Magmatic sources of porphyry deposits in postcollisional settings remain controversial. We have used new and published petrological and geochemical data for the Eocene-Oligocene porphyry Cu +/- Mo +/- Au deposits in the Sanjiang region, SW China, to address this outstanding issue. New data for three deposits (Machangqing, Tongchang, and Beiya) in the Ailaoshan-Red River porphyry Au-Cu-Mo belt (southern part of the Sanjiang region) suggest that ore-forming porphyries were emplaced at similar to 35 Ma, have high (Sr-87/Sr-86)(i) (0.7068-0.7071) and negative epsilon(N)(d)(t) (- 6.9 to - 5.0), low zircon epsilon(Hf)(t) (-5.3 to 4.5), and relatively high delta O-18 (5.9-9.0 parts per thousand). Magmatic amphibole phenocryst compositions indicate that the parental magmas are all relatively oxidized (Delta FMQ = 1.7 +/- 0.6), and H2O-rich (3.8 +/- 0.3 wt% H2O). These results are consistent with those estimated from zircon compositions (Delta FMQ = 1.8 +/- 0.8) and high whole-rock Sr/Y ratios (75 +/- 31), respectively. Based on the new and published data, we suggest that the parental magmas for the Ailaoshan-Red River porphyry Au-Cu-Mo belt were derived from a preserved juvenile arc lower-crust and the underlying metasomatized subcontinental lithospheric mantle (SCLM) attributed to a Neoproterozoic subduction event, whereas the parental magmas for the Yulong porphyry Cu-Mo belt (northern part of the Sanjiang region) originated from the Permian-Triassic juvenile arc lower-crust and metasomatized SCLM. Additionally, parental magmas for these porphyry deposits are all oxidized and H2O-rich, and we attribute such characteristics to inheritance from mixed mantle-crust sources that were modified by previous oceanic slab subduction.