Study On Daqingshan Ductile Deformation Zones Of Lower Crust In Inner Mongolia

Ductile deformation zones(DDZ) , also called high stain zones, exist extensively in the earth from supra-crust to lower crust. They sometimes have two common characters, one is they look like straight lines ; another is they have tectonites with high strain .More and more studies on DDZ indicate that we may resolve some puzzles related to structure and movement of crust lithosphere if we understand the relationship between the characters of ductile deformation zones and the deformation process, metamorphism, magmatic activity. A majority of DDZ reported distribute in supra-middle crust all the time, while few DDZ reported distributing in lower crust are recoginized, and people always consider mylonite as the representative of tectonite of DDZ. There are all kinds of high grade metamorphites ,including gneisses, metamorphic intrusive rock and so on , exposing in Daqingshan high grade metamorphic complexes area. This area have been deformed and metamorphosed for many times in Precambrian, so the structure style of the area is rather complicated, and the DDZ of lower crust strikingW-E formed during these stages. On the base of many researchers'work , Daqingshan DDZ would be studied in this thesis in term of three aspects which are the dimensional characters of DDZ, the structure traces of DDZ, and the microstructure characters and deformation mechanism of tectonites sampled from the DDZ, to open out how the DDZ formed and what is the deformation mechanism of rock in lower crust.1 . The dimensional characters of DDZ and theirs explanation of deformation partitioning The Daqingshan DDZ strike W-E, look like straight lines, and parallel with each other. As intense strain zones of foliation zones striking W-E and dipping S-N steeply, these zones charactered by developing dipping steeply dense straight gneissosity or bandings structure and S-C structure in cross-profile ; while those zones deforming gently laying between them charactered by developing upright folds dipping mildly. The dimensional characters of DDZ and the structure trace characters of high strain zones and low strain zones both accord with the theory of deformation partitioning put forward by T.H. Bell for the first time, and only this theory could explain reasonablely the geometry distributing characters of linear high stain zones and lentoid low stain zones.2.The temperature , pressure and chronology analysis of DDZDaqingshan DDZ formed at high amphibolite-granulite facies conditions which could be proved by deformaed and metamorphosed mineral aggregates, liquid inclusions in polycrystalline quartz banding and so on. The temperature is about 710℃and the pressure is about 0.6GPa, which are acquired by computing garnet- biotite geothermobaromete, quantitatively expaining the deformation conditions of DDZ. The chron of DDZ is between 1900-2200Ma through isotope analysis of single zircon grain selected from intrusives forming the same time as the DDZ.3.The movement mode of DDZThe structure traces of DDZ show that they are produced by the combination of pure and simple shear. The foliations and bandings striking W-E and dipping S-N steeply of the DDZ indicate that they were effected by horizontal S-N pure shear ,while W-E striking sinistral movemet indicates they are effected by W-E simple shear at the same time. The combination effect of pure and simple shear indicate the DDZ is a production of deformation partitioning which is resulted in oblique horizontal colliding between Yin Shan land mass and E Er Duo Si land mass.4.The microcosmic deformation mechanism of DDZ in lower crustWe could understand the microcosmic deformation mechanism of DDZ in lower crust through analyzing the microstructure characters of tectonites in DDZ. Microstructure study show the characters of tectonic gneiss of DDZ in lower crust are different from that of in supra-middle crust. Mylonite have smaller grain , made of porphyroclasts and groundmass, single grain is stretched to a banding; and most grains always exhibit intense intracrystalline deformation characters, for example, undulated extinction, deformed ripples, subgrain and so on . Compared with mylonite, the grain size of tectonic gneiss is near uniform; neither porphyroclasts nor groundmass exsit in tectonic gneiss; most grains touch each other with three-joint point configuration. Tectonic gneiss always develop banding foliation which may result in effect of rock partly molten , but have few intracrystalline deformation characters. These microstructure differences between mylonite and tectonic gneiss indicate that the microcosmic deformation mechanism of mylonite is different from that of tectonic gneiss. Mylonite laying in supra-middle crust may be controlled by dislocation creep during deformation while tectonic gneiss laying in lower crust may be controlled by diffusion creep and granula flow strengthened by melt, which indicate rock deformation mechanism in lower crust riched in quartzofeldspathic may be effected mainly by temperature, melt or liquid ,and differential stress.