Abstract The Variscan Querigut Pluton (eastern Axial Zone, Pyrenees), recently dated at 307 ± 2 Ma, is a classical example for the structural study of granitoids. We present a new structural analysis of this pluton using the powerful technique of magnetic susceptibility anisotropy (AMS). A model of pluton emplacement is proposed on the basis of complementary microstructural analyses allowing the determination of the temperatures of fabric acquisition in the magmatic units, and of the shear sense criteria in the surrounding country rocks. This pluton is constituted by two main units that have intruded metasedimentary rocks where regional metamorphic conditions decrease from southwest to northeast. A well-foliated southern granodioritic unit, rich in Devonian marble xenoliths, is bounded to the south by Cambro-Ordovician metapelites. A weakly foliated northern monzogranitic unit, bounded to the north by Devonian marbles, comprises two sub-types : an outer biotite-monzogranite and an inner biotite-muscovite leucomonzogranite. Abundant basic stocks of variable sizes and lithologies outcrop in the granodioritic unit and in the southern part of the monzogranitic unit. Mean magnetic susceptibility and magnetic foliation maps show a very good agreement with the previous compiled petrographic and structural maps, strengthening the validity of the AMS technique. The northern monzogranitic units display two unevenly distributed structural patterns : (a) a NE-SW-trending pattern of weakly to steeply dipping foliations, dominant in the outer biotite monzogranite, is associated to subhorizontal NE-SW lineations ; and (b) a NW-SE-trending pattern of steeply dipping foliations, dominant in the inner biotite-muscovite monzogranite, is concentrated in NW-SE elongated corridors, associated to subhorizontal NW-SE lineations. In the southern granodioritic unit, foliation patterns follow roughly both the main regional foliation pattern and the pluton boundary, with foliation dips increasing to the south. Subhorizontal NW-SE trending magnetic lineations in the inner parts of this unit, are progressively verticalized toward the southern pluton boundary. A progressive increase in total magnetic anisotropy is observed toward the border of the pluton, correlated with both an increase in solid-state deformation and a decrease of the final temperature of fabric acquisition. These features result from a pluri-kilometric shear zone localized in the western half of the granodioritic unit, decreasing in thickness in its eastern half and along N060oE trending contacts with the country rocks. In the northern monzogranitic unit, one can roughly correlate the magmatic microstructures to the NE-SW trending fabric, and the superimposed subsolidus microstructures to the NW-SE-trending corridors, where rather low-temperature (< 300 oC) fluid-assisted cataclastic microstructures may also appear. The country-rocks, half kilometer away from the pluton border, display the D2 regional Variscan pattern, with subvertical and N110oE-striking foliations and subhorizontal and E-W-trending stretching lineations associated to a dextral shear. Closer to the pluton, the country-rocks are subjected to the pluton influence, particularly along the southern border where a strong flattening is associated to subvertical lineations related to local thrusting of the pluton onto its country rocks. An emplacement model is proposed through the injection of three principal magma batches (granodiorite, biotite-monzogranite and biotite-muscovite monzogranite) that successively and progressively built up the pluton while the whole region was subjected to a dextral and compressive deformation regime, in agreement with AMS results obtained from several other plutons of the Pyrenees.
We summarise numerical and analogue models of shape fabrics, and discuss their applicability to the shape preferred orientation of crystals in magmas. Analyses of flow direction and finite strain recorded during the emplacement of partially crystallised magmas often employ the analytical and numerical solutions of the Jeffery's model, which describe the movement of noninteracting ellipsoidal particles immersed in a Newtonian fluid. Crystallising magmas, however, are considered as dynamic fluid systems in which particles nucleate and grow. Crystallisation during magma deformation leads to mechanical interactions between crystals whose shape distribution is not necessarily homogeneous and constant during emplacement deformation. Experiments carried out in both monoparticle and multiparticle systems show that shape fabrics begin to develop early in the deformation history and evolve according to the theoretical models for low-strain regimes. At large strains and increasing crystal content, the heterogeneous size distribution of natural crystals and contact interactions tend to generate steady-state fabrics with a lineation closely parallel to the direction of the magmatic flow. This effect has been observed in all threedimensional experiments with particles of similar size and for strain regimes of high vorticity. On the other hand, studies of feldspar megacryst sub-fabrics in porphyritic granites suggest that these record a significant part of the strain history. Thus, the fabric ellipsoid for megacrysts evolves closer to the strain ellipsoid than for smaller markers. This behaviour results from the fact that the matrix forms of the melt and smaller crystals behave like a continuous medium relative to the megacrysts. Consequently, in the absence of these markers, and because the fabric intensities of smaller particles such as biotite are stable and lower than predicted by the theory, finite strain remains indeterminate. In that case, strain quantification and geometry of the flow requires the addition of external constraints based on other structural approaches.
