PROBLEMS ON STRATIFIED STRUCTURES IN A METAMORPHIC TERRANE

The most important task in geological mapping in a metamorphic terranem is to recognize the types of stratified structures correctly.The author has sum- marized two types of stratified structures. 1.Blastoprimary stratified structures.This type of stratified structures is generally well preserved and can be easily recognized in low-and low-to me- dium-grade metamorphic terranes and slightly deformed sections.Of the indi- cators,the most important is blastoprimary structures（e,g.structures of psa- mite,cyclothem,bedding and fumarole）.These structures can sometimes be preserved in medium- to high-grade metamorphic terranes as well. 2.Secondary stratified structures.This type of stratified structures can be subdivided,in terms of genesis,into differentiation,structural transposition,and composite stratified structures. The differentiation stratified structure refers to the metamorphic tectonite belt that results in varying degrees of metamorphism and deformation of massive rocks.The layering is reflected by secondary foliation and lamellae and banded structures formed by metamorphic tectonic differentiation. The stratified structure formed by transposition of structures means the transposition of newly-formed foliation（S_n）to earlier foliation(S_n-1)during progressive deformation in an episode of folding,which acts in two ways, that is transposition of shape and that of composition.The transposition can be classified into W-type,N-type,and I-type from weaker to stronger. The composite stratified structure is the secondary stratified structure that originates from complicatedly composed rocks by metamorphism and deformation. The blastoprimary stratified structures and those formed by transposi- tion of structures through metamorphism and deformation should be distingui- shed by their composition and textures,distribution and extension,and chara- cteristics of their boundaries.Strain indicators,fabrics and textures of minerals should be analysed synthetically as well. Recognition of the nature of the stratified structures in a metamorphic ter- rane is the most important for restoring the order of rock formations and the stratigraphical sequence.However.the requirement should be varied beca- use the transformation varies in different tectonic fields.In a strongly trans- formed terrane,as the primary stratification is basically eliminated and the layering accumulation is the result of tectonic accumulation,the approximate order of rock formatians could only be established on the basis of a full structural analysis and synthetical recognition and analysis of the order of rock formations.On the contrary,in a slightly transformed terrane the primary structures are well preserved so that it is possible to establish regional or local stratigraphical sequence.