Nine subduction‐related magma suites are recognised from drillcore in the Upper Palaeozoic – Mesozoic volcanic–intrusive sequence of the Gympie Group, part of the tectonically anomalous Gympie Province, southeast Queensland. Magmas of the Gympie Group include Lower Permian submarine (later subaerial) island‐arc tholeiites, basaltic tuff‐breccias and lavas comprising the Highbury Volcanics, as well as andesites and dacites in the unconformably overlying Rammutt Formation. Mantle‐derived mafic–intermediate magmas also comprise several intrusive suites in the Gympie Group, some possessing alkaline and shoshonitic affinities. Most eruptives of the Highbury Volcanics and Rammutt Formation show uniformly high Nd‐isotope ratios [εNd(270 Ma) = +7.4 to +8.1] and geochemical features (e.g. high Zr/Nb, Sr/Nd and La/Nb) that reflect depleted asthenospheric mantle sources metasomatised by hydrous fluids from subducted lithosphere. Predominant andesites from the Rammutt Formation have higher Zr/Y, Nb/Y, Ti/V, La/Y, P/Nd and K/P than isotopically similar basalts of the Highbury Volcanics, as well as high Al2O3, Zr and SiO2. The andesites may represent island‐arc tholeiite magmas that assimilated as much as 40% isotopically primitive (young) terrigenous sediment, akin to turbiditic greywacke of Carboniferous accretionary complex origin in contiguous terranes of the New England Orogen. Rare dacites from the Rammutt Formation differentiated from island‐arc tholeiite magmas similar to those of the Highbury Volcanics without incorporation of substantial metasedimentary components. Lower initial Nd‐isotopic ratios characterise Gympie Group intrusive suites. In order of emplacement, these are: (i) the transitional–alkaline multiphase Langton Dolerite sill (εNd = +6.2 to +6.7); (ii) dolerite dykes intimately associated with gold mineralisation (εNd = +4.3 to +5.7); and (iii) microdiorite dykes (εNd = +4.5 to +4.7) that include some near‐primary magmas with features transitional toward those of shoshonites. Low εNd(T) values (εNd = +4.9 to +5.4) are also shown by a unit of ankaramitic basalts from the top of the Highbury Volcanics. In addition to a predominantly depleted asthenospheric mantle source, isotopically enriched source component(s) are required for these low‐εNd magmas. Higher TiO2, Nb, Zr, Ti/V, Nb/Y and Nb/Zr for samples from the Langton Dolerite than for earlier formed lavas of the Gympie Group are also compatible with an enriched mantle contribution. High Zr/Y, La/Y, Ba/Zr, Th/Yb and La/Nb compared to (asthenosphere‐derived) backarc basalts from the contiguous Cambroon beds (εNd = +9 to +10), probably owe their origin to interaction of asthenospheric melts with older (isotopically evolved) subcontinental lithospheric mantle. A further component is required to account for distinctive Th‐, K‐ and Si‐enrichments (e.g. Th/La up to 0.45) of the microdiorite–shoshonite dykes and some primitive ankaramites and dolerite dykes. These enrichments are independent of uniform isotopic compositions and favour hybridisation of the mantle...