The Late Hercynian-Indosinian Structural Characteristics Of The Zongwulong Tectonic Belt In North Qaidam Basin

Zongwulong tectonic belt, an S-shaped collisional orogenic belt with NW-SE orientation, is located at the north margin of the North Qaidam bain and is surrounded by North Qaidam tectonic belt, South Qilian block and West Qinling block. As the convergent and combining zone of different blocks and kinds of orogenic belt through various mechanisms and directions, the tectonic belt exhihits complex structural characteristics and evolution history. Comprehensive studies have given in this thesis on material composition, formation environment, formation age, deformation characteristics and tectonic evolution of Zongwulong tectonic belt in the north margin of Qaidam basin by the compound study methods of structural geology, petrology, rock geochemistry, isotopic geochronology. A few major developments and preliminary cognitions are acquired as follows:1.Material composition studies of Zongwulong tectonic beltThe stage of initial break-up in early Devonian, is mainly of basaltic magma deep intrusion in post orogenic environment. The Chahannuo amphibole gabbro of this stage is the outcome of intracontinental cracking after the post Caledonian orogenic period. The stage of extension-collapse to passive continental margin during the late Carboniferous-early Premian period, which is featured with the Carboniferous flysch in extension environment, the stable Permian marine carbonate deposits in passive continental margin and the Ercidegou N-MORB basalt and Baluogenguole E-MORB basalt. During the subduction and closing stage of Zongwulong limited oceanic basin in the late Late Permian-Middle Triassic period, the widely developed continental island arc type granites in the south rim of Northern Zongwulong fault and carbonate-clastic sediments with lots of limestone gravel are widely developed, which means the dramatic changing of tectonic setting from passive continental margin to active continental margin. The intra-continental shear orogenic evolution in Late Trassic casts the shearing-extension A1type granites and widely missing of the Upper Trassic. The event of early Early-Middle Jurassic fault-downlift deposition in extension environment was the closing of the Hercynian orogeny.2.The provenance of sediments in ZongWuLong tectonic beltResearch on petrology, geochemistry and zircon geochronology of clastic rocks in Zongwulong structure belt, some conclusions have been made.The Late Carboniferous Zongwulong Group sandstone has high clastic content and lower psephicity, which means the Carboniferous clastic rocks should be close to the source material deposition. According to geochemistry and zircon geochronology of Late Carboniferous clastic rocks, it shows that the source rocks are mainly felsic rocks from Oulongbuluke block in the North Qaidam basin deposited in active continental margin-island arc environment. Permian sandstone has lower feldspar and detritus content, relatively smaller particle size, meanwhile source rock characteristics show the Permian rocks derive from cyclic sedimentation in passive continental margin. Unconformities between the Permian and Silurian widely developed in the north of Zongwulong tectonic belt, it also shows the Permian clastic rocks mainly derive from the Silurian deposits in South Qilian which formed in passive continental margin environment. Contrarily, the Lower-Middle Triassic sandstone with extremely high feldspar and detritus content, poor mineral particles round, it deposited inrelatively unstable tectonic environment. Source rock characteristics of Lower-Middle Triassic sandstone shows it derives from the active continental margin-island arc of the North Qaidam.3.Study of the Late Hercynian-Indosinian structural features in Zongwulong tectonic beltZongwulong tectonic belt has undertaken poly-cycle and multi-type tectonic deformation and tectonic superposition. After detailed structure analysis, it is considered that the deformations can be divided into2systems as follows:ocean-continent subduction deformation under the control of compression-contraction in Late Permian-Early and Middle Triassic, dextral strike-slip ductile shear deformation in Late Triassic.The deformation of dextral strike-slip shear is obvious in the east part of Zongwulong tectonic belt and weaker to the west. Chronology studies on the deformed sericite in the metabasalt suggests that the time of subduction and dextral strike-slip are258.1Ma-236.0Ma and238Ma-202Ma respectively.4.The Late Hercynian-Indonian evolution and genesis of Zongwulong tectonic beltZongwulong tectonic belt has mainly undertaken4evolutional stages since Silurian, such as the initial break-up of Silurian, the extension and division-collapse of Late Carboniferous to passive continental margin evolution in Early Permian, Subduction and collision in Late Permian to Early-Middle Triassic, dextral strike-slip ductile shear orogeny in Late Triassic. Meanwhile, the evidence of Caledonian tectonothermal event is easily found in Zongwulong tectonic belt. All these can be proved that Zongwulong tectonic belt is a compound orogen. Through the comparison between Zongwulong and Elashan tectonic belts, they developed same type of magma rocks, same genesis and evolution, which are all caused by the Late Hercynian-Indonisian tectonic-magmatic activity. The similarities between these two structure belts show that the formation of the Zongwulong tectonic belt may have a close genetic relationship with the oblique collision between the West Qinling and Qaidam blocks...