Geochemical and Nd‐isotopic systematics of the Permo‐Triassic Gympie Group, southeast Queensland

Abstract

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 wedge source for these rocks by small‐volume, isotopically primitive K–Th–Si–rich partial melts, probably derived from eclogitic stalled slabs. The Gympie Province appears to have been the site of a continent — island‐arc accretion event in the Early to Middle Triassic. Geochemical and Nd‐isotopic compositions of volcanics from Gympie Group reflect the sequential involvement of asthenospheric and lithospheric sources attendant during the Late Palaeozoic — Early Mesozoic tectonomagmatic evolution of the palaeo‐Pacific Gondwana rim. Lower Permian eruptives of the Gympie Group were formed in a supra‐subduction zone (primitive island arc) setting, proximal to Gondwana rim accretionary elements, compatible with extension above a steepening Early Permian Benioff zone. Intrusive suites in the Gympie Group post‐date accretion of the Gympie Province to the eastern Australian continental margin by the Middle Triassic.