Partial Melting of the Deeply Subducted Continental Crust: Constraints From a Gneiss‐Felsic Vein System From the North Qaidam Orogenic Belt

ABSTRACT The North Qaidam ultrahigh‐pressure metamorphic belt represents a typical continental deep subduction orogenesis characterised by extensive partial melting during the Early Palaeozoic. In this study, a comprehensive analysis of petrography, geochemistry and isotopic chronology was conducted on gneisses and veins from the Lüliangshan area of the North Qaidam orogenic belt. The results indicate that zircon and monazite can be divided into three groups (inherited cores, newly grown cores and rims) and two groups (cores and rims), respectively. Newly grown zircon cores and monazite cores in the host gneisses are peritectic ones, whereas newly grown zircon rims and monazite rims in the host gneiss, as well as zircon and monazite in the vein, are crystallised from melt, denoting a melting age of ~420 Ma. Moreover, garnets in two gneisses display core‐rim structures. Garnet core in one gneiss indicate growth during the eclogite facies stage, whereas garnet core in the other gneiss suggests growth during the early granulite facies stage. Garnet rims in both gneisses represent growth during the granulite facies stage. These observations suggest that during the transition from UHP eclogite facies to granulite facies, the host gneiss underwent partial melting, during which garnet cores were consumed to generate peritectic zircon cores, monazite cores and plagioclase, followed by later crystallisation of zircon rims, monazite rims and euhedral plagioclase. In addition, the study of felsic veins indicates that dehydration melting tends to produce more oxidised melts. Moreover, the oxygen fugacity of the anatexis melt exhibits a progressive reduction during the transition from deep subduction to exhumation.