Reworking of juvenile crust beneath the Bangong–Nujiang suture zone: Evidence from Late Cretaceous granite porphyries in Southern Qiangtang, Central Tibet.

Crustal reworking in the collisional zones is commonly considered as an important process to result in continental crust differentiation and maturation. However, the reworking mechanism remains unclear. Here, a combined study of zircon SIMS U–Pb age, whole-rock major and trace element, Sr–Nd–Hf and zircon Hf–O isotopic geochemistry was carried out for the Duoma–Namuqie (DM–NMQ) granite porphyry intrusions, including DM, NMQ I, II and III intrusions, in southern Qiangtang, central Tibet. Zircon SIMS U–Pb dating yielded ages of 79–76 Ma, suggesting that they were formed in the later stage of Late Cretaceous. The DM granite porphyries are metaluminous to peraluminous with slightly low SiO 2 (69.2–71.4 wt%) and high MgO (0.67–0.79 wt%) contents. They are depleted in high-field-strength elements (HFSEs) and enriched in light rare earth elements (LREEs), large ion lithophile elements (LILEs), and show weakly negative Eu anomalies. The NMQ III granite porphyries show similar elemental features to the DM granite porphyries, but have relatively high SiO 2 (69.8–72.2 wt%) and low MgO (0.25–0.28 wt%) contents. Both of them belong to I-type granites. The NMQ I and II granite porphyries are relatively differentiated with high SiO 2 (76.4–77.9 wt%), low MgO (0.11–0.16 wt%) contents, and significantly negative Eu anomalies. Extremely low LREE contents and notable REE tetrad effect can be observed in the NMQ II granite porphyries. The NMQ I and II intrusions are fractionated I-type granites. The whole-rock Sr–Nd–Hf and zircon Hf–O isotope data exhibit an enriched trend from the DM granite porphyries ((87Sr/86Sr) i = 0.7049–0.7050, ε Nd (t) = 1.2–1.6, ε Hf (t) whole-rock = 9.9–12.0, ε Hf (t) zircon = 7.4–11.5, δ18O = 6.2–7.1‰) to the NMQ III granite porphyry ((87Sr/86Sr) i = 0.7058–0.7067, ε Nd (t) = −0.9 to −1.1, ε Hf (t) whole-rock = 7.0–7.4, ε Hf (t) zircon = 2.7–9.5, δ18O = 7.2–7.6‰) to the NMQ I and II granite porphyries (ε Nd (t) = −3.0 to −4.3, ε Hf (t) whole-rock = 5.3–6.6, ε Hf (t) zircon = 1.5–7.4, δ18O = 6.6–9.2‰). We suggest that the DM–NMQ granite porphyries were mainly derived by partial melting of the juvenile mafic lower crust with variable volumes of ancient crustal components involved in their magma sources. The way in which these granite porphyries were formed provides evidence for the crustal differentiation and maturation in a post-collisional extensional setting triggered by the far-field rollback of the subducted Neo-Tethys oceanic slab • The Duoma–Namuqie granite porphyries were formed at 79–76 Ma. • The Duoma–Namuqie granite porphyries were mainly derived by juvenile crust reworking. • The Duoma–Namuqie granite porphyries were formed in a post-collisional setting. [ABSTRACT FROM AUTHOR]