Geochemical, Sm–Nd isotopic characteristics and petrogenesis of Paleoproterozoic mafic rocks from the Georgetown Inlier, north Queensland: Implications for relationship with the Broken Hill and Mount Isa Eastern Succession

Abstract

Fe-rich tholeiites intruded the lower Etheridge Group of the Georgetown Inlier, north Queensland during the late-Paleoproterozoic (ca. 1685–1640 Ma). Emplacement of the tholeiites occurred during a prolonged period of rift-related sedimentation associated with extension. The tholeiites comprise two lithologically distinct units: the extrusive Dead Horse Metabasalt (DHM), a conformable 1000 m-thick series of fine- to coarse-grained basalts and pillow lavas, and the intrusive Cobbold Metadolerite (CMD), a multitude of sills and lesser dykes that range up to 500 m thick. The DHM and CMD are comagmatic, and have compositions typical of relatively evolved, low-K, Fe-rich continental (rift) tholeiites. Immobile major and trace element patterns suggest that the tholeiites were derived from a depleted upper mantle source (initial ɛ Nd values range between +2.6 and +5.3). The parental melts evolved along a trend of increasing Fe-enrichment (Fe 2O 3* 6.81–21.11 wt%) via closed-system fractionation, with little or no replenishment. Several samples show distinct REE patterns, consistent with limited crustal contamination. Trace element and isotopic data show that the crustal contaminants were derived from the host metasediments of the lower Etheridge Group. The age and geochemical characteristics of the Etheridge Group Fe-rich tholeiites show striking similarities to similar mafic sills in the southeastern Curnamona Province around Broken Hill in western New South Wales, and to basaltic lavas and dolerites of the Soldiers Gap Group in the eastern edge of the Mt Isa Block (some 400 km west of the Georgetown Inlier), suggesting a spatial link between these terranes during the Paleoproterozoic. They may have formed along a more than 2000 km segment of a ca. 1685–1640 Ma volcanic passive margin, tectonically and magmatically akin to the modern East Greenland margin.