TRACE ELEMENTS BEHAVIOR AND P-T-T EVOLUTION FOR PARTIAL MELTING DURING DIFFERENTIAL EXHUMATION OF NORTH QAIDAM UHP METAMORPHIC ROCKS, NW CHINA

The Paleozoic North Qaidam UHPM belt lies between the Qaidam and Qilian blocks at the northern margin of the Tibetan Plateau, NW China (see more details in the review paper of Zhang et al., 2013, Journal of Asian Earth Sciences 63, 98–111.). On the basis of time and temperature data from zircon and rutile we construct two distinct T-t paths with slow cooling rate of <10°C/Ma for the Yuka/North Dulan and the South Dulan sub-belts of the North Qaidam UHPM belt. These data indicate a differential exhumation and emplacement of UHP metamorphic rocks in the North Qaidam (Zhang et al., 2014, Lithos 205, 15-27.). Trace elements behavior was investigated of coexisting leucosomes, garnetite and residue granulitized eclogite (experienced later amphibolite-facies overprinting as amphibolitized eclogite) from a single outcrop of UHP rocks, with the aim of understanding the thermal evolution during partial melting of eclogite. South Dulan eclogites partially melted synchronously with granulite-facies overprinting during exhumation (Song et al., 2014, Geochimica et Cosmochimica Acta 130, 42–62.). Two episodes of partial melting occurred. First, Grt-free tonalitic–trondhjemitic leucosome melts with higher silica contents formed, which crystallized at 424.0 ± 2.7 Ma. Igneous garnets cumulated away from the leucosome, and coexisting Ti-rich phases (rutile and titanite), giving rise to a stronger adakitic signature. Later Grt-bearing leucosome melts with an age of 412 ± 3 Ma cross-cut boudins and layers of amphibolitized eclogite. Geochemical investigation of both bulk rocks and in-situ minerals verifies the genetic relationship of the amphibolitized eclogites and the associated tonalitic–trondhjemitic melts. Zircons from the amphibolitized ecologite record older (>700 Ma) protolith ages and subsequent eclogite facies and granulite facies overprinting/partial melting. The Grt-free leucosome melts show enriched Hf isotopes features (ε_Hf (424 Ma) = -6.3 to 0.4). Phase modeling, Zr-in-rutile thermometry calculations combined with the zircon geochronology data reveal the P-T-t evolution of the partial melting events and reflect a thickened crust tectonic environment during Paleozoic subducted continental crust exhumation at the North Qaidam subduction zone.