麻粒岩的研究进展与方法

近年来，有关麻粒岩的研究取得了长足进展，本文讨论了4个相关问题：（1）麻粒岩的大地构造环境与P-T轨迹。麻粒岩可以形成于4种大地构造环境中：（a）碰撞造山带以形成高压麻粒岩为特征，为中压相系，包括曾位于地壳浅部的岩石经历构造埋深达到变质峰期后再折返的过程，为顺时针型P-T轨迹；也包括曾经历洋壳或陆壳俯冲形成的高压-超高压榴辉岩相岩石折返叠加变质形成的麻粒岩，P-T轨迹以减压为主。（b）地壳伸展区以形成低压麻粒岩为特征，并可达到超高温条件，其P-T轨迹为减压加热至温度峰期，随后发生等压或降压冷却。（c）岛弧或陆缘岩浆增生区的下地壳多为高压麻粒岩相，其中侵入的辉长岩首先经历等压冷却，然后再经历升温升压进变质过程。（d）太古宙克拉通麻粒岩相表壳岩呈皮筏状分布于TTG片麻岩内部，多达到超高温条件，发育逆时针型P-T轨迹，受太古宙特殊的垂直构造体制控制。（2）麻粒岩的进变质过程与流体行为。按照流体行为，麻粒岩的进变质过程分为3种型式：（a）流体饱和进变质过程，指岩石在饱水固相线之前达到流体饱和，随后发生饱水固相线熔融与含水矿物的脱水熔融，以及阶段性熔体丢失，导致岩石中水含量降低，缺流体固相线温度升高；在峰期之后的降温过程中，发生熔融反应的逆反应，或结晶反应，形成含水矿物，结晶反应终止于缺流体固相线。（b）流体不饱和或缺流体进变质过程，指岩石在进变质过程中会处于流体缺失状态，不会发生变质反应，岩石中原来的矿物组合以亚稳定状态保留至缺流体固相线后，才开始变质演化，因此经常形成一些不平衡结构。（c）流体过饱和进变质过程，指有过量水参与的熔融反应过程，也称为水化熔融，与熔体注入或局部汇聚有关；水化熔融过程中会更多地消耗斜长石、石英及辉石等无水矿物，导致残余物中富集角闪石和黑云母等含水矿物。（3）确定麻粒岩P-T条件的视剖面图方法。利用视剖面图方法分析麻粒岩的变质条件时，首先需要通过岩相学观察区分出峰期组合和最终组合；然后通过计算T-M（H₂O）图解确定最终组合的含水量；最后利用所确定的水含量计算P-T视剖面图。利用P-T视剖面图分析麻粒岩的峰期变质条件时，首先找到峰期矿物组合在视剖面图上的稳定域，然后再结合有价值的矿物成分等值线确定P-T条件。特别需要注意的是，岩相学观察确定的峰期组合和最终组合都可能受局部结构域控制，与滞留熔体的不均匀分布或原地分凝有关，此时不能简单地用全岩成分模拟其相平衡关系。（4）相平衡模拟时需要选择有效的全岩成分。当选择实测全岩成分进行相平衡模拟时，首先需要检验其有效性，即检验实测全岩成分是否能够代表薄片中所观察到的相平衡关系。方法是计算有效全岩成分，并与实测全岩成分进行对比。对于成分不均匀的变质岩石，需要处理局部结构域的成分。分如下3种情况：（a）宏观尺度的结构域，可以分别取样；（b）微观尺度的结构域，需要在显微薄片中进行图像分析，针对不同结构域分别进行相平衡模拟；（c）由叠加或退变质形成的结构域，需要确定相应的变质反应，通过对反应配平，确定有效全岩成分。此外，文中还介绍了计算岩石中的水含量、O含量和各种矿物相含量的方法与注意事项。

Some advances and research approaches on granulite

In recent years, the research on granulite has made great advances. This article discusses four related issues:(1) The geotectonic environments and P-T paths of granulite. Granulite can be formed in four types of tectonic environments:(a) Collision orogens are characterized by the formation of high-pressure (HP) granulite of medium-pressure facies series. The HP granulites include two sub-types. The first sub-type refers to that rocks once located in shallow crust levels experience loading and burial to reach a metamorphic peak, and are subsequently exhumed, constructing clockwise P-T paths. The second sub-type includes that HP and UHP eclogite facies rocks that have undergone oceanic and continental subduction are exhumed to be involved in orogens with granulite overprinting, the P-T paths of which are dominated by decompression. (b) Crustal extension zones are marked by the formation of low-pressure granulite, generally up to UHT conditions. The P-T paths include decompressional heating to temperature peak, followed by isobaric or decompressional cooling. (c) The magma accretion zones in island arcs or continental margins generally reach HP granulite facies in their lower crust, where intrusive gabbros first undergo isobaric cooling, and then undergo progressive metamorphic evolution with increasing both temperature and pressure. (d) In Archean cratons, supracrustal rocks of granulite facies occur as rafts witin the domes of TTG gneisses, which mostly reach UHT conditions, and share counterclockwise P-T paths, indicating Archean unique vertical tectonic regimes. (2) The prograde processes and fluid behavior of granulite. According to the fluid behavior, the prograde processes of granulite can be divided into three types:(a) The prograde process with fluid-saturation, means that a rock is saturated with fluids under subsolidus conditions, and then melting on the water-saturation solidus and dehydration melting of hydrous minerals occur successively with heating. As staged melt loss occurs in the rock, its water content decreases but the temperature of its fluid-absent solidus increases. During the post-peak cooling process, back-reactions or crystallization reactions occur to form hydrous minerals, which are terminated at the fluid-absent solidus. (b) The prograde process with fluid-unsaturation or fluid-absence, means that a rock is fluid-absent during the subsolidus prograde process, where no metamorphic reactions may occur. As the original mineral assemblages in the rock commonly remain metastablly until temperature reaches the fluid-absent solidus, where metamorphic evolution begins, some disequilibrium textures are often developed. And (c) The prograde process with fluid oversaturation, refers to that the melting process is involved with excessive water, also known as water-flux melting, commonly related with melt injection or local segregation. Water-flux melting may consume more anhydrous minerals such as plagioclase, quartz and pyroxene, resulting in a residual accumulation of hydrous minerals such as amphibole and biotite. (3) The determination of P-T conditions of granulites using pseudosection. To document a metamorphic evolution of granulite using pseudosection approach, it is first to identify the peak and the final assemblages through petrographic observation; then to calculate the water content of the final assemblage using a T-M(H₂O) pseudosection; and finally to calculate P-T pseudoections using the determined water content. To analyze the peak P-T condition of a granulite using a pseudosection, it is first to match the observed peak assemblage with the predicted one in the pseudosection, and then to combine the useful mineral composition contours for further determining the P-T condition. It is important to note that both the petrographically observed peak and final assemblages may be controlled by local textural domains, related to the uneven distribution or in-situ segregation of retained melts. At this case, the phase relations cannot be simply modelled using the bulk-rock composition. (4) Effective bulk-rock compositions are required for phase equilibria modelling. Using a measured bulk-rock composition for phase equilibria modelling, it is first to check the validity of the composition, that is, to check whether the measured bulk-rock composition can represent the phase equilibria observed in a thin section. The method is to calculate an effective bulk-rock composition and to compare it with the measured one. For metamorphic rocks are commonly inhomogeneous, it requires addressing the compositions of local textural domains. There are three situations as follows:(a) macro-scale textural domains can be sampled separately; (b) micro-scale textural domains need to be imaged in thin sections, and thus, to be modelled separately; and (c) for the textural domains formed by overprinting or retrograde reactions, it requires determining the corresponding reactions, and the effective bulk-rock composition can be determined by balancing the reaction. In addition, the paper also introduces the methods and precautions for calculating the water content, O content and mineral modal proportion in a rock.