Early Cretaceous low-Mg adakitic granites from the Dabie orogen,eastern China: Petrogenesis and implications for destruction of the over-thickened lower continental crust

It is generally accepted that the low-Mg adakitic rocks were derived from the partial melting of metabasalts/eclogites. In this study, we demonstrate that the early Cretaceous low-Mg adakitic granites in the North Dabie Complex (NDC) were generated by the partial melting of the NDC orthogneisses. Here we present in-situ U–Pb and Lu–Hf isotopes in zircon with whole-rock geochemical and Sr–Nd isotopic compositions were carried out for the Tiantangzhai porphyritic monzogranites from the Dabie orogen, eastern China. The monzogranites are characterized by high Sr (576–988 ppm), low Y (7.3–19.0 ppm), and depletion in HREE (Yb: 0.50–1.78 ppm) (thus resulting in high Sr/Y (34.3–135.2) and (La/Yb)_N (17.0–105.2) ratios) without a negative Eu anomaly. They also exhibit high SiO₂ (66.5–73.5 wt.%) and K₂O (2.7–4.7 wt.%), and low MgO (0.4–1.6 wt.%) or Mg^# (28.2–45.3, mostly < 40) values. Whole-rock geochemical compositions suggest that the monzogranites represent low-Mg adakitic rock with high-Si and rich-K features equilibrated with residues rich in garnet. Sr–Nd isotopic compositions (ε_Nd (t) = ? 16.2 to ? 20.3, (⁸⁷Sr/⁸⁶Sr)_i = 0.707798–0.708804, t_DM2(Nd) = 2.3–2.6 Ga) of the monzogranites are distinct from that of the eclogites and amphibolites in the Dabie orogen, but similar to that of the Neoproterozoic (700–800 Ma) gneisses in the NDC. U–Pb dating of zircons gives a consistent age of 130.0 ± 3.4 Ma with discordia upper intercept age of 716 ± 34 Ma for inherited cores identified by CL imaging. Correspondingly, in-situ Lu–Hf analyses of early Cretaceous young age-spots from zircons yield initial ¹⁷⁶Hf/¹⁷⁷Hf ratios from 0.281898 to 0.282361, εHf(t) values from ? 28.1 to ? 17.6 and two-stage “crust” Hf model ages (t_DM2) from 2293 ± 89 to 2949 ± 108 Ma, which are generally in agreement with values of 0.281891 to 0.282218, ? 28.2 to ? 11.7 and 1927 ± 87 to 2963 ± 92 Ma for the pre-Mesozoic inherited cores, respectively. As for individual core-rim pairs in zircon, Th/U ratios increase from the inherited cores to the young growth rims possibly due to variable degrees of partial melting, whereas ¹⁷⁶Lu/¹⁷⁷Hf ratios greatly decrease because of the garnet effect in residues. Thus, we suggest that the early Cretaceous low-Mg adakitic granites were derived from the partial melting of the NDC Neoproterozoic (700–800 Ma) gneisses, and the foundering of the garnet-bearing residues could have caused the destruction of the over-thickened lower continental crust.