Study On The Relationship Between Sedimentation And Tectonism Of Late Paleozoic-Early Mesozoic Sediments In The North-Central North China Craton

Basins and orogenic belts are two basic tectonic units on the surface of continental lithosphere,the formation and evolution of basins is closely related to tectonic events in orogenic belts.Therefore,it becomes an effective method to study geotectonic evolution.This paper reconstructs the development and evolution history of orogenic belts by clastic sedimentary records of basins.Based on a combination with the previous research results,this thesis made an detailed analysis of the sedimentary characteristics,chronostratigraphic framework,sedimentary provenances and geotectonic background of Late Paleozoic-Early Mesozoic strata within the Ningwu-Jingle basin,Yungang-Pinglu basin and Fugu basin,with the aims of revealing the tectonic evolution process of the orogenic belts on the north margin of the NCC.Therefore,a study of sedimentary tectonic evolution in the north-central North China Craton（NCC）not only enriches our understanding of the tectonic evolution of the Yinshan–Yanshan Orogenic Belt（YYOB）and the Central Asian Orogenic Belt（CAOB）in the northern margin of the NCC,but also have important implications for understanding the relationship between sedimentation and tectonism of Late Paleozoic-Early Mesozoic strata in the north-central NCCand its geodynamic mechanism.The main conclusions are as follows:Based on isotopic chronology data of detrital zircons in sandstones,the maximum sedimentary age and age attribution of Late Paleozoic to Early Mesozoic strata in north-central NCC were determined,and the chronostratigraphic framework was established.The Shihezi Formation in the Yungang-Pinglu basin is assigned to a depositional age of Middle Permian on account of the presence of youngest detrital zircons of 286.2±5.5 Ma and266.9±5.7 Ma from the lower and upper part of the Shihezi Formation,respectively.The Shihezi Formation is generally developed in the north-central NCC.The Liujiagou Formations within the Ningwu-Jingle basin are considered as Early Triassic units according to two youngest zircon ages of 261±7 Ma,it is the same sedimentary strata with Ermaying Group of Jiyuan,Luoyang basin and Fenghuangshan Formation of Luxi basin.The Ermaying Formation in the Ningwu-Jingle basin proves to be of Middle Triassic age according to four youngest zircons with a weighted mean age of 261±7 Ma,it can be compared with Zhifang Formation of Ordos basin,Liujiagou Formation of Yanshan intermountain basin,Ermaying Group of Jiyuan,Luoyang basin and Fenghuangshan Formation of Luxi basin.The Tianchihe Formation in the Ningwu-Jingle basin belongs to Late Jurassic in age,four youngest detrital zircons confine the maximum sedimentary age of the strata to 149±5 Ma.It is similar to Anding Formation,Fenfanghe Formation of Ordos basin,Jiulongshan Formation of Yanshan intermountain basin and Santai Formation of Luxi basin.The provenance characteristics are studied by detrital zircon U-Pb geochronology,Hf isotopic composition and paleocurrent analysis.Results show that zircons from five sandstone samples of Middle Permian Shihezi Formation,Early Triassic Liujiagou Formation,Middle Triassic Ermaying Formation and Late Jurassic Tianchihe Formation are characterized by three major populations:Late Paleozoic and Early Mesozoic,Late Paleoproterozoic（with a peak at approximately 1.8 Ga）and Neoarchean（with a peak at approximately 2.5 Ga）.The U-Pb age populations of detrital zircons from the Middle Permian,Triassic and Upper Jurassic sandstones are consistent with that of the YYOB.The two Precambrian zircon ages broadly match the widely distributed age components of the NCC basement.All of the Hf isotope data of the Phanerozoic zircons from the Shihezi Formation and Ermaying Formation are characterized by negativeεHf（t）values.The majority of the Phanerozoic zircons from the Liujiagou Formation and Tianchihe Formation have negativeε_Hf（T）values with only three grains having positiveε_Hf（T）values,respectively.Hf isotope peculiarity of negativeε_Hf（T）value is consistent with that of the YYOB,while the positiveε_Hf（T）is consistent with that of the CAOB.In summary,the U–Pb age spectra,Hf isotope analyses of the detrital zircons and paleocurrent analysis,combined with published data,indicate that the Middle Permian sediments were sourced from the YYOB,the Early Triassic sediments were derived from the mixed provenance of primarily YYOB and secondarily CAOB,the Middle-Upper Triassic sediments were derived from the YYOB,the Middle Jurassic sediments were derived from a mixture of the YYOB and CAOB,and the Upper Jurassic sediments were mainly derived from the YYOB,the CAOB provides only a small number of clastic materials.The provenances of basins in the north-central NCC transitioned from the YYOB in the Middle Permian to a mixture of primarily YYOB and secondarily CAOB in the Early Triassic,indicating that the final amalgamation of the NCC and CAOB（closure of the Paleo-Asian Ocean）probably occurred between 267-261 Ma,constrained by the youngest detrital zircon ages.The provenances of the Jurassic sediments in the basins of the north-central NCC transitioned from a mixture of the YYOB and CAOB in the Middle Jurassic to primarily the YYOB in the Late Jurassic,and the CAOB provides only a small amount of materials.The marked changes in the sedimentary sources might be ascribed to the closure and southward subduction of the Okhotsk Ocean,resulting in the intense uplift of the YYOB during the Late Jurassic.The Late Paleozoic to Early Mesozoic tectonic evolution in north-central NCC can be divided into five stages.Middle-Late Permian syn-collisional compression stage:the subduction and closure of the Paleo-Asian Ocean resulted in the strong uplift and accelerated denudation of the YYOB under the compression environment.Early Triassic to early Late Triassic post-collisional extension stage:In the Early Triassic,the YYOB was continuously uplifted and denuded under the transition stage from compression to extension after the closure of the Paleo-Asian Ocean.In the Middle Triassic to early Late Triassic,the orogenic belts in the northern margin of the NCC was differentially uplifted under the post-collision extensional tectonic background.The late Late Triassic syn-collisional compression stage:the collision of the CAOB and the NCC in the Late Triassic might have caused compressional deformation of the north NCC and the intense uplift of the YYOB.Early-Middle Jurassic post-collisional extension stage:after the Indosinian Orogeny,the post-orogenic extension system began to develop.The late Middle Jurassic to Late Jurassic syn-orogenic compression stage:with the southward subduction closure of the the Paleo-Okhotsk Ocean,the tectonic regime in the north-central NCC was transformed into compressive state,resulting in the intense uplift,fold thrusting and magmatic activities of the YYOB.The tectonic evolution of the north-central NCC during the Late Paleozoic-Early Mesozoic might be related to the southward subduction closure of the Paleo-Asian Ocean in the Late Permian,the collision between the CAOB and the NCC in the late Late Triassic and the southward subduction of the Okhotsk Ocean and the collision between the Siberia Block and the CAOB in the Late Jurassic.