40Ar/39Ar Geochronology of Meso-Cenozoic magmatism and its aqueous alteration, northern Santos Basin, South Atlantic

Reliable ages for magma emplacement are essential for assessing geodynamic models controlling the evolution of the South Atlantic and its prolific hydrocarbon-bearing sedimentary basins. Laser ⁴⁰Ar/³⁹Ar incremental-heating geochronology of single crystals, crystal ensembles, and groundmass fragments from nine distinct magmatic rocks – 18 incremental-heating spectra – from well 1-BRSA-905-RJ, northern Santos Basin, reveals three distinct magmatic events affected by several aqueous alteration episodes that partially reset the K-Ar clocks in the magmatic units. The oldest magmatic event at 71.4 ± 0.6 Ma records the extrusion of E-MORB-type magmas either subaerially onto volcanic edifices emerged from the ocean floor or as subaqueous lava flows onto the ocean floor. A second event, with two roughly coeval pulses at 50.02 ± 0.30 Ma and 50.63 ± 0.40 Ma, is recorded by two lamprophyric bodies, one intruded into the lower Cretaceous pre-salt sequence at 4201 m and another into Albian/Cenomanian sedimentary rocks at 3117 m. A third phonolitic magmatic event intruded into pre-salt units in the 4080-4087 m interval does not yield reliable geochronological results, but its best estimated intrusive age ranges from ∼50 to 43 Ma. All magmatic units have undergone aqueous alteration after 50 Ma, with significant alteration events at 43–40 Ma and as recently as ∼28 Ma that partially reset the magmatic ages. Magmatic events in the northern Santos Basin are coeval with magmatism along the continental margins in southeastern Brazil, suggesting a direct link between magma generation and emplacement on and offshore.