RECONSTRUCTION OF A METAMORPHOSED NEOPROTEROZOIC TO DEVONIAN CONTINENTAL MARGIN, SEWARD PENINSULA, ALASKA, AND ITS ORIGIN

Seward Peninsula, NW Alaska, is underlain by a 150 x 200 km, penetratively deformed, Mesozoic blueschist-facies metamorphic terrane called the Nome Complex (NC). Protoliths of the NC, part of the Arctic Alaska terrane, were Neoproterozoic to middle Paleozoic sedimentary and igneous rocks. We use lithologic and geochemical characterizations calibrated by U-Pb zircon (Till et al. 2006), detrital zircon (Till et al. 2008) and conodont ages (Till et al. 1986) to construct a depositional history of these rocks for the first time.

Geologic and geochemical evidence indicates that the basement on which the NC protolith was deposited was Proterozoic. Small bodies of Neoproterozoic orthogneiss are folded in with the supracrustal sequence. Their Nd isotopes have crustal residence ages (TDM = 0.8 to 1.6 Ga) that suggest contributions from Middle to Early Proterozoic basement. Orthogneiss whole rock Pb isotopes are consistent with a young, uncratonized (<1.6 Ga?) crustal component in the source region.

The lithologically varied supracrustal sequence includes shallow water metacarbonate rocks of Cambrian through Middle Devonian age, most likely deposited on a passive margin. Carbonate deposition was interrupted during the Ordovician by mafic magmatism related to an incipient rift. The mafic rocks are subalkalic to mildly alkalic products of volcanism developed from mixtures of enriched mantle and crustal sources. Detrital zircon populations deposited in rift basins are dominated by 600-700 Ma zircons.

Deposition of shallow water carbonate rocks largely ceased during the Middle Devonian. Subsequent deposition was dominated by siliciclastic material and material derived from the margin itself (the carbonate platform). A 420-440 Ma detrital zircon population consistently appears in these younger metaclastic rocks.

The protolith history of the NC parallels continental margin history in other parts of the Arctic Alaska terrane; the NC was likely part of the same margin. However, no sources for major detrital zircon populations in the NC, such as the 600-700 Ma and 420-440 Ma populations, are known to occur in western Laurentia. Thus, the Neoproterozoic-early Paleozoic sequence of the NC was likely not part of the western Laurentian margin.