Tracking partial melting and protolith nature by zircon U-Pb and Hf-O isotope compositions of migmatites in the North Dabie terrane with emphasis on Paleozoic low-δ¹⁸O magmatism

An integrated study using cathodoluminescence (CL) images, U-Pb geochronology, and Hf-O isotopic compositions of zircons, and whole-rock geochemistry was carried out on migmatites from the North Dabie terrane in order to constrain the sources and processes of the melt during partial melting and implications for tectonics in the northern Yangtze block. CL imaging revealed that zircon grains in the migmatites occur as either new crystals or overgrowth rims around older inherited cores. The zircons display euhedral in shape with oscillatory or weak zoning and variable Th/U ratios, suggesting that they are of metamorphic origin and grew during partial melting. These zircons have concordia U-Pb ages from 129 ± 2 Ma to 140 ± 4 Ma, indicating partial melting of the North Dabie terrane due to the extension and collapse of the thickened orogenic root in the Early Cretaceous. The metamorphic zircons in two leucosome samples show higher δ¹⁸O but lower ε_Hf(t) values than the inherited zircons in the same sample or adjacent melanosome samples, suggesting that the leucosomes were derived from external melt. The large variation of zircon ε_Hf(t) values (–17.9 ± 0.5 to –4.6 ± 0.6) in one leucosome sample implies mixing between outside Hf from external melt and internal Hf from the dissolution-reprecipitation of the inherited zircons. The metamorphic zircons in one melanosome sample have higher ¹⁷⁶Hf/¹⁷⁷Hf ratios than, but similar δ¹⁸O values to those of the inherited zircons in the same sample, providing strong evidence for the generation of the metamorphic zircons by breakdown of a zirconium-bearing phase other than zircon in a closed system. The inherited zircon cores show oscillatory or weak zoning, resorption textures, and overprinting by overgrowth rims, suggesting that they are magmatic zircons inherited from the protoliths. Their U-Pb ages are ca. 450 Ma, providing unambiguous evidence for the presence of early Paleozoic magmatism in the Dabie-Sulu orogen. The inherited zircon cores have ε_Hf(t) values of 8.6–14.0 and depleted mantle model (T_DM2) ages of ca. 880–540 Ma, indicating prompt reworking of juvenile crust during their formation. Inherited zircons in two samples (a melanosome and a leucosome) show δ¹⁸O values of ∼5.3‰, suggesting no involvement of hydrothermal materials. In contrast, inherited zircons in a melanosome sample show δ¹⁸O values from 3.22‰ ± 0.24‰ to 5.00‰ ± 0.29‰, with a weighted average of 4.28‰ ± 0.23‰. These values are distinctly lower than the normal mantle zircon values, suggesting the occurrence of Paleozoic low-δ¹⁸O magma in the northern part of the Yangtze block. The Paleozoic low-δ¹⁸O magma is inferred to have been generated by remelting of preexisting ¹⁸O-depleted rocks in a rift setting. Combined with previous geological observation, we proposed a rift setting for the northern Yangtze block during the early Paleozoic, which might have resulted in the final opening of the Paleotethys Ocean along the Qinling–Tongbai–Hong’an orogenic belt.