Zircon U-Pb Age, Trace Element, and Hf Isotope Evidence for Paleoproterozoic Granulite-Facies Metamorphism and Archean Crustal Remnant in the Dabie Orogen

Abstract

Zircon U-Pb age, trace elements, and Hf isotopes were determined for granulite and gneiss at Huangtuling, which is hosted by ultrahigh-pressure metamorphic rocks in the Dabie orogen, east-central China. Cathodoluminescence (CL) images reveal core-rim structure for most zircons in the granulite. The cores show oscillatory zoning, relatively high Th/U and ¹⁷⁶Lu/¹⁷⁷Hf ratios, and high rare earth element (HREE)-enriched pattern, consistent with magmatic origin. They gave a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of (2 766 ± 9) Ma, dating magma emplacement of protolith. The rims are characterized by sector or planar zoning, low Th/U and ¹⁷⁶Lu/¹⁷⁷Hf ratios, negative Eu anomalies and flat HREE patterns, consistent with their formation under granulite-facies metamorphic conditions. Zircon U-Pb dating yields an age of (2 029 ± 13) Ma, which is interpreted as a record of metamorphic event during the assembly of the supercontinent Columbia. The gneiss has a protolith age of (1 982 ± 14) Ma, which is similar to the zircon U-Pb age for the granulite-facies metamorphism, suggesting complementary processes to granulite-facies metamorphism and partial melting. A few inherited cores with igneous characteristics have ²⁰⁷Pb/²⁰⁶Pb ages of approximately 3.53, 3.24, and 2.90 Ga, respectively, suggesting the presence of Mesoarchean to Paleoarchean crustal remnants. A few Triassic and Cretaceous metamorphic ages were obtained, suggesting the influences by the Triassic continental collision and postcollisional collapse in response to the Cretaceous extension. Comparing with abundant occurrence of Triassic metamorphic zircons in ultrahigh-pressure eclogite and granitc gneiss from the Dabie-Sulu orogenic belt, however, very limited availability of aqueous fluid or hydrous melt is evident for zircon growth in the Huangtuling granulite and gneiss during the continental collision. The magmatic protolith zircons from the granulite show a large variation in ¹⁷⁶Hf/¹⁷⁷Hf ratios from 0.280 809 to 0.281 289, corresponding to ɛ_Hf(t) values of -7.3 to 6.3 and Hf model ages of 2.74 to 3.34 Ga. The 2.90 Ga inherited zircons show the similar Hf isotope features. These indicate that both growth of juvenile crust and reworking of ancient crust took place at the time of zircon formation. It is inferred that the Archean basement of the Yangtze block occurs in the north as the Dabie orogen, with ca. 2.90–2.95 Ga and 2.75–2.80 Ga as two major episodes of crustal formation.