Large Size Ductile Strike-Slip Shearing And The Formation Of Qilian Caledonian Orogen

There are two subduction and collision related complex belts, which occur at the north borderand south border, respectively, of the Qilian microcontinent, formed by the collision between theAlxa micro-continent, the Qilian micro-continent and the Qaidam micro-continent in the EarlyPaleozoic period. The north one is called the North Qilian Caledonian subduction-collisioncomplex belt (NQCB), and the south one is called the South Qilian subduction-collision complexbelt (SQCB). The NQCB is characterized by the occurrence of ophiolite, subduction relatedgranites, island arc volcanic rocks, high pressure metamorphic rocks and accretionary wedges,whereas the SQCB is characterized by the occurrence of island arc volcanic rocks and subductionrelated granites, ultrahigh pressure rocks. These two belts were formed by the close of the Qilianocean basin, and the different is that ocean-continent subduction occurred at the NQCB whereas atthe SQCB the oceanic crust subduction was followed by continental subduction.This thesis is aimed to study the two boundary ductile shear zones of the Qilianmicrocontinent, both of them are large ductile strike-slip shear zones, and to understand thecharacteristics and nature of shearing, original mechanism, and their contribution to the orogen,through the investigation of their spatial distribution, geometry, structural analysis, chemicalcomposition of mylonite and syntectonic anatexis, and geochronology.The north boundary shear zone is called the North Boundary Ductile Strike-slipping ShearZone of Qilihn (NBSZ), and the south boundary shear zone is called the South Boundary DuctileStrike-slipping Shear Zone of Qilian (SBSZ). The foliations of mylonites in the two zones deeplydip to the north with nearly horizontal stretch lineation, dextral shear sense, and compressionaldeformation in the YZ face.The geochemical compositions show that the protolith of mylonite in NBSZ probably waspelite, sandstone and volcanic rocks. The compositional features of garnets, high TiO2 (about 2wt%) in biotite and sillimanite occurs in the mylonite suggest a high temperature environment ofthe shearing. The hightemperature prismatic glide system , middle temperature rhombic glidesystem and low temperature based glide system for the preferred orientation of quartz inNBSD indicate the shear zone formed in a high temperature (above 650℃), and a middle-hightemperature (400-600℃), after undergoing a retrogressive metamorphism, which is accordancewith the temperature T=542-753℃(P=6.2-8.4 kbar) of mylonite calculated by GBIMSPL andThermocalc geothermobarometers. The felsic bands in ductile shear zone are ubiquitous, distributeisolated along foliation, and the deformation features of felsic bands are the same with mylonite,which show that was formed by partial melting resulted from shear heat energy during shearing.They are products of syntectonic melting during dextral strike-slip shearing.Geochronology study shows that the formational age of NBSZ is 410-394Ma, dated by U-Pbmethod of zircon from felsic mylonite, and the age of SBSZ is 406-401 Ma, both suggest a lateCaledonian age. The NBSZ and SBSZ formed during transformation from normal to obliqueintrocontinental subduction between the micro-continents. Since large size shearing fault zones arecommonly developed in the Qinghai-Tibetan plateau, "oblique introcontinental subduction"probably is a mechanism for the formation of the shearing fault, and the latter causes the formationand uplift of mountains as well as plateau.