Permian-Mesozoic Tectonic Evolution In Central Jilin Region,Eastern Part Of Northern Margin Of The North China Block

The central region of Jilin is located on the eastern edge of the northern margin of the North China Block.It was mainly influenced by the Paleo-Asian Ocean tectonic domain in the Paleozoic,and then reformed by the superposition of the Paleo-Pacific tectonic domain during the Mesozoic.Its complex tectonic evolution history also makes this region an ideal area for studying the impacts and transitions of the two major tectonic domains.This paper takes the magmatic rocks and sedimentary strata in the Gongzhuling-Jilin City-Shancheng area as the research objects,and determines the tectonic background of magmatic rocks of different periods and sedimentary strata of different eras in this region through detailed field geological surveys,zircon U-Pb geochronology,geochemistry and other series of methods.The characteristics of rock combination and spatiotemporal distribution of magmatic rocks in the region are clarified by summarizing the previous achievements and data,and the tectonic evolution history of the eastern part of the northern margin of the North China Block from the Permian to Mesozoic is reconstructed,and the crustal growth process of the northern margin of the North China Block is also discussed.During this work,the geological units previously mapped as Silurian were redefined as Permian volcanic rocks and Early to Middle Jurassic intrusive rocks.The main understandings obtained in this paper are as follows:1.Early Middle Permian Volcanic Rocks:The rock types include greenschist metamorphosed tuff and metamorphosed basalt,etc.The original rocks are mainly composed of a set of intermediate-basic volcanic rocks.The metamorphic volcanic rock ages range from 284-272 Ma,which originated from the depleted mantle wedge that was altered by the dehydration and melting of the subducting slab,and was later possibly contaminated by crustal materials.The ages of Middle Permian intermediate-acidic volcanic rocks range from 272-262 Ma and originated from partial melting of the crust,accompanied by the fractional crystallization of late-stage magma.Both have typical continental arc volcanic rock characteristics,indicating that in the Early to Middle Permian,the study area was under a subduction background.2.Middle Permian Volcanic Rocks:The rock type mainly includes andesite,dacite,rhyolite and their clastic rocks,which is a set of intermediate-acidic volcanic rocks.The ages of this acidic volcanic rocks are 260-254 Ma,originating from the partial melting of mixed crust.Unlike the early Middle Permian,the rhyolite of this period began to show A-type granite features,indicating that the subduction action in the study area became weak,and the regional tectonic background began to transition from subduction-collision to intra-plate environment.3.Late Permian Sedimentary Strata:The maximum depositional age of the Shoushangou Formation in the study area is 253 Ma,and the maximum depositional age of the Fanjiatun Formation is 251 Ma,which are late Permian.They are located on both sides of the Solonker-Xar Moron-Changchun suture zone,and still have different zircon compositions at the end of the Permian,indicating that the Paleo-Asian Ocean was not completely closed at this time.The maximum depositional age of the Yangjiagou Formation is between 247-249 Ma,which is the early Triassic.At this time,the zircon combination contains components of the North China Block and the northern Songliao-Xilinhot block,indicating that the Paleo-Asian Ocean might have closed.4.Late Triassic Dajianggang Formation:The Dajianggang Formation is a set of clastic rocks deposited in river-lake environments with normal stratigraphic order.The maximum depositional age of this formation obtained in this paper is 226 Ma,indicating that the Dajianggang Formation represents foreland sedimentation during the closure and orogenic process of the Paleo-Asian Ocean.This marks the complete closure of the Paleo-Asian Ocean and the onset of inland tectonic evolution.The results of the apatite fission track indicate that the Dajianggang Formation experienced a relatively stable period since it formed to 180 Ma,a slow heating process from 180-80 Ma,and a rapid uplift at 17 Ma,indicating the cessation of the late Triassic orogeny in the study area and the transition towards post-orogenic extension.5.Early to Middle Jurassic Acid Intrusive rocks:The acidic intrusive rocks in the Shancheng area are mainly composed of granodiorite and monzonite granite.The formation time is 176-164 Ma.Theε_Hf（t）values range from-22.09 to-7.59,and the T_DM2ages range from 3622 to 2336 Ma.The samples collected in this paper all have the characteristics of I-type granite and adakite,and the magma originated from the partial melting of the thickened old lower crust.They were formed in an active continental margin environment,probably related to the westward subduction of the Paleo-Pacific Plate,and have no connection with the southward subduction of the Mongol-Okhotsk Ocean.6.Establishment of the Permian-Mesozoic tectonic model.Based on the data and regional information in this study,a reconstruction of the tectonic evolution pattern of the northern margin of the North China Block during the Permian-Mesozoic period is proposed.During the Early-Middle Permian,the Paleo-Asian Ocean subducted continuously southward,leading to the formation of a combination of intermediate-basic and intermediate-acidic calc-alkaline volcanic rocks in the study area.In the Late Permian-Early Triassic,the Paleo-Asian Ocean closed,resulting in the formation of A-type granites in the study area.During the Middle-Late Triassic,the suture zone of the Paleo-Asian Ocean entered a stage of ongoing orogenic and collisional processes,and the study area experienced foreland basin sedimentation.The northern Mongol-Okhotsk Ocean subducted southward,while the eastern Paleo Pacific subduction initiated.In the Early-Middle Jurassic,subduction of the Paleo Pacific Ocean continued,leading to the formation of adakitic granites derived from crustal thickening in the study area.From the Late Jurassic to Early Cretaceous,the tectonic movement of the Paleo Pacific Plate transitioned from subduction to strike-slip,and the study area entered a period of magmatic quiescence.The Paleo Pacific Plate continued to rollback,resulting in the formation of a large number of extension-related A-type granites.