Maures-Tanneron Massifs

Also known as “Provence Crystalline”, the Maures (to the SW) and Tanneron (to the NE) Massifs (Fig. 14) were not involved in the Alpine orogeny and preserve a fine example of the South Variscan crust as exposed at the end of the Variscan orogeny. They have been the focus of numerous studies in recent times. Only a brief summary of their geological history is given here to put the geological record preserved in the Argentera and Mont-Blanc-Aiguilles Rouges Massifs in a broader Variscan framework. This summary is based on Toutin-Morin et al. (1994) and on the papers by Crévola & Pupin (1994), Elter et al. (2004), Bellot (2005), Demoux et al. (2008) and Rolland et al. (2009), in which the reader will find references to earlier papers.

Maures Massif

The Maures Massif comprises two main tectono-metamorphic blocks, the Western Block and the Eastern Block, which are in tectonic contact along the Grimaud Fault (Fig. 14). The two blocks together display a complete metamorphic zoning, ranging from low greenschist-facies to upper amphibolites-facies conditions and including in the Western Block, from W to E, a chlorite zone, a garnet-chlorite zone, a biotite-staurolite zone, a biotite-kyanite zone, a biotite-muscovite-sillimanite zone, and in the Eastern Block a biotite-sillimanite zone characterized by partial melting (Fig. 14).

The Western Block

The Western Block consists of three metamorphic units: the Western metamorphic Unit, the middle Bormes Unit, and the eastern La Garde-Freinet Unit (Fig. 14).

The Western metamorphic Unit is mainly composed of phyllite and quartzite. Intercalated beds of black schist preserve the well known Llandovery graptolite fauna of Mont Fenouillet, near Hyères (Gueirard et al., 1970), discovered by Schoeller (1938).

The Bormes Unit consists of staurolite-kyanite-garnet two-mica micaschist and two-mica paragneiss, and of the Bormes granitic orthogneiss. Relicts of an aluminous porphyritic granite (“Barral granite”), which was metamorphosed under LP granulite-facies conditions (P = 0.67 GPa and T >850°C; Gueirard, 1976), are found in the Bormes orthogneiss. Geochronological data summarized by Bellot (2005) indicate a Precambrian age (~ 550-600 Ma) for the emplacement of the Barral granite. U-Pb dating of monazite from the Bormes orthogneiss yielded an age of 345 ± 3 Ma, interpreted as dating the regional intermediate P metamorphism (Moussavou, 1998). 40Ar/39Ar dating of biotite from a sheared orthogneiss yielded ages of 323-328 Ma, that have been interpreted to date the top-to-the NW shear deformation and retrograde low-P metamorphism (Gaubert, 1994, quoted by Bellot, 2005). Metapelites included in the Bormes orthogneiss preserve pre-D1 whiteschist assemblages (1.2-1.6 GPa and 480-550°C) retrogressed to 0.4-0.6 GPa and 600-650°C during shear deformation of the Bormes orthogneiss (Leyreloup et al., 1996, quoted by Bellot, 2005).

Figure 14. Maures and Tanneron Massifs: geotectonic map

Maures and Tanneron Massifs: geotectonic map

Geotectonic map of the Maures and Tanneron Massifs [from Elter et al. (2004) and Rolland et al. (2009), Crévola & Pupin (1994), modified].

The La Garde-Freinet Unit (“Cavalaire Unit” of Bellot, 2005) consists of a wide range of rock types, including acid, mafic, and ultramafic igneous rocks hosted by two mica-garnet-sillimanite micaschist and migmatitic paragneiss. In addition to migmatitic orthogneiss this unit is characterized by an association of amphibolites and “leptynites” (Seyler, 1986) and by the local presence of cordierite in the migmatitic paragneiss. Emplacement of the felsic volcanic protoliths of the “leptynites” is constrained at 548 +15/-7 Ma by U-Pb zircon ages (Innocent et al., 2003). A recent study of the ultramafic rocks (Bellot et al., 2010) shows that they are spinel peridotites and minor garnet–spinel peridotites. In particular, spinel peridotites represent both residual dunites and harzburgites typical of fore-arc upper mantle, and ultramafic cumulates, i.e. dunite adcumulates, harzburgite heteradcumulates and mesocumulates, melagabbro heteradcumulates and amphibole peridotites. The Maures spinel peridotites and melagabbros are interpreted as the lowermost parts of a crustal sequence and minor residual mantle of lithosphere generated in a supra-subduction zone during the Early Paleozoic (Bellot et al., 2010).

The Eastern Block

The Eastern Block (“Cavalières Unit” of Bellot, 2005) is mainly made up of migmatites derived from orthogneiss and biotite-sillimanite paragneiss containing amphibolitized eclogite lenses recording an early HP metamorphism (Fig. 14). Small bodies of biotite- and amphibole-rich orthogneisses are interpreted as primary calc-alkaline granodiorite and diorite, emplaced during the Precambrian (U-Pb zircon ages of 612-630 Ma; Lancelot et al., 1998, quoted by Bellot, 2005). A detailed study of eclogites (Caruba, 1983, Chapt. IX) shows that, even in the best preserved varieties (“amphibolites à grenats auréolés”) such as at Cap du Pinet, garnet is the only pristine eclogite phase that survived the subsequent retrogression. A notable exception is the Cavalières eclogite, in which kyanite also survived with garnet, and is surrounded by spectacular corona textures of sapphirine-anorthite symplectite (Bard & Caruba, 1982). The Cavalières kyanite eclogites are inferred to be derived from calc-alkaline gabbro emplaced in a back-arc setting (Buscail et al., 1999 quoted by Bellot, 2005) during Upper Ordovician (zircon U-Pb 452 ± 8 Ma; Lancelot et al., 1998, quoted by Bellot, 2005) and transformed into eclogite during the Lower Silurian (zircon U-Pb 431 ± 4 Ma; Lancelot et al., 1998, quoted by Bellot, 2005).

Four generations of synkinematic to postkinematic plutons intruded the central and eastern Maures from 336 to 297 Ma. The oldest intrusions are the Hermitan Granite in Central Maures emplaced at 338 ± 6 Ma and the Reverdit Tonalite in Eastern Maures emplaced at 334 ± 3 Ma (Moussavou, 1998). The largest granite body is represented by the Plan de la Tour Granite of Central Maures, 19 km-long and 5 km-wide (in its northern part), elongated N-S (Fig. 14). The granite is porphyritic—with K-feldspar phenocrysts up to a few cm-long—and hosts dark microgranular "enclaves" or mica-rich inclusions (“enclaves surmicacées”) with diffuse rims. Local varieties also contain cordierite crystals a few mm to 1-2 cm of size. The emplacement of the Plan de la Tour Granite is constrained by an U-Pb age of 324 ± 5 Ma (Moussavou, 1998) and by a Rb-Sr age of 313 ± 10 Ma (Maluski, 1971). The youngest granite intrusion is that of Cap Camarat which was emplaced at c. 300 Ma (Morillon et al., 2000).

The exhumation of the Maures Massif

A detailed 40Ar/39Ar geochronological study (22 plateau ages on single grains and bulk samples of amphibole, muscovite and biotite) of metamorphic and magmatic rocks from both sides of the Grimaud Fault (Morillon et al., 2000), shows that migmatites, micaschists, gneisses, pegmatites and granites display muscovite and biotite plateau ages which range from 317.2 ± 1.0 to 322.9 ± 1.7 Ma on the western side of the Grimaud fault, and from 300.2 ± 0.6 to 306.0 ± 2.4 Ma on the eastern side, respectively. In the Western Block, amphiboles yielded plateau ages of 328.1 ± 2.8 and 329.9 ± 2.1 Ma for amphibolites, whereas in the Eastern Block, amphibolite and magmatic bodies contain amphibole that yielded ages ranging from 307.9 ± 1.2 to 317.4 ± 2.4 Ma. These data demonstrate distinct cooling histories in the eastern and western side of the Grimaud Fault. Two distinct periods of fast cooling were recognized, at 320 Ma to the west and at 305–300 Ma to the east of the Grimaud fault, which is identified as a major crustal discontinuity during the late Variscan orogeny.

Post-metamorphic exhumation of the Maures Massif is also constrained by the Late Carboniferous sedimentation in the Plan de la Tour basin. This 16 km-long and 1 km-wide basin is located in the central Maures along the Grimaud fault (Bellot, 2005, and references therein) and is filled with a 400 m thick sequence of conglomerates and arkoses. The base of the sedimentary sequence is dated as Early Stephanian, whereas microgranite dykes emplaced within the basin are of Late Stephanian age (295.4 ± 2.4 Ma, 40Ar/39Ar on biotite: Morillon, 1997, quoted by Bellot, 20 05).

Tanneron Massif

The Tanneron Massif is the 40 x 15 km area of metamorphic rocks and granites sited NE of the Maures Massif, from which it is separated by a belt of Permian sedimentary and volcanic deposits (Fig. 14). The Tanneron Massif is divided by the N-S-trending Joyeuse (to the W) and La Moure (to the E) faults into three domains: the Western, the Central and the Eastern domain.

To the W of the Joyeuse fault (Fig. 14), the Western domain comprises a narrow band of sillimanite-rich migmatitic paragneiss and micaschist forming the basement of the small Late Carboniferous Pennafort basin (G. Crévola in Toutin-Morin et al., 1994; Demoux et al., 2008, and references therein).

To the E of the Joyeuse fault (Fig. 14), the Central domain comprises sillimanite-rich migmatitic gneiss intruded by the granite and tonalite of the Rouet-Prignonet magmatic complex. The Rouet granite, an aluminous porphyritic biotite granite with large crystals of altered cordierite, is comparable to the Plan de la Tour granite of the Maures Massif (Crévola & Pupin, 1994; Onezime et al., 1999). The migmatitic gneisses are sillimanite-rich, occasionally with cordierite, and are intruded by an E-W-trending tourmaline-bearing leucogranite body (Grime leucogranite) associated with a swarm of leucogranite dykes. More to the E, a thick sequence of orthogneisses and paragneisses, showing increasing partial melting from E to W, and a wide band of blastomylonitic orthogneiss of granodiorite composition (Bois de Bagnols orthogneiss) is found (G. Crévola in Toutin-Morin et al., 1994). The Bois de Bagnols orthogneiss is the basement of the Reyran Late Carboniferous basin, which is filled with 1000 m of sandstone, coal, pelite, and conglomerate dated as Upper Westphalian to Lower Stephanian and hosts intercalations of pyroclastic rhyolite (Basso, 1985, quoted by Bellot, 2005).

The Eastern domain, E of the La Moure fault (Fig. 14), is composed of a thick sequence of migmatitic gneiss and of biotite-muscovite-sillimanite-kyanite-garnet micaschist. A migmatitic orthogneiss (Tanneron orthogneiss), intercalated in this series as a N-S-trending bands, locally preserves granitic structure with garnet and cordierite, in spite of widespread transformation into blastomylonitic orthogneiss (Crevola & Pupin, 1994). Amphibolites lenses with eclogite relicts are found in all three domains (Fig. 14). However, even in the best preserved variety (“kelyphitoid eclogite”) omphacite is completely replaced by a plagioclase + diopside symplectite (G. Crévola in Toutin-Morin et al., 1994).

Recent isotope dilution U–Pb monazite dating (Demoux et al., 2008) indicates a pre-Variscan history in the Bois de Bagnols orthogneiss of the Central domain, with monazite ages from ~ 440 to 410 Ma. Monazites from a migmatitic paragneiss of the Central domain record a Late Carboniferous HT event at 317 ± 1 Ma (Demoux et al., 2008) In the eastern part, a migmatization event is recorded by monazites from a synkinematic leucogranite body and the Tanneron mylonitic orthogneiss, which yielded ages of 309 ± 5 and 310 ± 2 Ma, respectively (Demoux et al., 2008) Later post-collisional magmatism is recorded by the intrusion of the post-tectonic Rouet granite at 302 ± 4 Ma (Demoux et al., 2008) and of undeformed leucogranite dykes in the Eastern domain, which mark the final stage of the Variscan evolution at 297 ± 5 Ma (Demoux et al., 2008)

Thirty-two 40Ar/39Ar plateau ages on single grains of muscovite from metamorphic and magmatic rocks sampled along an east-west transect through the Massif range from 302 ± 2 to 321 ± 2 Ma, and reveal a heterogeneous exhumation that lasted about 20 Ma during the Late Carboniferous (Corsini et al., 2010). In the Eastern Domain, closure of the K-Ar isotopic system is at ~ 311-315 Ma (Corsini et al., 2010). whereas in the Central Domain the K-Ar system closed earlier (~ 317-321 Ma, (Corsini et al., 2010). These cooling paths are interpreted as resulting from differential exhumation processes of distinct crustal blocks controlled by the La Moure fault that separates the two domains (Corsini et al., 2010). In the Western Domain, the ages decrease from ~ 318 to 304 Ma approaching the Rouet granite, which provided the youngest age at 303.6 ± 1.2 Ma (Corsini et al., 2010).