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

Nine subduction-related magma suites are recognised from drillcore in the U pper Palaeozoic Mesozoic volcanic-intrusive sequence of the Gympie Group, p art of the tectonically anomalous Gympie Province, southeast Queensland. Ma gmas of the Gympie Group include Lower Permian submarine (later subaerial) island-are tholeiites, basaltic tuff-breccias and lavas comprising the High bury Volcanics, as well as andesites and dacites in the unconformably overl ying Rammutt Formation. Mantle-derived mafic-intermediate magmas also compr ise several intrusive suites in the Gympie Group, some possessing alkaline and shoshonitic affinities. Most eruptives of the Highbury Volcanics and Ra mmutt Formation show uniformly high Nd-isotope ratios (epsilon (Nd)(270 Ma) = + 7.4 to +8.1) and geochemical features (e.g. high Zr/Nb, Sr/Nd and La/N b) that reflect depleted asthenospheric mantle sources metasomatised by hyd rous fluids from subducted lithosphere. Predominant andesites from the Ramm utt Formation have higher Zr/Y, Nb/Y, Ti/V, La/Y, P/Nd and K/P than isotopi cally similar basalts of the Highbury Volcanics, as well as high Al2O3. Zr and SlO(2). The andesites may represent island-are tholeiite magmas that as similated as much as 40% isotopically primitive (young) terrigenous sedimen t, akin to turbiditic greywacke of Carboniferous accretionary complex origi n in contiguous terranes of the New England Orogen. Rare dacites from the R ammutt Formation differentiated from island-arc tholeiite magmas similar to those of the Highbury Volcanics without incorporation of substantial metas edimentary components. Lower initial Nd-isotopic ratios characterise Gympie Group intrusive suites. In order of emplacement, these are: (i) the transi tional-alkaline multiphase Langton Dolerite sill (epsilon (Nd) = +6.2 to +6 .7): (ii) dolerite dykes intimately associated with gold mineralisation (ep silon (Nd) = +4.3 to +5.7): and (iii) microdiorite dykes (epsilon (Nd) = +4 .5 to +4.7) that include some near-primary magmas with features transitiona l toward those of shoshonites. Low epsilon (Nd)(T) values (epsilon (Nd) = 4.9 to +5.4) are also shown by a unit of ankaramitic basalts from the top o f the Highbury Volcanics. In addition to a predominantly depleted asthenosp heric mantle source, isotopically enriched source component(s) are required for these low-epsilon (Nd) magmas. Higher TiO2, Nb, Zr, Ti/V, Nb/Y and Nb/ Zr for samples from the Langton Dolerite than for earlier formed lavas of t he Gympie Group are also compatible with an enriched mantle contribution. H igh Zr/Y, La/Y, Ba/Zr, Th/Yb and La/Nb compared to (asthenosphere-derived) backarc basalts from the contiguous Cambroon beds (epsilon (Nd) = +9 to +10 ), probably owe their origin to interaction of asthenospheric melts with ol der (isotopically evolved) subcontinental lithospheric mantle. A further co mponent 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 primit ive ankaramites and dolerite dykes. These enrichments are independent of un iform isotopic compositions and favour hybridisation of the mantle wedge so urce for these rocks by small-volume, isotopically primitive K-Th-Si-rich p artial melts, probably derived from eclogitic stalled slabs. The Gympie Pro vince appears to have been the site of a continent-island-arc accretion eve nt in the Early to Middle Triassic. Geochemical and Nd-isotopic compositions of volcanics from Gympie Group ref lect the sequential involvement of asthenospheric and lithospheric sources attendant during the Late Palaeozoic-Early Mesozoic tectonomagmatic evoluti on of the palaeo-Pacific Gondwana rim. Lower Permian eruptives of the Gympi e Group were formed in a supra-subduction zone (primitive island are) setti ng, proximal to Gondwana rim accretionary elements, compatible with extensi on above a steepening Early Permian Benioff zone. Intrusive suites in the G ympie Group post-date accretion of the Gympie Province to the eastern Austr alian continental margin by the Middle Triassic.