Permian Tectono-magmatic Activities And It's Geological Significance In Tumuertai Area,Inner Mongolia

The Central Asian Orogenic Belt is one of the oldest and most intense accretionary orogen in the world.Since Paleozoic,it has gone through a series of evolution processes such as subduction,collision and collapse,and remain a lot of magmatic records.The chronology and geochemistry of these magmatic rocks reflect their magmatic source and genetic characteristics,which is important to recover the tectonic evolution of this area.The research area lies in the central-northern margin of the North China Plate,and its geotectonic position is adjacent to the eastern part of CAOB-Xing'an Mongolia orogenic belt.Permian magmatic activity is well developed and shows good outcrop,especially near Tumuertai.Author and research team coworkers did a lot of detailed field observations of intrusive and volcanic rocks in this area,then we analyzed the samples through petrological,petrographic,chronological and geochemical method to get the information of rock type,origin and source area.Finally,combined the previous achievements to explore the Late Paleozoic,particularly the Permian tectonic evolution history of the study area.The main results are as follows:?1?Volcanic rocks from the Suzy formations:the main research object is andesite and basaltic andesite.Geochemical indexes indicate that the basaltic andesites belong chemically to medium-K cale-alkaline series,are poor in SiO₂?49.0%⁵5.2%?and HFSEs,rich in Al₂O₃?15.7%¹7.8%?,Mg?Mg^#=49.8⁶7.0?and LILEs,positive Eu anomalies??Eu=0.95¹.39?and display low REE abundances and flat REE pattern,that is similar to those from an island are setting.LA-ICP-MS zircon U-Pb age data indicate that the basaltic andesite from the Suzy formation formed in the Early Permian?279.0±3.1Ma?.Zircons form the Suzy volcanic rocks exhibit negative?_Hf?t?values form-19.3^-13.8,old Hf isotopic t _DM2 is between 2516.5Ma²177.4 Ma,indicating that Suzy volcanic rocks were derived from partial melting of the depleted mantle wedge metasomatized by the subducted slab-derived fluid.Taken together,we conclude that the basaltic andesites from Suzy formed in a subduction tectonic setting.?2?Granodiorite:The samples show the granodiorite is rich in SiO₂?65.6%⁶7.0%?,K and LILEs,poor in HFSEs.The rocks belong chemically to high K calc-alkali series with a medium A/CNK?0.94⁰.98?.The?REE is relatively high,and REE shows significant enrichment of LREE and flat distribution of HREE,Eu strong loss??Eu=0.61⁰.69?of the right inclined REE patterns.SHRIMP zircon U-Pb data indicate that the formation age of the granodiorite is 275.3±2.6 Ma,Early Permian.Combined with its common mafic enclaves and I-type characteristics,we consider that its primary magma could be possibly contributed by magma mixing at a continental arc setting.?3?Norite gabbro:Geochemical indexes indicate that the norite gabbro are poor in SiO₂?46.2%⁴9.8%?and HFSEs,rich in TFeO?15.7%¹7.8%?,Mg^#?59.2⁶1.6?and LILEs,and display low REE abundances and flat REE pattern with obviously positive Eu anomalies??Eu=1.02².41?,which are similar to mantle derived intrusive rocks.LA-ICP-MS zircon U-Pb data indicate that the formation age of the norite gabbro is 270.1±4.2 Ma,Middle Permian.The norite gabbro belongs chemically to cale-alkaline and subduction zone basalt series,showing its magmatic resource possibly derived from partial melting of the depleted mantle wedge metasomatized by the subducted slab-derived fluid.?4?The ages of magmatic rocks are very close in our study area,among early-middle Permian.The volcanic rocks did not show the bimodal features and it is unlikely to find a successive evolutionary between basic and acid rocks,there was possible a magma mixing process.Together with their arc volcanic rocks,volcanic arc granitoid and subduction zone basalt features,we think the central-northern margin of the North China Plate was still in a subduction setting in Permian.Combined with the petrological and chronological evidence from adjacent areas,we concluded that the tectonic evolutionary regime of the central-northern margin of the North China Plate in Permian were from ocean crust subduction?early-middle Permian?to syn-collision?late Permian?,then it move into a post-collision setting?after Permian?.