HEAVY MINERAL ANALYSIS OF THE NE YUKON-KOYUKUK BASIN SANDSTONES, ALASKA

The Yukon-Koyukuk basin (YKB) is a complex successor basin located south of the Brooks Range orogen of Alaska. Brookian orogenesis started in latest Jurassic to Early Cretaceous, in response to southward subduction of the passive margin of the Arctic Alaska terrane beneath an island arc which eventually collided. The early YKB is thought to be the forearc basin of this oceanic arc, but it continued to accumulate sediment through at least the mid-Cretaceous. Our samples were collected along >100 km of the middle course of the Koyukuk river from Allakaket to 35 km southwest of Hughes. They include the following units (Patton et al. 2009):

-Kvg consists of a very thick section of tuffaceous greywacke and mudstone with evidence of turbidite deposition. Albian molluscs are reported.

-Kv composed of interbedded volcanogenic greywacke, mudstone, volcanoclastic rocks, intermediate tuffs and andesitic to basaltic flows. K/Ar ages range from 137 to 120 Ma.

We used the Quantitative Evaluation of Minerals by Scanning Electron Microscopy (QEMSCAN) technique at the Rocktype laboratory (Oxford, UK) to define the heavy mineral population of 10 samples. A change in mineral morphology and composition is seen from N to S from relatively more rounded to relatively more angular, and from feldspathic litharenite to arkose. Heavy mineral assemblages, together with lithic fragments, document two distinct sources: 1) a metamorphic source dominated by garnet and epidote in Kvg samples north of Hughes, and 2) a volcanic source dominated by clinopyroxene and oxides in Kv sandstones south of Hughes. High amounts of unstable (clinopyroxene) and moderately stable minerals (epidote and titanite) suggests that all samples are relatively proximal and therefore may reflect their primary source regions. The clear differences between the two groups may reflect the shift in the basin from sedimentation dominated by a southern volcanic-rich source to a phase controlled by material eroded from the emerging Brooks Range. U-Pb analysis of detrital and volcanic zircon and chemical analyses of Cr-bearing minerals are in progress to better constrain the provenance and timing of deposition.