Constraining the protolith ages and tectonic settings of metamorphic rocks is key to understanding the nature and evolution of orogenic belts. This paper presents a systematic study of leptite interlayers within extensively exposed layered marbles in the central South Tianshan Orogenic Belt. Based on field investigations, petrology, zircon U–Pb geochronology, and zircon trace‑element geochemistry, we constrain the depositional ages of the leptite and marble protoliths and evaluate their tectonic environment. The leptites occur as thin interbeds within medium‑ to thin‑bedded and banded marbles. Petrographic observations reveal a granoblastic texture composed predominantly of recrystallized quartz and calcite, suggesting that the leptites were derived from acidic tuff protoliths. Zircon U–Pb dating of two leptite samples yields protolith ages of 401.9 ± 2.7 Ma and 392.3 ± 1.3 Ma, recording two discrete volcanic ash‑fall events near the Early–Middle Devonian boundary. These ages constrain deposition of the regionally extensive marble succession to the Middle–Late Devonian. Inherited zircon ages of 480–503 Ma and c. 420 Ma point to earlier tectono‑magmatic events in the region. Zircon trace‑element signatures are consistent with derivation from volcanic eruptions in a subduction‑related magmatic arc setting. Accordingly, we propose that the South Tianshan region experienced arc magmatism during the Early–Middle Devonian, linked to subduction of the South Tianshan Ocean.