Triassic diorites in the Qinling orogen: Underappreciated role for gold metallogeny in continental collision orogenic belt

Abstract

The Triassic granitoids and associated diorites in the Qinling orogenic belt reveal critical evidence of crust–mantle interaction during the terminal collision between the North China and Yangtze Blocks. This study presents new constraints from zircon U–Pb age, Lu–Hf isotopes, and amphibole-plagioclase-apatite geochemistry for the Maoerliang diorite in the Foping area. Zircon U–Pb dating yields a crystallization age of 212 ± 2.8 Ma, with εHf(t) values ranging from −8.6 to +3.0 and corresponding two-stage Hf model ages (T_DM2) of 886–1479 Ma, indicative of derivation from an evolved lithospheric mantle source. Petrogenetic indicators reveal a mantle affinity: amphiboles exhibit high MgO (9.8–11.2 wt%) and elevated Nb/Ta ratios (14.3–18.1), while apatites display F-rich (2.1–2.8 wt%) and Cl-poor (0.08–0.15 wt%) characteristics. Thermobarometric calculations based on amphibole chemistry constrain crystallization conditions of 805–866 °C and 211–383 MPa, corresponding to mid-crustal emplacement depths (8–14 km). Both amphibole and zircon indicate elevated oxygen fugacity (ΔNNO = −4.08 to −3.71; ∆FMQ = −1.14 to +3.96) and hydrous magma conditions (H₂O = 4.22–4.94 wt%). Late-stage plagioclase crystallization (An21–26.5) reflects prolonged fractional crystallization in a hydrous dioritic magma. These diagnostic features—mantle-derived signatures, high fO₂, and hydrous nature—exhibit remarkable convergence with gold-mineralized granites in the East Qinling. Our findings suggest that Triassic dioritic magmatism may have played an underappreciated role in facilitating gold enrichment processes within the South Qinling metallogenic belt.