TY - JOUR
T1 - 40Ar/39Ar thermochronometry of the Imataca Complex, Venezuela
AU - Onstott, Tullis C.
AU - Hall, C. M.
AU - York, Derek
N1 - Funding Information:
This research was supported by NSF grant EAR82-187-28 and NSF PYI program grant EAR84-51696 to T.C.O. and by a grant from NSERCC to D.Y. This paper is not only dedicated to Allan Gibbs whose friendship and assistance will be sorely missed, but also to Peter Kuybida, whose dedication and enthusiasm were an inspiration for us, and whose death in January, 1986, was a tragic loss. We are grateful for the assistance of John Kenyon and Peter Kuybida of the University of Toronto in the collection of the Ar data and the preparation of the mineral separates. We are also extremely indebted to C. Kulick for training in the acquisition of microprobe data. We thank J. Bial-kowski and S. Shaginaw for their diligent typing of the manuscript and B. Koblentz for her deft draftmanship. The final version of this paper benefited substantially from the diligent reviews of J. Hanes and R.D. Dallmeyer, whom we thank.
PY - 1989/3
Y1 - 1989/3
N2 - 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.
AB - 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.
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U2 - 10.1016/0301-9268(89)90014-4
DO - 10.1016/0301-9268(89)90014-4
M3 - Article
AN - SCOPUS:0024532860
SN - 0301-9268
VL - 42
SP - 255
EP - 291
JO - Precambrian Research
JF - Precambrian Research
IS - 3-4
ER -