Day 2.

Stop 1.

Locality: Campodonico (UTM 32T 528505 E4691631)

Themes: Introduction to the geology of the Monte San Petrone area

In this stop, a general introduction to the San Petrone area and its main lithological, structural and metamorphic features are given and shown in the landscape (Fig. 1.3, 1.4, 1.9, 1.10). This area is characterized by different lithologies, including ultramafic rocks, continental-basement rocks, metaophiolite and metasediments, which were already juxtaposed in a Mesozoic OCT zone (Fig. 1.11).

Figure 1.9. Monte San Petrone area

Monte San Petrone area

Simplified geological map of the Monte San Petrone area. Modified after Vitale Brovarone et al. (2011b).

Figure 1.10. E-W cross-section of the Monte San Petrone area

E-W cross-section of the Monte San Petrone area

Interpretative E-W cross-section of the Monte San Petrone area. See Fig. 2.9 for location.

Figure 1.11. Simplified reconstruction

Simplified reconstruction

Simplified reconstruction of the primary Mesozoic relationschisp among the different lithologies of the Monte san Petrone area.

Stop 2.

Locality: Track to Monte San Perone (UTM 32T 528040 E4691798)

Themes: The Mandriale shear zone

Climbing to the top, we will walk across the lower limb of a large-scale recumbent antiform formed under retrograde epidote-blueschist conditions (Fig. 1.10, 1.11). The core of this structure consists of a serpentinized basement, which occupies the lowest structural level in the Monte San Petrone tectono-stratigraphic association. The overturned sequence that underlies the ultramafic basement consists of ~50-100 meters-thick layer of highly strained metabasalts followed by a thick metasedimentary sequence. This metabasaltic layer, also known as the “Mandriale unit” or “prasinite unit” for the French authors, represents a major feature of the SL complex, hosting a major shear zone that can be followed throughout Alpine Corsica. To the north, in the Bastia area this shear zone coincides with a major tectonic contact separating the lawsonite-eclogitic unit from the overlying lawsonite-blueschist unit. Otherwise, in the Monte San Petrone area, this shear zone develops on the stretched overturned limb of large antiform within the lawsonite-eclogite unit.

Stop 3.

Locality: Punta Favalta (UTM 32T 526361 E4692271)

Themes: The Basal Tectonic Contact

The Basal Tectonic Contact is one of the main lithological interfaces within the Monte San Petrone Unit. It separates the serpentinized basement from the overlying lithologies, which include continental basement rocks, ophiolite-type rocks and metasediments. Relics of rift-related brittle tectonic structures, such as ophicalcites or serpentinite cataclasites, are locally observed along this contact. In the Punta Favalta area, along the upper limb of a recumbent synform, ophicalcites separate the serpentinized basement from the overlying metasediments (Fig. 1.12). This succession is interpreted as a primary stratigraphic sequence characterized by exhumed mantle rocks, capped by ophicalcites, directly overlain by oceanic sediments.

Figure 1.12. Outcrop of ophicalcite close to Punta Favalta.

Outcrop of ophicalcite close to Punta Favalta.

These rocks are interprete to correspond to the fractured and hydrothermalized top of the exhumed mantle during the Jurassic rifting.

Stop 4.

Locality: Aja Rossa (UTM 32T 526574 E4692900)

Themes: The lawsonite-eclogite facies metabasalts

Metabasaltic rocks are well exposed in the northern part of the Monte San Petrone unit, forming a thick body of pillow basalts, lava tubes or pillow breccias re-equilibrated under lawsonite-eclogite facies metamorphic conditions. Differently from the metabasaltic rocks observed during stop#2, these rocks preserve well the primary igneous structures. In the Aja Rossa area, a primary contact separating metabasalts from the underlying serpentinites is observed. Metabasalts consist of well-preserved pillow lava or lava tubes still preserving primary igneous structures at the meso- and micro-scale (Fig. 1.13). As an example, primary sites of the igneous plagioclase, now consisting of lawsonite ± actinolite ± garnet ± phengite aggregates, are common. Two main peak metamorphism mineral assemblages are observed. The first is the typical lawsonite-eclogite paragenesis consisting of omphacite + lawsonite + garnet + phengite + titanite, which characterizes the core of meta-pillows, unaffected by hydrothermal alteration at the seafloor. The second assemblage consists of glaucophane + actinolite + lawsonite + garnet + phengite + titanite and is typical of garnet-bearing lawsonite-blueschists. These two mineral assemblages equilibrated and coexisted at the same PT conditions (520 ± 20°C, 2.3 ± 0.1 GPa, Fig. 1.14). Their coexistence is related to minor differences in the bulk-rock composition acquired during seafloor hydrothermal alteration (Vitale Brovarone et al., 2011a).

Figure 1.13. Metabasalts from the Monte San Petrone area.

Metabasalts from the Monte San Petrone area.

(a) Preserved lava tube structure. (b) Typical relationships between coexisting lawsonite-eclogite (Law-Ecl) and lawsonite-blueschist (Law-Bs) assemblages. Note that the Law-Ecl occurs in the coreo of primary pillow structures, while the Law-Bs occurs in the outer portion. (c) Primary basaltic structure characterized by abundant preserved plagioclase sites, now mostly consisting of lawsonite (e). (d) Static growth of Law-Ecl minerals on a preserved volcanic structure. Note the assemblage of Omphacite + lawsonite + garnet. (f) Primary undeformed basaltic variolitic structures overgrown by the Alpine lawsonite (white prisms).

Figure 1.14. P-T pseudosection

P-T pseudosection

P-T pseudosection of coexisting Law-Ecl (a) and Law-Bs (b). Modified after Vitale Brovarone et al. (2011a).

Stop 5.

Locality: To the east of Punta Ventosa (UTM 32T 527193 E4691543)

Themes: The continental extensional allochthon

A continental basement sliver characterize the central part of the Monte San Petrone area. The sliver, which is interpreted as a rift-related continental extensional allochthons, is structurally interposed between the serpentinized basement and the metasedimentary cover rocks. This sliver consists of both polycyclic basement rocks, such as garnet-bearing micaschists, and metagranitic rocks. Relics of Permian HT metamorphism (age 290 Ma) are found in the micaschists, indicating that this sliver of continental crust has been exhumed from depth up to the ocean floor during Jurassic rifting (Martin et al., 2011).

The most common Alpine HP minerals are glaucophane + garnet + lawsonite + phengite ± omphacite in the polycyclic basement rocks and phengite + jadeite + glaucophane ± lawsonite in meta-granitoids.

Evidence of a multi-stage deformation history, characterized by both coaxial and non-coaxial flow, is observed in the continental basement sliver. A beautiful example of non-coaxial deformation occurs East of Punta Ventosa, where well-preserved HP sheath folds range in size from ten centimeters to several meters. As a result of this deformation event, HP fold axes are scattered, but mostly oriented NE-SW, while HP stretching lineations marked by minerals (e.g. glaucophane) and stretched quartz aggregates display little scatter and strike towards ~ 250° (Fig. 1.5). PT estimates via pseudosection modeling of a continental basement rock of mafic compositions yields minimum peak metamorphism conditions at around >T = 490°C and P = 2.2–2.6 GPa. U-Pb geochronology on zircon rims in equilibrium with the HP assemblage lawsonite + omphacite + garnet yields an Alpine age of ~ 34 Ma (Martin et al., 2011).

Stop 6.

Locality: To the east of Punta Ventosa (UTM 32T 527167 E 4691255)

Themes: Marble including clasts of continental basement rocks: evidence of a preserved paleo-scarp

Marble of the Monte San Petrone unit locally contains clasts of the underlying rocks (e.g. continental basement rocks or metabasalt). This is especially evident where the continental basement sliver thins to a few meters (e.g. to the east of Punta Ventosa or close to Monte Calleruccio) (Fig. 1.15a). To the east of Punta Ventosa, the thinner the continental basement sliver, the thicker is the marble, and clasts of continental basement rocks are found within this cover rock where the continental basement sliver is the thinnest. More precisely, clasts of both granitic (Fig. 1.15b) and polycyclic basement rocks (e.g. micaschists, Fig. 1.15c) are found. On the contrary, these clasts are never found where the continental basement sliver gets thicker, where particular lithologies, interpreted as meta-condensed sequences, are found. The relative distribution of the continental basement and metasediments described above suggests that syn-sedimentary (i.e. Mesozoic) topographic highs and lows morphologies controlled the deposition of sediments and the erosion of the basement allochthon (Fig. 1.15d). Such high-angle normal faults affecting the sea floor may be responsible for the formation of continental basement clasts found in the carbonatic cover described above. These Mesozoic structures can be still be recognized despite intense Alpine deformation.

Figure 1.15. Punta Ventosa

Punta Ventosa

(a) N-S cross-section to the east of Punta Ventosa. Modified after Vitale Brovarone et al. (2011b). Note the occurrence of marbles including clasts of granitic (b) and polycyclic basement rocks (c) on top of the thinnest part of the continental basement sliver. (d) Simplified recontruction of the inherited rifting-related morphologies.

Stop 7 (optional).

Locality: Track to Campodonico (UTM 32T 527534 E4692009)

Themes: Serpentinite metaconglomerate

Along the track to “Punta Favalta” or “San Petrone”, a thick body of serpentinite sedimentary breccia can be observed (Fig. 1.9, 1.16, 1.17a). The serpentinite clasts, mostly rounded, range in size from a few millimeters to several meters, and are embedded in a carbonate matrix (Fig. 1.16a, b). The matrix-clasts ratio is extremely variable, and primary sedimentary bedding can be still recognized (Fig. 1.16c). This body is interpreted to derive from a graben-type structure filled by the surrounding ultramafic rocks and carbonate-rich sediments, then inverted and deformed during subduction-related deformation (Fig. 1.17b).

Figure 1.16. Serpentinite sedimentary breccia

Serpentinite sedimentary breccia

In (c), note the primary sedimentary beddings.

Figure 1.17. S-E of Monte San Petrone

S-E of Monte San Petrone

(a) Interpretative cross-section to the S-E of Monte San Petrone. See Fig. 2.9 for location. Simplified reconstruction of the inherited Mesozoic morphologies.