变质流体研究新进展

ADVANCES IN THE STUDY OF METAMORPHIC FLUIDS

变质流体是变质过程的主要动力学因素之一。目前变质流体研究主要集中在下部地壳麻粒岩相变质流体，俯冲带高压－超高压变质流体和接触变质流体等方面。研究的主要问题是流体流动机制和元素迁移，流体－岩石相互作用和流体来源。下部地壳麻粒岩相变质流体以ＣＯ２为主，具有较低的ａＨ２Ｏ。δ１３Ｃ研究表明大约２／３ＣＯ２是深成的。富ＣＯ２流体流动是紫苏花岗岩形成和热扰动的原因之一，也是麻粒岩形成和大离子亲石元素亏损的主要因素。俯冲带是高压、超高压变质作用发生和流体活动最活跃的场所。流体富含Ｈ２Ｏ、ＣＨ４和ＣＯ２，可以诱导部分熔融反应和岛弧岩浆作用。高压变质条件下的矿物稳定性也与流体有关。同位素研究表明，在超高压变质期间没有化学上完全相同的流体大规模循环。流体－熔体系统模式能更有效地解释下插板片的元素再循环。接触变质流体研究主要集中在含有易于发生流体－岩石反应的不纯碳酸盐岩地区。硅灰石带中流体／岩石比率高达４０∶１，表明接触变质岩石中有大量流体存在。接触变质过程流体成分有较大差异，主要取决于流体来源、原岩性质和侵入体特征。流体流动和循环模式受控于构造变形，岩浆作用和变质过程的动力学条件及流体成分。

Metamorphic fluid is a major dynamic agent of metamorphic process. At present, the study of it focuses on lower crust and granulite facies metamorphic fluid, subduction zone and high ultrahigh pressure metamorphic fluid, contact metamorphic fluid. The main problems are fluid flow and elemental transportation, the fluid rock interaction and the source of the fluid. CO 2 is a main composition of lower crust and granulite facies metamorphic fluid, which has low H 2O activity ( a H 2O ). The study of carbon isotope (δ 13 C) indicates that 2/3 CO 2 is derived from deep crust and mantle. CO 2 rich fluid flow is a major cause for thermal perturbation during the granulite facies metamorphism and charnockitization, and results in LILE depletion in granulite. Subduction zone is the most active site for high ultrahigh pressure metamorphism and fluid flow which is rich in H 2O, CH 4 and CO 2. The fluids released during high pressure metamorphism play an important role in the evolution of subducted crustal rocks and may connect directly with igneous activity in island arc. The mineral stability is in close relationship with the fluid under high pressure metamorphic conditions. Isotopic data indicate that fluid mobility was limited during high pressure metamorphism. The fluid melt interaction model provides an attractive explaination for the instantaneous and selective transport of elements between the downgo slab and the overlying mantle wedge. Impure limestones with interstratified metachert layers where apt to happen fluid rock reactions are excellent for the study of contact metamorphic fluid. The fluid/rock ratio which is 40∶1 in wollastonite zone shows that there are a lot of fluid during contact metamorphism. Fluid compositions very in different contact metamorphic processes depending on the fluid source, the features of sedimentary rocks and intrusive rocks. The direction and mechanism of fluid flow are controlled by deformation, magmatism and dynamic conditions of contact metamorphism.