多期变质作用、多相变质作用与变质作用p-T-t轨迹

Polymetamorphism,polyphase metamorphism and metamorphic p-T-t paths

多期变质作用是指地质体经历过不止一次变质事件的变质过程。在某一期变质作用中,仅出现过一个温度(或压力)极值的过程称为单相变质作用,出现过不止一个温度(或压力)极值的变质过程则属于多相变质作用。研究变质作用,首要的工作是要正确区分不同期的变质作用,不过目前还缺少足够多的准则。但是,岩石中所保留的地球动力学背景不同的变质事件,必然属于不同期的变质作用。准确恢复变质作用p-T-t轨迹,须根据变质反应结构准确划分各变质期次,利用矿物温度计、压力计、变质相图等估算变质条件,根据矿物原位测年确定变质作用某个阶段的地质时间。这三方面还有一些不确定因素,相关理论及方法都有待深化。

Monometamorphism is resulted from one metamorphic event, whereas polymetamorphism resulted from two or more metamorphic events. If a metamorphic event involves only one phase of heating or pressing and then cooling or depressing, it is called monophase metamorphism. If a metamorphic process involves two or more climaxes in the changes of temperature and/or pressure, it is called a polyphase metamorphism. Although criteria are quite rare, it is very important to determine if a metamorphism belongs to a monometamorphism or a polymetamorphism. However, metamorphic processes preserved in one geologic entity with different geodynamic backgrounds are definitely of different metamorphic events. To accurately retrieve metamorphic p-T-t paths, metamorphic events should be discriminated by inspecting metamorphic reaction textures, p-T conditions should be determined by valid geothermometers and geobarometers, and finally, the geologic time should be determined by in-situ geochronology. Since uncertainties exist in all of these three aspects, refining and developing the fundamental theory and technology are much needed in the near future.