Identification of processes driving carbonate diagenesis requires an accurate chronology, particularly along volcanic margins where complex mineral parageneses develop. The Cretaceous “Pre-Salt” carbonates formed in the volcanic proto-South Atlantic and are now extensive hydrocarbon reservoirs offshore Brazil and Angola. Primary and diagenetic textures strongly affect reservoir properties, but timings of deposition (c. Barremian–early Aptian) and diagenesis are poorly constrained. This study provides the first direct age measurements from offshore Pre-Salt carbonates using in situ U–Pb geochronology. Depositional calcites from the Santos Basin (Brazil) do not preserve depositional ages, instead yielding an Albian recrystallization age of 106.9 ± 4.3 Ma. Dolomite rhombs and bridges have Palaeocene ages of 59.2 ± 5.3 Ma, and 58.0 ± 4.6 Ma, respectively. Calcite recrystallization was linked to the onset of oceanic spreading in the South Atlantic. Contrary to existing models, dolomites formed much later, during a time of enhanced tectono-magmatism in southeastern Brazil.