Evidences for a « hot » regional-scale Cretaceous Na-Ca metasomatic event in the Pyrenees
Résumé
In the Pyrenees, a Na-metasomatic event dated between 117 and 98 Ma
has been recognized and was related to the hydrothermal system associated
with the transtensive displacement of the Iberian plate relative to
Europe.
A set of geochemical and geochronological data has been acquired on
five new occurrences of albitized rocks, sampled both to the north and
to the south of the North Pyrenean fault (NPF). These albitized rocks
are associated with ductile shear zones, faults, or undeformed zones and
were developed at the expense of various protoliths (granite, migmatized
metasediments or migmatites). None of these samples are pure albite,
but their chemical and mineralogical evolutions demonstrate they
have undergone dequartzification and albitization and even a slight carbonatation
(CaO > 4%). The oxygen isotope compositions show that
each of them developed under a peculiar regime of fluid-rock interaction,
during which either the temperature or the fluid-rock ratio may
have been different. The temperature of the Na-Ca metsomatism, based
on the presence of newly formed andesine and sillimanite is estimated
to be above 550°C. U-Pb dating on titanite show that albitization took
place between 110 and 92 Ma, ie a period comparable with the known
ages for the LP-HT North Pyrenean metamorphism (110-85 Ma).
Na and Na-Ca metasomatisms are now recognized in the Northern Pyrenees,
both sharing a common spatial and temporal distribution suggesting
that they are two complementary records of the same long-lasting
hydrothermal event. In further detail, the Na-metasomatism is restricted
to the north of the NPF and is slightly older than the Na-Ca metasomatism
located both to the north and to the south of the NPF. Both the time
constraints and temperature estimates suggest that the Na-Ca metasomatism
is related to the low-P high-T North Pyrenean metamorphism,
whereas the Na-metasomatism might be related to the talc-