First U/Pb dating of rare-element magmatism from the north French Massif Central: A milestone in rare-element events of the Variscan belt.
Résumé
Rare
-elements
(Ta, Nb, Be, Cs, Li, Sn, etc.)
are critical raw material for the European Union. One
significant primary source of these metals is rare-
element
-enriched LCT pegmatites and high
phosphorus peraluminous granites. This type of magmatism is parti
cularly well expressed in the
Variscan belt,
mainly
in localized parts of the Iberian massif, the Bohemian Massif
and the French
Massif Central
(FMC)
. In the latter, an E
-W rare
-element magmatic belt
(
Cuney
et al
., 2002)
is
defined by three rare
-element pe
raluminous granites (Beauvoir, Montebras) or volcanic equivalent
(Richemont rhyolite) and the LCT pegmatite field of Chédeville. The main aims of this study are to
investigate the timing of this
magmatism and the chronological relationship with deformation
,
metamorphism and peraluminous magmatism in the FMC and to compare, at the scale of the
Variscan belt, these results with our previous studies. U
-Pb dating of columbite-
group minerals
from Beauvoir, Montebras and Chédeville rare-
elements magmatic bodies has been performed
using laser ablation system connected to a single collector magnetic Sector Field
- Inductively
Coupled Plasma -
Mass Spectrometer (LA
-SF
-ICP
-MS). The Beauvoir granite leads to an
emplacement age of 317 ± 6 Ma which is significantly older
than previous Ar/Ar on lepidolite
published one (308 ± 2 Ma, Cheilletz
et al
., 1992). Conversely our U/Pb ages of 314 ± 4 Ma and
309 ± 5 Ma obtained on Montebras granite and Chèdeville pegmatites are identical, within the
analytical uncertainties, from pr
evious published Ar/Ar age (Cuney
et al
., 2002; Cheilletz
et al
.,
1992) and also identical to the 313 ± 3 Ma Ar/Ar age of Richemont rhyolite (Cuney
et al
., 2002). In
the north of the FMC, several granites are slightly early to coeval with this rare element
magmatism; however, the spatial distribution of rare-
element magmatic bodies is by far smallest
than those of coeval granites. Migmatites sources of the
ses granites are unknown in the FMC
since there is a lack of Bashkirian to Moscovian migmatites (ca. 318
-307 Ma, e.g. Faure
et al
.,
2010). However, a more discrete migmatitic event seems to occur since cordierite-
bearing
migmatitic gneisses has been recent
ly dated at 316 ± 2 Ma in the North of the MCF
(Gébelin
et al
.,
2009)
. The Marche fault system seems to be a key
-structure for the rare-
element magmatism since
it appears as an hyphen between all bodies. Moreover the Marche fault system is coeval with the
rare
-element magmatism as evidenced by Ar/Ar ages ranging from 316 ± 5 to 312 ± 2 Ma
(Gébelin
et al
., 2007)
. We compare these results with our previous studies on the Moldanubian domain of
the Bohemian Massif (Melleton
et al
., 2012) and the Iberian Massif
(Melleton
et al
. submitted)
where rare-
element magmatic bodies display interesting relationships with metamorphic events
and crustal
-scale structures. In the light of these new results, rare-
elements magmatic bodies of
the MCF appear as a milestone in the polyphased rare
-metal events of the Variscan belt which
extend at least from 333 to 280 Ma.
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