40Ar/39Ar thermochronometry of the Imataca Complex, Venezuela

Tullis C. Onstott, C. M. Hall, Derek York

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The Imataca Complex of Venezuela, an Archean metamorphic belt located on the northern margin of the Amazonian Craton, underwent upper amphibolite to granulite facies metamorphism during the 2.0 Ga Trans-Amazonian Orogeny. 40Ar/39Ar plateau dates on hornblende separates from the Imataca Complex vary from 1972 to 1760 Ma and record post Trans-Amazonian Orogeny cooling. Biotite and feldspar pairs, which were separated from the same samples or from samples collected at the same exposure and should record identical thermal histories, yield distinct results. The biotite 40Ar/39Ar spectra are primarily plateau profiles with dates ranging from 2044 to 1254 Ma, whereas perthitic potassium feldspar yield stair-step 40Ar/39Ar spectra and integrated dates ranging from 1395 to 1166 Ma. The internally discordant perthite spectra appear to reflect cooling at rates of 0.1-0.2°C Ma-1, consistent with the cooling rates estimated from the blocking temperature and age differences between biotite-feldspar pairs from the same sample. Two plagioclase samples yield saddle-shaped 40Ar/39Ar spectra with 1235 and 1221 Ma minimum dates. These internally discordant spectra are caused by degassing of non-atmospheric, trapped Ar from the plagioclase and of radiogenic Ar sited in K-rich inclusion phases. The 2.0-1.7 Ga biotite dates are confined to the margins of the Imataca Complex and record the waning stages of Trans-Amazonian Orogeny uplift and cooling. The 1.4-1.2 Ga biotite dates, which are confined to the center of the Imataca Complex, are correlated with their 200-350°C Ar blocking temperatures as estimated from their average grain size and Fe/(Fe + Mg) composition. The 1.3-1.1 Ga feldspar ages are correlated with their 200-300°C Ar blocking temperatures as determined from analysis of the 40Ar/39Ar step-heating data. The extremely slow cooling rates implied by the biotite-feldspar pairs suggest that between 1.7 and 1.1 Ga, the Imataca Complex cooled isobarically at intermediate crustal levels. All Ar mineral systems closed by 1.1 Ga, perhaps reflecting a renewed phase of uplift associated with the mild tectonism of the Nickorie Episode.

Original languageEnglish (US)
Pages (from-to)255-291
Number of pages37
JournalPrecambrian Research
Issue number3-4
StatePublished - Mar 1989

All Science Journal Classification (ASJC) codes

  • Geology
  • Geochemistry and Petrology


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