Petrogenesis of early Paleozoic granitoids in the North Qinling Orogen, Central China: Implications for crustal evolution in an accretionary orogen

Accretionary orogens are sites of extensive continental crustal growth and modification. The mechanism by which mafic crust is transformed into silicic melts (i.e., maturation of continental crust) is important for understanding the formation of the continental crust. The North Qinling Orogen (NQO) is a composite orogenic belt and contains an early Paleozoic accretion-dominated orogenic system, which is ideal for investigating continental crustal maturation. We obtained zircon and monazite U–Pb age and O isotope data, zircon Lu–Hf isotope data, and whole-rock major- and trace-element and Sr–Nd isotope data for early Paleozoic granitoids of the NQO. The granitoids are divided into three groups. Group 1 includes the Taiping tonalite (445 ±3 Ma), the Manziying syenogranite (445 ±2 Ma), and the Huoshenmiao granodiorite (436 ±2 Ma). The Taiping and Huoshenmiao plutons have relatively high SiO₂ contents (68.64–71.67 wt.%) and Na₂O/K₂O ratios (1.15–4.19), with enrichments in Rb, Ba, Th, and U and depletions in Nb, Ta, P, and Ti, and they are geochemically similar to sodic arc magmas. The Manziying syenogranite is a peraluminous potassic granite with high K₂O contents (4.59–5.27 wt.%). Grantioids from Group 1 have similarly depleted Sr–Nd–Hf–O isotopic features (ε_Hft = +5.9 to +8.8; δ¹⁸O = 3.98‰–5.41‰), indicative of derivation via partial melting of oceanic arc crust, which suggests that partial melting of oceanic arc crust in a subduction system contributes to the generation of continental crust and causes its maturation. Group 2 consists of the Wuduoshan monzogranite (418 ±2 Ma) and the Sikeshu granodiorite (423 ±3 Ma). These plutons have relatively high SiO₂ (65.59–72.06 wt.%), K₂O (3.26–4.79 wt.%), and Al₂O₃ (14.65–16.12 wt.%) contents and Sr/Y (33–87) and (La/Yb)_N (23–48) ratios. The Wuduoshan monzogranite has positive zircon ε_Hf(t) (+0.4 to +3.1) and uniform δ¹⁸O (6.38‰–8.07‰) values, but the Sikeshu granodiorite has more variable isotopic compositions (ε_Hft = -1.9 to +5.0; δ¹⁸O = 6.37‰–10.60‰). The Wuduoshan monzogranite and Sikeshu granodiorite have similar whole-rock Sr–Nd isotopic compositions to basement rocks of the NQO. These features indicate that the two plutons formed by partial melting of basement rocks (i.e., subducted into the lower crust) of the North Qinling unit, along with juvenile crustal material. Group 3 is represented by the Xiaguan monzogranite, which formed at 434–430 Ma, and can be subdivided into heavy rare earth element (REE)-depleted and -enriched units. The former has high Sr/Y (56–98) and (La/Yb)_N (34–73) ratios and low MgO (0.13–0.24 wt.%), Cr (0.37–1.69 ppm), and Ni (0.32–1.09 ppm) contents, similar to adakites derived from metabasaltic sources. The heavy REE-enriched nature of the Xiaguan monzogranite may reflect modification of its source by melt or fluid. Our results show that partial melting of enriched oceanic arc crust contributed to crustal maturation in an accretionary orogen. The addition of evolved crustal material also facilitated this process; therefore, the basement rocks and crustal thickness should be considered when assessing crustal dynamics in an accretionary orogen.