The Paleozoic Tectonic Characters And Evolution Of The Middle Section Tianshan Orogenic Belt

The faults of the southern and northern margin of the middle Tianshan were important structure belts, but their attribute isn't entirely known. Furthermore, there is no common viewpoint about their characters , evolution and polarity of subduction . For this reason, we choose to work in these areas(Mishigou ,Gangou, Wusitegou ,Kekenaikegou, Wuwamen, Yushougou, and so on)with more ophiolite suits ,which belong to the faults. We get their sections or geostenogrames and find these ophiolitic melanges were made up of different rocks ( like serpentine rocks, basalt rocks, gabbro rocks, sillcalite rocks, limestone rocks,etc.) and chaotic matrixes . Geochemical analysis suggests that the composition of these peridotites and basalts being similar to the mid-ocean ridge ophiolite. Moreover ,another melange in Hongwuyue bridge area was discovered , and its geological characters is similar to the Mishigou-Gangou ophiolitic melanges . On the basis of the plate tectonic theory, through field investigation, in-door research and synthesizing former datum, we contrasted and concluded characters of sedimentary formation, volcanics, deformation and metamorphism. We find that the northern margin orogenic belt of the middle Tianshan is suture zone. So there was a ocean, which began in Early Cambrian. Its west part dived and closed from Late Cambrian epoch to Late Ordovician epoch. The middle part occurred same phenomenon in Silurian, the polarity of its subduction is north to south .In Late Silurian epoch, the ocean closed and started postorogenic evolution .The southern margin orogenic belt of middle Tianshan is a suture zone too, the ocean basin represented by it , according to our studies ,1 thinks it developed independently, that is to say ,it wasn't from back-arc basin . The action that the north Tianshan ocean dived to south only accelerated its development. Perhaps, we may consider that the ocean was developed by itself at earlier stage, but added some back-arc basin feature because of the north ocean's subduction . The south Tianshan ocean polarity of subduction is south to north.In Early Carboniferous, Tianshan mountains entered an intraplate evolution stage.