Geodynamic Implications Of Dyke Swarms In Sonidzuoqi Of Inner Mongolia

Sonidzuoqi in the north-central of Inner Mongolia, it belongs to the Inner Mongolia-Da Hinggan Mountains orogenic belt of the eastern segment of the central Asian orogenic belt. The study area is located in the erlian-hegenshan arc accretionary belt. This belt extends to NEE, its southern border is erlian-hegenshan fault zone and its northern border is chaganaobao-arongqi fault zone. Intensive dike swarms developing in this area intrude in the sandstone and conglomerate of niqiuhe group of Devonian and the late Paleozoic magmatic rocks, like the volcanic rock of boligaomiao group of carboniferous,carboniferous granodiorite, carboniferous monzonitic granite. Dike swarms types are mainly acidic granite porphyry dike and rhyolitic dike, and include a small amount of diorite porphyry dikes. Because of its small number and little research significance, we will not research it in this paper. Through the LA-ICP-MS method measured the zircons of granite porphyry U-Pb harmonic age was 289.2±2.8Ma, zircon of rhyolite U-Pb harmonic age was 282.4±7.4Ma, which can determine that the dikes inteuded in this era in early Permian. These ages conform to the the geological facts. Dike swarms parallel to each other,their directions is consistent with regional tectonic line as NNE. On the landscape, most of the dikes’ type is an s, part of the dikes are cut by the NNE faults. Because of having strong weathering resistance, they have obvious difference of weathering with surrounding rocks,they became ridge and remote sensing image displays as dense ribbons. The interval of adjacent dike is mainly concentrated in 70-100 meters, the minimum concentration areas is about 10-20 meters. A single dike exposed up to several kilometers in length, 20-30 meters in width, their directions between 75 °- 90 °and their occurrence is nearly vertical. Both of the granite porphyry dike and rhyolitic dike are porphyritic structure. Granite porphyry’phenocrysts are mainly potassium feldspar and quartz and little plagioclase. Rhyolitic’phenocryst are quartz and a small amount of feldspar, and it developes flow lines structure and spherolitic structure. The granite porphyry dike and the rhyolitic dike have the similar rock geochemistry characteristics. They have rich silicon, rich alkali, high aluminum and low calcium, at the same time, they have a relatively inrichment of the light rare earth elements. Their heavy rare earth elements display a flat distribution and their Eu has little negative anomaly.The rocks are enrichment of Rb, Ba, Th, U, K and Pb, but their Nb, Ta, Ti,P was significantly negative anomaly. These geochemical characteristics show that the dike swarms is made up of partial melting of crust which is made from the arc island magmatic rocks. The age of the A-type granite belt is 276Ma-286 Ma and the bimodal volcanic line is270Ma-281 Ma. Both of them in southern margin of the Siberia plate represent stretch tectonic of environment. By contrast with them, the dike swarms represent more stretchtectonic. And its age is slightly earlier than them which raises the age limit of stretch tectonic in southern margin of the Siberia plate. Above all, the dike swarms in the study area is made up of partial melting of crust which is made from the arc island magmatic rocks in early Permian when the tectonic background is extending.