Role of deep-Earth water cycling in the growth and evolution of continental crust: Constraints from Cretaceous magmatism in southeast China

The late Mesozoic igneous province in southeast China provides an excellent opportunity to understand the processes that controlled the growth and evolution of Phanerozoic continental crust. Here we report petrological, whole-rock geochemical and isotopic data, and in situ zircon U-Pb-Lu-Hf isotopic data from granitoids and associated gabbros in the Pingtan and Tong'an complexes, southeast China. Through combining the new results with published datasets in southeast China, we show that the Early Cretaceous magmatic rocks are dominated by juvenile Nd-Hf isotopic compositions, whereas the Late Cretaceous ones display less radiogenic Nd-Hf isotope signatures. Furthermore, Nd-Hf isotope systematics are coupled with decreasing abundance of hydrous minerals and an increase of zircon saturation temperatures. Compiled zircon Hf-O data indicates that the 117-116 Ma granites have zircon delta O-18 values ranging from mantle values (close to 53 parts per thousand) to as low as 3.9 parts per thousand, but with dominantly positive initial epsilon Hf (epsilon(Hf)(t)) values. Zircon grains from 105 to 98 Ma rocks have delta O-18 values plotting within the mantle-like range (6.5 parts per thousand - 4.5 parts per thousand), but mainly with negative epsilon(Hf)(t) values. Zircon grains from ca. 87 Ma rocks have positive epsilon(Hf)(t) values (+9.8 to +0.7) and a large range of delta O-18 values (6.3 parts per thousand - 3.5 parts per thousand). The variations in Hf-Nd-O isotopic compositions are correlated with decreasing abundance of magma water contents, presenting a case that water-fluxed melting generated large-scale granitic magmatism. Deep-Earth water cycling provides an alternative or additional mechanism to supply volatiles (e.g., H2O) for hydrous basaltic underplating, continental crustal melting, and magmatic differentiation. (C) 2018 Elsevier B.V. All rights reserved.