Genesis of zircons with core?rim structure from HP?UHP gneisses of the Dabie Mountains
Liu Wenyuan1,2,3 Liu Jingbo1 Liu Xiaoming2
1.Stae Key Laboratory of Lithospheric Evolution|Institute of Geology and Geophysics|Chinese academy of science|Beijing 100029|2.National Key Laboratory of Continental Dynamics|Northwest University|Xi'an 710069| 3.Graduate School of Chinese Academy of Sciences|Beijing 100039
Most of zircons in high?pressure and ultrahigh?pressure gneisses from the Dabie Mountains show the core?rim structures in their Cathodoluminescence(CL)images，consisting of inherited magmatic cores with oscillatory zoning and the rims of metamorphic origin with bright CL images. The researches，on the basis of the inclusion minerals determined by using Laser Raman microprobe，U?Pb ages and trace elements analyzed by using LA?ICP?MS suggest that the metamorphic rims of zircons was formed through two different mechanisms：recrystallization and overgrowth. Recrystallization could result in gradually decreasing U?Pb ages and total contents of rare earth elements(REE)，but the REE pattern remains unchanged. Comparing with the recrystallization zircons，the overgrowth zircons have no negative Eu anomaly，lower Th/U ratios and contents of Nb and Ta，more depleted HREE.