Neoarchean crustal thickness and geothermal gradient of northwestern Yangtze Craton, South China: Constraints from geochemistry and zircon U-Pb-Hf isotopes of TTGs

Abstract

The Neoarchean geodynamic regimes of the continental crust formation and evolution of the Yangtze Craton remain enigmatic. The thickness and geothermal gradient of the continental crust could provide direct clues for understanding the geodynamic regimes of the early Earth. The lower crust-derived tonalite–trondhjemite–granodiorite (TTG) is an ideal lithology because its melting pressure and temperature conditions can be used to estimate the minimum crustal thickness and Moho temperature. In the Yudongzi Complex of the northwestern Yangtze Craton, three episodes of Neoarchean TTG magmatism are recognized at ∼2.75–2.7 Ga, ∼2.62–2.60 Ga and ∼ 2.53–2.50 Ga. These TTGs are characterized by low MgO (0.28–1.07 wt%), Cr (3.80–10.64 wt%) and Ni contents (2.06–7.86 wt%) and Mg# values (21–45), high Sr/Y (41.2–235.3) and (La/Yb)_N (19.6–127.9) values and positive ɛ_Hf(t) values of +1.5 to +6.8 and are derived from the partial melting of juvenile thickened lower crust. The thermodynamic and trace element simulation results indicate that the geochemical compositions of these three episodes of crust-derived TTG magmas are best matched by the melts generated at 1.2 GPa and 830 °C, 1.4 GPa and 870 °C and 1.3 GPa and 840 °C, respectively. Together with the thermodynamic formula, we estimate the Neoarchean crustal thicknesses as 36–43 km with geothermal gradients of 14–21 °C/km for the ∼2.75–2.7 Ga TTG gneisses, 43–50 km with geothermal gradients of 12–17 °C/km for the ∼2.62–2.60 Ga TTG gneisses and 40–46 km with geothermal gradients of 13–18 °C/km for the ∼2.52–2.50 Ga TTG gneisses. These observations, in conjunction with the occurrence of coeval subduction-related igneous rocks in the adjacent region, suggest that the cratonic lithosphere has gradually stabilized and that slab subduction was the most likely geodynamic regime leading to the Neoarchean magmatism in the northwestern Yangtze Craton.