Post-collisional high-K calc-alkaline volcanism occurred in the Tengchong volcanic field in the southeastern Tibetan Plateau from 8 Ma to AD 1609. The volcanic rocks have chondrite-normalized rare earth element (REE) patterns characterized by enrichment in light REE (LREE), a flat heavy REE (HREE) pattern and weak negative Eu anomalies. Primitive mantle-normalized incompatible trace element diagrams indicate enrichments in large ion lithophile elements (LILE) relative to high field strength elements (HFSE) and negative Ta–Nb–Ti anomalies. Incompatible trace element and Sr–Nd–Pb isotope compositions reflect the suprasubduction-zone fluid enrichment of the asthenospheric mantle wedge beneath the Burma–Tengchong terrane as a consequence of eastward underthrusting of the Indian continental lithosphere following the India–Asia collision at 55 Ma. Sr–Nd isotope mixing modelling indicates that amounts of the Indian slab-derived fluid range from 1.0 to 4.6% in the asthenospheric mantle source. Incompatible trace element modelling calculation results show 16% melting of the enriched asthenospheric mantle wedge beneath the Burma–Tengchong terrane. Three parameters (i.e. India-derived fluid, melting degree and residual minerals in the mantle source) constrain the composition of the Tengchong volcanic field mafic magmas. Trace element and Sr–Nd isotopic mixing modelling indicate that the post-collisional mafic magmas in this volcanic field are the product of slab detachment of the eastward subducted Indian continental lithosphere, which may be linked to a significant change in the convergence angle between India and Asia since 8 Ma.