The Nome Complex is a large metamorphic unit that sits along the southern boundary of the Arctic Alaska–Chukotka terrane, the largest of several microcontinental fragments of uncertain origin located between the Siberian and Laurentian cratons. The Arctic Alaska–Chukotka terrane moved into its present position during the Mesozoic; its Mesozoic and older movements are central to reconstruction of Arctic tectonic history. Accurate representation of the Arctic Alaska–Chukotka terrane in reconstructions of Late Proterozoic and early Paleozoic paleogeography is hampered by the paucity of information available. Most of the Late Proterozoic to Paleozoic rocks in the Alaska–Chukotka terrane were penetratively deformed and recrystallized during the Mesozoic deformational events; primary features and relationships have been obliterated, and age control is sparse. We use a variety of geochemical, geochronologic, paleontologic, and geologic tools to read through penetrative deformation and reconstruct the protolith sequence of part of the Arctic Alaska–Chukotka terrane, the Nome Complex. We confirm that the protoliths of the Nome Complex were part of the same Late Proterozoic to Devonian continental margin as weakly deformed rocks in the southern and central part of the terrane, the Brooks Range. We show that the protoliths of the Nome Complex represent a carbonate platform (and related rocks) that underwent incipient rifting, probably during the Ordovician, and that the carbonate platform was overrun by an influx of siliciclastic detritus during the Devonian. During early phases of the transition to siliciclastic deposition, restricted basins formed that were the site of sedimentary exhalative base-metal sulfide deposition. Finally, we propose that most of the basement on which the largely Paleozoic sedimentary protolith was deposited was subducted during the Mesozoic.

The chemical character of mafic rocks from the Arctic Alaska–Chukotka terrane records rifting of continental crust during the early Paleozoic, possibly during the Ordovician. The mafic rocks are part of a metamorphosed Neoproterozoic to Devonian continental margin sequence preserved in a Mesozoic metamorphic terrane, the Nome Complex, of Seward Peninsula, Alaska. Protoliths of the mafic rocks include basalt and mafic clastic rocks, which were interlayered with calcareous, pelitic, and feldspathic sediments, and gabbro and diabase, likely feeder dikes and sills to the basalt. Major-element, trace-element, and rare-earth element (REE) analyses of these mafic rocks, together with analyses of Nd, Pb, and Sr isotopes, form two compositional groups. The two groups differ in Nb/Y (one plots as basalt, the other as alkali to subalkali basalt), TiO 2 , P 2 O 5 , and Nb (and other elements). The high-Ti group is characterized by enrichment of light REE; the low-Ti group lacks such enrichment. The trace-element and isotopic characteristics of the two groups resemble typical non-arc magmas derived from the mantle: the low-Ti group has compositions between normal mid-ocean ridge basalt (N-MORB) and enriched mid-ocean ridge basalt (E-MORB), while those of the high-Ti group are between E-MORB and ocean-island basalt (OIB). The two groups have overlapping positive values of ε Nd (+0.34 to +7.40). TiO 2 /Yb ratios suggest the high-Ti group formed from melts generated under normal thickness of continental crust, while the low-Ti group formed from melts generated at shallower conditions, presumably after rift-related crustal thinning had progressed. Geologic, paleontologic, and geochronologic characteristics of the Nome Complex support an origin along the NE margin of Baltica. The rift-related magmatism in the Nome Complex likely occurred during the opening of the Uralian ocean along that margin; by implication, related parts of the Arctic Alaska–Chukotka terrane may have experienced a similar origin.

Interpretation of the Central Metamorphic terrane metabasites as the lower-plate of a Devonian intraoceanic subduction zone is a basic tenet of paleotectonic models for the eastern Klamath province. Specifically, such models hold that the (1) east-dipping Central Metamorphic terrane represents a subducted piece of oceanic lithosphere accreted to the overlying Trinity subterrane; (2) Trinity and Redding subterranes represent mantle and magmatic-arc crust of the overriding lithospheric plate; and (3) east-dipping Trinity thrust, separating the Central Metamorphic terrane and Trinity subterrane, represents the paleo-plate boundary. The subduction-zone model predicts that mid-ocean ridge basalt (MORB)-type oceanic basalts formed the protolith of Central Metamorphic terrane metabasites. We present new major (X-ray fluorescence spectrometry) and trace (inductively coupled plasma-mass spectrometry) element data for twenty Central Metamorphic terrane metabasites; these geochemical data demonstrate that (1) seafloor hydrothermal alteration had little or no effect on protolith compositions; (2) original igneous fractionation trends are preserved by immobile, incompatible trace elements; and (3) immobile trace elements preserve a distinct normal-MORB (N-MORB)-type composition. Fluid-mobile elements show varying degrees of enrichment relative to MORB, suggesting post-magmatic metasomatism. Based on these results, we conclude that the Central Metamorphic terrane represents oceanic lithosphere accreted to the upper-plate of an intraoceanic subduction-zone. Two new Early Permian 40 Ar/ 39 Ar ages of 274 ± 2 Ma are reported for Central Metamorphic terrane amphibolite near the Trinity fault are considerably younger than the previously reported Devonian age. These ages reflect Early Permian uplift and cooling of the Central Metamorphic terrane during a proposed episode of supra-subduction extension that reactivated the Devonian subduction-margin (Trinity thrust).