Geochemical evolution of magmatic foliations in

IX Congreso Geológico de España

Geo-Temas, 16 (1) ISSN 1576-5172

Geochemical evolution of magmatic foliations in Blond granitic stock (Northwest Limousin, Massif Central, France) Evolución geoquímica de las foliacións magmáticas en el macizo granítico de Blond (Noroeste de Limousin, Macizo Central, Francia) M. Soufi1 and T. Remmal1 1

Hassan II University - Casablanca, Faculty of Sciences Aïn Chock, BP. 5366, Mâarif, Casablanca, Morocco. [email protected]; [email protected]

Abstract: Study of magmatic to plastic field structures help to constrain history of magmatism and tectonism within the Blond granitic stock and allow us to affirm the presence of three sets having different structural and petrological signatures as well as their succession. Structural patterns and strain paths indicate syntectonic emplacement of the Blond granitic stock during initiation of Oradour dextral fault, as a result of sub-meridian crustal shortening. Four distinct steeply dipping magmatic fabrics are documented at Blond. However, the composition of the dark micas in the granitic stock evolves from lithian siderophyllites to zinnwaldites and is characterized by relatively high F and Li2O contents (2.18-2.76 wt%, and 0.957-1.696 wt% respectively). Micro-structural evidences of recrystallization at the magmatic stage of Blond Li-Fe micas can be used as geochemical memories of the superposed magmatic fabrics and their temporal evolution. Based on indexing mineral orientation method, we show that the Li-Fe micas chemistry is related to structural micro-contexts and emphasizes four superposed directions of regional significance. Key words: Blond granitic stock / Northwest Limousin / French Massif Central / structural and geochemical microcontexts / lithium-iron micas.

Resumen: El estudio de las estructuras magmáticas y plásticas medido en el campo puede reconstruir la historia de magmatismo y tectonica en el macizo granítico de Blond y nos permite confirmar la presencia de tres grupos con diferentes características estructurales y petrológicos, así como su sucesión. El análisis estructural indica un emplazamiento sincinemático del macizo granítico de Blond durante la formación de la falla transcurrente dextrale de Oradour, como resultado del acortamiento sub-meridiano de la corteza. Cuatro fábricas magmáticas fuertemente inclinadas están presentes en Blond. Sin embargo, la composición quimica de las micas oscura en el macizo granítico evoluciona desde Li-siderofilita a zinnwaldita, y se caracteriza por un alto contenido de F (2,18-2,76%) y Li2O (0,9571,696%). Evidencias microestructurales de recristalización en la etapa magmática de las micas ferrolíticas de Blond se puede utilizar como recuerdos geoquímicas de las fábricas magmáticas superpuestas y su evolución en el tiempo. Basado en el método de indexación de la orientación mineral, se muestra que la química de las micas ferrolíticas está conectado a los micro-contextos estructurales y determinar cuatro direcciones superpuestas de importancia regional. Palabras clave: macizo granítico de Blond / Noroeste de Limousin / Macizo central francés / micro-contextos estructurales y geoquímicos / micas ferrolíticas.

by Zak et al. (2007) In fact, geometric configuration of magmatic foliations at Blond defined by preferentially oriented mica and K-feldspar megacrysts indicates imbricated planar flow structures (Blanchard, 1978). Micaceous phases are analyzed on oriented thin sections in close relation to magmatic fabrics of the host rocks, defining the structural and geochemical micro-contexts concept.

INTRODUCTION The Blond granitic stock is located in Northwest Limousin at 30 km northwest of Limoges (France). It consists of lithian siderophyllite and Li-muscovite leucogranite, Li-muscovite leucogranite and late-stage topaz-bearing granite, as well as related topaz-bearing microgranite or ongonite (Soufi, 1988). In contrast to two other granites, the lithian siderophyllite and Limuscovite granite is characterized petrographically by early andalusite crystallization, in addition to the presence of Li-Fe micas, and structurally by individualization of four magmatic fabrics N100E/90, N165E/80W, N15E/78W and N70E/90. This number of magmatic fabrics has been documented in a single granitic pluton since a long time (Gagny et al., 1984, Soufi, 1988). Other example was more recently given

STRUCTURAL PETROLOGY Field magmatic structures of Blond granitic stock were investigated in order to better constrain internal magmatic and regional tectonic processes (Soufi, 1988). The observation of the crystal shape preferred orientation of K-feldspar and micas in the lithian 29


Geo-Temas, 16 (1) ISSN 1576-5172

siderophyllite and Li-muscovite granite show that this facies presents four superposed fabrics of respective orientations: N100E/90, N165E/80W, N15E/78W and N70E/90 (strike/dip). The first fabric N100E that is parallel to the extension of the massif and marked by flattened enclaves corresponding to the gneissic host rocks should be related to the magma emplacement. The N165E, N15E and N60-70E trending fabrics are related to regional tectonic stresses, which are responsible for the initiation of N135 striking dextral Oradour fault (delimiting the Blond stock at its western part) and induce upright ascent of F-rich ongonitic melt.

Blond Li-Fe micas are much aluminous (20.5923.92 wt% Al2O3) and iron (15.28-20.98 wt% FeO) rich. In opposition, they are little magnesian (1.36-3.69 wt% MgO), more weakly manganesiferous (0.36-0.55 wt% MnO), poor in titanium (1.23-2.62 wt% TiO2) and moderately fluoric (2.41 wt% F). While, calculations of lithium contents for these micas, using equation tril [Li2O = (0.289 x SiO2) - 9.658] for the “trioctahedral” Li-Fe micas (Tischendorf et al., 1997), in order to fill up vacant octahedral sites with Li-ions, yield the estimates of 0.957 wt% Li2O for lithian siderophyllites and 1.696 wt% Li2O for zinnwaldites. Li-Fe micas compositions are shared into two slightly overlapping groups (figure 2): i) the first is relatively rich in Fe, Mg, Ti and Mn in the octahedral sites, links to isolated Li-Fe micas corresponding to lithian siderophyllites, according to the nomenclature of Rieder et al. (1998); and ii) the second is more rich in Si and Li, and much aluminous (AlVI), belongs to recrystallized Li-Fe micas and corresponds to zinnwaldites.

Leucogranite representing the zone of multiple fabrics involves some lithiniferous and ferriferous micas with spectacular configurations. Thus, two types of micro-textural relationships are distinguished across this granite: isolated Li-Fe micas (figure 1A), and recrystallized Li-Fe micas (figure 1B). The former micas oriented N165E remain under the shape of coarse-grained relics, and are generally destabilized and recrystallized in phyllitic minerals, frequently striking N70E.

1)

B

E 1) (00

E 65 N1

65 N1

Newly formed Li-Fe mica

1) (00

(00

A

1) (00

0E N7

ILi-Fe mica

0E N7

ILi-Ms

ILi-Ms Q

Relic Li-Fe mica Q

ILi-Fe mica

1mm

2mm

FIGURE 1: A: isolated Li-Fe micas. B : recrystallized Li-Fe mica. Q : quartz, Li-Ms : Li-muscovite.

However, compared to relic micas, newly formed ones correspond to the Si, Li and AlVI-richest, and the Fe, Mg, Ti and Mn-poorest compositions. Likewise, the relic Li-Fe micas are mostly N165E striking and the newly formed Li-Fe micas recorded the last magmatic fabric N70E. Blond Li-Fe micas recrystallization is in contrast with a post-magmatic origin, and the zoning of these micas is the result of the enrichment in AlVI, F and Li of the growing environment. These mineral phases proved to be themselves good structural and geochemical tracers of magmatic fabrics at Blond.

CONCLUSION The trioctahedral Li–Fe micas in the Blond granite belong to the siderophyllite-polylithionite join and constitute some true structural and petrogenetic indicators of multiple strain increments. A few magmatic foliations are preserved in the oldest pluton from Blond. The last magmatic and steeply dipping N70E fabric is geochemically recorded by newly formed Li-Fe mica or zinnwaldites.

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Geo-Temas, 16 (1) ISSN 1576-5172

Al VI


Isolated Li-Fe mica Recrystallized Li-Fe mica

3

Relic mica Newly formed mica

2.8 2.6 2.4 2.2 2 1.8 1.6 1.4 2 3 Fe+Mg+Ti+Mn

4

FIGURE 2: AlVI vs Fe+Mg+Ti+Mn in Li-Fe micas from Blond. Curved arrows indicate core-to-rim zoning trend of decreasing Fe+Mg+Ti+Mn and increasing AlVI in the octahedral sites.

(Massif Central français). Comptes Rendus de l’Academie des Sciences Paris, 298 : 459-462.

ACKNOWLEDGEMENTS We would like to thank the reviewer Drs Ana CrespoBlanc (University of Granada, Granada, Spain) and Francisco J. Fernández (University of Oviedo, Oviedo, Spain) for their detailed comments and constructive suggestions.

Rieder, M., Cavazzini, G., D’yakonov, Y.S., FrankKamenetskii, V.A., Gottardi, G., Guggenheim, S., Koval, P.V., Müller, G., Neiva, A.M.R., Radoslovich, E.W., Robert, J.L., Sassi, F.P., Takeda, H., Weiss, Z. & Wones, D.R. (1998) : Nomenclature of the micas. Canadian Mineralogist, 36 : 905-912.

REFERENCES: Blanchard, J.Ph. (1978) : Dynamique magmatique du granite porphyroïde des Ballons (Vosges méridionales). "Fluidalités planaires imbriquées" et "couloirs magmatiques". Phénomène de percolation. Bulletin de la Sociéte Géologique de France, 20 : 157-162.

Soufi, M. (1988): Etude des magmatismes leucogranitique et ongonitique de Blond (Haut Limousin-Massif Central Français). Relations avec une mise en place syntectonique du massif granitique. Unpubl. Ph.D. thesis, Henri Poincaré University, Vandoeuvre-lès-Nancy (France), 304p.

Gagny, Cl., Courrioux, G., Cuenin, O., Jacquot, T. & Leistel, J.M. (1984) : Contrôle structural de la métallogenèse dans la région d'Echassières par une zone de cisaillement régional orientée à N60E

Tischendorf, G., Gottesmann, B., Forster, H.-J. & Trumbull, R.B. (1997): On Li-bearing micas : estimating Li from electron microprobe analyses 30


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and an improved diagram for graphical representation. Mineralogical Magazine, 61: 809834.

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