Crustal basement controls granitoid magmatism, and implications for generation of continental crust in subduction zones: A Sr–Nd–Hf–O isotopic study from the Paleozoic Tongbai orogen, central China

Ascertaining the petrogenesis of granitoid rocks in subduction zones holds the key for understanding the processes of how continental crust is produced. The synchronous Taoyuan and Huanggang plutons occur in two different geological units of the Paleozoic Tongbai orogen of central China. They provide an optimal opportunity for a study to address the role of the crustal basement in generating voluminous granitoid magmatism in subduction zones. The Taoyuan and Huanggang plutons have identical U–Pb zircon crystallization ages of 440–444 Ma, which are temporally related to northward subduction of the Paleotethyan Ocean. The Taoyuan samples show high SiO 2 (73.36–79.16%) and low Al 2 O 3 (12.00–13.45%) contents, Mg numbers (20.6–38.2), and Sr/Y (2.04–10.1) and (La/Yb) N (2.34–7.32) ratios with negative Eu anomalies (Eu/Eu* = 0.33–0.93). They yielded positive e Nd (t) (+ 3.0 to + 6.7) and e Hf (t) (+ 11.8 to + 13.2) values, elevated initial Sr isotopic ratios (0.7040–0.7057) and relatively low zircon δ 18 O values of 4.62–5.39‰. These suggest that they were produced through partial melting of hydrothermally altered lower crust of the accreted Erlangping oceanic arc. In contrast, the Huanggang samples exhibit variable whole-rock geochemical and isotopic compositions with SiO 2 contents of 57.01–64.42 wt.%, initial Sr isotopic ratios of 0.7065–0.7078, and e Nd (t) values of − 5.7 to − 9.4. Additionally, they have high zircon δ 18 O values of 7.57–8.45‰ and strongly negative zircon e Hf (t) values of − 14.4 to − 10.5. They were suggested to have been mainly derived from ancient continental crust of the Kuanping crustal unit with the addition of 20–40% juvenile, mantle-derived material. Accordingly, the granitoids in both oceanic and continental arcs are likely to be mainly derived from intracrustal melting of their crustal basement. It is revealed by the Huanggang pluton that little net continental crust growth occurs in continental arcs, and addition of new volume of continental crust may be balanced by the loss of old mafic lower continental crust. Continental growth was likely achieved through the accretion and subsequent differentiation of oceanic arcs, as manifested by the Taoyuan pluton.