NEW ZIRCON U-PB AGES AND HF ISOTOPES FROM BROOKS RANGE OPHIOLITE AND KOYUKUK ARC, THE UPPER PLATE OF THE BROOKIAN OROGEN, NORTHERN ALASKA

Siniktanneyak Mountain (SM) is a klippe of the Brooks Range ophiolite, the structurally highest sheet in the N-directed Brooks Range thrust belt, in the central Brooks Range. SM layering dips steeply to the north-northwest and has a stratigraphic thickness of up to 12 km. From southeast to northwest, SM consists 2 km of ultramafic rocks, 6 km of layered cumulate gabbro, and 4 km of isotropic gabbro, diabase dikes and basaltic lavas. Intermediate to felsic intrusions cross-cut the isotropic gabbro and diabase dikes, but also cut the basal ultramafic rocks. Basaltic lavas and underlying diabase dikes have island arc tholeiite chemistry with low chondrite-normalized La/Yb (~2) and moderate normalized Nb-Ta depletion relative to La. Cross-cutting intermediate to felsic intrusions also have arc-type trace element chemistry but distinctly higher La/Yb (~5) and are not co-genetic with the basaltic lavas and diabase dikes. Scattered high-Mg boninite dikes also cut isotropic gabbro and cumulate gabbro, and rocks of this composition are characteristic of early magmatism in modern island arcs of the western Pacific Ocean. Three new SHRIMP-RG U-Pb zircon ages of intermediate and felsic intrusions at SM, as well as two from Misheguk and Asik Mountains 200 km to the west, range from 161.1 +/- 2.0 to 163.7 +/- 2.6 Ma, all within error of each other. Zircon eHf(t) are juvenile and range from +16 (Late Jurassic depleted mantle) to +7; the most felsic dikes have the lowest eHf(t). By comparison Late Jurassic tonalite (WSU LA-ICPMS zircon U-Pb age: 146.9 +/- 1.2 Ma) from the Koyukuk arc terrane south of the Brooks Range yielded similarly juvenile eHf(t) (+11 to +13) and a similar trace element signature to the dated Brooks Range ophiolite samples. The arc tholeiite lavas and sheeted dikes, along with boninite dikes, suggest the Brooks Range ophiolite was generated in an extensional setting during arc initiation (forearc of the Koyukuk arc system?). The dated intermediate and felsic intrusive phases in the Brooks Range ophiolite are younger than the main mafic intrusive and volcanic section, but how much younger is uncertain.