Low-δ18O and negative-Δ199Hg felsic igneous rocks in NE China: Implications for Early Cretaceous orogenic thinning

Magmatic rocks carry key information to understand the processes of material cycling in the Earth's interior and record crucial clues to reconstruct the important tectonic evolution of the Earth. Therefore, it is of great importance to constrain the magma source (such as origin from crust or mantle, oceanic crust or continental crust, lower or upper continental crust) accurately, which however remains to be unraveled because of the variety magmatic sources and complex magmatic processes. High-precision stable isotopic analyses provide new tools to decipher the genesis of igneous rocks and to reconstruct their associated tectonic settings. Here, we report a pioneering work using integrated O-Hg-Hf-Nd isotopes to understand the genesis of the Early Cretaceous A-type granites and rhyolites from the Paleozoic-Mesozoic accretionary orogen in NE China. Low δ18O zircon values (3.2–5.7‰) of the A-type rocks reveal a magma source affected by meteoric fluids. Their whole-rock negative Δ199Hg values (−0.24 to −0.01‰), which are similar with the terrestrial systems but clearly different from the marine systems, indicate that hydrothermal alteration took place in the continental setting. Positive zircon εHf(t) (4.2–10.7) and mainly positive whole-rock εNd(t) (−0.5 to 2.4, mean = 1.2 ± 1.0, 1SD) values indicate that the magma source was dominated by juvenile crustal materials. This study suggests an elevated geothermal gradient due to the lithospheric thinning triggered by rollback and foundering of a Paleo-oceanic slab. The specific thermal anomaly caused the remelting of the altered upper continental crust, resulting in the formation of the A-type granites and rhyolites, with the characters of low-δ18O and negative-Δ199Hg values. Our study shows that Hg isotope, coupled with O and NdHf isotopes, is a promising tool for petrogenetic studies.