Eastern Inner Mongolia Tectonic-magmatic Events: Evidence From The Detrital Zircon U-Pb Dating Of Benbatu Group

NE China is located between three sections： the Siberian plate, the North Chinaplate and the Pacific plate, and it has important significance for its extremely complextectonic environment and a long history evolution. Geologists have pay close attention tothe geological problems of this region, such as the split collision of the northeast regionmirco plate, the hyperplasia of the block in the Central Pacific tectonic domain and thedevelopment evolution process, the mechanism and the time of the two major tectonicdomain.The Late Carboniferous Benbatu Formation mainly outcrop at the Xiwuqi region ofInner Mongolia, tectonically located in the western Songliao basin and close to theXilamulun fault to south. The formation consists mainly of marine carbonate and clasticrocks. Due to the lack of the high quality chronological data, its timing and tectonicimplications of this formation have long been controversial. Based on the previousresearch, the present field geological characteristics, lithologies, rock assemblages,sandstone detrital clasts component analysis and zircon LA-LCP-MS U-Pb data from theBenbatu Formation to address this issue. Microscopic identification and sandstone detritalclasts component analysis show that the sandstone of the Benbatu group is feldsparlitharenite, which consists mainly of quartz, lithic clasts and feldspar. The lithic clasts arepredominately characterized by the volcanic lithic grains. And it contains a lot ofmonocrystalline quartz. The sorting and rounding is poor. However, the detrital compositions indicate that the sandstones are immature, probably due to the effects ofrapid transport, nearly supplying and the wide sources. In addition, the detrital modalanalysis applying the Dickinson’s triangle diagrams also indicate the sediments of BenbatuFormation are mainly derived from dissected arc and transitional arc. The zircons fromsandstone show oscillatory growth zoning or striped absorption in Cathodoluminescenceimages, together with their high Th/U ratios (0.06-1.03) and REE patterns similar to thatof the Chondrite-normalized REE pattern, indication a magmatic origin.The Zircon U-Pb dating results show that87analyzed spots obtained from100zircons can be divided into five groups, i.e.,308~455Ma (peak at392Ma),469~544Ma(peak at518Ma),770~983Ma (peak at945Ma),1010~1957Ma (peak at1571Ma) andtwo older ages2489±33Ma,2659±32Ma. These ages indicate that:(1) A group ofyounger detrital zircon ages peak at320Ma which are consistent with the fossil evidenceshows that Benbatu Formation should be deposited after320Ma;(2) the presence of thedetrital zircons with the ages of469~544Ma indicates the Pan-African tectonic event iswidely distributed in NE China and become a landmark event in this region;(3) thepresence of the detrital zircons with the ages of770~983Ma indicates the metamorphicbasement of NE China is the provenance of the Benbatu formation;(4) the1718~2659Mazircons of the Benbatu Formation reveals that the NE China hasPaleoproterozoic-Neoarchaean ancient provenance information;(5) Benbatu formation hasthe same detrital zircon ages with NE China and significantly different from the ages ofNorth China. Therefore, the suture location of the North China plate and the Siberian plateshould be the south of the Benbatu formation which specific location is theXarmoron-Changchun-Yanji fracture;(6) Synthesizing the relevant data indicate thatSongliao block is a stable block, rather than an active orogenic belt.