Zircon U–Pb geochronology

Zircon U–Pb isotopic analyses were performed using the laser ablation-inductively coupled plasma–mass spectrometry (LA-ICPMS), using an Agilent 7500s quadrupole ICPMS and a NewWave UP213 laser ablation system housed at NTU. The operating conditions and data acquisition parameters are summarized in Table 1.The laser ablation was performed with a helium carrier gas that can substantially reduce the deposition of ablated material onto the sample surface and greatly improve transport efficiency, and thus increase the signal intensities, as compared to “conventional” ablation using argon as the carrier gas (Eggins et al., 1998; Günther and Heinrich, 1999; Jackson et al., 2004). During the experiments, about 1 min was spent for measuring gas blank and the results indicate sensitivities of less than 1000 counts per second (cps) for all measured isotopes. Calibration was performed using the GJ-1 zircon standard (provided by the Australian Research Council National Key Centre for Geochemical Evolution and Metallogeny of Continents, at Macquarie University, Sydney), well established for a precise 207Pb/206Pb age and an intercept age using isotope-dilution thermal ionization mass spectrometry (ID-TIMS) at 608.5±0.4 Ma (2σ) and 608.5±1.5 Ma (2σ), respectively (Jackson et al., 2004). The Harvard reference zircon 91500 and Australian Mud Tank Carbonatite zircon (MT) were used as secondary standards for data quality control. All U–Th–Pb isotope ratios were calculated using the GLITTER 4.0 (GEMOC) software, and
common lead was corrected using the common lead correction function proposed by Anderson (2002). The weighted mean U–Pb ages and concordia plots were carried out using Isoplot v. 3.0 (Ludwig, 2003).

Our LA-ICPMS results for the secondary zircon standards 91500 and MT, obtained during April 2006 and October 2007, are shown in the concordia diagrams (Fig. 1). The intercept, 206Pb/238U and 207Pb/206Pb ages of zircon 91500 are 1061.8±2.1 Ma (2σ), 1062.1±1.7 Ma (2σ) and 1066.7±2.1 Ma (2σ), respectively (Fig. 1a–c), which are in perfect accordance with the consensus results reported by Wiendenbeck et al. (1995) using ID-TIMS method that gave 207Pb/206Pb age at 1065.4± 0.6 Ma (2σ) and 206Pb/238U age at 1062.4±0.8 Ma(2σ). The intercept, 206Pb/238U and 207Pb/206Pb ages of zircon MTare 736.8±5.2 Ma (2σ), 734.0±2.4 Ma (2σ) and 739.2±8.1 Ma (2σ), respectively (Fig. 1d–f), also well match the U–Pb concordia intercept age of 732±5 Ma reported by Black and Gulson (1978) using ID-TIMS method.
Additionally, before starting routine U–Pb dating analysis, we established three “in-house” secondary zircons, which are igneous zircons separated from three Gangdese I-type granites, samples ST129A, ST143A and ST147A, whose 206Pb/238U ages have been measured using the SHRIMP method that gave 94.1±2.4 Ma (2σ), 84.8±1.6 Ma (2σ) and 50.6±0.7 Ma (2σ), respectively (Wen et al., 2008a). Our LA-ICPMS results of 206Pb/238U ages of these three inhouse zircon standards are 96.4±0.6 Ma (2σ), 86.2±0.6 Ma (2σ) and 49.9±0.2 Ma (2σ), respectively (Fig. 2), which are all in good consistency with the SHRIMP results.