极低级变质作用及其研究方法

VERY LOW-GRADE METAMORPHISM AND ITS RESEARCH METHODS

近年来国际上极低级变质作用的研究进展包括综合运用了伊利石结晶度等各项指标 ,将极低级变质带划分为成岩带、近变质带和浅变质带 ,成岩带和近变质带又可进一步划分为低级成岩带、高级成岩带、低级近变质带和高级近变质带 ;研究了伸展背景、阿尔卑斯碰撞背景和增生背景下的极低级变质作用 ;进一步理解了矿物的转变过程 ,建立了变质泥质岩石的矿物反应系列。即变质泥质岩石的二八面体矿物反应系列 ,为蒙皂石—伊 /蒙混层 (I/S)—伊利石—白云母 ;对应变质镁铁质岩石的三八面体矿物反应系列 ,为蒙皂石—绿泥石 /蒙皂石混层 (C/S)—绿泥石。岩石微构造也发生同步变化 ;明确了广泛应用的伊利石结晶度的本质是雏晶大小的一种量度 ,也与晶格应变有关 ,主要受反应动力的控制 ;建立了系统的研究技术方法 ,包括野外采样方法、室内X光衍射法 (XRD)、透射电镜法 (TEM )、地质温度计、地质压力计、岩石学方法。

The major progresses of very low-grade metamorphism are as follows: very low-grade metamorphic belts have been subdivided into Early diagenetic zone, Late diagenetic zone, Low anchizone, High anchizone, Epizone through integrally using of XRD and TEM techniques.The research mainly has focused on pelitic and metabasic rock types where there have been major advances in terms of phase transitions, processes, and also in being able to link to the metamorphic styles with different geodynamic settings.The main geotectonic settings of very low-grade metamorphism, Extensional, Alpine collisional and Accretionary, have been studied; The mineral reaction progress in the metapelitic zone has been identified.Pelites and metapelites formed at lower grades commonly display regular, predictable patterns of changes in mineral assemblages and textures.Among dioctahedral clay minerals the sequence involves smectite-mixed-layer illite/smectite-illite-muscovite.The sequence of changes for trioctahedral Mg, Fe-rich clay minerals is analogous to that for smectite-mixed-layer chlorite/smectite(C/S)-chlorite.These changes are coupled with a regular pattern of increasing crystal size and crystal perfection as commonly measured by illite crystallinity, and changes of microfabrics.There are clearer understandings of the widely used illite crystallinity.Certainly the revelation that changes in crystallite are fundamentally due to changes in crystallite size means that the principal forces driving these types of changes in fine-grained layer silicates-grain boundary and strain energies-would appear to be clearly linked to kinetic effects.A series of research methods to very low-grade metamorphism, such as: field sampling methods, XRD, TEM, geothermometry and geobarometry, organic maturation and fluid inclusions, lithologic method and isotopic methods have been developed.A review of major advances in very low-grade metamorphism in recent years is presented.