Journal of the Virtual Explorer

The recent geochronological-geochemical studies of Rubatto et al. (2001, 2010) and the petrological study of Ferrando et al. (2008), combined with previous data, well constrain the evolution of the GSV Terrane of the Argentera massif before and during the Variscan orogeny (Fig. 15). On the contrary, the geological history of the Tinée Terrane is less documented, but the presence of eclogites and HP granulites suggests it may have been similar to that of the GSV Terrane. A major difference between the two terranes is that in the Tinée muscovite was stable during the Carboniferous amphibolite-facies event (see below) and the P-T path ended in the kyanite stability field, not in the cordierite stability field as in the GSV Terrane. Though each massif has its peculiarities and has been studied from somewhat different perspectives, the main geological events recorded in the GSV Terrane of Argentera are also recorded in the Mont Blanc-Aiguilles Rouges and Maures-Tanneron (Table 1).

Figure 15. Argentera Massif: Evolution of the GSV Terrane

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Evolution of the GSV Terrane before and during the Variscan orogeny [from Rubatto et al. (2001), modified].

The GSV Terrane of the Argentera Massif underwent in the Paleozoic a complex magmatic and metamorphic evolution beginning in the Early Paleozoic (or possibly still in the Late Proterozoic) with the intrusion of large granitoid bodies into a (meta-) sedimentary sequence of graywackes, pelites and rare limestones. In the Maures-Tanneron Massifs, this event is documented by the emplacement of the Barral granite (550-600 Ma) and of the felsic volcanic protoliths of the “leptynites” (~ 548 Ma). However, evidence of this event appears to be lacking in the Mont Blanc-Aiguilles Rouges Massifs (Table 1).

These early granitic intrusions were followed in the Early Ordovician by the emplacement of mafic magmas (now represented by eclogites and HP granulites) in the Argentera (at 480-460 Ma; Fig. 15), in the Maures-Tanneron (at 452 ± 8 Ma), and in the Mont Blanc-Aiguilles Rouges Massifs (at ~ 450 Ma). In the Argentera Massif the crustal contamination shown by the Laghi del Frisson mafic sequence supports an extensional setting, in agreement with what proposed for other External Crystalline Massifs (Ménot & Paquette, 1993; Guillot & Ménot, 2009).

A new magmatic cycle, at the Ordovician – Silurian boundary is documented by shallow sheet-like intrusions of dacite with granulitic xenoliths at 443 ± 3 Ma in the GSV Terrane (Fig. 15), by the emplacement of both the S-type and I-type calc-alkaline “Ordovician granitoids” at ~ 440 Ma in the Mont Blanc-Aiguilles Rouges Massifs, and of the granodioritic protolith of the Bois de Bagnols orthogneiss at 440-410 Ma in the Tanneron Massif (Table 1). Granulite xenoliths in the GSV Terrane metadacite are the only preserved evidence of a high-grade metamorphic basement already present in the area during Late Ordovician. Another relic of a LP granulite-facies metamorphism (P = 0.67 GPa and T >850°C;) is reported from the “Barral granite”, an aluminous porphyritic metagranite preserved in the “Bormes orthogneiss” (Gueirard, 1976).

These crystalline basements were later subjected to HP–HT metamorphism with formation, depending from the protolith bulk chemical composition, of either eclogites or HP-granulites. P-T-t conditions of this event are well constrained at ~ 1.38 GPa, 735 ± 15 °C and 340 ± 4 Ma in the GSV Terrane (Fig. 15). Similar peak metamorphic conditions (1.4 GPa and 700°C) are recorded in the Aiguilles Rouges Massif.

Peak conditions reached by the Laghi del Frisson mafic sequence indicate a geothermal gradient of ~ 16 °C/km, compatible with subduction zone environments and, in particular, with the conditions reported by O'Brien (2000) for subduction during continental collision. Relatively high exhumation rates can be inferred for the GSV Terrane from the preservation of peak mineral compositions and the recording of an almost isothermal decompression in the Laghi del Frisson mafic sequence. It is noteworthy that a similar P-T evolution has been reported from the Aiguilles Rouges Massif, where the decompression melting observed around eclogite boudins has been dated at ~ 340 Ma (Table 1). Also the Barrovian amphibolite-facies metamorphism of the Maures “leptynites” at 348 Ma possibly represents decompression following a HP metamorphic event, but this speculation awaits to be confirmed by further studies.

Exhumation of the HP rocks was followed—after a few million years—by the emplacement of characteristic Mg-K monzonite and granite. In particular, a limited magmatism of likely extensional nature is represented in the GSV Terrane by the intrusion of small dykes and bodies of K-rich monzonite dated at 332 ± 3 Ma (Fig. 15). A more extensive syn-tectonic magmatism is represented in the Mont Blanc-Aiguilles Rouges Massifs by the Pormenaz monzonite (332 ± 2 Ma) and Montées-Pélissier granite (331 ± 2 Ma), and in the Maures Massif by the the Hermitan granite (338 ± 6 Ma) and Reverdit tonalite (334 ± 3 Ma) (Table 1).

Only some 10–20 Ma after the Carboniferous HP metamorphism, an amphibolite-facies metamorphism with extensive development of migmatites occurred not only in the Argentera Massif (323 ± 12 Ma; Fig. 15) but elsewhere in the External Crystalline Massifs (Mont Blanc-Aiguilles Rouges: 327-317 Ma) and Variscan Provence (Maures-Tanneron: 317-310 Ma) (Table 1). The tight succession of HP metamorphism and amphibolite-facies anatexis suggests that, at least in the Argentera Massif, the two stages are part of the same metamorphic cycle, where intense melting occurred upon decompression and advective heat transfer. However, further research is needed to better establish the P-T path followed by the GSV and Tinée Terranes during the Carboniferous amphibolite-facies event and its timing.

The Variscan igneous cycle is closed by the emplacement at shallow crustal levels of syntectonic peraluminous granites (Table 1)—the Valle Stura leucogranite (299 ± 10 Ma) in the GSV Terrane (Fig. 15), the Vallorcine and Montenvers granites (~ 307 Ma) in the Mont Blanc-Aiguilles Rouges Massifs, possibly the Plan de la Tour granite of the Maures massif, which however seems to be older (324 ± 5 Ma), and of post-tectonic, granites (Table 1), i.e. the Central Granite (cooling ages at 299-292 Ma) in the GSV Terrane (Fig. 15), the Mont Blanc granite (303 ± 2 Ma) in the Mont Blanc Massif, the Camarat and Rouet granites (~ 300 Ma) in the Maures-Tanneron.

The final exhumation of the Massifs is marked by the deposition on the crystalline basements of Westphalian D-Stephanian A (306-303 Ma, according to the geological time scale of Gradstein & Ogg, 2004) continental sediments (Fig. 15 and Table 1). Currently available geochronological data seem to suggest that in Argentera the Tinée Terrane may have been exhumed a few My earlier than the GSV Terrane, but this has to be corroborated by further investigations.