变质地质学研究中的一些困难问题

变质地质学研究中,首先需要准确判明变质过程是属于多期或单期、多峰或单峰变质作用。研究中应该对同一研究区不同类型的变质岩石开展综合研究,查明各种变质反应结构,以期对变质反应结构的解释尽可能准确,并对变质反应进行定量论证。同一变质岩内前、后两个世代变质矿物组合之间,并不存在所谓的热力学局部平衡。在应用方面,矿物温度计与压力计属于反演方法,热力学视剖面图模拟属于正演方法。本文指出了它们之间的相同之处和各种区别。众所周知,目前反演出的变质作用压力并无方向性,差异应力对总应力的贡献尚不明朗。最好根据同一视域或同一块变质岩石,来反演变质作用P-T-t轨迹。实际上,P-T-t轨迹一般不应该是圆滑的曲线,其构造意义的解释也还存在不确定性。同一地区变质作用PT条件的突变、P-T-t轨迹的明显差异,表明地质体之间存在断层接触关系。变形作用与变质作用的耦合研究需要加强,变质作用的定年也不能仅仅局限于变质副矿物。特别值得注意的是,同一造山带内范围有限的地域,也未必属于同一变质相系。

Current problems in metamorphic geology

Accurately determining polymetamorphism or monometamorphism, or polyphase or monophase metamorphism of the studied metamorphic terrane, is the most important work in metamorphic study. Valid interpretation of metamorphic reaction textures should be based on integrated micropetrographic research on different kinds of metamorphic rocks within the metamorphic terrane and naturally, quantitative analysis of mass balance between reactants and products are quite necessary. It should be stated that the so-called local thermodynamic equilibrium between the reactant and product assemblages in a metamorphic rock does not exist, therefore, estimation of metamorphic P-T conditions of the symplectic or coronitic assemblage should include only the mineral compositions of the assemblage itself. In principal, the metamorphic P-T-t path should be retrieved from the exact one micro-domain or ideally from the same one rock sample. In deciphering metamorphic P-T conditions or P-T-t paths, geothermobarometry is an inverse method, whereas pseudosection modeling is a forward one. The estimated thermodynamic pressure has been always treated as isotropic in which the deviatoric stress cannot be separated from the total pressure until today. Also in principal, any one metamorphic P-T-t path should be a kinked curve although the most retrieved P-T-t paths are smooth in shape. Abrupt interrupt of metamorphic P-T conditions among different metamorphic units suggests the existence of faults, and quite different P-T-t paths retrieved from different blocks within a metamorphic terrane may imply tectonic contact relationship. Presently, interpretation of the relationship between metamorphism and deformation is somewhat ambiguous, and geochronologic work should be done on both the main and accessory metamorphic minerals. Finally, it should be noted that even in a narrow geographic area within one same metamorphic terrane, different metamorphic facies series may exist, which is in contrast to traditional theory of metamorphism.