DATING THE LOST ARC OF ALASKA: CONSTRAINING THE TIMING OF INITIATION OF THE WRANGELL ARC WITH A NEW 40AR/39AR DETRITAL GEOCHRONOLOGY APPROACH ON MODERN RIVER 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 glacial cover. 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 the area’s basin-wide magmatic history.

It is common to constrain a region’s magmatic history by U-Pb dating on modern river sand zircons. Age bias related to dating < ~2 Ma zircons with ICP-MS U-Pb geochronology and mineral fertility in mafic rocks, like the common andesitic volcanic products of the Wrangell Arc, can lead to a failure to capture a watershed’s true magmatic history. 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 fertility issues and additional alteration concerns.

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. We 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 500 micron 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 and U-Pb zircon geochronology on 15 Wrangell Arc rivers (N = ~2000 grains) demonstrating the utility of the combined use of both techniques. Based on this data set and new regional bedrock sample age data, 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.