The Maures Massif (Figure 1) comprises two main tectono-metamorphic blocks, the Western Block (WB) and the Eastern Block (EB), which are in tectonic contact along the Grimaud Fault (Pin & Peucat, 1986, Pin, 1990, Onezime et al., 1999, Morillon et al., 2000).
Figure 1. Geological sketch map of the Maures Massif
Geological sketch map of the Maures Massif and cross section (after Elter et al., 2004; modified from Morillon et al., 2000). The inset map shows the position of the Maures Massif in the Variscan Belt of France and respect to the Corsica-Sardinia block.
The Western Block consists of three metamorphic units: 1) the Upper Western Unit (UWU) composed of phyllites; 2) the Middle Bormes Unit (MBU) characterized by the Bormes orthogneiss dated 344 15 Ma, (Mossavau, 1998), paragneiss, micaschists, and minor amphibolites; 3) the Lower La Garde-Freinet Unit, made up of orthogneiss, leptynites, meta-gabbros, meta-serpentinites and garnet-spinel meta-peridotites, and sillimanite-bearing micaschists and migmatites. The geochemical trend of the ultramafites (Bellot et al., 1998) show a provenance from Lower Oceanic Crust.
The Eastern Block is mainly made up of of migmatites derived from orthogneisses and paragneisses containing amphibolitic eclogite lenses (Le Cartelle eclogite: Bodinier et al., 1986, Crevola et al., 1991). The migmatization process dates to 325 ± 8 Ma and 333 ± 3 Ma (Morillon et al., 2000). Four generations of syn-kinematic to post-kinematic plutons intruded EB since 334 ± 10 Ma to 297 ± 5 Ma (Amezou, 1988, Mossavau, 1998) fig. The metamorphic, magmatic and tectonic evolution of EB suggests a continuous orogenic history (Vauchez & Bufalo, 1988) in an area of rapid crustal thickening determined by large-scale thrusting within continental crust. In particular, the following metamorphic-magmatic evolution can be pointed out for EB (see P-T paths in Figure 2):
Figure 2. Summary of P-T-t-d paths
Summary of P-T-t-d paths (redrawn from Bellot et al., 2003); Western Maures = Western Block (WB); LAC = Leptyno-Amphibolitic Complex = Eastern Block (EB).
an early HP metamorphism which is linked to relict eclogites and to white kyanite+talc-bearing micaschists (Buscail & Leyreloup, 1998). Moreover, Bellot et al. (2003) recognized coarse-grained amphibolites characterized by P= 7-8 kbar and T= 750°-700° C;
a HT/LP metamorphism involving anatectic melting and syn-kinematic non-coaxial deformation (Vauchez & Bufalo, 1988), dated at 345 ± 15 Ma - 339 ± 16 Ma (U/Pb age on zircon: Moussavou et al., 1998);
the intrusion of the first generation of anatectic granitoids at 334 ± 10 Ma (Amezou, 1988; Moussavou, 1998);
a MT/LP to LT/LP metamorphism (e.g. the fine-grained amphibolites from P 4-6 kbar, T 550° - 650° C to P 2 - 4 kbar, T 350°-500° C of Bellot et al., 2003), syn-tectonic with sinistral strike-slip shear zone (Grimaud Fault) in which the early anatectic granitoids were mylonitized (Vauchez & Bufalo, 1988);
a moderate cataclastic reactivation of the Grimaud Fault with a dextral sense of shear; syn-tectonic to the emplacements of the Plan-de-la Tour and Rouet granites (Onezime et al., 1999) whose emplacement is dated 334 ± 5 Ma (Moussavou et al., 1998).
the intrusion of the anatectic granites at 320-325 Ma (Vauchez & Bufalo, 1988; Moussavou et al., 1998);
the beginning of exhumation (320-330 Ma: Morillon et al., 2000) with diachronous cooling ages on both sides of the Grimaud Fault (320 Ma for WB and 305 to 300 Ma for EB) ;
The late exhumation in the Maures Massif was probably associated with two orthogonal stages of extension (Faure, 1995) and/or with a transfer fault zone (Burg et al., 1994).
The Variscan Sardinia (Figure 3) consists of four Northwest to Southeast - trending tectono-metamorphic zones : the External Zone (Foreland area), the Nappe Zone, the Posada Valley Zone ("Inner Nappes" of Carmignani et al., 1992; "Low to Middle Grade Metamorphic Complex of the Axial Zone" of Oggiano & Di Pisa, 1992) and the High Grade Metamorphic Complex (HGMC, "High Grade Metamorphic Complex of the Axial Zone" of Oggiano & Di Pisa, 1992; Axial Zone of Elter et al., 2004 and Corsi & Elter, 2005). An evident northeastward increase of the metamorphic grade is recognizable going from the sub-greenschist/greenschist facies of the External Zone /Nappe Zone, to the high grade amphibolite facies and migmatites within the Axial Zone (Franceschelli et al., 1982) including scattered retrogressed granulite and eclogite lenses. The isograds of Barrovian regional metamorphism are parallel to the Northwest-Southeast strike of the tectono-metamorphic zones. The stratigraphic and tectono-metamorphic features of each zone are shown below (the readers are referred to cited authors and Carmignani et al., 2001 for details):
Figure 3. Regional distribution of the Variscan tectono-metamorphic zones
Regional distribution of the Variscan tectono-metamorphic zones in Sardinia (modified from Elter et al., 2004).
The Paleozoic successions of the External Zone (Foreland area), cropping out in the southwestern corner of Sardinia, are weakly deformed and metamorphosed (anchizone to lowermost greenschist facies) and include neritic Early Cambrian to Early Ordovician carbonates and siliciclastics which are unconformably overlain (Sardinian Unconformity) by continental/coastal to epicontinental marine sediments of Middle Ordovician to early Carboniferous age;
The Nappe Zone successions crops out in the central part of Sardinia and consists of Late Cambrian-Early Ordovician basinal marine siliciclastics which unconformably (Sardinian Unconformity or Sarrabus Unconformity) rest below Middle Ordovician mostly acidic volcanites, Late Ordovician neritic and Silurian-Devonian marine epicontinental sediments and Early Carboniferous "Culm"-like syn-orogenic siliciclastics. The Nappe Zone can be divided into two sub-zones based on their structural - metamorphic evolution: an External Nappe Zone (ENZ) and an Internal Nappe Zone (INZ). The ENZ was affected by lower greenschist facies metamorphism, whereas the INZ is characterized by an increase in metamorphic grade up to the passage high-grade greenschist facies/amphibolitic facies, notwithstanding that, according to Carosi et al. (1992) and Carmignani et al. (2001), the staurolite zone of the amphibolitic facies was attained in the deepest Mt. Grighini Unit. In particular two tectono-metamorphic complexes can be recognized in the INZ: the Lula Complex (albite-biotite zone: Elter et al., 1986) and the S.Lucia Complex (oligoclase + biotite zone: Elter et al., 1986) The INZ is in tectonic contact with the Axial Zone across the Posada Valley shear zone (Elter et al., 1990, 1999, 2004).
the Posada Valley Zone is characterized by MT/MP condensed isograds and by a metamorphic evolution syn-kinematic with non-coaxial deformation (Elter et al., 1999, 2004 and references therein). The Posada Valley Zone is here considered a transitional metamorphic complex from the Nappe Zone to HGMC. The northern border of the Posada Valley Zone is represented by a segment of the most important regional tectonic line, the so-called Posada - Asinara Line (Figure 3). This about 3 km-thick, dextral strike-slip shear zone developed in greenschist facies conditions which overprinted the condensed isograds metamorphism. An earlier amphibolite facies, extensional, non-coaxial deformation event (Corsi & Elter, 2005) can be identified in some scattered pods of leptyno-amphibolitic rocks with eclogitic relics (Torpè amphibolites: Memmi et al., 1983; Cappelli et al., 1992; "eclogite B" in Cortesogno et al., 2004) hosted in the ultramylonite. The thermometric calibrations provide T in the range 610° - 700° C for P about 1.3-1.5 Gpa for the eclogitic event (Cortesogno et al., 2004).
Figure 4. P-T- t paths for some metamorphic zones
P-T- t paths for some metamorphic zones of the Variscan basement in Sardinia: A-A: P-T-t path of the granulites from NE sardinia; B-B, C-C, I-I: P-T-t path of the sil+kfs zone in the Barrabisa, Golfo Aranci and Porto Ottiolo, and Anglona areas respectivelly; D-D: P-T-t path of the sil+ms zone in the Porto Ottiolo area; E-E/F-F: P-T-t path of the ky+st+bt zone in the Posada Valley area; G'-G' and G"-G": P-T-t path of the gt+bt zone (INZ); H-H: P-T-t path of the bt zone in the Grighini area (ENZ) (after Elter et al., 1999).
In particular, the Posada Valley Zone can be divided into two Units, an Upper Unit (UU) and Lower Unit (LU, Elter et al., 2004). Two complexes are identified in the UU: the Siniscola - Monte Longu Complex (metasediments in the albite+oligoclase+biotite zone) and the Lodè-Mamone Complex (Ordovician orthogneisses). The four complexes distinguished in the LU are (from top to bottom): 1) the Punta Gortomedda Complex (metasediments in the staurolite + biotite zone), 2) the Bruncu Nieddu Complex (metasediments in the kyanite + biotite zone), 3) the Upper Punta Figliacoro Complex (an assemblage of metasediments in the kyanite + biotite zone and N-T MORB eclogitic leptyno-amphibolites in a mylonitic context: Torpè amphibolites in Memmi et al., 1983; Cappelli et al., 1992) and 4) the Lower Punta Figliacoro Complex (sillimanite-bearing micaschists).
The Axial Zone is the High Grade Metamorphic Complex (HGMC, Elter et al., 2004) which comprises the sillimanite+muscovite and the sillimanite+K-feldspar metamorphic zones (Elter et al., 1986). The HGMC consists mainly of migmatites deriving from paragneiss and orthogneiss that contain lenses of amphibolites with relics of eclogitic (Punta de Li Turchi eclogites of 957 Ma: Miller et al., 1976; Ricci & Sabatini, 1978; "eclogite A" of Cortesogno et al., 2004) and/or granulitic (Montiggiu Nieddo amphibolites and P.ta Scorno amphibolites: Ghezzo et al., 1979, 1982; Castorina et al., 1996) metamorphism. Cortesogno et al. (2004) defined for the Punta de Li Turchi eclogites geothermobarometric parameters of T= 690° - 760° C for minimun pressure about 1.3 Gpa.
Post- orogenic fluvial and lacustrine siliciclastic successions, Westphalian/Stephanian-Autunian (including anthracite bodies: e.g. San Giorgio basin in Iglesiente) and Late Permian-Middle Triassic in age, with local intercalations of calc-alkaline volcanites are also present in several part of the island (Carmignani et al., 2001 and references therein).
The Paleozoic metamorphic-magmatic evolution of Sardinia is characterized by (Figure 4):
A syn-collisional crustal shortening event, characterized by a first prograde migmatization event with T 700°-650° C and P 9-10 kbar (Cruciani et al., 2002; Corsi & Elter, 2005), that develops in a time span of 380-350 Ma in the HGMC (Carosi & Palmeri, 2002);
Late Carboniferous - Early Permian extensional tectonics that affected the whole belt and was due to gravitational collapse of the previously thickened crust (Elter et al., 2004, Corsi & Elter, 2005). The geometric and kinematic features of extensional tectonism include a complex network of composite shear zones syn-tectonic to a second migmatization event (Corsi & Elter, 2005) which took place in a time span from 350 - 300 Ma with T600°-450° C and P 4-6 kbar (Ferrara et al., 1978; Ricci, 1992).
The complicate shear zone network was generated by two shear events (Elter et al., 1999): an Early Shear Event and a Late Shear Event. The early shear zones are not yet radiometrically dated, but they are cut by the later ones (Elter et al., 1999, 2004) which develop during a time span between 320 - 300 Ma (Muzio, 2003).
The early shear zones shows pervasive HT/LP metamorphism, coeval with non-coaxial deformation and variably trending foliation planes, is consistent with Northwest-Southeast mineral stretching lineations and a "top-to-the South-Southeast" shearing. This event may be in relation to extensional doming (Elter et al., 1999, 2004, Corsi & Elter, 2005). By contrast, the late shear zones are characterized by dominantly dextral strike-slip movement (Elter et al., 1990, 1999, 2004, Corsi & Elter, 2005). Some of them are associated with syn-kinematic granodiorites and tonalities whose emplacement ages range from 320 Ma to 300 Ma (Bralia et al., 1982/83; Poli et al., 1989; Muzio, 2003). The metamorphic conditions of the LSE show different patterns (Elter et al., 1999). In fact, those associated with synkinematic intrusions are characterized by HT texture, while the LSE shear zones characterized by retrograde metamorphism show metamorphic conditions in the greenschist facies (e.g., the Posada Valley Shear Zone) or in the low-grade amphibolitic conditions (e.g. the Ottiolu Shear Zone).
Extensional tectonics gave rise to the fast exhumation of amphibolite facies rocks, with the thermal perturbation due to tectonic unroofing causing HT/LP metamorphism and anatexis (Elter et al., 1999) which produced late-tectonic granitoids (307-299 Ma granodiorites, tonalites and monzogranites, Del Moro et al., 1975; Bralia et al., 1982/83; Poli et al., 1989) with rare gabbroic masses (307 Ma, Bralia et al., 1982/83; Poli et al., 1989). The fast uplift also quenched the stability of the amphibolitic facies paragenesis in the upper levels where deformation developed through brittle processes. During this event, large scale late- to post-tectonic intrusive magmatism (298-289 Ma leucogranites, Del Moro et al., 1975; Bralia et al., 1982/83; Poli et al., 1989) took place.
The post-tectonic granitoids are not foliated, and their Northeast-Southwest direction of emplacement is orthogonal to the axis of the Variscan belt. In contrast, the earlier magmatic bodies are generally foliated and sub-parallel to the belt axis.
The Variscan Belt in Corsica (Figure 5) (Carmignani & Rossi, 2001; Durand Delga et al., 2001) is mainly represented by a wide composite granitoid batholith and volcanics which mostly hide the original relationships within the units of this Variscan segment. The magmatic evolution of the Variscan Corsica consists of the polyphase emplacement of three magmatic associations occurred during Late Carboniferous-Permian-?Early Triassic times (Durand-Delga et al., 2001; Del Moro et al., 1975; Cocherie et al, 1984, 1992, 1994; Bonin et al., 1998 and references therein). At the boundary Late Carboniferous-Early Permian, especially the north-western part of the Variscan Corsica was extensively covered by calc-alkaline volcanites of andesite-dacite and dacite-rhyolite compositions (Vellutini, 1977). According to Paquette et al. (2003) four emplacement stages of the magmatic bodies can be distinguished: a) Post-collisional peraluminous granites at about 346 Ma; b) Syn-orogenic (Palagi et al, 1985; Menot & Orsini, 1990), high Mg-K calc-alkaline granitoids at about 338 Ma; c) late orogenic Calc-alkaline granitoids and volcanics at about 305 Ma during the extension and shearing of the Corsica Variscan segment; d) Layered mafic-ultramafic complexes, and metaluminous and (per-) alkaline granitoids at about 280-285 Ma. The same authors also suggest that d) magmatic event was linked to the pre-Alpine continental rifting.
Late Carboniferous, anthracite bearing siliciclastics are present at Osani, Mausoleo-Asco and Punta di L'Acciolu.
Pre-Late Carboniferous Variscan successions are locally recognizable as host rocks of the granitoid composite batholith in some places of Variscan Corsica (e.g. Belgodere-Castifao-Corscia, Aiaccio, Osani, Porto Vecchio, Solenzara, Zicavo and Galeria).
In particular, the pre-granitoids rocks can be attributed to two metamorphic domains (Durand-Delga et al., 2001 and references therein):
A meso-to-catazonal domain, which is the continuation of the HGMC of NE Sardinia and crops out at Porto Vecchio-Solenzara, Zicavo, west of Aiaccio, Belgodere-Castifao-Corscia and in the Valinco Gulf (Bellini, 1971; Palagi et al., 1985). It is characterized by polymetamorphic and polydeformed orthogneisses, paragneisses and migmatites (Rossi, 1998). A peculiar feature is the presence of: a) a "leptyno-amphibolitic complex", consisting of: amphibolites with a "continental" tholeiitic and T- MORB affinity with local cumulitic structures (Palagi et al., 1985a), and associated orthogneisses (e.g. the 338 Ma Belgodere orthogneiss: Paquette et al., 2003), possibly deriving from Ordovician granitoids (Rossi, 1998); b) a migmatitic complex, included in a Barrovian metamorphic sequence, with eclogitic relics providing evidence of early HP metamorphism (Belgodere eclogites: Palagi et al., 1985b)
An epizonal domain, which is characterized by sericite-chlorite micaschists and amphibolite bands and by the Paleozoic succession of Galeria consisting of pre-Cambrian metasiliciclastics with levels of metabasites which underlies a ?Cambro-Ordovician to Early Carboniferous sequence (similar to those of the Central Sardinia). This succession is characterized by a low metamorphic grade (Ricci e Sabatini, 1978).
The Paleozoic and Triassic successions of the Northern Apennines (see locations in Figure 6) were strongly involved in the Alpine Orogeny in the Oligocene-Miocene times (27 to 12 Ma). In particular, these rocks suffered polyphase greenschist facies tectono-metamorphism and were dissected in several tectonic units (Carmignani & Kligfield, 1990; Elter & Pandeli, 1990, 1993, 1994, 1996; Bertini et al., 1991; Conti et al., 1991; Pandeli et al., 1994; Franceschelli et al., 2004; Pandeli et al., 2005). Moreover, the Paleozoic successions, as part of the overlying covers, underwent HT-LP metamorphism during the emplacement of the post-tectonic, Late Miocene-Quaternary granitoids in southern Tuscany (e.g. in the subsurface of the Larderello-Travale geothermal field: Batini et al., 1983; Elter & Pandeli, 1990; Franceschini, 1998; Carella et al., 2000; Gianelli & Ruggieri, 2002; Musumeci et al., 2002; Pandeli et al., 2005) and Tuscan Archipelago (e.g. in the eastern Elba Island: Barberi et al, 1969; Duranti et al., 1992; Bortolotti et al., 2001; Garfagnoli et al., 2005). The Alpine metamorphism and tectonics largely obliterated the fossiliferous content and textures of the pre-Carboniferous rocks and juxtaposed different portions of the Variscan crust (Elter & Pandeli, 1996; Pandeli et al., 2005). The restoration of the original stratigraphic successions was mostly made through lithostratigraphic and petrographical-geochemical correlations with the correspondent, well-dated stratigraphy of Central Sardinia (Bagnoli et al., 1979; Conti et al, 1991; Pandeli et al., 1994 and references therein).
Figure 6. Regional distribution of the outcrops
Regional distribution of the outcrops of the Paleozoic-Triassic metamorphic successions (in black) in the Northern Apennines.
In particular, the outcropping stratigraphic successions (e.g. Punta Bianca, Apuan Alps, Mts. Pisani, Monticiano-Roccastrada Ridge, eastern Elba Island, Mts. Romani) are made up of ?Late Cambrian-to Devonian units similar to those of the Central Sardinia. These rocks often preserve pre-Alpine, greenschist facies schistosity relics which Conti et al. (1991) and Pandeli et al. (1994) referred to the Sudetian Event of the Variscan Orogeny. At places, Middle Carboniferous foredeep siliciclastics and Late Carboniferous-Permian, continental to marine successions also occur (e.g. Mts. Pisani, Iano, Monticiano-Roccastrada Ridge; Mts. Leoni, Mt.Argentario, eastern Elba Island: Pandeli et al., 1994, 2004; Pandeli, 2002 and references therein).
In the subsurface of the Larderello-Travale geothermal field (Figure 7), the ?Late Cambrian/Ordovician rocks lie onto two mica, garnet-bearing micaschists which includes OFB (=Ocean Floor Basalts) hornblende amphibolite levels (Micaschist Complex: Puxeddu et al., 1984; Elter & Pandeli, 1990). The same rocks also crop out at Cerreto Pass (northeastward of the Apuan Alps: Di Sabatino et al., 1979; Molli et al., 2002) and in southeastern Elba Island (Garfagnoli et al., 2005), and were crossed by the Pontremoli 1 oil well northward of the Apuan Alps (Pandeli et al., 2005). The coarse-grained, medium-grade Barrowian foliation of the mostly OFB amphibolites in the Cerreto outcrops is dated about 330 Ma by Molli et al. (2002). An Early Permian HT event was also defined in the micaschists of the Larderello field (about 285Ma age of a muscovite associated with pre-Alpine andalusite: Del Moro et al., 1982).
Figure 7. Geological sketch of the Larderello
Geological sketch of the Larderello geothermal field (after Elter & Pandeli, 1996).
Alpine quartz-mylonites separates the Micaschist Complex, pervasively affected by the Alpine tectono-metamorphism (and thus included in the Alpine Monticiano-Roccastrada Unit by Bertini et al., 1991), from the underlying Gneiss Complex, which preserves its Variscan framework (Elter & Pandeli, 1990; Pandeli et al., 1994, 2005). The Gneiss Complex consists of muscovite-biotite gneisses with interlayered orthogneisses, OFB amphibolites and minor calc-silicate rocks (Puxeddu et al., 1984; Pandeli et al., 1994). The pre-Alpine Barrovian minerals and textures in the Micaschists Complex are recognizable in the main foliation of the Gneiss Complex which suggest a similar Variscan metamorphic evolution (Elter & Pandeli, 1996; Franceschelli et al., 2004; Pandeli et al., 2005 and references therein)(Figure 8). In particular besides a higher peak P-T values for the gneisses (up to sillimanite-in), it passed through an earlier Barrovian-type intermediate P event (P 6-7 kb and T=550°-650° according to Bertini et al., 1994) to a subsequent lower P syn-tectonic event (P 2-3.5 kb and T=550°-600° according to Bertini et al., 1994). These P-T paths are comparable to ones of other well-known Variscan units of southwestern Europe and northern Africa (e.g. Central Massif, Maures Massif, NE-Sardinia, Calabrian-Peloritan arc, Bethic-Rifean belt: see Figure 6 in Elter & Pandeli, 1996 and Figs. 2 and 4 in this paper).
Figure 8. P-T paths of the Micaschist Complex
P-T paths of the Micaschist Complex and Gneiss Complex (after Franceschelli et al, 2004).