Two-stage deformation of the Mont-Louis-Andorra granite pluton (Variscan Pyrenees) inferred from magnetic susceptibility anisotropy

Abstract

The Mont-Louis-Andorra granodiorite pluton, Eastern Pyrenees (France, Andorra and Spain) was the subject of a thorough study of its internal structures, principally the measurement of the anisotropy of its magnetic susceptibility and the classification of its microstructures. Regularly spaced sampling (281 sites) allowed the precise mapping of the susceptibility magnitudes, consistent with mapped rock types. The magnetic structures fall into two distinct families which occur in separate parts of the pluton and have different microstructures. Family I is observed almost everywhere and has close to horizontal lineations, remarkably constant in NE-SW azimuth. Magmatic to submagmatic microstructures characterize these rocks. In contrast, family I1 is confined within NW-SE-[rending corridors, where moderate to steep NW-plunges of the lineations are recorded, and subsolidus microstructures are observed. Magnetic structures of family I are argued to equate with the kinematic framework of the deforming magma during its emplacement. Family I1 structures represent a late to post-consolidation reworking, and therefore record a second stage of deformation in the pluton. It is concluded that magma emplacement was related to a NE-SW-trending strike-slip and/or thrust fault system, coeval with the SW-verging thrust episode documented in the Pyrenees. The second stage corresponds to dextral shear movements with a thrust component on NW-SE-striking planes.Corridors containing family I1 structures are shear zones responsible for dissection of the massif into three blocks, the eastern block representing the former base, and the western block the top of the pluton. Restoration of the pluton's shape along these corridors demonstrates the importance of the early Variscan NE-SW kinematic direction. Because the family I1 structures that form shear zones affecting the pluton have similar orientation and kinematics as the compressive, steep main-phase of deformation ubiquitous in the Pyrenees, the pluton of Mont-Louis-Andorra was probably emplaced before this main-phase of deformation. NE 04" mean lineation 308" NW 35" mean foliation 60" NW 44" mean foliation 92" NW 53' Fig. 3. Magnetic structures of the Mont-Louis-Andorra pluton in map view and orientation diagrams, distinguishing family I and family 11. (a) Lineations; (b) foliations.at University of St Andrews on March 23, 2015 http://jgs.lyellcollection.org/ Downloaded from B mattriel dtvonien htttropique dans les Pyrtntes aritgeoises. Comptes Rendus de I'AcadLmie Sciences, Paris, S&. 11, 302, 969-974. BORRADAILE, G.J. 1988. Magnetic susceptibility, petrofabrics and strain.

Carboniferous normal faults in the Eastern Pyrenees: evidences and age constrains of syn-orogenic Variscan extension

“…Thus, it can be proposed that granite emplacement took place under the same extension direction that is related to the normal fault movement. This contradicts the syntectonic character of the emplacement of the Andorra-Mont Lluís granite proposed by some authors [21,5] but is in strong agreement with the structural [23,11] and radiometric data [30] that points out that the granodiorite was emplaced well after the formation of the main Variscan contractional structures.…”

“…From ASM and microstructure data Gleizes and Bouchez [21] and Bouchez and Gleizes [5] found two different microstructures affecting the Andorra-Mont Lluís granite. Family I is defined by a near horizontal NE-SW trending magnetic lineation, related to NW-SE oriented magnetic foliation ( [21] Plate I; [5], Figure 3). These authors interpreted this fabric as being due to near horizontal NE-SW shearing during magma emplacement.…”

An early extensional event of the South China Block during the Late Mesozoic recorded by the emplacement of the Late Jurassic syntectonic Hengshan Composite Granitic Massif (Hunan, SE China)

“…Ferre et al, 2014). In a granitic pluton, the magnetic fabric pattern can be used to simulate the tectonic regime during the emplacement of the pluton (Bouchez and Gleizes, 1995;Aurejac et al, 2004;Talbot et al, 2005a and b;De Oliveira et al, 2010;Turrillot et al, 2011;Charles et al, 2012). In the Hengshan massif, a previous study has shown that the AMS fabric pattern of the mylonite is consistent with the mineral fabrics observed in the WHBF (Zhang and Zhu, 1989).…”

The method of anisotropy of magnetic susceptibility: theory and applications. A case study from the Rhodope massif

“…Thus, it is obvious that the suscepti bility in paramagnetic rocks depends on the iron content. As a result, it is possible to map the main phases in a paramagnetic granitic pluton based on the content of ferrous, silicate minerals, as in the case of the MontLouis-Andorre (Bouchez & Gleizes, 1995). Generally K m <50(tySI.…”

“…Balsley & Buddington, 1960;Bouchez et al, 1990;Archanjo et al, 1994;Cruden & Launeau, 1994;Saint-Blanquat & Tikoff, 1997). Moreover, AMS has proved to be useful for the recognition of multiple deformation episodes in granitoids (Bouchez & Gleizes, 1995;Borradaile & Henry, 1997). Finally, it can be used in order to approach some environmental problems.…”