Anatexis of ultrahigh-pressure eclogite during exhumation in the North Qaidam ultrahigh-pressure terrane: Constraints from petrology, zircon U-Pb dating, and geochemistry

Plagioclase-rich leucosomes and K-feldspar-rich veins are widely distributed within retrogressed eclogite in the Xitieshan area of the North Qaidam ultrahigh-pressure metamorphic terrane, western China. In this contribution, a combined study of petrology, zircon internal structure, zircon U-Pb age, zircon trace-element composition, whole-rock geochemistry, and Sr-Nd isotopes in plagioclase-rich leucosomes and K-feldspar-rich veins provides insight into the nature and timing of partial melting in these rocks. Petrological evidence for partial melting is provided by elongated, highly cuspate feldspars, quartz along grain boundaries (e.g., muscovite, garnet, and clinopyroxene), and felsic veinlets composed of quartz and feldspar along the boundaries of garnet, clinopyroxene, and clinopyroxene-plagioclase symplectite in the metabasite. Whole-rock geochemistry suggests that the plagioclase-rich leucosomes have lower contents of both rare earth elements (REEs) and high field strength elements (HFSEs) but higher large ion lithophile elements (LILEs; e.g., Rb, Ba, K, Sr, and Pb) than the metabasite hosts. The plagioclase-rich leucosomes may be divided into two subgroups according to their distinct REE patterns: (1) higher total REE content with or without weak negative Eu anomalies, and (2) lower total REE content with conspicuous positive Eu anomalies. The Eu-rich group also shows higher Sr content than those in the Eu-poor group. However, the K-feldspar-rich veins show higher REE, HFSE, and LILE concentrations than the plagioclase-rich leucosomes and metabasites. The inherited cores of K-feldspar-rich veins are of typical magmatic origin (oscillatory zoning with high Th/U ratios, enriched heavy [H] REEs, and negative Eu anomalies), yielding Pb-206/U-238 ages ranging from 904 +/- 8 Ma to 915 +/- 28 Ma. The zircon cores of plagioclase-rich leucosomes and mantles of K-feldspar-rich veins exhibit no zoning or weak zoning, with flat HREE patterns and no negative Eu anomalies, and they contain mineral inclusions of garnet and clinopyroxene, implying that these zircon domains were formed during an episode of eclogite-facies metamorphism with Pb-206/U-238 ages ranging from 444 +/- 10 Ma to 452 +/- 9 Ma. Internal textures and mineral inclusions (quartz, plagioclase, and K-feldspar) and trace-element systematics (steep HREE patterns with negative Eu anomalies and low Th/U ratios) of the zircon rims from plagioclase-rich leucosomes and K-feldspar-rich veins are similar to anatectic zircon. These anatectic zircon domains yielded Pb-206/U-238 ages ranging from 430 +/- 10 Ma to 440 +/- 12 Ma, and 406 +/- 12 Ma to 430 +/- 13 Ma, respectively. An integrated study of petrology, geochronology, and geochemistry demonstrated that the metabasites of the North Qaidam ultrahigh-pressure terrane likely experienced initial partial melting with plagioclase-rich leucosome formation under eclogite-facies conditions and partial melt crystallization during the granulite-facies stage, triggered by dehydration melting involving zoisite and rare muscovite. The K-feldspar-rich veins within the metabasites may be the result of injected melts that were derived from partial melting of nearby gneisses.