Provenance Analysis Of The Permian Sandstones And Tectonic Significance In Solonker And Linxi Areas Of Inner Mongolia

Solonker and Linxi area are located at the eastern part of the Central Asian Orogenic Belt?CAOB?.The geotectonic evolution of CAOB was extremely complex,so it was an ideal laboratory to study tectonic evolution characteristics of giant orogenic belts.I undertook an integrated approach incorporating petrology,framework petrography,whole-rock geochemistry and detrital zircon SHRIMP U-Pb geochronology of the Permian sandstones?Huanggangliang Formation,Zhesi Formation and Linxi Formation?with the aim of understanding the tectonic evolution of the eastern segment of the Central Asian Orogenic Belt.The Permian sediments in the study area?Solonker area,Linxi area?are mainly feldspathic lithic sandstone and greywacke.The sandstones are predominantly quartz,lithic fragments,feldspar,and a small amount of micas in composition.Most of the quartz grains are monocrystalline quartz,while the lithic clasts are volcanic fragments and a small number of sedimentary fragments,revealing a close relationship between the sedimentary and magmatic rock suites in the study region.Sandstones of the Permian sandstones are immature,indicating the characteristics of the rapid erosion rate of these source areas,with short transport distance.Studies from whole-rock geochemistry analysis of the Zhesi Formation show that the major source area was tectonic background of continental island arc,its original substance mainly came from felsic rock of upper crust,indicating that the provenance may came from arc orogenic belt of surrounding areas.In the Cathodoluminescence?CL? images of the zircons,the zircons are mainly sub-angular to sub-rounded,and most zircons show clear oscillating zonings,with the ratios of Th/U mostly higher than 0.4,indicating that most of the zircons are magmatic in origin.Samples of the Huanggangliang Formation yielded detrital zircon U-Pb ages ranging from 2653 Ma to 265 Ma,with three age populations at: 2653?2443 Ma,1935?1764 Ma,and 482?265 Ma;the ages of Zhesi Formation are distributed between 255 and 2634 Ma,with main peaks of 288 Ma,452 Ma and some other older ages of 2634?543 Ma;and samples from the Linxi Formation yielded detrital zircon U-Pb ages ranging from 3363 Ma to 257 Ma,with four age populations at: 2705?2403 Ma?age peak?2497 Ma?,2011?1203 Ma?age peak ?1881 Ma?,464?435 Ma?age peak ?445 Ma?,and 309?255 Ma?age peak ?271 Ma?.The youngest age of sample HLS3 from Lower Formation is 271 ± 4 Ma,and the youngest age is 255 ± 9 Ma for sample CGH6 and 255 ± 6 Ma for sample CGH7,respectively,indicating that it was deposited since the Middle Permian and continued into the Late Permian;the weighted mean ²⁰⁶Pb/²³⁸ U ages of eleven zircon U-Pb ages from the sedimentary to volcaniclastic unit is 256.7 ± 1.1?Ma,suggesting that the Linxi Formation was mainly deposited during Late Permian.The dominant provenances of the Huanggangliang Formation and Zhesi Formation sedimentary rocks in the south is the magmatic arc of Paleozoic Southern Orogenic Belt?SOB?along the northern margin of North China,and the materials from the north margin of North China plate may have also made some contribution;whereas the provenance of Linxi Formation in the north is mainly the Northern Orogenic Belt and some metamorphic basement rocks distributed in the orogenic belt.In addition,by the contrast the study on chronological characteristics of the detrital zircons and the tectonic setting of the Permian sandstones,the results support the view that the final closure of eastern part of the PAO occurred during Late Permian to Earliest Triassic times.