Compositional Variation And Strain Texture In The Ductile Shear Zones With The ⁴⁰Ar-³⁹Ar Chronology Analysis

Ductile shear zone has been studied nearly 50 years.However,there are a series of basic scientific problems that have not been solved,for example:formation time of the ductile shear zone,characteristics of fluid-flow during deformation,features and distributions of new-growth minerals,relationships between deformation and strain texture,fragmentation of the original minerals,compositional changes during fragmentation process and so on.In this Ms thesis,an east-west trending ductile shear zone at the north of Yunmengshan,Beijing,has been studied systematically,in order to understand the exact time of ductile shear zone,as well related new-growth minerals and strain characteristics.According to the study of quartz-feldspar mylonite and carbonate mylonite on the outcrop scale,the characteristics of different types of tectonite have been analyzed.In the field,from slight to extensive deformation,there are tectonic breccias,cataclasite,protomylonite,mylonite,a small amount of ultramylonite and carbonate mylonite on the same scale.The mineral composition and structure of the rocks have changed regularly during ductile shear deformation.Feldspar mineral occurs from brecciated to fragmentation,more fine-graining and rotated prophyroclast in the mylonite scale,etc.Main elements of feldspar have been dragged in and out in the process of deformation,but there is no new-growth feldspar.Quartz transfers fragmentation to fine-graining at beginning and starts to appear heterogeneous recrystallization in the early stages of mylonite,then the newly formed quartz makes up strip which is orientation and parallels to structural lineation strip.Minerals such as biotite and hornblende,also experience fracture,fine-graining and gradual disappearance.In addition to a small amount of chlorite,a great deal of new crystallized sericite and muscovite emerge in the rocks.Sericite and muscovite are new-growth minerals which form in the state of protomylonite and mylonite.We have dated the muscovite which is fresh and parallels to mylonitic lineation by the ⁴⁰Ar-³⁹Ar technique and have obtained ⁴⁰Ar-³⁹Ar plateau age of?124Ma.K-Feldspar(potassium feldspar)selected from mylonite yields ⁴⁰Ar-³⁹Ar plateau age of?114Ma.Based on the analytical results made by means of electron probe(EPMA)and BSE images,it indicates that ⁴⁰Ar-³⁹Ar plateau age of K-feldspar represents the end time of ductile shear deformation and that age of new formative muscovite mineral indicates the initial time of deformation.Deformation in ductile shear zone demonstrates generally from slight to extensive on the same level.Structural deformation of rock texture characteristics shows diverse strain features during the same tectonic deformation,rather than deformation distinction of disparate depth levels.Various tectonites in one location suggest that brittle and ductile deformation may be formed at different depth in the same time.