Early Paleozoic crustal anatexis in the intraplate Wuyi-Yunkai orogen, South China

Early Paleozoic amphibolite- to granulite-facies metamorphism, crustal anatexis and coeval magmatism are extensively developed in the Wuyi-Yunkai intraplate orogen in the South China block. However, the exact timing of granulite-facies partial melting and its link with orogenesis have not been well constrained. In this study, the charnockites, gneissic migmatites and Al-rich gneisses (Grt-Sil-Bt gneiss and Bt-Pl gneiss) from the Gaozhou Complex of the Yunkai uplift in the Cathaysia block were selected for the analysis of whole-rock major elements and zircon U-Pb dating, trace elements and Lu-Hf isotopes. The Gaozhou Complex experienced early Paleozoic regional high-temperature (up to 850°C), low- to medium-pressure (4-7kbar) metamorphism accompanied by crustal anatexis. The melts were produced through the dehydration of mica, such as biotite+quartz+plagioclase=orthopyroxene+K-feldspar+melt and biotite+quartz+plagioclase+sillimanite=garnet+K-feldspar+melt in the charnockites, and muscovite+quartz+plagioclase=sillimanite+K-feldspar+melt in the Grt-Sil-Bt gneisses. The charnockites, gneissic migmatites and gneisses are felsic with SiO₂>64% and peraluminous with A/CNK>1.0, reflecting protoliths with affinities to sedimentary rocks. Zircons from these rocks partly show clear core-rim structure and yield concordant ages mainly around 440-425Ma, with minor groups at 2.8-2.4Ga, 1.5-1.25Ga, 1.2-0.9Ga, 850-540Ma and 460-450Ma. The 440-425Ma grains are euhedral, oscillatorily-zoned and have steep slopes from the LREE to the HREE with a positive Ce anomaly and clear negative Eu anomaly, suggesting they (re-) crystallized in the melts. These early Paleozoic zircons have negative ε_Hf (t) (-34.1 to -1.5) and much older T_CRUST (3.6-1.5Ga), demonstrating they were formed by re-melting of old crustal materials (>1.5Ga). The zircons with ages of 2.8-2.4Ga, 1.6-1.2Ga and 1.2-0.9Ga have relatively high ε_Hf (t) values (up to +10.2-+15.2). The 850-540Ma zircons show variable ε_Hf (t) values of +9.0 to -24.0 with T_DM (depleted mantle Hf model ages)=2.2-1.0Ga and T_CRUST (crustal Hf model ages)=3.1-1.1Ga. Combined with the published data, we suggest that the Cathaysia block contains Archean materials as old as 3.6Ga and has had a complex evolution, including the addition of juvenile materials at ca 2.7Ga, 1.6-1.2Ga and 1.2-0.9Ga. Reworking of old crustal components dominated at ca 850-750Ma, 750-540Ma, 460-450Ma and more intensively at ca 440-425Ma. Synthesizing the obtained results, we argue that the Yunkai charnockites, gneissic migmatites and Bt-Pl gneisses were formed due to the early Paleozoic high-T crustal anatexis, which may have been triggered by crustal shortening and thickening during the intraplate Wuyi-Yunkai orogeny in the South China block.