Petrogenesis And Geological Significance Of The Late Permian Igneous Rocks In The Jingtai-leshan Area,Jilin Province

Northeastern(NE)China,which has been tectonically referred to as the Xing'an-Mongolia(or Xing-Meng)Orogenic Belt(XMOB)in the past,forms part of the eastern segment of the Central Asian Orogenic Belt.The XMOB is located between the Siberian Craton,North China Craton,and Pacific Plate.This region witnessed the evolution and final closure of the Paleo-Asian Ocean during the Paleozoic,and also witnessed overprinting by the Paleo-Pacific tectonic domain and the Mongol-Okhotsk tectonic domain during the Mesozoic.Therefore,the study of the tectonic evolution of this region has great scientific significance.The Daheishan Horst,which is located in the southern margin of the XMOB,is a key geological unit that links the Solonker-Xar Moron-Changchun and Changchun-Yanji sutures.This thesis studies the geochemistry and geochronology of the Daheishan volcanic rocks and Doushantouzi granite which crop out in the Jingtai-Leshan area,Yitong Man Autonomous County,using LA-ICP-MS zircon U-Pb dating and Hf isotopic analyses,whole-rock geochemical and Sr-Nd isotopic analyses,and discusses the ages,petrogenesis of the igneous rocks,and the Permo-Triassic tectonic evolution of the eastern segment of northern margin of North China Craton.Petrographical study of the igneous rocks indicates that the Daheishan volcanic rocks are mainly consist of rhyolite,rhyodacite and andesite with minor basaltic lava,while the Doushantouzi granite is a mylonitized syenogranite.Cathodoluminescence images indicate that zircons from the above rocks exhibit complex internal structures,with core-mantle structure,structureless grains,transgressive zones,and curved alteration shape,reflecting multiple growth stages,recrystallizati-on and hydrothermal alteration.This conclusion is also supported by the evidence from zircon rare earth element(REE).There are two types of chondrite-normalized REE patterns for the dated zircons,with one group of zircons display distinct positive Ce anomalies and negative Eu anomalies,which are characteristics of magmatic zircons,another group of zircons are characterized by relatively high concentration of light rare earth elements(LREE)and show a flat LREE distribution pattern with distinct Eu anomalies and no significant or weak Ce anomalies,which are considered to be the geochemical imprints of hydrothermal activity.The above characteristics and the zircon U-Pb age data suggest that the igneous rocks formed during the Late Permian and suffered hydroth-ermal alteration during the Early Triassic.In addition,the abundant of Early-Middle Permian magmatic zircons reflect multiple phases of magmatism in the study area.Geochemically,most of the Late Permian igneous rocks in this study belong to calc-alkaline series,while minor of them belong to high-K calc-alkaline or shoshonite series.They have right dip chondrite-normalized REE patterns and are characterized by enrichment in large ion lithophile elements(LILEs),depletion in high field strength elements(HFSEs,e.g.,Nb,Ta,and Ti)and depleted Sr-Nd isotopes.Furthermore,they have Meso-Neoproterozoic Nd and Hf two-stage model ages.The above geochemical characteristics and the rock assemblage character-istics indicate that the Daheishan basaltic rocks,which are similar to high-Al basalt,were derived from the partial melting of a depleted mantle wedge that was metasomatized by fluids from a subducted slab,while the Daheishan intermediate-felsic rocks and Doushantouzi syenogranites likely originated from the partial melting of a Meso-Neoproterozoic metabasaltic lower crust,with hornblende and garnet as the major residue mineral phases in the source region.In addition,part of the andesitic magmas were contaminated by some old crustal materials,a certain amount of carbonatite materials,and metasomatized mantle melts.The above discussions suggest that the Late Permian igneous rocks in the Daheishan Horst formed in an active continental margin setting.These subduction-related igneous rocks link those in the Solonker-Xar Moron-Changchun and Changchun-Yanji sutures,thus the Changchun-Yanji suture is the eastern extension of the Solonker-Xar Moron-Changchun suture,and the Paleo-Asian Ocean was not closed during the Late Permian.On the basis of comprehensive study of the tectonic position of the study area,previous field investigations,lithofacies palaeogeographic and paleomagnetic data,we suggest that the formation of the Changchun-Yanji suture was related to continental-continental collision that caused by the biodirectional subduction of the Paleo-Asian oceanic plate,and the Late Permian igneous rocks have a close relationship with the northward subduction of the Paleo-Asian Oceanic plate to the combined NE China block.In addition,based on the recent study of the Paleozoic tectonic evolution of western(Northeastern margin of the Tarim Craton)and middle(northern margin of the Alxa block)segment of the Paleo-Asian Ocean,we propose that the final closure of the Paleo-Asian Ocean might have taken place along the northern margin of the Tarim-Sino Korean cratons in a scissors style,and the closure time of the western-middle segment is earlier than that of the eastern segment.