Fission track, K–Ar, and Rb–Sr mineral dates for the Ecstall and Quottoon plutons were determined to resolve the different cooling histories indicated by Rb–Sr whole-rock and mineral isochrons and published K–Ar dates.Comparison of mineral dates with previously assigned closure temperatures for the various isotopic chronometers has allowed temperature–time plots for the thermal history of individual samples and sample suites to be constructed.The Quottoon pluton, emplaced approximately 51 Ma ago, cooled rapidly during initial uplift that ended ~46 Ma ago with the present-day land surface at a depth of ~4 km. A second episode of uplift at an average rate of 0.05 cm year−1 (measured in Kasiks pluton that lies immediately to the east) began in the late Miocene. Cooling of the Ecstall pluton, following emplacement approximately 80 Ma ago, was disrupted by a thermal event approximately 65 Ma ago. Biotite in the Ecstall pluton appears to have incorporated extraneous argon during this event so that the calculated K–Ar dates lie between the time of emplacement and time of cooling through the closure temperature for Ar in biotite.The cooling curves and observed dates yield estimates for closure temperatures, at an intermediate cooling rate, for Ar in plagioclase and K-feldspar of ~260 °C and ~160 °C, respectively. For fission tracks in epidote the closure temperature estimate is ~240 °C.

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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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New U–Pb, K–Ar, and Rb–Sr dates from the Eagle Plutonic Complex and adjacent map units place timing constraints on intrusive and deformational events along the southwestern margin of the Intermontane Belt. U–Pb zircon minimum dates for Eagle tonalite and gneiss (148 ± 6, 156 ± 4, and 157 ± 4 Ma) document previously unrecognized Middle to Late Jurassic magmatism and syn-intrusive deformation along the eastern margin of the Eagle Plutonic Complex and the southwestern margin of the Intermontane terrane. Widespread mid-Cretaceous (Albian–Cenomanian) resetting of K–Ar and Rb–Sr isotopic systematics in Jurassic and older rocks is coeval and cogenetic with emplacement of plutons of the Fallslake Plutonic Suite (110.5 ± 2 Ma, U–Pb) which crosscut Jurassic plutons and structures but were themselves ductilely deformed along the Pasayten fault during sinistral, east-side-up, reverse displacement. K–Ar and Rb–Sr cooling dates for the Fallslake Suite of ca. 100 Ma, including dates from mylonites along the Pasayten fault, suggest that uplift, cooling, and unroofing of the Eagle Plutonic Complex occurred in mid-Cretaceous time along the Pasayten fault. Regional geologic evidence suggests that this thermal and unroofing event affected much of the southwest margin of the Intermontane Belt. Initial 87Sr/86Sr ratios and U–Pb geochronometry for the Fallslake Plutonic Suite suggest that it was derived, in part, from preexisting and relatively nonradiogenic Paleozoic to Mesozoic crust. K–Ar dating of several stocks demonstrates widespread Early Eocene plutonism in the Coquihalla area, and dating of the Needle Peak pluton indicates plutonism continued into Middle Eocene time.