Origin, alteration and geochemical correlation of Late Permian airfall tuffs in coal measures, Sydney Basin, Australia

摘要

The Late Permian Illawarra Coal Measures of the southern Sydney Basin contain tuffs derived from altered pyroclastic airfall material. These units represent ideal stratigraphic time planes because, as the products of airfall volcanic ashes, they are distributed over wide areas in short periods of geological time. Geochemical correlation of the tuffs of the Illawarra Coal Measures with similar units in the stratigraphically equivalent Newcastle Coal Measures offers a unique opportunity to understand the temporal and spatial relations during deposition of coal-bearing units in the Sydney Basin. Thirty five core samples comprising 12 samples from each of the Burragorang and Farmborough Claystone Members and 11 samples from the Huntley Claystone Member were analysed for major and trace elements by X-ray fluorescence and instrumental neutron activation, and the results were subjected to stepwise discriminant analysis. The elements that served as the best discriminators between tuffs were, in order of atomic number, Ti, V, Sn, Hf and Th. Samples for basin-wide correlation were taken from thick interseam tuffs of the Newcastle Coal Measures and treated as unknowns in the discriminant model. The Burragorang Claystone Member shows a strong geochemical correlation with the Awaba Tuff, and the Farmborough Claystone Member correlates with the Warners Bay Tuff. The Huntley Claystone Member, however, has proved difficult to correlate with the Nobby's Tuff, the proposed stratigraphic equivalent in the Newcastle Coal Measures. Petrographic data for tuffs of the Illawarra Coal Measures indicate that they were deposited by airfall mechanisms and consist mainly of kaolinite and mixed-layer illite/smectite clays, quartz and plagioclase crystal fragments, lithic fragments and secondary calcite and siderite. Zr/TiO_2 and Nb/Y ratios, tectonic discrimination diagrams and chondrite normalised REE patterns (La/Yb=3.3 to 11.9) are identical to those for tuffs of the Newcastle Coal Measures and are interpreted as being derived from magma that was calc-alkaline, rhyodacitic to rhyolitic in composition and derived from a continental volcanic-arc tectonic setting. The source of the tuffs is interpreted to be an active volcanic arc to the east of the present coastline (Currarong Orogen), but a lack of evidence from below the continental shelf makes it difficult to make concrete conclusions on its likely extent and composition.