⁴⁰Ar/³⁹Ar evidence that formation of blueschists in continental crust was synchronous with foreland fold and thrust belt deformation, western Brooks Range, Alaska

Abstract

40Ar/"Ar ages from white mica in rocks of the internal zone of the Brooks Range contractional orogen indicate that the Nanielik antifomal duplex developed at about 120Ma and was remobilized on its southern boundary at c. 108 Ma. Blueschist facies metamorphism accompanied development of the antiform. The timing of the blueschist facies event and creation of the antiform overlap the period of shallow-seated deformation in the foreland fold and thrust belt and sedimentation in the foreland basin of the Brooks Range. Blueschist facies P -T conditions may therefore characterize the thicker parts of orogenic wedges in some orogenic systems; ancient blueschists need not necessarily be interpreted as indicators of active subduction or continent-continent collision.Microprobe analysis using quantitative wavelength-dispersive and electron backscattered electron imaging methods was used to characterize the composition of white micas in the dated samples. None of the samples was compositionally homogeneous; many contained 2-3 populations of white mica, including both potassic and sodic varieties. Samples which had undergone (in sequence) amphibolite, albite-epidote amphibolite and blueschist facies metamorphic events retained muscovites relict of the amphihiite facies event. Samples that had undergone only the blueschist facies event also contained multiple populations of mica, some probably from detrital sources.

Mechanisms and timing of exhumation of collision‐related metamorphic rocks, southern Brooks Range, Alaska: Insights from 40Ar/39Ar thermochronology

“…Other studies have suggested that a minimum age for blueschist‐facies metamorphism is given by oldest step ages in hump‐shaped 40 Ar/ 39 Ar white mica spectra [ Gottschalk and Snee , 1998]; however, Hannula and McWilliams [1995] presented similarly shaped 40 Ar/ 39 Ar spectra from the Seward Peninsula and noted that unreasonably old step ages are likely due to excess argon. Till and Snee [1995] suggested that a 120 Ma white mica 40 Ar/ 39 Ar plateau age effectively dates a younger blueschist metamorphic event that affected basement rocks in an antiformal stack in the western Brooks Range.…”

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

“…Phengite 40Ar/"Ar dates (c. 120 Ma) from the Mt Angayukaqsraq region are interpreted to represent the time of peak high-P /low-T metamorphism in this portion of the Central Belt (Till, 1995).…”

Episodicity of Mesozoic terrane accretion along the Pacific margin of Gondwana: implications for superplume-plate interactions

“…Range (Till et al 1993;Till & Snee 1995) and major thrusting and tectonic transportation of ophiolite terranes was coeval in the northern Brooks Range (Cole et al 1997).…”