Petrogenesis and geodynamic implications of late Cenozoic basalts in NorthQueensland, Australia: Trace-element and Sr-Nd-Pb isotope evidence

Radiogenic isotopic (Sr-Nd-Pb) and trace-element compositions of late Cenoz oic basalts from two discrete geographical regions in North Queensland, Aus tralia, can be used to identify contributions from geochemically distinctiv e mantle source components. The North Queensland basalts have positive Delt a7/4Pb and Delta8/4Pb values (relative to the Northern Hemisphere Reference Line), and high (206)Pg/Pb-204 and Sr-87/Sr-86 at given epsilon (Nd) relat ive to Tertiary basalts in new South Wales, eastern Australia. The northern most Cooktown nephelinites in North Queensland are isotopically depleted wi th low Sr-87/Sr-86 and Pb-206/Pb-204 and high Nd-143/Nd-144 compared with t he more southern enriched Atherton-Nulla basalts. Sr-Nd-Pb isotopic data fi t with two-component mixing between an isotopically depleted Indian Ocean m id-ocean ridge basalt source component and an enriched mantle component wit h an EM2 signature. The geochemical characteristics of the isotopically enr iched Atherton-Nulla basalts are consistent with contributions from a subco ntinental 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 (PB)-P -206/Pb-204 and high epsilon (Nd) of these depleted basalts are not correla ted with their respective high U/Pb and low Sm/Nd parent/daughter element r atios. This decoupling implies that the formation of the metasomatic amphib ole and apatite assemblages must be a close precursor of the magnetism, pos sibly connected with the eastward migration of the Indian Ocean asthenosphe re and/or subduction at the northeastern margin of the Australian plate dur ing early Tertiary time.