NEW U-P SHRIMP AGES FROM DEVONIAN FELSIC VOLCANIC AND PROTEROZOIC PLUTONIC ROCKS OF THE SOUTHERN BROOKS RANGE, AK

The Ambler schist belt, southern Brooks Range, contains mid-Paleozoic volcanic-hosted massive sulfide (VMS) occurrences that are similar to VMS deposits in other pericratonic and arc terranes along the Cordilleran margin. Felsic volcanic sequences in the Ambler belt are associated with Proterozoic-lower Paleozoic metaclastic, mafic metavolcanic, and carbonate rocks and Proterozoic and mid-Paleozoic metaplutonic rocks of the Coldfoot and Hammond subterranes of the Arctic Alaska terrane. All of these units were deformed and metamorphosed at greenschist to blueschist facies conditions such that primary relationships between lithologic units are rarely preserved. Metarhyolite samples from the Arctic, Tom-Tom, and Sun/Picnic Creek occurrences in the Ambler VMS district yield 207Pb-corrected, weighted mean 206Pb/238U ages of 378 ± 2 Ma (MSWD = 1.0), 381 ± 2 Ma (MSWD = 0.7), and 386 ± 2 Ma (MSWD = 1.8), respectively (determined by SHRIMP-RG analysis at the USGS-Stanford analytical facility). The Arctic sample contained a bimodal population of igneous grains. The younger population, ca 378 Ma (n=6), is consistent with ages determined for other units at the deposit. The older population yielded an age of 405 ± 3 (MSWD = 1.6; n=9). A second Sun/Picnic Creek sample yielded a bimodal population as well with ages of 387 ± 2 (MSWD = 0.9; n=4) and 379 ± 2 (MSWD = 0.3; n=6). The range in ages across the district and older magmatic zircon components in the Arctic and Sun/Picnic Creek samples suggest that magmatism was relatively long-lived (20-30 my) and generally older than that in the Yukon Tanana terrane.

Orthogneiss bodies in the Coldfoot and Hammond subterranes are largely Devonian in age but Proterozoic ages have been obtained. Zircon from granitic orthogneiss north of Ernie Lake yielded a weighted mean 206Pb/238U age of 971 ± 5 (MSWD = 0.75), which is interpreted as the emplacement age. Some of the carbonate rocks in the Ernie Lake area are Proterozoic in age, based on intrusive relationships. The age of the Ernie Lake orthogneiss is similar to that determined for felsic volcanic units in the Nixon Fork subterrane of the Farewell terrane (e.g., 979 ± 2 Ma, TIMS), supporting comparison and correlation between pericratonic fragments in central and northern Alaska.