Role of deep-Earth water cycling in the growth and evolution of continental crust: Constraints from Cretaceous magmatism in southeast China
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Â© 2018 Elsevier B.V. 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 Uâ€“Pbâ€“Luâ€“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 Ndâ€“Hf isotopic compositions, whereas the Late Cretaceous ones display less radiogenic Ndâ€“Hf isotope signatures. Furthermore, Ndâ€“Hf isotope systematics are coupled with decreasing abundance of hydrous minerals and an increase of zircon saturation temperatures. Compiled zircon Hfâ€“O data indicates that the 117â€“116 Ma granites have zircon Î´ 18 O values ranging from mantle values (close to 5.3â€°) to as low as 3.9â€° but with dominantly positive initial epsilon Hf (Îµ Hf (t)) values. Zircon grains from 105 to 98 Ma rocks have Î´ 18 O values plotting within the mantle-like range (6.5â€° âˆ’ 4.5â€°), but mainly with negative Îµ Hf (t) values. Zircon grains from ca. 87 Ma rocks have positive Îµ Hf (t) values (+ 9.8 to + 0.7) and a large range of Î´ 18 O values (6.3â€° âˆ’ 3.5â€°). The variations in Hfâ€“Ndâ€“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., H 2 O) for hydrous basaltic underplating, continental crustal melting, and magmatic differentiation.
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