Chris M. Hall

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

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. n nThe 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. n nThe 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.

Constraints on the motions of South American and African Shields during the Proterozoic: I. 40Ar/39Ar and paleomagnetic correlations between Venezuela and Liberia

The Encrucijada pluton of Venezuela and amphibolites near Harper, Liberia, are located on opposite sides of the Liberian Pan-African mobile belt, when South America is restored in a fit described in the literature with respect to Africa. Both units yield stable, bipolar, high-temperature magnetizations that, on the basis of thermal demagnetization data, 40Ar/39Ar hornblende and biotite radiometric results, and Rb-Sr whole-rock and biotite radiometric results, appear to be indistinguishable in age at 1.9 to 2.0 Ga. With South America in this reconstruction, the corresponding paleomagnetic poles suggest that ∼1,000 km of right-lateral strike-slip motion has occurred between the West African and Guayana Shields, probably along the Liberian Pan-African Belt. Furthermore, the Encrucijada and Harper poles are distinct from the 1.9- to 2.0-Ga paleomagnetic poles from the Kalahari Shield and tentatively suggest that relative motion has occurred between the Kalahari Shield and the West African and Guayana Shields since that time.

Direct dating of Plio-Quaternary pumices by 40Ar/39Ar step-heating and single-grain laser fusion methods: the example of the Monts-Dore massif (Massif Central, France)

Abstract K-feldspars separated from Plio-Quaternary pumice flows of the Monts-Dore massif (Massif Central, France) give 40 Ar/ 39 Ar saddle-shaped age spectra. Laser-probe analysis of hand-picked single grains gives ages in agreement with those of overlying and underlying lava flows previously dated by the conventional K-Ar method. The laser-probe ages are lower than the minimum ages measured on the populations of grains by the step-heating method. As previously suggested by Lo Bello and co-workers for the pumice flow of Neschers belonging to the same volcanic massif, the saddle-shaped age spectra are ascribed to the different Ar-release patterns of two populations of K-feldspars: (1) young sanidine phenocrysts of the pumices; and (2) old K-feldspars plucked from the Hercynian basement during the explosive eruption. Measurements of both the granitic rocks from surrounding areas and the granitic xenoliths included in pumices give ages around 330 Ma and show that most of the xenocrysts included in the pumices did not lose significant amounts of argon during the eruption. With the 40 Ar/ 39 Ar step-heating method, we were able to detect contamination of Quaternary K-feldspars by Hercynian K-feldspars as low as 0.25ℵ. Because pumice flows of the Monts-Dore massif crop out over large areas, these new ages will be useful for establishing a precise stratigraphy of the European Plio-Quaternary.

Eruption ages and magma supply rates during the miocene evolution of Gran Canaria

the estimation by some workers [5], the age of Copernicus is 0 . 8 1.5 x 109a. The K-Ar age of some Apol lo 12 samples gave an age of about 0.8 x 109a, which was interpreted as the age of Copernicus format ion [7]. However, since this crater is located in a mare region and as there is little or no mare component in Y-82192, it can be excluded as a possible source. Other youngray craters, such as Kepler or Aristarchus, also exist in the maria. On the other hand, the age of the Tycho crater is (700 + 300 -60 J x 106a [5] and this crater exists in the highland. Therefore it is probable that Y-82192 derived from near Tycho. However, according to another estimate [6], the age of formation of Tycho is around 100 x 106a. It should, however, be noted that these estimates have considerable uncertainties, so that it is, therefore, difficult to identify the origin of this meteori te uniquely by comparison of our radiometric age with those estimated f rom cratering rate. For example, one cannot rule out the possibility that this meteorite might have been derived from the far side of the moon. But this possibility is low, because the craters on the far side appear old. As far as the currently available data are concerned, it may be conceivable that the glassy sample studied by us came from the Tycho crater. Further studies of lunar glassy samples would throw clearer lights on the ages of crater format ion and on the sources of glassy materials. Moreover, a further problem to be studied is whether the samples examined here were ejected into space at the time of crater formation [8].

Laser probe 40Ar/39Ar dating studies on sub-milligram whole-rock basalt samples: the age of the Steens Mountain geomagnetic polarity transition (revisited)

The Steens Mountain geomagnetic polarity transition is arguably the most comprehensively studied geomagnetic field reversal trapped in igneous rocks. Over twenty years ago, this reversal was dated at 15.5 ± 0.3 Ma by the KAr method. We have now tested the feasibility of carrying out 40Ar/39Ar dating on < 1 mg samples of fine-grained whole-rock basalt utilizing a laser probe, i.e. on samples ∼ 1000 times smaller than have been traditionally analyzed. Twenty samples taken from lava flows below, within and above the reversal (N/R) in Steens Mountain, Oregon, have been studied by this laser incremental heating technique. Eighteen of these specimens yield acceptable plateau ages with a weighted mean age of 6.2 ± 0.1 Ma (standard error on the mean). This result is in excellent agreement with the results obtained on two bulk samples of whole-rock basalt that average 16.1 ± 0.1 Ma. An attempt to confirm these ages by fusion of plagioclase laths taken from a few of the individual lava flows, was hampered by their low K content (0.2–0.3%); our best estimate for the age of six plagioclase laths each < 1 mg weight, is 16.5 ± 0.5 Ma. We suggest the techniques utilized herein could be extended to carry out dating of basaltic material of age < 1 Ma.