Mont Blanc – Aiguilles Rouges Massifs
The Mont Blanc, as the Argentera Massif, straddles the boundary between Italy and France in the NW Alps. Elevations in the Mont Blanc Massif are much higher than in the Argentera, culminating at 4810 m in Mont Blanc itself, the highest point in the European continent. This summary is largely based on an extensive compilation of previous works that can be found in von Raumer & Bussy (2004), in Bussy et al. (2001), and in the web site at: http://www.unil.ch/webdav/site/igp/shared/stampfli_research/field_trips/field_trip1.pdf
The Mont Blanc Massif (Fig. 13) consists of a complex pile of tectonic units with contrasting peak P-T metamorphic conditions. These units are separated by major, steeply dipping NNE-SSW faults and/or mylonitic zones. Most of these tectonic contacts probably formed during the late Variscan strike-slip regime and were reactivated during the Alpine orogeny. One such fault is the faille de l'Angle (“Angle fault”; Fig. 13) that separates the Massif in two portions, each one with its sedimentary cover: an internal portion, essentially granitic, to the E, and a more composite external portion to the NW (Epard, 1990).
Unlike the Argentera, the Mont Blanc Massif mainly consists of Variscan granitic rocks, with only minor paragneiss sequences and Ordovician metagranitoids. Its pre-granitic history may be better understood in the nearby Aiguilles Rouges Massif, where a polymetamorphic basement is extensively exposed (Fig. 13).
Like the Argentera, the Mont Blanc Massif was only marginally reworked during the Alpine tectonometamorphic cycle, developing low-T greenschist-facies mineral assemblages (albite, stilpnomelane, green biotite, epidote, actinolite: von Raumer & Bussy, 2004)
Figure 13. Mont Blanc - Aiguilles Rouges Massifs: geotectonic map
Geotectonic map of the Mont Blanc Massif, from Bussy et al. (2001) and von Raumer & Bussy (2004), modified. Distribution of Carboniferous sedimentary rocks in the Mont Blanc Massif after Gidon (1976).
Polymetamorphic basement and Ordovician metagranitoids
According to the geochronology of its polymetamorphic basement, the Mont Blanc Massif underwent at least three orogenic cycles: Late Precambrian, Ordovician and Variscan (von Raumer et al., 1999).
Magmatic ages of ~ 450 Ma have been obtained from zircons of MORB-like HP mafic rocks (Paquette et al., 1989), and from S-type and I-type calc-alkaline metagranitoids (“Ordovician granitoids”: Bussy & von Raumer, 1994). The “Ordovician granitoids” intruded detrital sequences (now paragneisses) of supposedly Late Precambrian to Ordovician age and mainly consisting of sandstones and graywackes, with minor carbonate intercalations and tholeiitic basaltic layers (von Raumer & Bussy, 2004). It has been suggested that flysch-type sediments enriched in Cr and Ni, which are associated to mafic rocks (now eclogites) and ultramafic rocks (Aiguilles Rouges), represent a former accretionary prism (von Raumer, 1998).
The subsequent HP event has not been precisely dated. Although observed in metapelites and paragneisses as relicts of staurolite and kyanite (the latter being sometimes rimmed with reaction coronas of cordierite and spinel), the HP assemblage (1.4 GPa and 700°C) is better preserved in the retrogressed eclogites of Lac Cornu (Aiguilles Rouges, Fig. 13; Liégeois & Duchesne, 1981). An isothermal decompression to ~700 °C and 0.8 GPa and a later amphibolite-facies overprint is preserved in retrogressed eclogites from Val Bérard, Aiguilles Rouges (Fig. 13; Schulz & von Raumer, 1993). Leucocratic rocks associated with some eclogitic boudins have been interpreted as early decompression melting products linked to the exhumation from HP conditions (von Raumer et al., 1996). A preliminary dating of zircons from one of these leucosome yielded an age of ~340 Ma (Bussy & Schaltegger, quoted by Bussy et al., 2001).
The paragneisses display a succession of deformation events attributed to the Variscan orogeny, with thrust tectonics (Dobmeier, 1998) and nappe stacking, which led to the development of Barrovian metamorphism (von Raumer et al. 1999). Metapelites of the Aiguilles Rouges experienced a typical Barrovian evolution, recording a clockwise P-T path characterized by sillimanite-bearing assemblages (biotite + plagioclase + quartz +sillimanite ± garnet ± cordierite ± muscovite ± K-feldspar) and by late stage andalusite found in quartz ± K-feldspar tension gashes (von Raumer, 1984). Metagraywackes and metagranites show evidence of partial melting of variable degree, ascribed to isothermal decompression during exhumation processes. Typical mineral assemblage of these migmatites is plagioclase + quartz + K-feldspar + biotite ± sillimanite ± muscovite ± garnet ± cordierite. A decompressional event is also documented in polymetamorphic paragneiss collected in the French part of the Mont Blanc Tunnel (Borghi et al., 1987), where an earlier amphibolite-facies mineral assemblage with garnet I, oligoclase, white mica I, is formed before the development of the Variscan regional foliation defined by biotite, white mica II, K-feldspar and garnet II. The thermal peak has been dated at 327 ± 2 Ma by U-Pb analyses on monazites from Lake Emosson micaschists (Fig. 13; Bussy et al., 2000). Monazite from a leucosome vein of a migmatitic graywacke at Lake Emosson yielded a crystallization age of 320 ± 1 Ma, whereas monazite from a migmatitic granite from the Mont Blanc Massif yielded 317 ± 2 Ma (Bussy et al., 2000). These ages are similar to that suggested by Rubatto et al. (2001) for migmatization of the Argentera GSV Terrane (≤ 323 ± 12 Ma) on the basis of a zircon lower intercept age obtained for the Meris eclogite.
Greenstone Unit and Visean volcano-sedimentary sequence
The SW part of the Aiguilles Rouges Massif (Fig. 13) is characterized by the presence of the so-called “Greenstone Unit” and of low grade metamorphic sediments and volcanics (Dobmeier, 1996; Dobmeier et al., 1999). The Greenstone Unit is composed of greenish, calc-alkaline volcanic rocks of rather low metamorphic grade (von Raumer & Bussy (2004), presumably representing an Early Paleozoic arc environment (Dobmeier et al., 1999), though a Devonian or Silurian age cannot be excluded (von Raumer & Bussy (2004). The low grade metasediments consist of variably deformed pelites, graywackes and sandstones (Bussy et al., 2001), which were metamorphosed at greenschist-facies conditions [chlorite zone, Dobmeier, 1996, 1998)] An Early Carboniferous deposition age was obtained using palynological data (Late Visean acritarchs, Bellière & Streel (1980). Meter-thick bands of green metabasalts (Fe-basalts of E-MORB affinity; Dobmeier, 1996) and meta-andesite are found interlayered with the metagraywackes. These rocks possibly record Early Carboniferous transtension linked to the opening of the sedimentary basin and to the 330-340 Ma high-K magmatic pulse.
Carboniferous magmatism
Carboniferous magmatic rocks mainly consist of non- to weakly-metamorphosed intrusions, locally associated to subvolcanic rocks (Fig. 13). Three magmatic pulses are recognized in the Mont Blanc – Aiguilles Rouges area: a) high-K calc-alkaline to shoshonitic, b) peraluminous, and c) metaluminous, ferro-potassic and alkali-calcic (Bussy et al., 2001).
The high-K calc-alkaline to shoshonitic magmatism
The 330-340 Ma high-K calc-alkaline to shoshonitic event that is well known throughout the European Variscan belt is documented by the Pormenaz monzonite (333 ± 2 Ma; Bussy et al., 2000) and the Montées-Pélissier granite (331 ± 2 Ma; Bussy et al., 2000), both in the southwestern Aiguilles Rouges.
The Pormenaz monzonite was emplaced at shallow depth, along the transpressive border fault of the Visean volcano-sedimentary basin. The main facies is porphyritic to equigranular with pink or white cm-sized K-feldspar megacrysts in a dark grey-green amphibole-rich matrix. Euhedral crystals of brown titanite are clearly visible in hand specimen. Metre-sized bodies of durbachite are found as dark-green microgranular magmatic enclaves (von Raumer & Bussy, 2004).
Like the Pormenaz monzonite, the Montées-Pélissier granite was emplaced syntectonically along ductile shear zones during the transcurrent stage, and recorded the subsequent vertical movements during cooling (Dobmeier, 1996). It is a foliated fine-grained two-mica monzogranite, which hosts rare biotite-rich restites, mafic microgranular enclaves and biotite-bearing lamprophyres (von Raumer & Bussy, 2004). The lamprophyre-type mafic magmatism is systematically associated to the Mg-K granitoids: the isotopic composition of Sr, Nd and Hf, and the episodic zircon inheritance all point to an essentially high-K lithospheric mantle source, metasomatized during an earlier subduction event, mixed with variable amounts of lower crust material (von Raumer & Bussy, 2004).
The peraluminous magmatism
The second magmatic pulse is represented by peraluminous granites intruded simultaneously into the Aiguilles Rouges and Mont Blanc Massifs at ~ 307 Ma: the Vallorcine and Montenvers granites, and the Fully granodiorite, respectively (Fig. 13).
The Vallorcine granite (Aiguilles-Rouges Massif) is a syntectonic pluton, which intruded at 306.5 ± 1.5 Ma (Bussy et al., 2000) along a steeply dipping, NE-SW trending, dextral strike-slip shear zone. During Late Variscan, the Vallorcine granite was affected along its SE contact by intense low-T ductile shearing subsequently reworked during the Alpine orogeny. The Vallorcine intrusion is a typical S-type granite with Al-rich minerals, such as cordierite, muscovite and andalusite, abundant zircon inheritance and a crustal stable isotope signature (Brändlein et al., 1994). Two facies have been differentiated in the Vallorcine granite (Brändlein et al., 1994): 1) the topographically lower facies is a biotite-rich monzogranite hosting numerous enclaves (up to 30 cm in size), including xenoliths of country gneiss, hornfelses, micaceous restites with sillimanite and hercynite, cordierite-bearing leucogranites, and mafic microgranular enclaves; 2) the upper facies is finer grained, with less biotite and almost devoid of enclaves. Aplites and other types of dykes are concentrated within the wall rocks of the upper contact.
The peraluminous Montenvers granite (Mont Blanc Massif) was emplaced syntectonically at 307 ± 3 Ma (Bussy & von Raumer, 1994) as a sheet-like intrusion, similarly to the Vallorcine granite. It is a S-type granite now strongly deformed and often converted to a mylonitic leucocratic orthogneiss hosting both microgranular and restitic enclaves.
The Fully granodiorite (Aiguilles-Rouges Massif; Krummenacher, 1959) is a highly heterogeneous, coarse-grained granodiorite of migmatitic appearance (presence of Schlieren and nebulitic structures) and characterized by clots of pinitized cordierite, scattered K-feldspar megacrysts, and numerous small biotite-rich restitic enclaves. Cordierite-bearing leucogranitic dykes and stocks crosscut the main plutonic facies. The Fully granodiorite is a typical peraluminous, anatectic granitoid with a significant restitic component and abundant Al-rich minerals (garnet, cordierite, muscovite, hercynite). It might represent a deeper and less evolved equivalent of the Vallorcine granite. Micaschists, gneisses, marbles and amphibolites are found as dm-long xenoliths. Enclaves of magmatic rocks include microgranular quartz-diorite to granodiorite and angular to rounded pieces of fine- to coarse-grained gabbros up to one meter in size (Bussy et al., 2000). Zircon and monazite, extracted from granodiorite, leucogranite and gabbro, yielded ages of 307 ± 2 Ma for all these rock types (Bussy et al., 2000). Although solid at time of its incorporation into the anatectic mass, the gabbro enclaves are coeval with the acid magmatism, suggesting large-scale dehydration melting of crustal units in close association with mantellic magmas (von Raumer & Bussy, 2004). These field relations have been related to mantellic basic magmatism that was active at the time of crustal anatexis (bimodal magmatism: Bussy et al., 2001).
The metaluminous, ferro-potassic, alkali-calcic magmatism
The last magmatic pulse within the Mont Blanc – Aiguilles Rouges area is represented by the voluminous 303 ± 2 Ma Mont Blanc granite (Fig. 13), a foliated, porphyritic monzo- to syenogranite with K-feldspar megacrysts and Fe-rich biotite (Marro, 1988; Bussy, 1990). It hosts numerous mafic microgranular enclaves, calc-alkaline micro-monzodioritic stocks and syn-plutonic dykes of mantellic origin, which record magma mingling processes (Bussy, 1990). The Mont Blanc granite is a metaluminous, ferro-potassic, alkali-calcic intrusion characterized by high K, Y, Zr contents and Fe/Mg ratios, and a low 87Sr/86Sr initial isotopic ratio of about 0.706 (Bussy et al., 1989). Emplacement of the Mont Blanc granite is the last major magmatic event recorded in the area. However, ash-fall deposits, embedded at different levels of the Salvan Dorénaz basin of the Aiguilles Rouges Massif, have been dated at 295 + 3-4 Ma (Capuzzo & Bussy, 2000). Such ash deposits probably derive from volcanic centers located in the Aar-Gotthard Massifs of Central Alps (Capuzzo & Bussy 2000).
Carboniferous sedimentation
Upper Carboniferous sedimentation is best documented in the Salvan-Dorénaz basin of the Aiguilles Rouges (Fig. 13) where it started at the end of the Westphalian. These coarse-grained clastic sediments formed an alluvial fan system and were deposited on top of the exhumed polymetamorphic basement intruded by Variscan granitoids (Capuzzo, 2000, quoted by Bussy et al., 2001). Subsequent sedimentation of braided, anastomozed and meandering river deposits records a sedimentary evolution in a strike-slip tectonic regime. Volcanic layers within the sediments are dated to the Late Carboniferous (Capuzzo & Bussy, 2000), with an age of 308 ± 3 Ma for basal dacitic flows and of 295 ± 3 Ma for a tuff layer from the upper levels of the basin sequence.