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.