Rb-Sr and K-Ar study of metamorphic rocks of the Seward Peninsula and Southern Brooks Range, Alaska

Abstract

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.

High‐pressure‐low‐temperature metamorphism of granitic orthogneiss in the Brooks Range, northern Alaska

“…K-Ar, 40Ar/3vAr from phengite) are likely to record the timing of recrystallization events that occurred during exhumation and/or heating of the terrane, rather than the time that peak pressure was attained. Blueschist facies rocks from the western Schist Belt yield an age of 120 f 7 Ma by a six-point Rb-Sr phengite isochron (Armstrong et al, 1986). This age is consistent with the range of K-Ar and 40Ar/3qAr white-mica cooling ages of 100-130 Ma (Turner et al, 1979;Blythe et af., 1990;Till & Patrick, 1991;Patrick et af., 194).…”

Characteristics and Tectonic Significance of the Late Proterozoic Aksu Blueschists and Diabasic Dikes, Northwest Xinjiang, China

“…Another possible Precambrian blueschist belt occurs at Mudangjang in northeastern China (Dong et al, 1986) in a presumably Paleozoic orogen; however, there are no strong geologic constraints to sub stantiate a Precambrian age of metamorphism (Liou et al, 1989b). Similarly, the Precambrian age for the Brooks Range blueschists (e.g., Armstrong et al, 1986) recently has been dis credited in preference for a Cretaceous age for blueschist-facies metamorphism (Hanula and McWilliam, 1994).…”

Blueschists and Eclogites of the World and Their Exhumation

“…The metamorphic grade increases northward with a thermal peak at P = 12 kbar at T = 460° C (Patrick and Evans, 1989). K-Ar mineral dates are 141 to 162 Ma (phengite), 122 Ma (paragonite), 164 to 175 Ma (glaucophane), and 194 Ma (actinolite) (Armstrong et al, 1986). The Jurassic age of glaucophane on the Seward Peninsula should be noted, as all K-Ar dates of glaucophane in the Brooks Range are of Precambrian age (Turner et al, 1979).…”