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
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.