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A Study On The Tectonic Environment Of The Mesozoic Granitiod In Laoling Area, Southern Jilin Province

Posted on:2011-05-25Degree:MasterType:Thesis
Country:ChinaCandidate:Y QinFull Text:PDF
GTID:2120360305454972Subject:Structural geology
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The study area is located in the east segment of Liaoji palaeoproterozoic rift zone which is in the eastern North China Plate. At the Mesozoic,a large number of granites developed in the study area. to define the geochemical characteristics of the rocks,the ages and periods of the rock formation,genetic types of rocks and tectonic environment of rock formation by researching those granite's the major element,rare earth element,trace element,zircon U-Pb dating.The Mesozoic granitoids at Laoling area inclued Longtou granite mass,Caoshan granite mass,Laotudingzi granite mass,Laoling granite mass,Laoling Diorite stock,Gaotaizi granite mass,Shanglvshuiqiao granite mass,Shihu granite mass and Toudaogou granite mass. According to lithological characteristics,Laoling granitoid can be divided into two categories: the one is the diorite magma intrusion event that the main rock types are diorite,quartz diorite,the other is granite magma intrusion event that the main rock types are granites,adamellite,moyite,granite porphyry,miarolitic moyite and homo-porphyritic biotite moyite.The granitoid masses are mainly I-type and A-type granite.I-type granite masses include Longtou granite mass,Caoshan granite mass,Laotudingzi granite mass and Laoling Diorite stock.While Gaotaizi granite mass,Laoling granite mass,Shihu granite mass,Shanglvshuiqiao granite mass and Toudaogou granite mass are A-type granite.I-type granites are metaluminous to peraluminous calc-alkaline series,having the feature of higher silica,rich in alkali,higher calcium,the similar content of potassium and sodium,sightly to unobvious negative Eu anomaly and right type of REE distribution pattern,loss of P,Ti,Sr,Nb,Ba and so on. A-type granitoids are metaluminous to peraluminous series,having the feature of higher silica,rick in alkali,higher potassium,lower iron magnesium calcium,moderate to strong negative Eu anomaly and right type of REE distribution pattern, loss P,Ti,Sr,Nb and Ba,rich in K,Th,Rb etc.Zircons in the granite rocks at the area are the typical magmatic zircons for they have the crystal shape of columnarand strip, the typical magmatic oscillatiry zoning and the Th/U ratios of 0.5-1.1. It means that the zircons age can represent the age of magma emplacement.According to LA-ICP-MS dating ,combing with the geochemical characteristics and tectonic evolution, granitoids in this area can be divided into three periods .The first period is Late Triassic (221-226Ma),the second period is Early-Middle Jurassic(177-129Ma) and the third period is the Early Cretaceous (117-125Ma).The first period of granitoid rocks formed in the collision of the extrusion tectonic environment.in the R1-R2 diagram ,Late Triassic rocks which fall into 6 areas is syn-collisional granite.One hand, Geochemistry is similar to C-type adakites,reflecting the rock from the partial melting of thickened crust.On the other hand,subduction and collision of the yangtze plate with the North china plate formed the Dabie-Sulu orogenic belt,which is found coesite-bearing or diamond-bearing gnesis and eclogite.The latest coesite-bearing eclogite age is 210Ma±,which reflect The Late Triassic rock has feature of thicken crust.Therefore,The Late Triassic rocks on the study area are outcome of subduciton and collision of the yangtze plate with the North china plate. The second period of granitoid rocks also formed in the collision of the extrusion tectonic environment. in the R1-R2 diagram , Early Jurassic rocks which fall into 6 areas is syn-collisional granite.The geochemical characteristics similar to the same period granite in Liaodong Peninsula and can be compared with each other.wu fuyuan etc consider that the Jurassic granites in southern liaoning that its formation may be involved in fluid belongs to low temperature granite.it highly reflects a subduction–related tectonic setting. Meanwhile,The Early Jurassic rocks and Late Triassic rocks with similar characteristics,shows that the tectonic environment of this periods rocks relate to South China Plate subduction and collision of North China Plate,but this time it has no thickened crust characteristics.The third period of granite formed in non-orogenic extensional tectonic environment. in the R1-R2 diagram , Early Cretaceous rocks which fall into 5 areas,the back of 4 areas and a part of 7 areas is non-orogenic granite.Meanwhile,the Early Cretaceous magmatic mainly formed A1-type aluminum granite,also illustrated that this period is in the non-orogenic extensional tectonic environment.The Early Cretaceous granites in study area are KCG class in the Barbarin classification,represents a tectonic environment conversion from compression to extension,The peak age of this transformation is about 117-126 Ma.
Keywords/Search Tags:Laoling Area in southern Jilin province, I-type granite, A-type granite, LA-ICP-MS zircon dating, tectonic environment
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