TECTONIC SIGNIFICANCE OF U-PB DETRITAL ZIRCON RESULTS FROM PRE-MISSISSIPPIAN BASEMENT ROCKS OF THE NORTHEAST BROOKS RANGE, ARCTIC ALASKA TERRANE, NORTHERN ALASKA

A record of the Arctic Alaska terrane (AAT) during the Neoproterozoic and Paleozoic is provided by uplift and exposure of the northeastern Brooks Range during the Tertiary. Here and elsewhere the AAT is characterized by a Mississippian to Jurassic platform sequence (Ellesmerian sequence) that rests on a regional angular unconformity. Although little studied, rocks below the unconformity underwent contractional deformation in the Early Devonian. This stratigraphy contrasts with that of adjacent northwestern Laurentia, indicating that the ATT is probably displaced relative to this part of North America.

Detrital zircons from eighteen rock samples in the northeastern Brooks Range were dated by U-Pb LA-ICPMS to constrain the origin of AAT. The results fall into three main groups: (1) six samples of Neoproterozoic rocks in the Romanzof Mountains (mostly the Neruokpuk Quartzite) having zircon populations with major peaks at 1.8-2.0 and 2.5-2.8 Ga and with few or no zircons younger than 900 Ma; (2) three samples of Neoproterozoic strata from the Shublik and northern Franklin Mountains having peaks at 1.0-1.2, 1.3-1.5 Ga in addition to the peaks of group 1; and (3) eight samples of Devonian sandstones from both areas having major peaks at 400-470 Ma and typically peaks at 0.9-1.2 and 1.6-1.7 Ga as well. A granite from the post-tectonic Okpilak Batholith was also dated and yielded an age of 370±2.7 Ma.

The group 1 zircon populations (Neruokpuk Quartzite) display similarities to zircons reported from the northwestern part of Laurentia. Although previously interpreted to be of Siberian origin, group 2 populations can also be interpreted as having Laurentian affinity. Group 3 populations reflect a non-Laurentian provenance. The age of the Okpilak Batholith indicates that deformation was completed by the latest Devonian. The detrital zircon populations of the Neruokpuk samples indicate that the AAT originated near northwest Laurentia during the Neoproterozoic. Zircons of non-Laurentian affinity and Ordovician to Devonian age, coupled with Early Devonian deformation, suggest that the AAT may include a part of the Caledonian Orogen that once extended along the northern margin of Canada. The AAT was subsequently rotated into its present position by counter-clockwise opening of the Canada Basin during the Early Cretaceous.