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Blueschists of the Nome Group in the Seward Peninsula formed in Jurassic time (prior to -160 Ma ago) in rocks of early Paleozoic to latest Precambrian age (approximately 360 to 720 Ma old). The Sr whole-rock isotopic signature on a plot of^Sr/^Sr vs 87 Rb/ 86 Sr ratio-a fan shaped array of orthogneiss points lying between 720 and 360 Ma isochrons and paragneiss points showing a similar scatter and spread toward lower ages-is much like that of the Yukon Crystalline Complex and Cariboo-Omineca Belt in Canada; partial lithologic and historical similarity support the hypothesis of a common origin and tectonic setting marginal to Paleozoic North America. The areas were overridden during latest Triassic to Jurassic time by oceanic and exotic allochthons, and portions are studded with middle to Late Cretaceous plutons. At the same time all have experienced widespread resetting of K-Ar dates and regional uplift. The southern Brooks Range shares many characteristics with the Seward Peninsula-late Precambrian to mostly(?) Paleozoic protoliths, including extensive pelitic and metavolcanic schists, Jurassic (prior to -120 Ma) blueschist development, and comparable tectonic setting. A late Precambrian metamorphic mineral isochron date for muscovite schist (686 ±116 Ma) in the Baird Mountains Quadrangle confirms previous K-Ar dating of the same rock by Turner and others (1979). This may be a tectonic fragment of an older blueschist terrane enclosed in a younger blueschist complex, but this area needs further study.185 on June 29, 2015 memoirs.gsapubs.org Downloaded from 186 Armstrong and Others Figure 1. Generalized geologic map of the Seward Peninsula showing sample locations: (a) 80-113-1, (b) 80-117-1, (c) 80-118-1, (d) 80-142-3, (e) SP81-333F and 80-19-4, (f) 80-15-5, (g) 80-151-1, (h) 80-155-1 (i) SL81-94-1, (j) SL81-5-2, (k) SL81-41-4, (1) 81-119, (m) 80-91-2, (n) 80-93-1 (o) 67-ASn-595 (collected by C. L. Sainsbury). Light stipple: Nome Group and other low-grade metamorphic rocks; dark stipple: Cenozoic sedimentary and volcanic rocks; diagonal striping: Paleozoic and Precambrian(?) sedimentary rocks; wavy lines: gneiss complexes; granite symbol-hatchures: Cretaceous intrusive rocks.

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Blueschist‐facies rocks on the Seward Peninsula constitute a structurally coherent terrane measuring at least 100 × 150 km. Radiometric age data indicate that high‐pressure metamorphism probably occurred in Jurassic rather than in Palaeozoic or Precambrian time, as previously suggested. Protolith sediments (Nome Group) are of intracontinental basin or continental margin type, and of lower Palaeozoic and possibly late Precambrian age, thus predating the high pressure metamorphism by more than 200 m.y. Blueschist‐facies mineral assemblages were developed in almost all lithologies of the Nome Group, and are best preserved in FeTi‐rich metabasites (glaucophane + almandine + epidote) and pelites (glaucophane + chloritoid + phengite). A lawsonite–crossite subfacies was developed in possible Nome Group rocks on the east flank of the Darby Mountains. Albite–epidote–amphibolite facies assemblages characterize Nome Group rocks in the southwestern part of the Peninsula. Metamorphism in the central zone of the terrane passed from early lawsonitic to subsequent epidote–almandine–glaucophane schist subfacies with the local development (east of the Nome River) of eclogitic assemblages. The high pressure metamorphic minerals were synkinematic with the development of mesoscopic‐scale intrafolial isoclinal folds and a flattening foliation of consistent orientation. Initiation of uplift probably corresponded to the growth of barroisite rims on earlier sodic and actinolitic amphiboles, and partial post‐kinematic greenschist facies replacements record later stages of decompression. Ophiolites and melange are not associated with the Seward Peninsula blueschists. The high‐pressure metamorphism was caused by tectonic loading of a continental plate by an allochthon of indeterminate origin. The PT conditions of high pressure metamorphism were approximately 9–11 kbar, 400–450°C, thus falling between the PT paths of the Shuksan and Franciscan terranes.