Insights on the Permian tuff beds from the Saint-Affrique Basin (Massif Central, France): an integrated geochemical and geochronological study
Résumé
The Permian marks the transition between the end of the accretion of the supercontinent
Pangea and the beginning of its dislocation. In the Eastern Pangea intertropical domain (i.e. the
present-day Western Europe), the late-Paleozoic (i.e. uppermost Carboniferous–Permian) history remains
poorly constrained due to the lack of precise radiometric data. This is particularly true for Permian
basins from the southern part of the FrenchMassif Central, making it difficult to determine correlations
between basins and therefore robust timings and constraints on the environmental and climate
events described in these basins, and to compare them with the larger-scale settings. This article
focuses on the Saint-Affrique Basin, via an integrated petrological, geochemical and geochronological
study of eight of the volcanic-ash levels interbedded in the sedimentary succession. It highlights
the existence of two different groups of felsic volcaniclastic rocks. The first group, located at the
base of the basin and attributed to the Stephanian continental stage, is related to a late-orogenic volcanic
setting and corresponds to calc-alkaline trachy-andesitic tuffites that could not be dated due
to the lack of volcanic zircon. The second group, located towards the top of the succession, is composed
of calc-alkaline dacitic ash beds and tuffites yielding Kungurian ages, i.e. late early Permian
(Cisuralian, 283.5§0.6 to 273.01§0.14 Ma), and are attributed to a post-orogenic deposition setting.
These ages show that the sedimentary filling of the basin is younger than hitherto expected (i.e. Artinskian,
290.1§0.26 to 283.5§0.6 Ma). The elemental geochemistry, the presence of inherited detrital
zircons and the Hf signatures of the volcanic ones indicate the involvement of an old (Proterozoic and older) basement in the magma genesis; this crustal contribution becomes more prominent towards
the top of the sedimentary succession.
Domaines
GéochimieOrigine | Fichiers éditeurs autorisés sur une archive ouverte |
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