Metamorphic-anatectic reworking of continental arc crust: Insights from the North Wulan metamorphic complex in the North Qaidam tectonic belt (northern Tibet)

Abstract Anatexis is a key process linking deep crustal metamorphism, tectonic deformation, and magmatic activity in orogenic systems. Understanding continental arc crustal metamorphism and anatexis is crucial for comprehending crustal differentiation and reworking. The North Wulan metamorphic complex, located along the northern margin of the Qinghai-Tibet Plateau, northern Tibet, contains a rock sequence that outcrops from deep to shallow crustal levels of a continental arc. In this paper, we present systematic studies on different types of migmatite in the North Wulan metamorphic complex to constrain the pressure-temperature-time conditions of metamorphism and partial melting within the deep crust of continental magmatic arcs. The biotite-amphibole gneiss formed through the remelting of preexisting Cambrian arc rocks, whereas the felsic gneiss originated from the partial melting of the Paleoproterozoic basement within the arc crust. Zircon U-Pb geochronology reveals that the igneous protoliths of the biotite-amphibole gneiss crystallized at 503–500 Ma. U-Pb data and Hf isotopic data from zircons indicate that these Cambrian arc rocks and the Paleoproterozoic basement underwent contemporaneous metamorphism and anatexis at 465–458 Ma. Based on both petrographic and geochemical evidence, the leucosomes in the migmatites formed from water-fluxed melting. Petrographic analysis shows diffuse boundaries between the leucosome and gneiss, along with an absence of anhydrous peritectic minerals in the leucosomes. Geochemical analysis supports this conclusion, with data showing specific correlations in element ratios (Rb/Sr versus Sr, Rb/Sr versus Ba, Ta versus Nb, and U versus Th). Phase equilibrium modeling indicates that partial melting of Cambrian arc rocks and felsic gneiss occurred under water-saturated conditions (with 1.48 wt% and 1.74 wt% H2O, respectively). The zircon Eu/Eu* data reveal that the switch from compression to extension occurred at ca. 480 Ma. As previous studies have concluded, we suggest that asthenosphere upwelling through thinned lithospheric mantle introduced high heat flow into the lower crust due to the rollback of the subducted oceanic plate. This caused water-fluxed melting in low-pressure/high-temperature granulite facies and the reworking of the continental arc crust during the subduction of the Qaidam oceanic slab in the early Paleozoic.