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TitleDeciphering the Paleoproterozoic cooling history of the northeastern Trans-Hudson Orogen, Baffin Island (Canada), using 40Ar/39Ar step-heating and UV laserprobe thermochronology
AuthorSkipton, D R; Schneider, D A; Kellett, D AORCID logo; Joyce, N L
SourceLithos vol. 284-285, 2017 p. 69-90,
Alt SeriesEarth Sciences Sector, Contribution Series 20160226
PublisherElsevier BV
Mediapaper; on-line; digital
File formatpdf
NTS26A; 26B; 26G; 26H; 26I; 26J; 26O; 26P
Lat/Long WENS -68.0000 -64.0000 66.0000 62.0000
Subjectsgeochemistry; geochronology; geophysics; igneous and metamorphic petrology; structural geology; orogenies; orogenesis; crustal evolution; argon argon dating; argon argon dates; metamorphism; metamorphic rocks; radiometric dating; thermal history; biotite; Proterozoic
Illustrationslocation maps; geological sketch maps; graphs; tables; microphotographs
ProgramCanada-Nunavut Geoscience Office, Funding Program
AbstractThe previously unstudied cooling and exhumation history of mid-crustal rocks exposed on southeastern Baffin Island (Canada) provides new insights into the post-orogenic evolution of the Paleoproterozoic Trans-Hudson Orogen (THO). New 40Ar/39Ar step-heat analyses of biotite, muscovite and phlogopite grains and core-to-rim intragrain 40Ar/39Ar analyses of muscovite show a range of apparent ages compatible with slow regional cooling following syn-collisional peak metamorphism. Twenty-nine amphibolite- to granulite-facies rocks were dated using the 40Ar/39Ar step-heating laser (CO2) method. 40Ar/39Ar spot analyses were performed along transects within large muscovite grains from three samples using an ultraviolet (UV) laser to investigate within-grain 40Ar/39Ar age variations. The step-heating apparent ages range from 1788 ± 3 to 1622 ± 2 Ma for biotite, 1720-1630 ± 3 Ma for phlogopite and 1729-1657 ± 3 Ma for muscovite. UV spot 40Ar/39Ar analyses in the three targeted muscovite grains range from 1661-1640 ± 3-4 Ma, 1675-1645 ± 3 Ma and 1680-1652 ± 6 Ma, with core-to-rim apparent age gradients of 20-30 Myr. Previous studies have resolved peak metamorphism in this region of the THO to between ca. 1860-1820 Ma and identified zircon and monazite populations at ca. 1800-1750 Ma that imply protracted late- to post-THO tectonism. Numerical diffusion models for Ar in muscovite were conducted to test different Proterozoic cooling and exhumation scenarios. Comparisons with the 40Ar/39Ar ages attest to cooling rates of ~1-2°C/Myr following peak metamorphism and ~1.5-2.5°C/Myr after ca. 1740 Ma. Anomalously old apparent 40Ar/39Ar ages, in cases equivalent to U¿Pb zircon crystallization ages, likely result from variable incorporation of excess Ar. The results suggest that the mid-crustal rocks on southeastern Baffin Island remained hotter than ~420-450°C for ~150-200 Myr after peak metamorphism, with subsequent slow cooling and denudation rates that are typical of Proterozoic orogens. The apparent absence of orogenic collapse implies that, despite high temperatures and estimated maximum crustal thicknesses comparable to those of large, hot orogens, the THO remained gravitationally stable during its terminal phase.
Summary(Plain Language Summary, not published)
The Trans-Hudson orogeny is a large hot continent-continent collision that occurred during the Paleoproterozoic era. Much of the resource potential of the large swaths of Canada impacted by this event can be understood through the study of this major collision. Towards that end, we investigated the post-collisional evolution of a region of the mountain belt on Hall Peninsula, Baffin Island, to understand how the rocks cooled and exhumed in the aftermath of the main collisional event. To do this, we used a isotopic method that dates the time rocks cooled through the middle crust, about 300-450 degrees Celsius called 40Ar/39Ar dating, paired with forward modeling. We present evidence that in this region rocks in the mountain belt cooled slowly at about 1-2.5 degrees Celsius per million years.

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