NEW PALEONTOLOGIC AND GEOCHRONOLOGIC PROTOLITH AGES FOR THE PALEO-CONTINENTAL MARGIN OF ARCTIC ALASKA

The Nome Group schists of Seward Peninsula, schist belt of the southern Brooks Range, and schist of the Ruby geanticline are thought to represent part of the Arctic Alaska continental margin that was subducted during the Jurassic and exhumed during the Brooks Range orogeny. New data from Seward Peninsula improve our ability to compare protoliths of these sequences and ultimately reconstruct pre-Jurassic paleogeography.

Detrital zircon data (SHRIMP & LA-ICP-MS) tied to specific, regionally mapped lithologic packages in the Nome Group show that (1) an extensive unit of quartz-mica schist contains a population of Late Devonian zircons (youngest peak: 382 ± 14 Ma, 5 grains), similar to zircons recovered from a comparable rock type in the schist belt; (2) the youngest population of zircons in a unit of mixed carbonate and quartz-rich lithologies is Silurian (about 430 Ma, 3 grains), and no correlatives are known; and (3) a unit that contains mafic metavolcaniclastic material is possibly Ordovician (youngest zircon: 466 ± 7 Ma, one grain) and not Devonian, the purported age of mafic metavolcanics in the Ambler district of the schist belt. Additional samples are being run to evaluate the reliability of these dates. Conodont ages from Nome Group marbles, most of which have obscure relationships to mapped lithologic packages, are Cambrian to Devonian.

SHRIMP ages from granitic orthogneiss and a metafelsite from Kiwalik Mountain, Seward Peninsula, are 391 ± 3 and 392 ± 5 Ma. These ages correspond closely to ages of Brooks Range and Ruby geanticline orthogneiss bodies, but not to metafelsites in the Ambler district. Several Late Proterozoic orthogneisses are known on Seward Peninsula; an additional body on Basin Creek is 669 ± 5 Ma, slightly younger than SHRIMP and TIMS ages of ~ 680 Ma previously reported. Late Proterozoic orthogneisses in the southern Brooks Range are generally older (>700 Ma).

The most surprising result is a single Late Permian to Triassic (probably Triassic) conodont collected from a marble near Nome. The CAI of the conodont (5) is consistent with the conditions of Nome Group metamorphism and we know of no possible source of sample contamination. Triassic protolith ages have not been reported from the Nome Group, schist belt, or Ruby schist. Aside from this age, the youngest protolith ages from the Nome Group are Devonian.