Petrology, geochemistry, zircon U–Pb dating and Lu–Hf isotope of granitic leucosomes within felsic gneiss from the North Qaidam UHP terrane: Constraints on the timing and nature of partial melting

Abstract

Granitic leucosomes are widely distributed within felsic gneiss in the North Qaidam ultrahigh-pressure (UHP) metamorphic terrane in western China, which is crucial to understanding the relationships between partial melting, metamorphic evolution and orogenic processes. We have applied petrology, whole-rock geochemistry and Sr–Nd isotope, zircon U–Pb geochronology, trace element composition and Lu–Hf isotope of these granitic leucosomes to determine the nature and timing of partial melting of these rocks. Anatexis of the felsic gneiss is evidenced by (1) highly cuspate, elongated feldspar grains along quartz–quartz and quartz–feldspar boundaries, (2) cuspate wedge-shaped pockets of K-feldspar+quartz+plagioclase±muscovite along the boundaries of quartz and/or plagioclase, and (3) felsic veinlets of K-feldspar+quartz±plagioclase±muscovite along grain boundaries. Major elements (FeOT, MnO, MgO and TiO2) as well as LREEs, HREEs and HFSEs are mainly retained in the melanosomes, whereas the large-ion lithophile elements (LILEs, e.g., Rb, Ba, K, Sr, Pb) are preferentially partitioned into the granitic leucosomes. Three discrete U–Pb ages are recorded in the zoned zircons from the melanosomes and granitic leucosomes. The inherited magmatic (pre-metamorphic) zircon cores from melanosomes and granitic leucosomes contain quartz+feldspar inclusions and record a Neoproterozoic protolith age of approximately ~950Ma. The unzoned zircon mantles in the melanosomes and granitic leucosomes show characteristics similar to metamorphic zircons, in terms of such as remarkably flat heavy rare earth element (HREE) patterns, an absence of obviously negative Eu anomalies, and low Th/U ratios. These zircon mantles record an eclogite-facies metamorphic age of 444–449Ma. The last discrete age at 433–435Ma is preserved in anatectic zircon rims, which display pronounced oscillatory zoning, and contain felsic mineral inclusions of K-feldspar+plagioclase+quartz. The anatectic zircons have negative Eu anomalies and pronounced positive Ce anomalies and their low Th/U ratios and Lu–Hf isotope features that are indistinguishable from those of the metamorphic domains. The similarity in Lu–Hf signatures indicates a closed system, whereby no Hf was added either from the outside via fluids or, surprisingly, from the pre-metamorphic cores during partial melting. Muscovite relicts coexist with cuspate feldspars in the granitic leucosomes, suggesting that the anatectic melts originate from dehydration melting due to breakdown of the muscovite in the felsic gneiss. In conclusion, an integrated study of the petrology, geochronology and geochemistry demonstrates that partial melting of felsic gneiss in the NQD UHP terrane could mainly occurred during the initial retrogression stage and anatectic melt crystallized probably under granulite-facies conditions.