Effects of partial melting and crystallization differentiation on zircon U-Pb and Lu-Hf isotope systems in early Cretaceous high-K and low-Mg adakitic granite during the collapse of over-thickened Dabie orogen, China

A combined study of in-situ U-Pb and Lu-Hf isotopes in zircons with whole-rock geochemical and Sr-Nd isotopic compositions was carried out for Tiantangzhai foliated porphyritic monzogranite from the Dabie orogen in China. The granite is characterized by high Sr, low Y and HREE (thus resulting in high Sr/Y and La/Yb ratios) without negative Eu anomaly. It also exhibits high K2O and low MgO or Mg# values with very negative ɛNd (t) value (-18.6) and high initial 87Sr/86Sr ratio (0.708025). Whole-rock geochemical and Sr-Nd isotopic compositions suggest that the granite is a high-K and low-Mg adakitic rock equilibrated with melt residues rich in garnet. LA-ICPMS U-Pb dating of zircons gives consistent ages ranging from 116±1 Ma to 140±1 Ma with two groups of discordia upper intercept ages of 716±34 Ma and 1895±15 Ma for inherited cores identified by CL imaging, respectively. Correspondingly, in-situ Lu-Hf analyses of Mesozoic age-spots from zircons yield initial 176Hf/177Hf ratios (ɛHf(t) values) from -28.1 to -17.6 and two-stage “crust” Hf model ages (tDM2) from 2293±89 to 2949±108 Ma, in agreement with values of -33.7 to -11.0 and 1927±87 to 3303±98 Ma for the pre-Mesozoic inherited cores, respectively. These results suggest that the adakitic granite was produced by partial melting of the basement at the orogenic crustal root when it was still thick (>50 km), and experienced from the start to the end during the collapse of the Dabie orogen. The crustal root could be an archaeozoic basement involving multiple steps of repeated melting and/or extraction of juvenile crust from the depleted mantle at Late Paleoproterozoic (~1900 Ma) and Middle Neoproterozoic (~715 Ma). The initial partial melting and subsequent crystallization differentiation are distinguished by a combined study of U-Pb and Lu-Hf isotopes in zircons. The U-Pb and Lu-Hf isotopic systems of zircons are affected more greatly by partial melting than by crystallization differentiation. From pre-Mesozoic inherited core to Mesozoic magmatic mantle/rim, the Mesozoic magmatic domains are characterized by elevated Th/U ratios relative to pre-Mesozoic inherited core because of diffusion Pb loss at variable degrees of partial melting or alteration, and display high 176Hf/177Hf ratios (rim(or mantle)/core: 1.00090-1.00116) and low 176Lu/177Hf ratios (rim(or mantle)/core: 0.20-0.49) because of the effects of garnet in relicts during partial melting. In contrast, Th/U ratios decreased from core to mantle/rim during zircon crystallization growth/overgrowth in Mesozoic, due to partition coefficient for U and Th in zircon/melt. Correspondingly, 176Hf/177Hf ratios increased (rim(or mantle)/core: 1.00003-1.00008) and 176Lu/177Hf ratios decreased (rim(or mantle)/core: 0.54-0.65) due to the incorporation of additional radiogenic 176Hf formed by 176Lu decay in the rock's matrix during crystallization growth/overgrowth. But incorporation of 176Hf into zircon domains is negligible due to subsequent crystallization differentiation process compared with the initial partial melting process.