The timing of magmatism and subsequent alteration of basaltic rocks cored at the base of IODP Site U1513, Naturaliste Plateau, southwestern Australia

An 82.2 m thick sequence of basaltic rocks was recovered from a deep-sea core on the eastern flank of the Naturaliste Plateau, offshore southwestern Australia during International Ocean Discovery Program (IODP) Expedition 369. The basaltic rocks were cored at the base of IODP Site U1513 and represent the acoustic basement of the Mentelle Basin. The recovered materials consist of subaerial to shallow-water basalt flows interbedded with volcaniclastic beds and cross-cutting dolerite dykes, all of which have been altered to some degree. Existing paleomagnetic data obtained from the overlying sedimentary sequences indicate that the basaltic sequence cored at Site U1513 was deposited before 130.9 Ma. Here we present the results of 40Ar/39Ar step-heating experiments conducted on whole-rock samples of two basalt flows and six dolerite dykes to provide greater certainty of the timing of magmatism. Each sample was characterised using optical and scanning electron microscopy together with x-ray fluorescence (XRF) and x-ray diffraction (XRD) analyses. The dolerite dykes are the only rocks to record the primary cooling age of the magmatic event (135.4 ± 4.0 Ma). This magmatism was contemporaneous with rifting that preceded the break-up of Greater India from Australia/Antarctica and is potentially associated with the Greater Kerguelen large igneous province. The basalt flows record younger ages (117.9–110.4 Ma) that make no sense when considering the available stratigraphic data from the overlying sequences. Instead, these younger ages most likely reflect a phase of hydrothermal alteration and resetting of the argon systematics. This is because the vesicular basalts are more porous than the dolerite dykes, meaning that hydrothermal fluids preferentially flowed through the basalt, not the dolerite. This interpretation was validated by examining structural features within the overlying sedimentary sequences, which showed that hydrothermal veins are only found in the basaltic sequence as well as the overlying Hauterivian– early Aptian¬¬¬¬ volcaniclastic-rich sedimentary sequence. This indicates that hydrothermal alteration occurred after the deposition of the volcaniclastic sequence, most likely at 115.0 ± 1.7 Ma. A tectonic reconstruction that shows the distribution of magmatism associated with the Greater Kerguelen large igneous province indicates that the earliest phases of magmatism occurred within an ellipse with a NW–SE oriented long-axis. The reconstruction also shows that the Naturaliste Plateau was some distance from known sources of magmatism, which suggests that the hydrothermal activity on the Naturaliste Plateau may have been driven by a local source of magmatism that remained active during the Aptian.