Origin and HFSE enrichment potential of a Late Cretaceous basalt to peralkaline rhyolite suite in central Queensland, Australia

Peralkaline magmatic systems can host significant enrichment in high-field-strength elements (HFSE) (e.g. HREE, Zr, Nb, Ta), potentially yielding large-tonnage resources. Primary enrichment is interpreted to result from low-degree melting of a metasomatised mantle source followed by extensive fractional crystallisation. Secondary processes may modify resource potential, but primary magmatic enrichment remains critical, highlighting the importance of investigating these systems from source and tectonic setting through to emplacement. To understand controls on HFSE enrichment, we combined field mapping, petrography, geochemistry, isotopic analysis and geochronology to investigate an alkali basalt to peralkaline rhyolite suite in central Queensland consisting of the Alton Downs Basalt and Mount Hedlow Trachyte. The suite comprises a widespread magmatic field of basaltic lava flows, shallow mafic intrusions and felsic cryptodomes emplaced contemporaneously in the Late Cretaceous at ca 77 Ma. Geochemical trends and restricted isotopic compositions suggest derivation from a common mafic parent magma with minimal crustal assimilation. Fractionation progressed through plagioclase- to alkali feldspar-dominated assemblages, producing felsic endmembers evolving towards relative HFSE enrichment and agpaitic indicators (e.g. aenigmatite), or less HFSE enrichment and miaskitic assemblages (e.g. zircon, monazite). Sm–Nd isotopic compositions (ƐNd(77Ma) = 6.07 to 7.34) indicate a depleted mantle source distinct from younger, plume-related elements of the Eastern Australia Volcanic Province. Trace-element enrichment in the most primitive basalts relative to MORB is consistent with a continental rift setting, which may ultimately relate to northward propagation of Gondwana breakup and development of failed rifts along the Queensland margin. Felsic rocks did not reach the extreme degrees of fractionation interpreted for more strongly HFSE-enriched suites in central Queensland (e.g. southern Peak Range Volcanics), and this may be the main limiting factor resulting in modest HFSE content. More fractionated bodies may exist at depth and would be more prospective. The Alton Downs Basalt and Mount Hedlow Trachyte were emplaced contemporaneously as an alkali basalt to peralkaline rhyolite suite at ca 77 Ma. These units form a widespread magmatic field of basaltic flows, shallow mafic intrusions and felsic cryptodomes, rather than a central volcano. Magma differentiation followed two evolutionary paths, influencing accessory phase mineralogy and HFSE content that was ultimately limited by the degree of fractionation achieved. Emplacement occurred in a failed continental rift during northward propagation of Gondwana breakup.