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Scientific Value of Coring the Proposed Southern Appalachian Research Drill Hole (1987) / Chapter Skim
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GEOCHRONOLOGY
Pages 28-31

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From page 28...
... . REGIONAL GEOCHRONOLOGY The tectonothermal evolution of the southern Appalachians is complex, with effects of at least three major events locally recorded: Lower-Middle Ordovician (broadly termed "Taconic")
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From page 29...
... The drill hole would provide a complete profile through many of the southern Appalachian tectonostratigraphic terranes. Fresh continuous sections across tectonic contacts, usually deeply weathered in typically poor surface exposures, would be a major focus of geochronologic and petrologic research.
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From page 30...
... With drill-core samples and the sophisticated mineral separation and analytical techniques that are now available, it is possible to separate and analyze the rare minerals that are essential to our obtaining the complete range of age information available in the mineral assemblage of a rock. Mineral separates for Rb-Sr, K-Ar, and 40Ar/39Ar analyses and fission track dating may be prepared from the material processed for zircon and other accessory mineral concentration.
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From page 31...
... The observed track density represents approximately the time elapsed since the temperature dropped below a value at which 50 percent of the tracks were retained. Experimental studies suggest the following 50 percent track retention temperatures at cooling rates of between 10 and 100°C/Ma: epidote = 550 ± 25°C, zircon = 425 ± 25°C, sphene = 375 ± 25°C, phlogopite = 175 ± 25°C, and apatite = 125 ± 25°C.
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Key Terms

  • drill hole
  • fission track
  • daughter products
  • mineral ages
  • radiogenic daughter

  • closure temperature
  • broadly termed
  • closure temperatures
  • age information
  • tectonic contacts

  • mineral concentration
  • dropped below
  • mineral closure
  • continuous sections
  • geologic unit

  • geochronological investigations
  • multiple fractions
  • crystalline terranes
  • deep drill
  • appalachian tectonostratigraphic

  • similar multifaceted
  • perature drops
  • thermal annealing
  • events locally
  • tectonothermal evolution

  • geologic units
  • accessory mineral
  • geologic context
  • major geologic
  • observed track


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