Mid-Devonian basaltic magmatism and associated sedimentation: the Ooloo Hill Formation, central-eastern South Australia.

We report composition and age data from a restricted <370 km², volcano-sedimentary basin beneath Mesozoic cover to the northwest of the Flinders Ranges, South Australia. This volcano-sedimentary basin comprises two components: a sedimentary sequence that includes shales, siltstones and sandstones, and a basalt–sedimentary member. Together, these components are defined as the Ooloo Hill Formation. Whole-rock K–Ar geochronology for five basalt samples yields a preferred mean K–Ar age of 392 ± 8 Ma. Two samples of detrital zircons from the sedimentary rock yield SHRIMP U–Pb maximum depositional ages of ca 527 and 510 Ma. These ages are significant, as they rule out a Neoproterozoic age for the sedimentary basin and demonstrate that this basin is not part of the Adelaide Superbasin. Lower eruptive sequences are calc-alkaline and mafic in composition (SiO₂ = 46.1–47.8 wt%, Na₂O + K₂O = 3.7–6.1 wt%, MgO = 6.85–8.37 wt%, Mg# = 57–65 and ε_{Nd(390 Ma)} = 3.4–4.0) and may represent two small-scale melting events from compositionally distinct mantle reservoirs. The uppermost eruptive sequence is tholeiitic andesite (SiO₂ = 51.6 wt%, Na₂O + K₂O = 8.1 wt%, MgO = 2.2 wt%, Mg# = 27 and ε_{Nd(390 Ma)} = 3.0) and displays geochemical affinity to the first two eruptive sequences, possibly representing a differentiated magma from a similar source region. Incompatible trace-element ratios and isotopic compositions suggest the source was a heterogeneous oceanic island basalt-like parent magma derived from partial melting of the asthenospheric mantle. We envisage decompressional melting was linked to large-scale tectonic processes inboard of the Tasman Orogen, with magma ascent facilitated by crustal faulting. First record of a mid-Devonian intracontinental volcano-sedimentary basin in South Australia. Basalts have an oceanic island basalt-like mantle source that represents at least two small-scale melting events from compositionally distinct mantle reservoirs. A localised mid-Devonian backarc basin or transpressional/transtensional basin linked to intracontinental faulting formed inboard of the Tasman Orogen. [ABSTRACT FROM AUTHOR]