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 degrees C), low- to medium-pressure (4-7 kbar) 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 chamockites, 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 440-425 Ma, with minor groups at 2.8-2.4 Ga, 1.5-1.25 Ga, 12-0.9 Ga, 850-540 Ma and 460-450 Ma. The 440-425 Ma 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 epsilon(Hf) (t) (-34.1 to -1.5) and much older T-CRUST (3.6-1.5 Ga), demonstrating they were formed by re-melting of old crustal materials (>1.5 Ga). The zircons with ages of 2.8-2.4 Ga, 1.6-1.2 Ga and 1.2-0.9 Ga have relatively high epsilon(Hf) (t) values (up to +10.2-+15.2). The 850-540 Ma zircons show variable epsilon(Hf) (t) values of + 9.0 to -24.0 with T-DM (depleted mantle Hf model ages) = 22-1.0 Ga and T-CRUST (crustal Hf model ages) = 3.1-1.1 Ga Combined with the published data, we suggest that the Cathaysia block contains Archean materials as old as 3.6 Ga and has had a complex evolution, including the addition of juvenile materials at ca 2.7 Ga, 1.6-12 Ga and 12-0.9 Ga. Reworking of old crustal components dominated at ca 850-750 Ma, 750-540 Ma, 460-450 Ma and more intensively at ca 440-425 Ma. Synthesizing the obtained results, we argue that the Yunkai chamockites, gneissic migmatites and Bt-Plgneisses 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 (C) 2013 The Authors. Published by Elsevier B.V. All rights reserved.