Reliability of LA-ICP-MS U-Pb dating of zircons with high U concentrations: A case study from the U-bearing Douzhashan Granite in South China

Abstract

Granite-hosted uranium ore deposits are the most important commercial U-producers in South China. These U-bearing granites generally contain high-U-concentration zircons. Zircons in the U-bearing Douzhashan Granite show black rims in cathodoluminescence images, which have very high U concentrations of up to >25,000ppm. The high-U black rim domains of zircons show similar REE patterns to those of low-U mantle domains with oscillatory zoning, but have higher REE concentrations and weaker positive Ce anomalies. The high-U black rim domains of zircons may have formed from late-stage evolved U-rich and H2O-rich magma. We carried out a SHRIMP U-Pb dating for these zircons and the results show that the high-U (12,007–26,706ppm) rim domains of the zircons always yield older 206Pb/238U ages (268±9Ma, n=5, MSWD=2.7) than the low-U (558–3667ppm) mantle domains (228±11Ma, n=4, MSWD=3.1), and both of these ages are older than the emplacement age (211±3Ma) of the U-barren Xiangchaoping Granite, which was intruded by the Douzhashan Granite. Hence, these results demonstrate a matrix effect associated with SHRIMP U-Pb analyses of high-U zircons. The reverse discordance for SHRIMP results of high-U zircons was produced by analytical artifact. However, our LA-ICP-MS analyses show that the black rim domains (12,840–26,018ppm U) of the zircons have the same U-Pb ages as the low-U mantle domains (1215–4075ppm U). Both domains yield a weighted mean 206Pb/238U age of 203±4Ma (n=13, MSWD=1.4), which is consistent with the 40Ar-39Ar age (207±4Ma) of muscovite from this granite. It thus indicates that LA-ICP-MS U-Pb analyses for high-U zircons (up to 26,000ppm U) show an insignificant matrix effect and likely yield more reliable U-Pb ages for the high-U zircons than SHRIMP analyses. In order to get reliable U-Pb ages using the LA-ICP-MS method for the high-U zircons, a pulse-analog cross calibration for the ICP-MS collector should be performed. Our precise U-Pb dating and geochemical studies indicate that the U-bearing Douzhashan Granite was emplaced during the late Indosinian stage and formed in a post-collisional setting of the Indosinian Orogeny. It is distinct from most U-bearing Granites in South China, which were largely emplaced during the early Indosinian stage and formed in a syn-collisional setting.