Multiple episodes of anatexis in a collisional orogen: Zircon evidence from migmatite in the Dabie orogen

Abstract

A combined study of mineral inclusions, U–Pb ages and trace elements was carried out on zircon from migmatites in the Dabie orogen. The results provide insights into multistage anatexis of ultrahigh-pressure metamorphic rocks in the continental collision orogen. Zircon grains in thin sections and mounts record four episodes of magmatic, metamorphic and anatectic events: (1) middle Neoprotrozoic U–Pb ages for domains that contain Qz ± Ap ± Pl inclusions and exhibit high Th/U ratios (> 0.1), steep HREE patterns with marked negative Eu anomalies, dating protolith emplacement; (2) late Triassic U–Pb ages of 212 ± 5 to 219 ± 4 Ma for domains that contain Cpx ± Grt inclusions and show low Th/U ratios (< 0.1), flat HREE patterns without negative Eu anomalies, dating quartz eclogite-facies metamorphism during the early exhumation of deeply subducted continental crust; (3) U–Pb ages of 192 ± 4 to 200 ± 4 Ma for domains that contain multiphase solid inclusions of Qz + Pl + Hem + Cal, Qz + Kfs + Ep, Qz + Bt + Ap and Qz + Pl + Bt, and exhibit low Th/U ratios (< 0.1), steeper HREE patterns with negative Eu anomalies and high Nb and Ta contents, dating the first episode of anatexis in association with granulite-facies overprinting during the late exhumation; and (4) early Cretaceous U–Pb ages of 124 ± 1 to 140 ± 4 Ma for domains that contain Qz inclusion and show variable Th/U ratios, typical magmatic REE patterns, dating the second episode of anatexis which occurred during the Cretaceous in the postcollisional stage. The zircon domains in the two episodes of anatexis exhibit a large range of U–Pb ages, suggesting protracted durations of anatexis in both exhumational and postcollisional stages. There are considerable differences in the compositions of zircons between the two episodes of anatexis, suggesting differential behaviors of their anatexis. The first episode of anatexis is caused by dehydration melting in association with decompressional exhumation, resulting in coprecipitation of zircon and REE-rich minerals such as allanite/epidote and monazite from the anatectic melt. The second episode of anatexis is caused by both dehydration and hydration melting due to breakdown of monazite and hydrous minerals such as allanite/epidote and amphibole with local focus of aqueous solutions. This leads to variable Th/U ratios but relatively consistent magmatic zircon-like trace element compositions, with the widespread occurrence of poikilitic amphibole in the leucosome. In either case, the anatectic zircons record the partial melting of ultrahigh-pressure metamorphic rocks at different times and P–T conditions.