EVALUATING THE AGE OF THE OLD CROW PLUTONIC SUITE, YUKON AND NORTHEASTERN ALASKA, WITH SIMS, LA-ICPMS, ID-TIMS AND CA-TIMS ANALYSIS

The Upper Devonian Old Crow suite intrudes the North Slope subterrane of the Arctic Alaska terrane in northern Alaska and Yukon. Establishing the emplacement age of the suite has been a high priority due to its importance on limiting timing of regional deformation and basin development. U-Pb zircon data was generated for plutons in Yukon by isotope dilution thermal ionization mass spectrometry (ID-TIMS) on air-abraded multi-grain fractions (Mortensen and Bell, 1991; Lane et al., 2018), secondary ion MS (SIMS) on grains in epoxy mounts (Ward et al., 2019) and laser ablation inductively coupled plasma MS (LA-ICPMS) using laser line raster (Lane et al., 2018) and pit techniques (Ward et al., 2019), and finally by chemical abrasion ID-TIMS (CA-TIMS) on grains previously analyzed by SIMS and LA-ICPMS (this study). Results of all techniques suggest emplacement ages between c. 360 and 375 Ma, the presence of inherited components and widespread Pb-loss. End-member characteristics are exemplified by the relatively simple systematics of the Dave Lord syenite and complicated systematics of the Old Crow granite. Interpreted ages for the Dave Lord syenite range from 362.6 ± 1.1 Ma (multi-grain ID-TIMS; n=6) to 369 ± 3 Ma (SIMS; n=26). Several analyses suggest minor Pb-loss and minimal inheritance. Most estimates are within uncertainty of a CA-TIMS age of 365.99 ± 0.42 Ma (n=6). The Old Crow granite defines a larger range of dates (280 to 420 Ma), presence of Precambrian and c. 390 Ma inherited components and evidence for Pb-loss. Age estimates range from 364.8 ± 2.7 Ma (n=18/20) and 374.3 ± 2.1 Ma (n=22/52) by LA-ICPMS raster and pit methods, respectively, to 375 ± 2 Ma (SIMS, n=27/34). CA-TIMS results from 7 grains are uniformly distributed between 373.15 ± 0.25 Ma and 371.17 ± 0.25 Ma, preventing age assignment.

Evaluating data from samples with complex zircon systematics using different methods is challenging. One common approach is to assume older and younger outliers to the main distribution of analyses are due to inheritance and Pb-loss, respectively. How far to “trim” Pb-loss and inheritance from the population is problematic in the absence of additional data. CA-TIMS analysis tests these assumptions in samples with simple systematics. For more difficult cases, the number of CA-TIMS analyses needs to be increased to define the presence and true variation in Pb-loss and inheritance components.