1 181105.1.pdf Geochronology Record 1266 181105: quartz sandstone, Rainbow Valley (Tumblagooda Sandstone, Southern Carnarvon Basin) Location and sampling Ajana (SG 50-13), Kalbarri (1742) MGA Zone 50, 217735E 6926406N Sampled on 25 September 2003 This sample was collected from a cliff adjacent to a walking trail at Rainbow Valley, about 11.0 km north- northeast of Bluff Point, 6.8 km southwest of Kalbarri town site, and 1.3 km south of Red Bluff. The sample comes from close to 36 m in the section measured there by Evans et al. (2007, Fig. 6). Tectonic unit/relations The unit sampled is a cross-bedded quartz sandstone of the Tumblagooda Sandstone (Hocking, 1991). The formation consists mainly of fine-grained sandstone, minor coarse-grained and pebbly sandstone, and rare siltstone grading into very fine grained sandstone. An Ordovician – earliest Silurian age (c. 485 to 420 Ma) is inferred for the formation based on Early Silurian conodont faunas in the overlying Ajana Formation (Mory et al., 1998). Petrographic description The sample is a medium- to coarse-grained quartz sandstone, consisting of about 80–90% quartz, 10% illite or sericite, 3–5% K-feldspar, 3–5% lithic clasts, and minor zircon, tourmaline, altered biotite, and limonite. Quartz is bimodal in size, with about 30% of the rock occupied by rounded grains from 0.8 to 1.6 mm (coarse to very coarse sand), commonly with narrow optically continuous overgrowths, and more abundant grains from 0.1 to 0.5 mm in diameter (fine to medium sand), possibly with optically continuous overgrowths and defining a mosaic texture. Most of the coarser sand is composed of single-crystal quartz grains with minor microcline and rare polycrystalline quartz as well as rare clasts of very fine grained sandstone containing quartz, microcline, and possible illite–kaolinite aggregates. The fine to medium sand is more quartz-rich. Interstitial porous decussate illite or sericite is common, and there are disseminated voids, including crystal-lined cavities where there are euhedral optically continuous overgrowths on larger grains. Zircon, up to 0.1 mm long, occurs as inclusions in quartz, partly in optically continuous overgrowths, with tourmaline also in optically continuous overgrowths. Altered biotite and patches of limonite-stained colloform clay are disseminated in the rock. Zircon morphology Zircons isolated from this sample are mainly colourless, anhedral to subhedral, and strongly rounded. The crystals are up to 400 μm long, and equant to elongate, with aspect ratios up to 4:1. Some crystals have pitted outer surfaces and, in cathodoluminescence (CL) images, most exhibit concentric zoning truncated at grain edges, features consistent with abrasion during sedimentary transport. A CL image of representative zircons is shown in Figure 1. Analytical details This sample was analysed on 21–22 October 2005 and 24–25 October 2005, using SHRIMP-A. Analyses 1.1 to 21.1 (spot numbers 1–23) were obtained during the first session, together with 12 analyses of the CZ3 standard. Significant drift of standard 238 U/ 206 Pb* dates during the session was addressed by fitting a LOWESS curve (Cleveland, 1979) with a smoothing window of six analyses, implemented using the program Squid 2.50 (Ludwig, 2009; Wingate and Kirkland, 2015). The analyses indicated an external spot-to-spot (reproducibility) uncertainty of 0.50% (1σ), which is included in the errors of 238 U/ 206 Pb* ratios and dates listed in Table 1. Analyses 22.1 to 44.3 (spot numbers 24–60) were obtained during the second session, together with 27 analyses of the CZ3 standard, which indicated an external spot-to-spot (reproducibility) uncertainty of 1.51% (1σ) and a 238 U/ 206 Pb* calibration uncertainty of 0.32% (1σ). Calibration uncertainties are included in the errors of 238 U/ 206 Pb* ratios and dates listed in Table 1. Common-Pb corrections were applied to all analyses using contemporaneous isotopic compositions determined according to the model of Stacey and Kramers (1975). Dates from analyses for which 204-corrected 238 U/ 206 Pb* ratios indicate ages <1000 Ma are based on 207-corrected 238 U/ 206 Pb* ratios; those >1000 Ma are based on 204-corrected 207 Pb*/ 206 Pb* ratios. Results Sixty analyses were obtained from 55 zircons. Results are listed in Table 1, and shown in concordia diagrams (Figs 2 and 3), and a probability density diagram (Fig. 4). Interpretation The analyses are concordant to strongly discordant (Figs 2 and 3). Ten analyses >1000 Ma are >5% discordant, and