Triassic Retrograde Metamorphism and Anatexis in the Sulu Orogenic Zone, Central China: Constraints from U–Pb Ages, Trace Elements, and Hf Isotopic Compositions of Zircon

We report information on the protolith, the Triassic retrograde metamorphism, and anatexis recorded in zircons extracted from granitic gneiss and biotite schist in the Sulu orogenic zone, central China. The schist is enclosed within the granitic gneiss in the form of a lens. Zircon grains from the schist sample indicate anatexis occurred at 214.6 ± 3.6 Ma (MSWD = 5.1), with εHf (t) values ranging from −22.6 to −18.3, corresponding to TDMC(Hf) ages between 2675 Ma and 2407 Ma. The granitic gneiss originated from magmatic rock formed at 774 ± 32 Ma (MSWD = 5.7) and subsequently underwent metamorphism at ~211 Ma. Three zircon cores from the granitic gneiss exhibit εHf (t) values ranging from −13.6 to −6.3, with TDM C(Hf) ages spanning 2487–2075 Ma. Six zircon rims from the gneiss yield εHf (t) values of −14.7 to −13.3, and TDM C(Hf) ages ranging from 2176 to 2092 Ma. We believe that the protolith of granitic gneiss is the Neoproterozoic magmatic rock, whose tectonic affinity is the northern margin of the Yangtze craton. The granitic gneiss experienced Triassic collisional orogeny-related metamorphism and subsequent retrograde metamorphism, with the timing of retrograde overprinting consistent with zircon-recorded anatexis in the schist. In addition, the protoliths of both the gneiss and schist exhibit close affinity to Archean-Paleoproterozoic crustal sources.