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

摘要

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 (576-988ppm), low Y (7.3-19.0ppm), and depletion in HREE (Yb: 0.50-1.78ppm) (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 SiO _2 (66.5-73.5wt.%) and K _2O (2.7-4.7wt.%), and low MgO (0.4-1.6wt.%) 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, (~(87)Sr/ ~(86)Sr) i=0.707798-0.708804, t _(DM2)(Nd)=2.3-2.6Ga) of the monzogranites are distinct from that of the eclogites and amphibolites in the Dabie orogen, but similar to that of the Neoproterozoic (700-800Ma) gneisses in the NDC. U-Pb dating of zircons gives a consistent age of 130.0±3.4Ma with discordia upper intercept age of 716±34Ma for inherited cores identified by CL imaging. Correspondingly, in-situ Lu-Hf analyses of early Cretaceous young age-spots from zircons yield initial ~(176)Hf/ ~(177)Hf 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±108Ma, which are generally in agreement with values of 0.281891 to 0.282218, -28.2 to -11.7 and 1927±87 to 2963±92Ma 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 ~(176)Lu/ ~(177)Hf 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 (700-800Ma) gneisses, and the foundering of the garnet-bearing residues could have caused the destruction of the over-thickened lower continental crust.