Abstract

Using several isotopic techniques, we have determined the ages of selected metamorphic rocks in the Yukon-Tanana terrane (YTT) of east-central Alaska. U-Pb zircon data from an augen gneiss body in the Big Delta quadrangle indicate that the granitoid protolith of the gneiss was intruded 341 ± 3 m.y. ago (lower intercept age). An upper intercept age of 2,136 ± 31 m.y. indicates an inherited early Proterozoic component in these zircons. This inheritance age is substantiated by a Sm-Nd whole-rock model age of 2.09 ± 0.08 b.y. from the Big Delta augen gneiss body. Detrital zircons from quartzitic wall rocks to this body were also derived from an early Proterozoic (∼2.1 to 2.3 b.y. old) crustal source(s). Zircons from three other augen gneisses occurring in an east-west belt which extends into the southern Yukon Territory, Canada, have similar Mississippian and early Proterozoic intercept ages. A Rb-Sr whole-rock isochron from widely separated bodies of augen gneiss has an age of 333 ± 26 m.y. and an initial 87Sr/86Sr ratio of 0.728 ± 0.002, confirming the Mississippian intrusive age for the protolith. The high initial 87Sr/86Sr ratio further indicates an old crustal component in these rocks. A Rb-Sr mineral isochron (115 ± 4 m.y.), K-Ar data from hornblende and micas (128 to 107 m.y.), and U-Pb data from sphene (134. m.y.) from augen gneiss and related rocks are similar to many K-Ar ages in this region and confirm the occurrence of an early Cretaceous thermal event. U-Pb ages of zircons from three metavolcanic units in the YTT suggest that extrusion of the protoliths of these rocks occurred 360–380 m.y. ago. Scatter in the data is caused by ubiquitous inheritance and multiple lead-loss events.

Significant Devonian-Mississippian igneous activity (380-340 m.y.) took place in the YTT, followed by one, or possibly two, metamorphic episodes (Mississippian? and Cretaceous). As yet, no early Proterozoic source rocks for the Paleozoic magmas have been identified in the YTT, but rocks of similar early Proterozoic ages occur to the east and south in the Northwest Territories and British Columbia, Canada.

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