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Hannula, K.A., 1993

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Bibliographic Reference

Hannula, K.A., 1993, Relations between deformation, metamorphism, and exhumation in the Nome Group Blueschist-Greenschist Terrane, Seward Peninsula, Alaska: Palo Alto, California, Stanford University, Ph.D. dissertation, 171 p., illust., maps.

Abstract

An oblique crustal section through a series of polydeformed, polymetamorphosed rocks is found on the southwestern Seward Peninsula, Alaska. The rocks experienced an early, high pressure/low temperature metamorphism accompanied by D1 deformation, followed by a greenschist-facies overprint. A second deformational event, D2, is responsible for the prominent gently-dipping foliation, NW-SE trending stretching lineations, and abundant recumbent isoclinal folds throughout the section. Metamorphism during D2 varied from extremely low grade at the shallowest structural levels to upper amphibolite-facies within the Kigluaik gneiss dome. Apatite fission track ages from the shallowest structural levels range from 70-100 Ma, during which time high-grade metamorphism and pluton intrusion occurred at depth. Significant vertical attenuation of the crustal section apparently took place during D2 deformation, resulting in unusually close spacing of both older M1 and syn-D2 M2 isograds. This extensional deformation played an important role in the exhumation of the blueschist-facies rocks, but may have followed an earlier period of erosional unroofing of these rocks. Twenty-one 40Ar/39Ar step-heating and 11 single-grain laser fusion ages from phengites from the Nome Group fall into two groups. Samples from the upper part of the structural section and the Cape Nome orthogneiss yield plateau ages of 116 to 125 Ma, while more intensely overprinted samples yield hump-shaped spectra with maximum ages as old as 334 Ma. Unreasonably old maximum ages from some of the disturbed spectra suggest that they result from the incorporation of excess 40Ar. Excess 40Ar can also explain the fact that the hump-shaped spectra are derived from samples from a greater structural depth and which experienced a higher grade of M2 metamorphism than most samples that yielded plateau ages. Since the maximum temperatures achieved were above the blocking temperature of Ar in phengite, the 116 to 125 Ma plateau ages are a minimum age for blueschist-facies metamorphism on the Seward Peninsula.

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