Petrogenesis of late-Variscan high-K alkali-calcic granitoids and calc-alkalic lamprophyres: The Aber-Ildut/North-Ouessant complex, Armorican massif, France
Résumé
The Aber-Ildut/North-Ouessant Variscan granitoid complex in the Armorican Massif is an
example of high-K alkali-calcic zoned pluton, c. 304 Ma in age. A first magmatic batch
intruded through a northern EW-trending sinistral transcurrent shear zone, before injecting
southwards as a huge horizontal zoned sill, with moderately peraluminous muscovite-free
granitoids in the north and strongly peraluminous muscovite-bearing leucogranites to the
south. The second magmatic stage resulted in the intrusion of a large two-mica leucogranitic
body from a root zone along the same shear zone, prior to the end of crystallization of the first
injection. Finally, ultrapotassic dykes, including calc-alkalic lamprophyres (leucominettes),
intrude the complex. The strongly peraluminous granites are interpreted as pure melting
products of crustal clay-rich pelitic material. All the other petrographic types, including
leucominettes, are thought to result from mixing of crustal melts and mantle-derived mafic
liquids. Highly silicic tourmaline-bearing leucogranites are significantly affected by a tetrad
effect, in relation with REE complexing behavior. Aber-Ildut/North-Ouessant granitoids
probably formed during the Variscan late-orogenic stage associated with exhumation and
lithospheric thinning, at low pressure and by advective heating of a ≤ 50 km-thick crust from
hot metasomatized asthenosphere.
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