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

Abstract

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 εHf (t) (−34.1 to −1.5) and much older TCRUST (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 εHf (t) values (up to +10.2–+15.2). The 850–540Ma zircons show variable εHf (t) values of +9.0 to −24.0 with TDM (depleted mantle Hf model ages)=2.2–1.0Ga and TCRUST (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.