The Mesozoic Tectonic Evolution Of Helong Area In Jilin Province

The study is located within the intersections of the eastern segment of thenorthern margin of the North China Block and Xing Meng Orgenic Belt, In thePre-Mesozoic, the study area was mainly influenced by the Paleo-Asian Oceanictectonic regime. In the Meso-Cenozoic, it was affected by the Mongolia-Ochotskocean tectonic systems and the Paleo-Pacific tectonic dynamic systems. In this paper,we have studied the formations in the Helong and Fudong basins and magmatic rocksformed in Mesozoic around the basins. Based on the researches of the typicalformation sections and petrology, geochronology, Geochemistry of magmatic rocks,we further discuss the late Paleozoic to Mesozoic tectonic evolution in Yanbian area.The formations in Fudong basin are composed of Changcai Formation,Quanshuicun Formation and Dalazi Formation. The arkose is the main type of rocksin Changcai formation. Zircon U-Pb dating results indicate that the major of theprovenance are mainly from Early Jurassic magmatic rocks, and others come fromJinanyan Formation formed in the Late Archean. The weathering indexes of originalrock including CIA, CIW and PIA values and A-CN-K diagrams, show that theprovenance of Changcai Formation has been suffered weakly weathering andpotassium metasomatism. In the geochemistry characteristics, the samples inChangcai Formation are strongly enriched in LILE and LREE, and are depleted inHFSE and HREE. All the geochemical characteristics are similar to the ones ofcontinental island arcs and active continental margin. Quanshuicun Formationbimodal volcanic rock association is composed of rhyolite and andesite. The uppermember of Quanshuicun Formation is composed of rhyolite volcanic rocks, and the lower member of Quanshuicun Formation is composed of andesite. The analyses ofzircons from Quanshuicun Formation yield weighted mean206Pb/238U age of116.8±1.4Ma. In the geochemistry characteristics, the samples of Quanshuicun Formationare strongly enriched in LILE, and are depleted in HFSE. The geochemicalcharacteristics are similar with igneous rocks formed in the active continental margin.The Liudong pluton, which is composed of miarolitic monzogranite, is locatedaround Helong basin and Fudong basin. The analyses of zircons yield weighted mean206Pb/238U age of118.58±0.87Ma. In the geochemical features, the samples ofLiudong pluton are enriched in LILE and LREE, and depleted in HFSE and HREE,and the samples have the characteristics of highly fractionated I-type granites. InMajor and Trace elements discrimination diagrams, all samples fall into the area ofPOG.The Early Jurassic magmatic rocks around Helong basin and Fudong basin arecomposed of Shasongdingzi pluton, Yueshandong pluton, Gaoling pluton andShiguoshuiku pluton. The analyses of zircons from Shasongdingzi pluton yieldweighted mean206Pb/238U age of175.6±1.1Ma, in the geochemical characteristics,they are enriched in large ion lithophile elements and depleted in high field strengthelements, have the characteristics of island-arc tholeiite, in the La/Nb-Ba/Nb diagram,all slamples fall into the range of volcanic arc, in the La/Yb-Nb/Y diagram, allslamples fall into the area of fluid metasomatism. The analyses of zircons fromYueshandong pluton yield weighted mean206Pb/238U age of178.4±1.2Ma, they arealso depleted in high field strength elements and enriched in large ion lithophileelements. the geochemical features are similar with igneous rocks formed in theactive continental margin in the subducted environment. in the La/Nb-Ba/Nb diagram,all samples fall into the range of volcanic arc. The analyses of zircons from Gaolingpluton yield weighted mean206Pb/238U age of180.47±0.7Ma, in the Ta-Th diagram,all samples fall into the range of active continental margin, in the (Y+Nb)-Rb andY-Nb diagrams, all samples plot into the volcanic arc granitoid field, and in the R1-R2diagram, most of samples fall into the area of the uplift granites after plate collision.These characteristics show that they are formed in the setting of plate subduction.The analyses of zircons from Shiguoshuiku pluton yield weighted mean206Pb/238Uage of193.06±0.8Ma. Geochemically, the samples of Shiguoshuiku pluton are high in SiO2and Al2O3, but low in MgO, they are enriched in LILE and depleted in HFSE.The geochemical features are similar with igneous rocks formed in the activecontinental margin.The Triassic magmatic rocks are composed of Yongxin pluton. The analyses ofzircons from Yongxin pluton yield weighted mean206Pb/238U age of243.2±1.1Ma.Yongxin plutons have the characteristics of I-type granite in geochemistry andpetrography. They are enriched in large ion lithophile elements and depleted in highfield strength elements, in the (Y+Nb)-Rb and Y-Nb diagrams, all samples are plottedinto the volcanic arc granitoid field, the features mentioned above show thecharacteristics of the active continental margin.In the Early-Middle Triassic, the study area is in the setting of subduction andclosure of Paleo-asian Ocean, which resulted in the Yongxin pluton, and in the lateperiod of Middle Triassic to Late Triassic, the study area is in the conversion stagefrom continental collision to extensional collapse. During the Early Jurassic, thestudy was competely controlled by the subduction of Paleo-Pacific ocean, whichresulted in Shasongdingzi pluton, Yueshandong pluton, Gaoling pluton andShiguoshuiku pluton. In the Late Jurassic-early Early Cretaceous, the subducion ofpaleo-pacific ocean became weak, and in the late Early Cretaceous, the subducion ofPaleo-Pacific ocean once began, which resulted in a series of basin with north-southdirection and similar bimodal igneous rocks of Quanshuicun Formation and Liudongpluton.Above all, the final closure time of paleo-Asian ocean is Middle Triassic, thestudy area is affected by the paleo-Pacific ocean from Early Jurassic, in the LateJurassic-early Early Cretaceous, the subduction of paleo-pacific ocean becameweaker, and in the late Early Cretaceous, the subduction of paleo-pacific ocean oncebegan.