Experimental determination of the fluid-absent melting relations in the pelitic system
Experimental determination of the fluid-absent melting relations in the pelitic system
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DOI：
10.1007/bf00375178
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发表时间：
1988-03
影响因子：
3.5
通讯作者：
D. Vielzeuf;J. Holloway
中科院分区：
文献类型：
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作者：
D. Vielzeuf;J. Holloway
为了给普通变质沉积岩部分熔融模型提供额外的约束条件，我们对天然变质沉积岩在无流体条件下的熔融进行了实验研究。起始成分包括石英、斜长石、黑云母、白云母、石榴石、橄榄石和蓝晶石。在750°至1250°C的温度范围内，在750°至1250°C的10 kbar条件下，在7、10和12 kbar条件下进行了实验。以下反应解释了在750°至1250°C之间观察到的矿物学变化：Bi+Als+Pl+Q=L+Gt+(Kf), Ky=Sill, Gt+Als=Sp+Q, Gt=L+Sp+Q和Sp+Q=L+Als。在扫描电子显微镜下，用能量色散系统测定了相的组成（温度在T bb0 ~ 875℃）。通过质量平衡和扫描电镜图像处理计算了熔体和晶体的相对比例。利用这些约束条件，结合其他可用的实验数据，提出了一系列p -t、T-XH2O和液相图，这些图代表了2-20 kbar和600°-1250°C范围内偏长岩有流体和无流体熔融的模型。我们证明，即使在无流体条件下，由于Bi+Als+Pl+Q=L+Gt（+/-Kf）的反应，在850°-875°C的狭窄温度范围内产生了大部分（≈40%）s型花岗岩液体。这些液体，或至少其中的一部分，可能会从源头分离出来，留下由石英、石榴石、硅线石、斜长石组成的残留物，代表了铝粒岩的特征组合。850℃左右大量熔体的产生对缓冲变质温度也有重要作用。在经历进一步变质的弹性再循环下地壳中，由于没有这种缓冲作用，温度可能达到接近1000℃的值。部分熔融是导致陆内分异的主要过程。我们在实验和模拟的背景下讨论了北比利牛斯带暴露的地壳截面。
In order to provide additional constraints on models for partial melting of common metasediments, we have studied experimentally the melting of a natural metapelite under fluid-absent conditions. The starting composition contains quartz, plagioclase, biotite, muscovite, garnet, staurolite, and kyanite. Experiments were done in a halfinch piston-cylinder apparatus at 7, 10, and 12 kbar and at temperatures ranging from 750° to 1250° C. The following reactions account for the mineralogical changes observed at 10 kbar between 750° and 1250° C: Bi+Als+Pl+Q=L+Gt+(Kf), Ky=Sill, Gt+Als=Sp+Q, Gt=L+Sp+Q, and Sp+Q=L+Als.The compositions of the phases (at T>875° C) were determined using an energy-dispersive system on a scanning electron microscope. The relative proportions of melt and crystals were calculated by mass balance and by processing images from the SEM. These constraints, together with other available experimental data, are used to propose a series ofP-T, T-XH2O, and liquidus diagrams which represent a model for the fluid-present and fluid-absent melting of metapelites in the range 2–20 kbar and 600°–1250° C.We demonstrate that, even under fluid-absent conditions, a large proportion (≈40%) of S-type granitic liquid is produced within a narrow temperature range (850°–875° C), as a result of the reaction Bi+Als+Pl+Q=L+Gt(+/-Kf). Such liquids, or at least some proportion of them, are likely to segregate from the source, leaving behind a residue composed of quartz, garnet, sillimanite, plagioclase, representing a characteristic assemblage of aluminous granulites.The production of a large amount of melt at around 850° C also has the important effect of buffering the temperature of metamorphism. In a restitic, recycled, lower crust undergoing further metamorphism, temperature may reach values close to 1000° C due to the absence of this buffering effect. Partial melting is the main process leading to intracontinental differentiation. We discuss the crustal cross-section exposed in the North Pyrenean Zone in the context of our experiments and modelling.