Study On The Variation Characteristics Of Clay Minerals In The Folds Of Permian Linxi Formation In Suolun Area And Its Hydrothermal Experimental Simulation

Folds are a kind of geological phenomenon that rock strata originally nearly straight bend and deform into an arc shape under the actions of stresses.In the folding process,elements migrate because of differences in physical and chemical conditions.Meanwhile,during the formation of folds,changes in mineral constituents and elements of rocks in the folds are affected by a large amount of fluid actions,mineral pressure solution,and so on.Therefore,research on migration and change characteristics of internal elements of fold is of great significance to the formation and distribution of various mineral deposits and the tectonic history of Earth crust.A small recumbent fold 3.2 kilometers away from S203 Highway beside Solun Prairie is taken as the study object in this paper.The microscopic structure,chemical composition,and clay mineral phase of the folded rocks are analyzed and tested.The geochemical changes and clay mineral changes in rocks caused by the folding process are explored.An indoor high temperature and high pressure hydrothermal experiment is carried out to explore how changes in physical environments(mainly pressure and temperature)of the folds affect the conversion among clay minerals.The main findings of this paper are as follows.(1)The main elements in the fold are Na,Mg,Al,Si,K,Ca,Ti,and Fe.In the core of the compression zone,Mg,Fe,and Ca are depleted while Si and Na are enriched.From the core to the two limbs on the neutral surface,the contents of Si increase gradually.The contents of K,Mg,Fe,Ca,and Al are relatively high in the core.The Mg,Si,Fe in the core of extension zone have a higher content.The contents of Si and Al show an overall negative correlation.And the contents of Mg,Ca and Fe are relatively high in places with small stress and strain(2)In different parts of the fold,the distribution of minerals is different.From the core to the limbs in the compression zone,quartz increases gradually while feldspar shows a gradual decrease.However,the distribution features of feldspar in the extension zone is opposite.Meanwhile,the change features of clay mineral contents are also varying within different parts of the fold.The contents of illite and mixed-layer minerals of illite and smectite both show a decrease trend from the limbs to the core in the compression zone but an increase trend in the same direction in the extension zone.And the chlorite show a increase trend from the limbs to the core in the compression zone.(3)Rock metamorphic grade is related to stress state.Due to the inconsistency of deformation degree and stress state among different parts of the fold,the rock metamorphic grade varies from one position of the fold to another.The metamorphic grades of different parts of the fold are divided based on the crystallinity value of illite.The following characteristics of the fold are found.Illite in compression zone(extension zone)has a low crystallinity level and the highest rock metamorphic grade,able to reach low-grade metamorphism;the neutral surface comes second(but the metamorphic grade on the axial zone here is the lowest);the extension zone is the weakest,where the majority is at an extremely low metamorphic grade.(4)Simulation study of the hydrothermal experiment shows that illitization of smectite is affected by the ambient temperature and pressure conditions.The higher the temperature is,the easier the illitization is.In addition,it is proved that as the increasing of temperature and pressure,the transformation series can be described as follows: from smectite to disordered I/S,to ordered I/S and to illite.