Lithology: subdivision of units

The Maures massif consists of Pre-Permian low- to high-grade metamorphic rocks of uncertain age intruded by granites and overlie by coal basins. A new subdivision is proposed here based on (1) lithology, (2) radiochronometric or stratigraphic age, and (3) large-scale geometric relationships between units and ductile shear zones. We have paid particular attention to the presence of high-pressure (HP) mineral assemblages and mafic-ultramafic rocks which may be markers of major thrusts. For a long time, three mafic-ultramafic units of the Maures massif were considered to be equivalent and therefore used as a marker-horizon (Bordet, 1957, 1969; Bordet and Gueirard, 1967; Caruba and Turco, 1976; Bard and Caruba, 1981; Seyler and Crévola, 1982; Seyler, 1983; Seyler, 1986; Crévola and Pupin, 1994; Morillon, 1997; Briand et al., 2002). The resulting interpretation has emphasized orogen-parallel, regional-scale isoclinal folds, following the hypothesis of Demay (1926a, 1926b). Unfortunately, this interpretation is in opposition to (1) geochemical analyses of the mafic rocks that emphasize their differences in nature and origin (Ricci and Sabatini, 1978; Avice, 1995), (2) the asymmetrical structure of the Maures-Tanneron massif inferred by geological mapping (Gueirard, 1960; Buscail, 2000) and structural analyses (Bronner, 1986; Vauchez, 1987; Morillon, 1997). Several units are distinguished here and are presented from west to east (Figures 2 and 3). They mainly correspond to the zones defined by Gueirard (1957) and detailed by Orsini (1968), Bordet (1969), Seyler (1975), Bordet (1976), Le Marrec (1976), Maquil (1976), Crévola (1977), Conti (1978), Seyler and Boucarut (1979), Olives Banos (1979), Seyler and Crévola (1982), Golberg (1983), Caruba (1983), Seyler (1986), Vauchez (1987) and Buscail (2000).

Figure 2. Simplified geological map of the Maures massif

Simplified geological map of the Maures massif

Simplified geological map of the Maures massif (after Gueirard, 1960; Orsini, 1969; Crévola, 1977; Seyler, 1986; Bronner, 1998; Buscail, 2000) with trace of cross-sections of Figure 3.

Figure 3. Simplified cross-sections

Simplified cross-sections

Simplified cross-sections through northern, central, and southern parts of the Maures massif. See Figure 2 for location.

The Cap Sicié Unit

The Cap Sicié Unit corresponds to very low-grade metamorphic rocks outcropping along the coast southwest of the Maures massif. This peculiar unit has northeastward thrusted the Permian-Trias during Pyrenean-Provencal events (Mattauer and Proust, 1963). By its anchizonal metamorphism of probable Paleozoic age, this unit may view as the uppermost unit of the Maures massif. From bottom to top, the Cap Sicié Unit is made of quartzite (intruded by dolerite sills) and conglomerates, then volcano-sedimentary deposits made of green schist and black schist that include black phtanite and radiolarian, then quartzite and black schist (Gouvernet, 1963).

The Fenouillet Unit

The Fenouillet Unit outcrops in the core of the Porquerolles synform (Bronner et al., 1971; Bellot, 2004) and in the Presqu'île de Giens (Gueirard, 1960; Bordet, 1976). It consists of dominant schist (metapelites) and quartzite (microconglomerate and sandstone) with frequent normal or reverse graded-beddings (Olives Banos, 1979). Metamorphism is anchizonal and no HP rock was found.

The Maurettes Unit

The Maurette Unit, 10 to 12 km wide, consists to the East, of dominant limonite-bearing schist (Fe-rich metapelites) with minor quartzite, calc-schist, crinoids-bearing limestone, arkoses (Tempier, 1978), and levels of metadolerite (Gueirard, 1960). The westernmost Maurette Unit consists of massive quartzite associated with graptolites, graphite-bearing schist, crinoids-bearing limestone and sandstone, indicating a Middle Silurian (Upper Llandoverian to Lower Tarannonian) age for its unit (Schoeller, 1938; Gueirard et al., 1970). Stable biotite and chloritoid indicate epizonal metamorphism (Buscail, 2000) and no HP rock was found (Olives Banos, 1979).

The Loli Unit

The Loli Unit consists of a monotonous flysch-like formation made of quartzite and schist, fining and thinning eastward (Gueirard, 1960). However, its base, occurring in the Bagaud island and in the westernmost Port-Cros island, shows an uncommon formation called Bagaud-Malalongue (Bronner, 1986). In this formation, short, low-dipping normal limb of P2 folds preserved sedimentary features as cross-beddings, drop stones, mud balls, slumps and graded-beddings, indicating that the whole unit is overturned due to P2 folding. Tempestite and turbidite occurrences suggest a distal platform palaeoenvironment. Biotite-rich quartzites include very abundant carbonated nodules with marine acritarchs and rare phosphorous nodules with radiolarians (Bronner and Bellot, 2000). Biotite and garnet indicate mesozonal metamorphism (Buscail, 2000) and no HP rock was found.

The Collobrières Unit

The Collobrières Unit consists of mica-schist and quartzite that includes a variety of Fe-rich rocks. At their bottom, several meters of an iron ore (the so-called Collobriérite) consisting of abundant magnetite, almandine garnet, fayalite, and Fe-amphibole, is regarded as a metamorphosed sedimentary iron ore (de Groulard, 1982). Upward, mica-schist becomes Fe-rich, hyper-aluminous, and concentrated abundant graphite, white micas, garnet and staurolite. They include levels of quartzites, marbles, gneiss with silicate calcite, amphibolites (alkaline basalt), and blue quartz-bearing orthogneiss (alkaline trachyte), and are regarded as volcano-sedimentary products emplaced during early stages of continental rifting (Seyler, 1986). Zircon U-Pb dating on alkaline meta-rhyolite and alkaline trachyte yielded 561-495 Ma and 498 12 Ma, respectively (Lancelot et al., unpub), indicating a Cambrian to Late Cambrian age for rifting assumed to be formed during plume activity (Briand et al., 2002). Garnet, staurolite, and biotite indicate mesozonal metamorphism (Buscail, 2000) and no HP rock was found.

The Bormes Unit

The Bormes Unit consists of an orthogneiss that includes mica-schist and relics of a aluminous porphyritic granite (Barral granite) metamorphosed under LP granulites facies conditions (6.7 kbar/>850°C; Gueirard, 1976). Zircon Pb-Pb (~605 Ma; Chessex et al., 1967), whole rock Rb-Sr (~560 Ma; Maluski, 1971), and biotite 40Ar-39Ar (575 ± 8 Ma; Maluski and Gueirard, 1978) dating converge into a Precambrian (550-600 Ma) age for the emplacement of the Barral granite, interpreted as an undeformed equivalent of the Bormes orthogneiss (Gueirard, 1982). Monazite U-Pb on the orthogneiss yielded 345 ± 3 Ma, interpreted as the age of regional IP metamorphism (Moussavou, 1998). Biotite 40Ar-39Ar dating on the western boundary of sheared orthogneiss yielded 323-328 Ma, interpreted as the age of top-to-the NW shear deformation and retrograde LP metamorphism (Gaubert, 1994). Metapelites included in the Bormes orthogneiss preserve pre-D1 white-schist assemblages (12-16 kbar/480-550°C) that retrograded to 4-6 kbar/600-650°C during shear deformation of the Bormes orthogneiss (Leyreloup et al., 1996).

The Cap Nègre Unit

The Cap Nègre Unit consists of paragneiss, aluminous kyanite-garnet-staurolite micaschist, orthogneiss, and quartzite (Gueirard, 1960).

The Cavalaire Unit

The Cavalaire Unit consists of a tectonic melange composed of a wide range of rock types, including acid, mafic, and ultramafic igneous rocks hosted by migmatitic paragneiss and migmatitic orthogneiss. Paragneiss (meta-siltstone) includes micaschist levels and quartzite with calcic silicates. Orthogneiss are metamorphosed and deformed aluminous cordierite-garnet bearing granite, gabbros to diorite and syenite with monzonitic composition, cordierite-bearing aplite, and pegmatite (Seyler, 1975). They are evidence for continental magmatism due to crustal extension (Seyler, 1986).

A layered formation of amphibolite (meta-tholeiites with oceanic affinities; Seyler, 1986), pink orthogneiss (metamorphosed alkaline lavas; Seyler, 1986), and amphibole-biotite orthogneiss is interpreted as evidence for bimodal magmatism related to an extensional setting (Seyler, 1986) due to plume activity (Briand et al., 2002). The layered formation includes small lenses of meta-igneous rocks as parts of a supra-subduction zone lithosphere (Bellot et al., 2000b). A first group of abundant spinel peridotites (Gueirard, 1956; Bard and Caruba, 1981; Lasnier, 1977; Laverne et al., 1997), garnet-spinel peridotites (Bellot, 1998; Bouloton et al., 1998), coronitic gabbros (Lasnier, 1970), garnet amphibolites (meta-andesites), felsic amphibolites (meta-dolerites), and fine-grained amphibolites (metamorphosed transitional to tholeiitic lavas with arc affinities; Ricci and Sabatini, 1978; Seyler and Boucarut, 1979; Bard and Caruba, 1981) was interpreted as portions of lithosphere generated in a supra-subduction zone during Early Palaeozoic time. A second group of gabbros (Seyler, 1982; Caruba, 1983; Avice, 1995; Bouloton et al., 1998) and fine-grained amphibolites (meta-tholeiites with oceanic affinities; Seyler, 1986; Briand et al., 2002) was interpreted as portions of an oceanic lithosphere generated at the Cambrian-Ordovician boundary. A consensual hypothesis is to interpret all these rocks as parts of a back-arc lithosphere. This hypothesis is supported by the presence of minor limestones (Seyler, 1986). Only garnet-spinel peridotites (Bellot 1998; Bouloton et al., 1998) are evidenced for HP metamorphism (P>2.8 GPa/T>850°C). Coronitic gabbros (0.6-0.7 GPa/750-850°C; Caruba, 1983) and garnet amphibolites (0.5 GPa/550°C; Bellot et al., 2003), previously interpreted as HP rocks (Bard and Caruba 1981, 1982), most likely reflect LP granulites and amphibolites facies metamorphisms, respectively (Seyler, 1982).

Zircon U-Pb dating on alkaline orthogneiss defines a Cambrian age for bimodal magmatism (498 17 Ma and 507 ± 5 Ma; Lancelot et al., unpub; 548 +15/-7 Ma; Innocent et al., 2003). Whole rock Rb-Sr dating on amphibolite gives 348 ± 7 Ma, interpreted as the age of regional metamorphism (Innocent et al., 2003), while 40Ar-39Ar dating on amphibolites (330 ± 2 Ma and 328 ± 3 Ma), on garnet micaschist (322,9 ± 1,7 Ma), on biotite micaschist (321,1 ± 1,3 Ma and 319,5 ± 0,3 Ma), on migmatite (317,2 ± 1,0 Ma), and sheared migmatite (319,7 ± 1,3 Ma and 320,5 ± 1,4 Ma) suggest cooling of the central Maures in relation with its exhumation during the Namurian (Morillon et al., 2000).

This unit is interpreted here as relics of a Cambrian (550-500 Ma) back-arc lithosphere involved in the Silurian continental subduction and the Carboniferous continental collision.

The Cavalières Unit

The Cavalières Unit consists of migmatitic orthogneiss, migmatitic paragneiss and minor mica-schist that include lenses of eclogites (Le Marrec, 1976; Maquil, 1976; Crévola, 1977; Vauchez, 1987). Kyanite-sapphirine-bearing eclogites are inferred to be calk-alkaline meta-gabbro (Avice, 1995) emplaced in a back-arc setting (Buscail et al., 1999) during the Upper Ordovician (452 ± 8 Ma, zircon U-Pb; Lancelot et al., 1998), and buried at mantle depths (15-25 kbar/800-950°C; Bard and Caruba, 1982; Caruba, 1983) during the Lower Silurian (431 ± 4 Ma, zircon U-Pb; Lancelot et al., 1998). The surrounding biotite- and amphibole-rich orthogneiss are interpreted as calk-alkaline diorite and granodiorite emplaced during the Precambrian (zircon U-Pb, 612-630 Ma; Lancelot et al., 1998) and intruded by mafic magma (forthcoming eclogites). Even though HP metamorphism has not been discovered in orthogneiss, these rocks have probably been buried with metagabbros during the Silurian. They have experienced partial melting during the Upper Visean (zircon U-Pb, 334 ± 3 Ma; Lancelot et al., 1998) in relation to their exhumation (Le Marrec, 1976; Vauchez and Buffalo, 1988).

The Petites Maures Unit

The Petites Maures Unit consists, from bottom to top, of metagabbros, felsic granulites, various migmatitic paragneiss, monotonous, layered or nodules-rich, that includes abundant calcic silicate-bearing gneiss, minor migmatitic orthogneiss, lenses of marbles, eclogites and serpentinites (Le Marrec, 1976; Maquil, 1976; Crévola, 1977; Vauchez, 1987).

Coal basins

Two coal basins of Late Palaeozoic age are located along major orogen-parallel faults. The Plan-de-la-Tour basin, 16-km-long and 1-km-wide, occurs in the central Maures along the Grimaud fault (Wallerant, 1889; Demay, 1927a and b; Gueirard, 1960; Bordet, 1967). The basin is filled by 400 m of conglomerates and arkoses (Wallerant, 1889; Bordet, 1967; Basso, 1985; Bégassat, 1985) those the base is dated Lower Stephanian (Masurel, 1964; Basso, 1985). Pebbles of mylonitic rocks from the Grimaud fault are frequent, while those of the Plan-de-la-Tour granite are lacking (Demay, 1927). Microgranite dykes emplaced within the basin are dated Upper Stephanian (290 ± 10 Ma, Rb-Sr on minerals: Roubault et al., 1970b; 295,4 ± 2,4 Ma, biotite 40Ar-39Ar; Morillon, 1997). These dates, in addition to ASM analyses (Edel, 1999), suggest that magmatism and conglomerates deposition were coeval during the Stephanian.

The Reyran basin, 12-km-long and 1-km-wide, occurs in the eastern Maures along the La Moure fault (Basso, 1985 and references therein). The basin is filled by 1000 m of sandstone, coal, pelites, and conglomerates dated Upper Westphalian to Lower Stephanian (Basso, 1985). Pyroclastic rhyolite took place during sedimentation.

Permian basins

End of the Variscan orogeny in the Maures massif is evidenced from deposition of Lower to Upper Permian sediments that unconformably overlain both basement and Upper Carboniferous coal basins in the western and northern Maures massif (Toutin-Morin et al., 1988). Basin development is associated with acid then mafic volcanisms in the Estérel area (Bordet, 1951; synthesis in Crévola and Pupin, 1994), and hydrothermal activity that generates some of the F-Ba-Pb-Zn deposits (Solety, 1964; Vervialle, 1975; Mari, 1979). 40Ar-39Ar dating on plagioclase of a mafic dyke and on adularia of a barite-fluorite lode have yielded 278 ± 0.4 Ma and 264 ± 0.7 Ma, respectively (Zheng et al., 1991-1992). In the northern massif, Thuriangian basin allows final exhumation of the Plan-de-la-Tour granite (Delfaud et al., 1989). Permian and post-Permian events have produced polyphased movements along E-W faults (Zheng, 1990; Toutin-Morin et al., 1992) between which some basement blocks are rotated, especially the Hyères-Bormes-Cavalaire block (Bronner, 1996).