DETRITAL ZIRCON GEOCHRONOLOGY AND PROVENANCE OF THE SOUTHEASTERN YUKON-TANANA TERRANE

Two samples of late Paleozoic grit and Late Mississippian quartzite-chert conglomerate were collected from southeastern Yukon-Tanana terrane (YTT), where it forms a composite thrust sheet resting structurally above North American paratochthonous strata and intervening imbricate sheets of the late Paleozoic oceanic Slide Mountain terrane. The samples yielded, respectively, 89 and 74 concordant or nearly concordant (<20% discordant) U-Pb ages on single detrital zircons.

Zircons in the grit range from 1770 to 2854 Ma, with a well-defined Early Proterozoic peak between 1800 and 2100 Ma. Precambrian zircons in the conglomerate also show a dominant peak between 1800 and 2100 Ma and smaller peaks between 2200 and 3200 Ma (Archean - Early Proterozoic), as well as a few 3.5 Ga grains and five grains with ages of 998 Ma, 1219 Ma, 1255 Ma, 1256 Ma and 1417 Ma. The conglomerate also yielded three Devonian grains, with ages of 366+23, 373+12 and 379+23 Ma. Their ages are approximately coeval with some of the oldest felsic to intermediate arc- and rift-related magmatism in the YTT.

The two age spectra from southeastern YTT units compare closely with those from Mississippian and older pericratonic units in the Coast Mountains, confirming observed lithologic correlations. They also strongly resemble detrital zircon populations from craton-derived Paleozoic units of the northern North American autochthon. This robust U-Pb data set lends support to the idea that the allochthonous, pericratonic YTT once formed part of the outer, active margin of the North American continent, prior to Mississippian rifting and marginal ocean basin development.