Multiple Metamorphic Events and Petrogensis of HighT / UHP Eclogites in the Dabie Orogen , Central China

of the Qinling−Dabie−Sulu orogenic belt formed by the collision of the North China Block and South China Block in the Triassic. It is generally subdivided into five major lithotectonic units from north to south (Xu et al., 2005; Liu et al., 2007a, 2011b): (1) the Beihuaiyang zone; (2) the North Dabie complex zone (NDZ); (3) the Central Dabie UHP metamorphic zone (CDZ); (4) the South Dabie low-T eclogite zone (SDZ); and (5) the Susong complex zone. The NDZ is one of three eclogite-bearing units in the region. Rare eclogites in this zone have been previously documented to be transformed from mafic lower continental crust of the South China Block by petrology and zircon U-Pb dating investigations (Liu et al., 2007a) and underwent ultrahigh-pressure (UHP) and HP eclogite-facies metamorphism, and subsequent HP granulite-facies overprinting and amphibolite-facies retrogression during continental subduction and exhumation (Liu et al., 2005, 2007a, 2011a). However, because multiple decompression and multistage metamorphic overprinting, the precise ages of subduction and exhumation processes of the NDZ are poorly constrained, rendering knowledge on the exhumation mechanism of UHP rocks and tectonic evolution in the Dabie orogen. In order to better constrain the ages of peak UHP metamorphism and subsequent retrogression as well as the petrogensis of the granulitized eclogites, a combined petrological, zircon U-Pb geochronological and whole-rock element and isotope geochemical study has been investigated on the eclogites from the Luotian dome in the NDZ. The zircon U-Pb age results indicate that the studied eclogites experienced the same metamorphic events and have a identical multiple Pb/U concordant age-groups within analytical uncertainly, i.e. Neoproterozoic, 234±3 − 243±15 Ma, 222±2 − 227±2 Ma, 210±3 − 216±5 Ma, 204±2 − 209±7 Ma, 176±2 − 199±2 Ma and 138+1 Ma (Liu et al., 2007a, 2011b; this study). Neoproterozoic ages defined by relic igneous cores represent their protolith time and the other age-groups reflect their metamorphic records by CL images, low Th/U ratios and mineral inclusions. The weighted mean ages for age-groups of 226±4 − 227±2 Ma and 210±3 − 216±5 Ma are 226±2 Ma (MSWD = 0.33, n = 6) and 214±2 Ma (MSWD = 0.39, n = 7) (Fig. 1), respectively. Trace element compositions of these two kinds of metamorphosed mantle domains in zircon exhibit obviously consistent REE patterns with clearly flat HREE and no significance negative Eu anomaly, which is compatible with absence of feldspar and contemporaneous growth of zircon and garnet, suggesting that both metamorphic domains formed under eclogite-facies conditions (e.g., Hermann et al., 2001; Rubatto, 2002; Rubatto and Hermann, 2003; Sun et al., 2002; Whitehouse and Platt, 2003). However, the mineral inclusion assemblages suggest that the former domains usually comprise the UHP assemblage of garnet, Na-rich omphacite, rutile, aragonite and coesite, and the latter contain garnet, rutile, quartz and Na-poor omphacite inclusions. The Raman spectrum of the coesite inclusion contains the typical peak of 521 cm associated with the quartz peak (466 cm), suggesting that coesite was evidently transformed to quartz accompanying a fracture through it during decompression. In addition, the garnets within the former mantle domains of zircon are rich in grossular and poor in spessartine with their end members of 16.99–21.91 mol% and 0.93–2.28 mol%, respectively; while the second are relatively rich in spessartine (1.76– 5.67 mol%) and poor in grossular (11.94–16.26 mol%). These are quite consistent with the above petrographic observations (Liu et al., 2007a, 2011c). Combined with trace element and mineral inclusion in zircon from the eclogites, 226±2 and 214±2 Ma were best estimated as UHP and HP eclogite-facies metamorphic ages, GU Xiaofeng, LIU Yican, 2013. Multiple Metamorphic Events and Petrogensis of High-T/UHP Eclogites in the Dabie Orogen, Central China. Acta Geologica Sinica (English Edition), 87(supp.): 459-461.