A LINK BETWEEN PALEO-PACIFIC SUBDUCTION AND ARC MAGMATISM ALONG THE MARGIN OF NORTH AMERICA IN THE BROOKS RANGE, ARCTIC ALASKA: REVEALED BY THE SEDIMENTS OF THE YUKON KOYUKUK BASIN (AK)

The Brooks Range (BR) orogenic belt resulted from the collision of an island arc with the Arctic continental margin of North America during the Middle Jurassic to Early Cretaceous. This led to the emplacement of oceanic and arc affinity thrust plates at the highest structural levels of the orogen. Subsequent extensional fragmentation of the BR left little to no evidence of the collided volcanic arc. Recent work on mafic-rich sediments in the adjacent Early to Late Cretaceous Yukon Koyukuk successor basin, developed along the south flank of the BR, yields clues to the chemistry and magmatic history of this now-removed arc. Mafic-rich sedimentary units contain gabbroic, chert and crystalline quartz vein clasts in coarse conglomerates. Petrographic observations reveal mafic rock fragments (gabbro and basaltic andesite lithics) and low-grade phyllite and quartzite lithic fragments. Samples contain monocrystalline Qtz + Pl + Hbl + Cpx + Cr-Spl within the matrix. Single-grain U-Pb geochronology of detrital and gabbro zircon separates from these samples are characterized by a distinct population at 160-220 Ma, with other populations at 110, 300, 310-500 Ma and varying proportion of Proterozoic zircons. εHf_t signatures for the 160-220 Ma zircons are characterized by juvenile values (+12 - +16), significant of MORB or juvenile oceanic island arc signatures (OIA).

These field and petrographic observations coupled with geochronology and Lu-Hf signatures suggest the erosion into the basin of a Late Triassic to Middle Jurassic primitive OIA, which earlier collided with the Artic Alaskan margin. The encroachment and collision of this arc led to the closure of the Angayucham oceanic basin (Slide Mountain equivalent). This new data confirm a clear link between the North American margin represented by the BR and its stratigraphy to Mesozoic subduction of the paleo-Pacific plate. Along the western North American Cordillerian margin, the destruction of the paleo-Pacific ocean involved the accretion of volcanic island arc to the craton. Permo-Triassic plate reconstructions of Pangea depict an alignment of Arctic Alaska with the North American Cordillierian margin. The continuation of Mesozoic subduction into northern Alaska suggests a link in the paleo-Pacific subduction and magmatic history between the two areas.