Petrogenesis and geodynamic implications of late cenozoic basalts in North Queensland, Australia: trace-element and Sr-Nd-Pb isotope evidence

Abstract

Radiogenic isotopic (Sr-Nd-Pb) and trace-element compositions of late Cenozoic basalts from two discreate geographical regions in North Queensland, Australia, can be used to identify contributions from geochemically distinctive mantle source components. The North Queensland basalts have positive Δ/4Pb and Δ8/4Pb values (relative to the Northern Hemisphere Reference Line), and high 206Pb/204Pb and 87Sr/86Sr at given εNd relative to Tertiary basalts in New South Wales, eastern Australia. The northernmost Cooktown nephelinites in North Queensland are isotopically depleted with low 87Sr/86Sr and 206Pb/204Pb and high 143Nd/144Nd compared with the more southern enriched Atherton-Nulla basalts. Sr-Nd-Pb isotopic data fit with two-component mixing between an isotopically depleted Indian Ocean mid-ocean ridge basalt source component and an enriched mantle component with an EM2 signature. The geochemical characteristics of the isotopically enriched Atherton-Nulla basalts are consistent with contributions from a subcontinental lithospheric mantle modified by subduction-related metasomatism. The isotopically depleted Cooktown nephelinites show HIMU-like incompatible element signatures that can be attributed to contributions from amphibole- and apatite-bearing assemblages in the lithospheric mantle. The low 206Pb/204Pb and high εNd of these depleted basalts are not correlated with their respective high U/Pb and low Sm/Nd parent/daughter element ratios. This decoupling implies that the formation of the metasomatic amphibole and apatite assemblages must be a close precursor of the magmatism, possibly connected with the eastward migration of the Indian Ocean asthenosphere and/or subduction at the northeastern margin of the Australian plate during early Tertiary time.