Orogeny, migmatites and leucogranites: A review
Orogeny, migmatites and leucogranites: A review
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DOI：
10.1007/bf02702898
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发表时间：
2001-12
影响因子：
1.9
通讯作者：
Michael Brown
中科院分区：
文献类型：
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作者：
Michael Brown
P-T-t轨迹类型和流体（富H_2O变质挥发相或熔体）的可利用性是变质作用的重要变量。碰撞造山带的特征是顺时针P-T演化，这意味着在核心，温度超过普通地壳岩石的湿固相线，熔体可能存在于整个演化的重要部分。对侵蚀造山带的野外观测表明，下地壳是混合岩，地球物理观测表明活动造山带中存在熔体。这些结果的一个后果是，在成熟造山带的造山塌陷可能是由一个部分熔融层，从俯冲岩石圈的弱地壳，这样一个弱层可能使深埋地壳折返控制。混合岩记录了部分熔融的地壳物质的熔融分离和自然条件下的同重熔变形。晶界流动和晶内、晶间断裂流动是主要的晶粒尺度熔体流动机制。对古造山带混合岩的野外观察表明，浅色体定向产于变质组构中或位于扩张部位。这些观察结果解释表明，熔体偏析和提取同构造过程，熔体迁移途径通常涉及到岩石组构和结构。因此，亏损混合岩中的浅色体记录了残余渗透率网络，但渗透率网络的演化和异常的放大作用却知之甚少。部分熔融岩石的变形是由熔体增强颗粒流来调节的，而体积应变是由熔体损失来调节的。熔体偏析和提取可以是循环的或连续的，这取决于所施加的差应力水平和熔体压力累积速率。在逆时针P-T演化过程中，H_2O通过脱水熔融反应从原岩向熔体中迁移，如果亏损原岩中仍有云母存在，则H_2O可能通过超固相线减压脱水反应在低P时向固相线以上演化。溶解在熔体中的H2O通过地壳输送，在结晶时脱溶。这种再循环的H2O可以促进在超固相线条件下的湿熔融和在亚固相线条件下的回归。混合岩中“晚期”白云母相对于硅线石的共同生长可能是这一过程的结果，外源H2O的流入可能不是必需的。然而，总体而言，地壳演化中的交代作用仍然是一个有争议的问题。最低地壳的过程可以从研究熔岩中带到地表的捕虏体套件中推断出来。根据地球化学数据，我们可以使用统计方法和建模来评估混合岩是否是花岗岩的源或供给带，或者简单地分离熔体，停滞在残留物中，并比较推断的下地壳与暴露的深地壳捕虏体。上地壳花岗岩是下地壳常见的熔融亏损麻粒岩的必要补充，但镁铁质岩浆在地壳熔融中的作用仍不确定。深圳湾岩体出现在地壳中脆性-粘性过渡层之上和之下的不同深度处，并具有各种各样的三维形状，可能随深度而系统地变化。从上升到侵位的转换可能是由于上升柱内部（渗透率、岩浆流速）或周围（应力强度或状态）的不稳定性放大，或者上升岩浆与地壳中的某些不连续面相交，从而使岩浆水平侵位，然后在深成岩膨胀期间增厚。反馈关系...
The type ofP-T-tpath and availability of fluid (H2O-rich metamorphic volatile phase or melt) are important variables in metamorphism. Collisional orogens are characterized by clockwiseP-Tevolution, which means that in the core, where temperatures exceed the wet solidus for common crustal rocks, melt may be present throughout a significant portion of the evolution. Field observations of eroded orogens show that lower crust is migmatitic, and geophysical observations have been interpreted to suggest the presence of melt in active orogens. A consequence of these results is that orogenic collapse in mature orogens may be controlled by a partially-molten layer that decouples weak crust from subducting lithosphere, and such a weak layer may enable exhumation of deeply buried crust. Migmatites provide a record of melt segregation in partially molten crustal materials and syn-anatectic deformation under natural conditions. Grain boundary flow and intra-and inter-grain fracture flow are the principal grain scale melt flow mechanisms. Field observations of migmatites in ancient orogens show that leucosomes occur oriented in the metamorphic fabrics or are located in dilational sites. These observations are interpreted to suggest that melt segregation and extraction are syntectonic processes, and that melt migration pathways commonly relate to rock fabrics and structures. Thus, leucosomes in depleted migmatites record the remnant permeability network, but evolution of permeability networks and amplification of anomalies are poorly understood. Deformation of partially molten rocks is accommodated by melt-enhanced granular flow, and volumetric strain is accommodated by melt loss. Melt segregation and extraction may be cyclic or continuous, depending on the level of applied differential stress and rate of melt pressure buildup. During clockwiseP-Tevolution, H2O is transferred from protolith to melt as rocks cross dehydration melting reactions, and H2O may be evolved above the solidus at lowPby crossing supra-solidus decompression-dehydration reactions if micas are still present in the depleted protolith. H2O dissolved in melt is transported through the crust to be exsolved on crystallization. This recycled H2O may promote wet melting at supra-solidus conditions and retrogression at subsolidus conditions. The common growth of ‘late’ muscovite over sillimanite in migmatite may be the result of this process, and influx of exogenous H2O may not be necessary. However, in general, metasomatism in the evolution of the crust remains a contentious issue. Processes in the lower-most crust may be inferred from studies of xenolith suites brought to the surface in lavas. Based on geochemical data, we can use statistical methods and modeling to evaluate whether migmatites are sources or feeder zones for granites, or simply segregated melt that was stagnant in residue, and to compare xenoliths of inferred lower crust with exposed deep crust. Upper-crustal granites are a necessary complement to melt-depleted granulites common in the lower crust, but the role of mafic magma in crustal melting remains uncertain. Plutons occur at various depths above and below the brittle-to-viscous transition in the crust and have a variety of 3-D shapes that may vary systematically with depth. The switch from ascent to emplacement may be caused by amplification of instabilities within (permeability, magma flow rate) or surrounding (strength or state of stress) the ascent column, or by the ascending magma intersecting some discontinuity in the crust that enables horizontal magma emplacement followed by thickening during pluton inflation. Feedback relations between …