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New 40Ar/39Ar incremental heating analyses and kspar diffusion modeling shed new light on the post-metamorphic cooling history of the upper amphibolite to granulite-facies gneissic rocks of the Kigluaik Mountains and on the relative timing of metamorphism and plutonism. The metamorphic rocks yield 40Ar/39Ar hornblende (Tc~500C-550C) ages that range from 86.5 Ma at high structural levels to 84 Ma at deepest levels. White mica (Tc~350C), present only at highest structural levels, yields consistent 83.5 Ma ages. Biotite (Tc~300C) data are mostly disturbed - preferred ages range from 83.5 to 84.5. Detailed kspar analyses and diffusion modeling document cooling from 325C to 150C between 83 and 77 Ma at high structural levels and from 250C to 125C between 82 and 75 Ma at deeper levels. This combined data suggest that the metamorphic complex cooled very rapidly ( > 50C/m.y.) from peak conditions (700C-800C) at ~91 Ma to mica closure and then cooled more slowly (20C to 30C/m.y.) from 83 to 76 Ma by which time most of the complex had cooled to 100C-150C. Apatite fission track data from the complex indicate that these rocks then resided at or slightly above 100C until the Eocene and a renewed period of rapid uplift (Dumitru, et al., Tectonics v.14, p. 549-563).
In contrast, hornblendes from the Kigluaik Pluton and associated dikes consistently yield 82-83 Ma plateau ages regardless of structural level, and are essentially concordant with coexisting biotite. This rapid cooling, together with epizonal textures, suggest the pluton was emplaced ~ 83 Ma.
Amato et al (1994, Tectonics v. 13, p. 515-527) concluded that emplacement of the Kigluaik plutonic complex and peak metamorphism of the enveloping gneisses were synchronous based on concordant U-Pb zircon ages from the pluton and monazite ages from the metasedimentary rocks. We speculate that the 91 Ma zircons from the Kigluaik pluton may be xenocrystic zircon incorporated from the slightly older metamorphic/migmatitic country rocks. Our new data seem to indicate that the Kigluaik Pluton was emplaced 8 Ma after peak metamorphism, when the country rocks had already been uplifted to shallow levels and cooled to about 300C. If this is true, it reopens the question of what was the ultimate source of heat and cause of uplift for the Kigluaik gneiss dome.
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