Comparison of Sn-related granitoids in subduction and collision settings by accessory mineral geochemistry: A case study in the Tengchong-Lianghe tin belt, SW China

Abstract

Magmatic-hydrothermal Sn deposits can develop in both subduction and collision settings. However, a systematic comparison of the geochemical fingerprints and petrogenetic evolution of Sn-related granitoids from both settings is still lacking. The Tengchong-Lianghe tin belt hosts numerous Sn deposits but of different metallogenic ages, such as the Xiaolonghe and Lailishan Sn deposits, which provide an ideal study area for addressing this issue. Zircon U–Pb dating suggests that the Xiaolonghe and Lailishan Sn-related granitoids were emplaced at 74.2–75.5 and 50.0–51.1 Ma, respectively. Given the initial India-Asia collision commenced at ca. 65–60 Ma, these ages, consistent with available Sn metallogenic ages, suggest the formation of the Xiaolonghe and Lailishan Sn deposits in subduction and collisional settings, respectively. New zircon and apatite compositions, and apatite Sr isotopic data, combined with previously reported whole-rock geochemical results, all indicate that the Xiaolonghe and Lailishan Sn-related granitoids share similar magma sources, evolution processes, redox states, and volatile contents. The presence of hornblende, high apatite NdN/NdN* (mostly > 1), and whole-rock evolutionary trends, together indicate that they represent fractionated I-type granites. Distinctly elevated apatite 87Sr/86Sr ratios (0.70988–0.71430) suggest that they originated from an ancient lower crust. Subsequently, they underwent an extensive fractional crystallization dominated by feldspar, as revealed by whole-rock, apatite, and zircon elemental variation trends. The low zircon CeN/CeN* (mainly < 200) and ΔFMQ (–1.9 to 1.7), together with elevated apatite F content (2.30–3.69 wt%), reveal that they crystallized from reduced and F-rich magmas. The relatively high whole-rock zircon saturation temperatures (mainly > 800 ℃) and Ba/Pb ratios imply that they were produced at a higher temperature by biotite-dehydration melting, which requires additional heat from the mantle. The specific mechanism that triggers mantle upwelling (oceanic slab subduction or break-off) could be the most significant difference between Sn-related granitoids formed during subduction and collision processes.