The Upper Carboniferous-Lower Triassic Sedimentary Environment And Tectonic Setting Of Southeast Inner Mongolia

This dissertation is mainly about the research of sedimentology, geochronology, and geochemistry focused on volcanic-sedimentary strata and granite of late Carboniferous-early Triassic to solve the issue of the late Paleozoic tectonic nature of Inner Mongolia region. The results are showed as follows:(1)This paper revised stratigraphic sequence of late Carboniferous-early Triassic of southeastern Inner Mongolia by using fossil data and geochronology, which deposited into an EW trending basin. In the northpart of the basin, there are late Carboniferous Baoligaomiao Formation, Carboniferous-Permian Gegen Obo Formation and middle Permian Zhesi Formation from old to the new. In the central part of the basin, namely steppe region of Inner Mongolia, there are late Carboniferous Amushan Formation, early Permian Shoushangou Formation, Dashizhai Formation, middle Permian Zhesi Formation, late Permian Linxi Formation and early Triassic Xingfuzhilu Formation. In the south part of the basin, there are early Permian Sanmianjing Formation, early-middle Permian Elitu Formation, middle-late Permian Yujiabeigou Formation and late Permian Tieyingzi Formation. In the north part of the basin, late Carboniferous stratumis continental facies and marine-continental alternative facies.Early Permian stratumis marine-continental alternative facies.Middle Permian stratumis littoral neritic facies.Late Carboniferous stratumis carbonate platform facies in the central. Permian stratumis shallow marine facies at the beginning and continental facies in the end.Early Triassic stratumis lake-rive facies.Early Permian is littoral neritic facies in the southern part, but early-middle Permin has changed into continental facies.Middle-late Permian is marine-continental facies and late Permian is continental facies.(2)Late Carboniferous-early Triassic volcanic rocks are tested by using zircon LA-ICP-MS U-Pb geochronology. This paper has obtained zircon ages of 311.2±0.9Ma and 311.6±1.3Ma from volcanic rocks of the Gegen Obo Formation. The age of quartz diorite porphyry intrusion from Xingfuzhilu Formation is 246.2±1.7Ma and 255.5±0.82 Ma, so the gigantic conglomerate stratum(lower part of Xingfuzhilu Formation) is put into the upper part of Linxi Formation.There are volcanic rocks in the upper part of Sanmianjing Formation, the age of which is 283.7±2.3Ma. The Sanmianjing Formation covers on Ordovician gneissic quartz diorite(462.7±4.3Ma). The age of volcanic rocks of upper part of Elitu Formation is 268.7±2.0Ma, so the time of Elitu Formation belongs to early-middle Permian. The age of andesite gravel of Yujiabeigou Formation is 252.6±1.3Ma, and then the upper of Yujiabeigou Formation may belong to late Permian or early Trassic.(3)Clastic sedimentary rocks from early Permian Gegen Obo Fromation, Sanmianjing Formation, Elitu Formation, late Permian Linxi Formation and early Triassic Xingfuzhilu Formation are studied by using detrital zircons geochronology. The U-Pb age spectras of Sanmianjing Formation and Elitu Formation are the same, both of which have peak age 1.8Ga and 2.5Ga. The spectrum of Gegen Obo Formation indicates sedimentary provenance which came from intraorogenic. The Phanerozoic detrital zircons of Linxi Formation have two groups(260-345 Ma and 443-528Ma), while Precambrian detrital zircons have no crest. The peak age of early Triassic Xingfuzhilu Formation is between 248-257 Ma. Combined with other detrital zircons geochronology data, we found the spectra of Zhesi Formation and Linxi Formation are the same with Devonian and Carboniferous clastic sedimentary rocks, which indicate the sedimentary provenance, came from North China Craton. In addition, the different spectra of late Permian and early Triassic detrital zircons mayreflect a depositional hiatus.(4)Late Permian Luotuochang granite(254.18±0.7Ma) and Wenducun granite(259.2±1.1Ma) are found near Balinzuoqi and Linxi respectively. Geochemical characteristics indicate that they are I-type granite and are formed of Lower crust melting in extrusion environment. The Triassic granite is divided into two stages: the first stage(249-241Ma) is I-type granite that formed in compressive orogeny environment, while the second stage granite(235-229Ma) is the product of syn-orogeny. About 229 Ma later, the study area stepped into post-orogenic extensional stage.(5)The Paleo-Asian Ocean closed before late Carboniferous or earlier. Since the late Carboniferous, the Siberian Craton and North China Craton jointed together.The sediments and magmatism are intracontinental. Especially after the Late Permian, the orogeny may be a new beginning of another tectonic cycle(Indosinian cycle).