Geology of the Monte San Petrone area
The Monte San Petrone (MSP) area is located in southern Alpine Corsica and belongs to the lawsonite-eclogite unit (Péquignot & Potdevin, 1984; Vitale Brovarone et al., 2011 b). In this area, serpentinized basement, which is overlain by a laterally variable lithostratigraphy comprising: i) a sliver of continental basement, extending for several kilometers in the NS direction, ii) “ophiolite-type” rocks, and iii) a metasedimentary cover (Fig. 1.3, 1.4). The description of these lithologies reported hereafter is largely taken from Vitale Brovarone et al. (2011b).
Figure 1.3. Monte San Petrone
(a, b) View of the northern part of the Monte San Petrone (MSP) unit (a), and line drawing (b) showing the MSP architecture. (c) Stratigraphic section of the MSP and simplified column (on the right). (d) Simplified geological map of MSP. Modified from Vitale Brovarone et al. (2011b).
Figure 1.4. Cross Section
(a, b) N-S interpretative cross section (b) and stratigraphic columns (a) showing the lateral variability of the MSP tectono-stratigraphy (vertical scale = 2X). The geologic section crosses the main MSP summits reported in Fig. 2.2d. In 2.3b, light blue indicates the undifferentiated metasedimentary cover.
i) Serpentinized oceanic basement
Mantle-derived rocks crop out in the eastern part of the MSP unit as a N-S elongated body that occupy the lowermost structural position of the tectono-stratigraphic sequence. This composite basement mainly consists of massive or foliated serpentinites, serpentinized peridotites and small bodies of metagabbros. The top of the serpentinized basement is defined by a tectonic contact, which will be referred to as Basal Tectonic Contact (BTC in Fig. 1.3, 1.4). This contact has been interpreted by Péquignot and Potdevin (1984) as an Alpine tectonic contact, but ample evidence for its pre-Alpine origin are observed. We interpret this contact as a pre-alpine extensional detachment locally reworked by alpine compressional tectonics (e.g., Vitale et al., 2011).
A serpentinite breccia crops out as lenticular bodies at the top of the serpentinized basement. It is clast-supported with local evidence of jigsaw clasts of serpentinite, indicating in situ cataclastic flow. Importantly, this brittle structure pre-dates the development of the Alpine ductile fabrics. Again, this structure has been interpreted by Vitale Brovarone et al. (2011) to derive from the pre-Alpine, rifting-related extensional tectonics.
Ophicalcites are locally exposed along the BTC, only where the serpentinized basement is directly in contact with metasediments. These rocks are both tectono-sedimentary breccias (sensu Lemoine et al., 1987) or calcified serpentinites comparable to those described in the Platta Nappe of the Central Alps (Manatschal, 2010). The matrix of this rock consists of carbonates, diopside, green amphibole, chlorite, and rare green uvarovitic garnets grown around detrital Cr-spinels. In some localities (e.g. Punta Favalta), reddish clasts of hematite-rich fibrous carbonates, which are often observed in OC2-type ophicalcites, are also found (e.g. Framura breccia, Folk and McBride, 1976).
ii) Continental basement
Slivers of continental basement, ranging in thickness from less than 2 meters to ~200 meters, rest directly upon the serpentinized ultramafic basement. Their lower contacts coincide with the BTC.
A lithological layering parallel to the BTC is observed, with the lower part of the sliver consisting of polycyclic basement with Permian granulite facies garnet-bearing micaschists and meta-mafic rocks (~290 Ma, Martin et al., 2011). In these rocks, the Alpine assemblage consists of garnet + lawsonite + phengite + chlorite + pumpellyite + quartz. Alpine garnets occur as small crystals or overgrowths on the relict pre-Alpine garnets.
The most common rocks in the continental sliver are metagranitoids and orthogneisses, whose protoliths range in composition from granite to granodiorite. The gneisses, which are generally exposed in the upper part of the continental sliver, usually show an augen structure with porphyroclasts of relict igneous K-feldspar wrapped around by the Alpine foliation mainly consisting of white mica and quartz. In metagranitoids, Alpine HP-LT assemblages, mainly represented by Na-clinopyroxene, Na-amphibole, phengite and lawsonite, are locally well preserved. Alpine garnet is found only occasionally in these rocks.
Locally, along the lower boundary of this continental basement sliver, relics of brittle structures pre-dating the Alpine fabrics are observed, such as angular clasts of granitic rocks wrapped around by HP fabrics. These rocks are especially well exposed in the Bocca di Querciole area, to the south of Monte Calleruccio. These rocks are interpreted as related to the juxtaposition of the continental sliver of the Monte San Petrone on top of the serpentinite basement by brittle faulting related to the rifting extensional tectonics.
Along the BTC, continental basement rocks are locally transformed into a Ca-rich metasomatic rim formed under HP conditions and mainly consisting of lawsonite, garnet and omphacite. Zircons from this particular rock show Alpine rims associated with this HP metasomatic event giving an age of about 34 Ma (Martin et al., 2011). The same metasomatic rim is also observed in metasediments overlying the serpentinized basement.
The upper part of the continental basement is locally characterized by a phengite-rich rock that is considered to represent the metamorphic equivalent of the so-called “tectono-sedimentary breccia” described along the top of the continental extensional allochthons at Ocean-Continent Transition zones.
iii) “Ophiolite-type” rock units
This group of rocks consists mainly of: i) meta-ultramafic debris, including mostly sedimentary breccias (“OC2-type” of Tricart and Lemoine, 1989), and ii) metabasaltic rocks, such as pillow basalts and pillow breccias, lying above the Basal Tectonic Contact as defined in the previous sections.
Serpentinite sedimentary breccias are particularly abundant in the northern part of the Monte San Petrone, along the track connecting the village of Campodonico and Punta Favalta. They consist of clasts of serpentinites embedded within a carbonate matrix. Serpentinite clasts are mostly rounded and vary in size from a few millimetres to several metres and the clast/matrix ratio is extremely variable suggesting local deposition close to fault scarps.
Metabasaltic rocks are also found in the same structural position as the continental basement, where the latter wedges out both to the north (Monte San Petrone) and to the south (Punta Caldane). They consist both of pillowed and massive lavas and basaltic breccias. Both types of metabasalts are common in the northern and southern parts of the Monte San Petrone unit, where they form a body with a maximum thickness of about 200 meters. Relics of pillows and igneous microstructures, such as varioles and plagioclase phenocrysts are commonly preserved (Vitale Brovarone et al., 2011a).
iv) Metasedimentary cover
A wide range of metasediments is found in the Monte San Petrone Unit. These metasediments stratigraphically overlie the serpentinized basement, the continental basement and the “ophiolite-type” rocks. The lower boundary of the metasedimentary cover, which coincides with both the BTC or the Upper Allochthon Surface depending on the absence/presence of continental basement, respectively, is has been labelled ‘Basal Unconformity Surface’ (BUS in Fig. 1.3, 1.4).
The cover sediments, which display a marked lateral heterogeneity, are subdivided into two main groups: i) early post-rift metasediments, consisting of metaradiolarites and marbles ranging in thickness from a few centimeters to several meters; ii) post-rift metasediments, which consist of calcschists sensu lato and metaconglomerates of both continental and ophiolitic origin (“Morosaglia metaconglomerates”; Sedan, 1983). The two groups are separated by a transitional contact characterized by interlayering of both types of lithologies (i.e. marbles and calcschists) for a minimum thickness of several tens of centimeters.
Metaradiolarites are overlain by marbles, which are widely distributed all along the Monte San Petrone unit and vary in thickness from a few tens of centimeters to several meters. The thickest portions commonly contain detrital layers with pebbles ranging in diameter from a few centimeters to a few tens of centimeters (e.g., east of Punta Ventosa, Monte Calleruccio and Aja Rossa). These pebbles, which consist of meta-granitoids and polycyclic basement (to the east of Punta Ventosa and Monte Calleruccio) or basaltic clasts (in the Aja Rossa area), are chemically and petrographically comparable to the rocks of the nearby continental sliver.
The post-rift cover, which is exposed in the western part of the Monte San Petrone unit, consists mainly of calcschists. They are relatively homogeneous, apart from the presence of impure marble layers folded at the hundred-meter scale. Calcschists commonly contain lawsonite porphyroblasts pseudomorphically replaced by aggregates of white micas.
P-T-t-d evolution of the Monte San Petrone Unit
The Monte San Petrone unit is characterized by polyphase HP deformation, leading to the formation of west-dipping fabrics (dip direction ~ N270/30) (Fig. 2.5). These fabrics are marked by HP minerals, i.e. omphacite, garnet, lawsonite and glaucophane in metabasalts, phengite, lawsonite and jadeite in metagranitoids, indicating lawsonite eclogite-facies conditions. Stretching lineations associated with HP conditions, mostly striking at ~ N250/20, are defined by jadeite and Na amphibole in metagranitoids and by elongated quartz rods in calcschists. Metagranitoids are commonly deformed by plurimetric sheath folds (Fig. 2.6). Non-cylindrical folds are typically found within the thickest parts of continental basement, giving rise to meter-scale sheath folds. On the other hand, hundred meter-scale isoclinal folds are commonly observed in the post-rift cover. All folds formed under HP conditions are characterized by highly scattered fold hinges, with a maximum orientation coinciding with the mineral stretching lineations, while the orientation of the axial planes is significantly more uniform (poles = ~ 90/70). A late folding event developed under epidote-blueschist facies conditions is then responsible for the formation of large-scale anticlinal and synclinal open folds.
Figure 1.5. Stereoplot
(a) Stereoplot of foliations (circles) and high-pressure stretching lineations (triangles) of continental and metasedimentary rocks. (b) Stereoplot of fold hinges (circles) and axial planes (triangles) of continental and metasedimentary rocks.
Figure 1.6. High-pressure sheath folds
a: High-pressure sheath folds in metagranite. Note the curved fold hinges. b, c: detail of high-pressure sheath folds in metagranite. Modiefied from Vitale Brovarone et al. (2011b).
From a metamorphic point of view, the different lithologies of the Monte San Petrone unit underwent a common evolution characterized by a prograde path under HP/LP conditions culminating at T = 490-550°C and P = 2.2–2.6 GPa (Vitale Brovarone et al., 2011b). The retrograde evolution is characterized by a pervasive retrogression at lawsonite-blueschist to epidote-blueschist facies conditions, followed by more localized re-equilibration at greenschist facies conditions. Eclogitic assemblages are often well preserved in meta-basalts, and consist of coexisting omphacite + lawsonite + garnet + phengite + titanite or glaucophane + actinolite + lawsonite + garnet + phengite + titanite, equilibrated at 520 ± 20°C and 2.3 ± 0.1 GPa (Vitale Brovarone et al., 2011a). The coexistence of these two mineralogical assemblages is interpreted as related to the different petrological behaviour of primary basaltic composition with respect to basaltic rocks variably hydrothermalized at the seafloor (Vitale Brovarone et al., 2011a). Peak metamorphism assemblages are more rarely preserved within continental basement rocks, where they allow to estimate T ≥ 490°C and P ~ 24 ± 2 Gpa. Calcschists are commonly strongly retrogressed. Raman Spectroscopy of Carbonaceous Material (RSCM) on these metasediments gives a peak metamorphism T at ~510-550°C.
Recent U-Pb dating on zircon from this unit provides a Late Eocene age (~34 ± 0.8 Ma; Martin et al., 2011) for eclogite facie metamorphism.