicated shapes are observed in some layered intrusions. Layering displays various characteristics in terms of layer thickness, homogeneity, lateral continuity, stratigraphic cyclicity, and the sharpness of their contacts with surrounding layers. It also often has similarities with sedimentary structures such as cross-bedding, trough structures or layer termination. It is now accepted that basaltic magma chambers mostly crystallize in situ in slightly undercooled boundary layers formed at the margins of the chamber. As a consequence, most known existing layering cannot be ascribed to a simple crystal settling process. Based on detailed field relationships, geochemical analyses as well as theoretical and experimental studies, other potential mechanisms have been proposed in the literature to explain the formation of layered igneous rocks. In this study, we review important mechanisms for the formation of layering, which we classify into dynamic and non-dynamic layer-forming processes.Dynamic processes occur during filling of the magma chamber or during its crystallization. They include differential settling or flotation of crystals with contrasted densities and/or grain sizes, flow segregation of crystal-laden magma and crystal segregation during convective liquid movement into the magma chamber. Double diffusive convection, which produces a stratified liquid column in the magma chamber, can also produce layering. Other dynamic processes include magma injection into the chamber, which results in magma stratification or magma mixing, and silicate liquid immiscibility either in the main magma chamber or within the solidifying crystal mush.Non-dynamic layer-forming processes mainly include rapid changes in intensive conditions of crystallization (e.g. pressure, oxygen fugacity) that disrupt and change the stable liquidus assemblages, and transitory excursions about cotectic curves. Layering can also result from variation in nucleation rates and from mineral reorganization in a crystal mush through grain rotation, dissolution-precipitation due to initial heterogeneity in terms of grain size distribution, mineral modes or differential pressure. Many of these processes are driven by dissipation of energy and can be referred to as equilibration or self-organization processes.

Igneous Layering in Basaltic Magma Chambers

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