63 research outputs found
Recherche sur les volcanoclastites des Séries Rouges Initiales en presqu'île de Crozon : Premier âge radiométrique de l'Arenig
National audienc
The verdine facies off French Guiana
This chapter reviews green grains from the shelf of French Guiana as a regional example of sedimentologic process occurring on the whole stable continental margin from the Amazon to the Orinoco River. Green grains have been observed and analyzed off the Orinoco delta and on the continental shelf of Surinam. These green grains were identified as “chamosite” and “glauconite.” The muddy coast of French Guiana is generally very flat and occupied by wet swamps and mangrove as a result of the equatorial climate. Most green grains on the continental shelf represent the verdine facies. Green grains are ubiquitous on the shelf and top of the slope off French Guiana. Two sedimentological facies exist: glaucony deeper than 150 m and verdine at shallower depths. The verdine facies has mainly developed from mineral debris and especially chloritized biotite. Carbonate bioclasts and faecal pellets are also utilized. The mica flakes were never wholly replaced by authigenic clay and the phenomenon leads to mixed grains where authigenic and substrate remains are recognizable. Carbonate substrates lead to mainly clay pure green grains becasue the initial carbonate has been dissolved. The formation of verdine can be located in a general marine environment at a comparatively warm sea-water temperature and at a depth probably shallower than 60 m
Le faciès verdine : néoformation d'une phyllite en milieu subrécifal
Cet article présente l'étude minéralogique et chimique qui a permis de caractériser une nouvelle argile phylliteuse. Compte tenu des connaissances actuelles, les auteurs définissent les conditions et l'environnement général de formation de cette phyllit
1st radiometric dating of a paleontologically dated Bathonian level from Georgia (USSR) - Use of the cathodoluminescence for selection of suitable plagioclases
Jurassic volcanic formations interlayered with (ammonite-bearing)
sediments are common in the Caucasus area; this situation is of interest
for the numerical calibration of the poorly documented Jurassic portion
of the time scale. However, following petrographic study on thin
sections no whole-rocks can be considered reliable geochronometers due
to subsequent alteration; from about 20 samples, two were selected for
plagioclase dating; one (V134) is probably early Kimmeridgian in age;
the other (V136) is probably located in the Lower Bathonian stage
according to diagnostic ammonites.
Cathodoluminescence (CTL) study has shown that sample V136 was similar
to usual volcanic feldspars (blue to green colour); however, the lack of
CTL of the V134 plagioclase is a character common to diagenetic
feldspars; consequently, in spite of a good optical preservation, this
geo-chronometer cannot give an age representative of the time of
emplacement of the lava flow. We have combined CTL observation with
microprobe analysis in order to document the poorly known CTL behaviour
of volcanic feldspars; cations Ti4+ and Fe2+ play a major role in the
CTL colour of plagioclases and are able to document the growing history
of these feldspars ; phenocrysts are initially rich in Fe2+ (core of the
crystals, green in colour), then richer in Ti toward the exterior;
microcrysts are even richer in Ti (blue to bright blue). We have also
observed that natural CTL colour was modified resulting from acid
``cleaning'' of the separated feldspars : the initial blue or green
colour tends to change to yellow or violet, respectively, after acid
treatment probably due to oxydation of Fe2+ toward Fe3+.
X-ray and microprobe analyses both indicated that plagioclases from
sample V134 was near the sodic end member (albite) suggesting a
diagenetic origin in this andesitic basalt; In contrast, sample V136
contains a calcic plagioclase of common composition for a doleritic
basalt.
The K-Ar conventional technique was applied as a preliminary tool for
radiometric analysis. The Kimmeridgian Na-plagioclase sample gave a
``rejuvenated'' (85 Ma) apparent age which confirms a late genesis for
the separated plagioclase phase; this interpretation is based on CTL
observation, X-ray analysis, and microprobe analysis ; these techniques
are able to distinguish samples which have been submitted to diagenetic
alteration from those which have not. An age consistent with the
stratigraphic location has been obtained from sample V136. This age of
161 +/- 3 (2-sigma) Ma, is the first one available from a sample
palaeontologically located with reasonable precision within the mid
Jurassic time
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