DATING THE LOST ARC OF ALASKA: CONSTRAINING THE TIMING OF INITIATION OF THE WRANGELL ARC WITH A NEW 40AR/39AR GEOCHRONOLOGY APPROACH ON MODERN RIVER DETRITAL LITHIC GRAINS

The Alaska portion of the Wrangell Arc covers > 15,000 km². Constraining the timing of initiation for such a large arc is complicated by obstacles like glacial cover, bears, and never ending swamp. For context, the previous age of initiation for the Wrangell Arc was based on a single bedrock sample (~26 Ma) making a strong case for a means to capture basin-wide magmatic history.

It is common to constrain a region’s magmatic history by U-Pb dating on modern river sand zircons. Age dispersion related to accuracy issues with high output U-Pb geochronology and mineral fertility in mafic rocks, like the common andesitic volcanic products of the Wrangell Arc, can be an issue. Detrital ⁴⁰Ar/³⁹Ar geochronology of mineral phases is also a common approach to determine a region’s magmatic history, but this can also be problematic with the same fertility issues as U-Pb detrital geochronology.

Ground mass, phenocryst free, ⁴⁰Ar/³⁹Ar dating of mafic lavas has been applied to numerous volcanic belts producing high precision results, but it has not previously been applied to modern river lithic grains due to fears about potential alteration. We designed and implemented a pilot study to test whether ⁴⁰Ar/³⁹Ar geochronology could be applied to detrital volcanic lithic grains from modern rivers draining the Wrangell Arc.

We selected phenocryst free ground mass grains for ⁴⁰Ar/³⁹Ar geochronology from the Chisana and Nabesna Rivers and step-heated single grains from each river to evaluate for alteration. We determined that on some grains the first low-temperature heating step (0.5 watts) demonstrated evidence of alteration with anomalous ages and high atmospheric ⁴⁰Ar contents. If the low temperature heating step was not used when calculating a plateau age, the altered samples produced a more robust age determination, and for the unaltered samples, the age determination was the same. Based on the step-heating we created a protocol where we degassed each grain for 60 seconds at 0.5 watts and pumped off the released gas.

To date, we have performed lithic grain geochronology on 15 Wrangell Arc rivers (N = ~1600 grains) with 8 other river samples in progress. Based on this data set and new regional bedrock samples, it is clear the Wrangell Arc initiated by ~29 Ma with the implication that the Yakutat microplate was undergoing flat slab subduction by this time.