4.3 Zircon U-Pb dating
After standard mineral separation, zircon grains were sprinkle-mounted onto double-sided tape on 1” acrylic discs and analyzed at random using depth-profiling LA-ICP-MS U-Pb geochronology [Marsh and Stockli , 2015]. Although this method differs from the more common analysis of cut and polished grains there is no indication that this profiling approach yields results that differ significantly from earlier work, including work done in the Himalaya [Colleps et al. , 2019]. For each sample at least 120 zircons were analyzed to obtain provenance datasets that resolve all components that comprise >5% of the total population [Vermeesch , 2004]. The analyses were completed using a PhotonMachine Analyte G.2 Excimer laser (30 μm laser spot size) with a large-volume Helex sample cell and a Thermo Element2 ICP-MS using procedures described in Hart et al. [2016] at the UTChron facilities at the Jackson School of Geosciences at the University of Texas at Austin. GJ1 was used as the primary reference standard [Jackson et al. , 2004] and a secondary in-house zircon standard (Pak1 with a TIMS206Pb/239U age of 43.0 Ma). The data from the analyses were then reduced using the Iolite data reduction software VizualAge [Paton et al. , 2011; Petrus and Kamber , 2012]. For analyzed detrital zircons, the206Pb/238U age was used for grains younger than 850 Ma and the207Pb/206Pb age was used for grains older than 850 Ma [Gehrels et al. , 2008]. All ages reported use 2σ absolute propagated uncertainties.207Pb/206Pb ages are less than 30% discordant, and 206Pb/238U ages are less than 10% discordant [Gehrels et al. , 2011]. The discordance reported is calculated with the206Pb/238U and207Pb/235U ages if <850 Ma and the 206Pb/238U and207Pb/206Pb ages if >850 Ma. Although some studies have suggested older crossovers between the 206Pb/238U and 207Pb/235U ages and the206Pb/238U and207Pb/206Pb ages (e.g., 1.5 Ga [Spencer et al. , 2016]) blindly picking a crossover at 1.5 Ga leads to culling of discordant206Pb/238U ages, or unacceptable smearing and loss of age mode definition between 800–1500 Ma for many samples. Picking a 1500 Ma crossover cutoff and a 20% discordance filter would result in the loss of 80% of the data between 850 and 1500 Ma making the data bad provenance proxies. The 850 Ma crossover was chosen in accordance with the approach of Spencer et al. [2016] and Marsh et al. [2019]. The data are reported in Table 3.
When accurately dating a geological event, high concordance is a requirement but when assigning grains to broad age populations for provenance work the emphasis is on high numbers of grains rather than on high precision in order to improve the statistical reliability. The appropriate level of discordance filter needs to be determined for each data set in light of the goals of the study and the complexities encountered. If a study yields a mix of Phanerozoic and Archean ages, and the relative proportions of these ages are important, a generous (e.g., 30%) discordance cutoff is appropriate so that most Precambrian ages are retained [Gehrels , 2012].
5 Results