Pliocene and Pleistocene deep-sea trench sediments cored near the Chile Triple Junction (Ocean Drilling Program Leg 141) were analysed for major and trace element concentrations, and for Sr, Nd and Pb isotopic ratios. Comparisons with the potential source rocks of these sediments suggest little alteration during sediment transport and diagenesis. The sediment compositions reflect exposed area fractions and different erosion rates of upper-crustal rock units of the Southern Andes, indicating that denudation, transport and deposition formed an almost closed system since 2.5 Ma. Pelagic sediments cored farther from the continent (>1000 km) on the Antarctic Plate also contain a significant terrigenous component, mixed variably with hydrogenous precipitates (high Fe–Mn–Th), biogenic barite (high Ba) and opal, but little biogenic carbonate (low CaO and Sr). Our sediment data allow us to estimate the subducted sediment input to southern Andean magmas. Mantle sources of basalts from the Andean southernmost Southern Volcanic Zone (41–47°S) were contaminated by 3–5 vol.% of a terrigenous sediment melt with variable amounts of Ba-rich pelagic sediments, but were not contaminated by slab-derived fluids. Adakites of the Andean Austral Volcanic Zone (49–55°S), formed by melting of a relatively hot subducted slab, contain a variable amount of subducted terrigenous sediment (0–20 vol.% sediment melt) and in some cases Ba-rich pelagic sediments.