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