Characteristics Of Metamorphism And Zircon Isotopic Geochronology For The Felsic Rocks In The Dabie Mountains

Dabie orogenic belt is located between the Sino-Korean and Yangtze cratons,belongs to the eastern part of the Qinling orogenic belt.It is the largest ultrahigh-pressure(UHP)metamorphic belt in the world.It is of great significance for studying the UHP metamorphism and revealing the orogenic processs of the continental crust.So far,there are still controversies about the genetic mechanism of UHP metamorphic rocks and the accurate distribution area of the UHP terranes in the Dabieshan area,especially the occurrence of low-grade metamorphic rocks within the UHP terranes.In this paper,different types of metamorphic rocks in the Bixiling area,Ganghe area,Pailou area and Luotian area are selected as research objects and we focus on the felsic gneiss which is less studied.A systematic study of field geological mapping,petrology,zircon U-Pb isotope geochronology,and zircon mineral inclusion Raman spectroscopy was carried out.The following results and new understanding are finally obtained: 1)It is confirmed that the UHP metamorphism in the Bixiling area is characterized bynon-integral and heterogeneity,and the country gneiss has not experienced UHPmetamorphism.In addition,even the same type of rock,taking fine grained granite as anexample,shows two distinct grade of metamorphism in the Bixiling and Ganghe areas,theformer zircon is euhedral thus has not undergone UHP metamorphism;the later zircon withcoesite inclusion thus has experienced UHP metamorphism.2)It is discovered for the first time that the Indosinian granite existed in the Dabieshan area,andits age is 221.0±2.6Ma.More significant is the discovery of the coesite microcrystallineinclusions preserved in the magmatic zircon core in the granite,indicating that the coesitewas formed before the Indosinian granite,thus determining that the UHP metamorphimoccurred before ~220Ma.3)The ages of metamorphic events recorded by the felsic gneiss are all younger than ~220Ma inthe Dabieshan Core Metamorphic Complex(Northern Dabie complex).Combined with thelowermost age of UHP metarmophism constrained by the Indosinian granite,it is suggestedthat the Dabieshan Core Metamorphic Complex has not experienced UHP metamorphism.The UHP metamorphic rocks should be a tectonic sheet located above the Dabieshan CoreMetamorphic Complex.4)The UHP metamorphic rocks may be formed in the seismic source area featured with UHPcondition,which are result of frequent earthquakes caused by intense shearing activities.Therocks in the Dabieshan area might not have experienced deep continental subduction.