Metamorphism, Composition And Forming Age Of The Qinling Group In Danfeng Area

The Qinling Group,the most essential part of the North Qinling tectonic belt,recorded prolonged tectonic evolution history.It is an important target to decipher the formation of the tectonic framework of Central Orogenic system and its evolution,thus it had drawn worldwide concern and been extensively studied for more than two decades.Recently,with the improvement of research methods and discovery of different kinds of HP-UHP rocks,extensive studies have been carried out on the Qinling Group to investigate its formation age,whether it experienced UHP metamorphism,and its subduction and exhumation history.However,there is still a hot controversy.Based on detailed field investigation,we conducted an integrated study of petrography,geochemistry,LA-(MC)-ICP-MS zircon U-Pb geochronology and Lu-Hf isotope on main rock types,felsic vein and enclaves-bearing granitic vein in Dasigou area of Qinling Group,which is located in northeast of Danfeng county,Shannxi province.In order to determine the distribution of HP-UHP rocks,the thesis also studied the metamorphism and geochronology of amphibolites from Fenshuiling and Fangzhuang area,as well as granitoid intrusions and their direct county rocks.The main achievements of this thesis are as follows:(1)Coesite,omphacite,garnet and rutile inclusions have been identified by laser Raman spectroscopy in an amphibolite sample collected from the Qinling Group outcropping in Dasigou area for the first time.The identification of these inclusions provide convincing evidence that the amphibolite is a retrograded product of the UHP eclogite.The Qinling Group indeed experencied UHP metamorphism.(2)LA-ICP-MS zircon U-Pb dating yielded formation ages of 920-953 Ma for the Niujiaoshan,Fangzhuang,Zhaigen and Dehe granitoid intrusions.The inherited zircons show different age peaks at about 1.0 Ga,1.08 Ga,1.2 Ga,1.39 Ga,1.57 Ga and 1.65 Ga.Detrital zircons from their host felsic gneisses/schists yield several age peaks at 1.0 Ga,1.08 Ga,1.2 Ga,1.27 Ga,1.45 Ga,1.56 Ga,1.7 Ga,1.77 Ga,1.86 Ga,respectively.This result is similar to that of Dasigou paragneiss which have age peaks at 1.0 Ga,1.08 Ga,1.29 Ga,1.42 Ga,1.52 Ga,1.69 Ga.Taken into together,the depositional age of the Qinling Group can be constrained at ca.0.96-1.0 Ga.We can conduct that the Qinling Group formed in the north continential margin of the Laurentia.(3)Combined the results of zircon inclusions,CL images,trace-element compositions and U-Pb ages,the protolith age of Dasigou amphibolite and garnet-amphibolites is 771±12 Ma,and they underwent peak metamorphism at ca.496-499 Ma,followed by 451-453 Ma granulite facies retrograde metamorphism and 413-425 Ma amphibolite facies metamorphism.Dasigou paragneiss experienced peak metamorphism at 491-500 Ma,and two stages of retrograde metamorphism at 453±4 Ma and 408±6 Ma.The peak metamorphic age of Fenshuiling amphibolite is refined to 491±6 Ma,and two weighted mean ages of 458±6 Ma and 415±8 Ma are interpreted as the timing of retrograde metamorphism.The protolith age of Fangzhuang amphibolite is 789±7 Ma,three different stages metamorphic ages are 495±5 Ma,445±7 Ma and 415±6 Ma.The possible peak metamorphic age of Fangzhuang paragneiss is 489±5 Ma.These age data provides strong evidence that these rocks experienced the same tectonic event.The Qinling Group experienced two stages of exhumation process at ca.450 Ma and ca.420 Ma after the peak metamorphism.(4)Geochemical data show that the amphibolites from Dasigou,Fangzhuang and Fenshuiling area have very uniform LREE riched chondrite-mormalized REE patterns.On the primitive mantle-mormalized spidergram,these amphibolite are enriched in LILEs and don't show any negative anomaly of Nb,Ta and Ti.The amphibolites form Dasigou and Fangzhuang have high ?Nd(t)and ?Hf(t)values,respectively,which indicate a lithospheric mantle source.These features indicate that protoliths of the amphibolites in the Qinling Group are probably formed in continental rift setting.(5)According to geochemistry,geochronology and mineral chemistry results of felsic vein and enclaves-bearing graintic vein in the Qinling Group of Dasigou area,we ascertained that these veins might be formed by the partial melting of host paragneiss at 445±3 Ma and 417±3 Ma,respectively,which are identical to the retrograde ages for the HP-UHP rocks found in the Qinling Group,suggesting that they are formed during exhumation of the deep subducted continental crust.(6)Genetic mineralogy research on garnet,muscovite and zircon in Neoproterozoic granitoid intrusions and their country rocks show that these rocks didn't experience high grade metamorphism except the Fangzhuang paragneiss.We suggest that the contact relationship between these rocks and HP-UHP rocks is structural.The Qinling Group didn't experience overall subduction.(7)Combined the achievements of this thesis and previous published data,we established the formation and evolution history of the Qinling Group:the Qinling Group formed in the north margin of the Laurentia continent at 0.96?1.0 Ga;later it divorced from the Laurentia continent with the break up of the Rodinia supercontinent.In early Paleozoic,dragged by the dense oceanic lithosphere of Shangdan ocean,it subducted under the North China Block and underwent the peak metamorphism at?500 Ma and subsequently two stages of retrogression at?450 and?420 Ma.The Neoproterozoic granite and its county rocks was at the upper hanging side during continental subduction,thus they only experienced transformation but not HP-UHP metamorphism and finally they mixed with exhumated HP-UHP rocks and formed present-day Qinling Group.