Geology of the Montblanc massif

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This article deals with the geology of the Montblanc massif .

introduction

Structural map of the Montblanc massif (right) and its surroundings
Location of the external crystalline massifs of the Alps

Together with the adjacent Aiguilles rouges , the Belledonne , the Pelvoux and the Mercantour represents the Mont Blanc an external solid ( English External Crystalline massif abbreviated or ECM ) of the French West Alps . It consists of polymetamorphem crystalline basement (mainly gneiss and granite), the was pressed out in the course of the alpine orogenesis towards the foreland. The age of its protoliths goes back to the early Paleozoic , possibly also to the Neoproterozoic . Mainly geophysical investigations confirm its allochthonousness (strangeness). Further to the northeast, in Switzerland, are the Aar and Gotthard massifs .

The early Alpine development of the later Montblanc massif was determined by the deposition of Triassic to Paleogenic sediments, which had transgressed over thresholds from the Triassic and left behind the characteristic strata of the Helvetic and Ultrahelvetic up to the Middle Jurassic in basin areas on the southern edge of the former European continent. The end of the sediment cycle and the subsequent onset of alpine tectonics in the Middle Oligocene are marked by the thrust of the ultra-Helvetian and Penninic nappes .

The arched shape of the Western Alps in comparison to the more or less straight running Eastern Alps has occupied the minds of geologists for some time. In the meantime, three theories have been established for their development:

  • The first and oldest theory prefers radial and isochronous movements, which find their echo in the current arch shape and go back to an original arch shape that was already paleogeographically laid out (Argand, 1916).
  • the second theory sees only a lateral bulge in the western alpine arc ("side blow"), which occurred perpendicular to the generally north-south narrowing of the alpine orange (Goguel, 1963 and Boudon and colleagues, 1976).
  • the third theory is that the two branches emerged at different times, with ceiling tectonics and side shifts equally involved. The direction of transport was north-south in the initial phase, but then turned east-west in the final phase (Ricou, 1980).

Terrain findings seem to support the latter theory.

description

View of the Val Ferret . Wedged in below the Montblanc massif (left), a steeply standing ultrahelvetic. Right Pennine of the internal zone.

The Montblanc massif has roughly the shape of a weaver shuttle, stretched in a northeast direction and at the same time bent, which, due to this shape, already indicates a supra-regional right-hand shear . Along its maximum length in northeast-southwest direction - which roughly corresponds to the curvature of the northern Alpine arc - it measures 50 kilometers. The maximum width in the southeast direction is almost 15 kilometers. Spatially, it represents a dome-shaped, anticlinal bulge in the basement, which reaches its apex near the Montblanc summit. This anticline structure descends at 15 to 20 ° to the north towards Valais and at 10 to 15 ° to the south into the Beaufortain .

The autochthonous (local) Triassic covers the southern part of the gneiss , otherwise the Montblanc massif is framed by the allochthonous Mesozoic and Old Tertiary , which is still assigned to the Ultra-Helvetic . For example, in the non-metamorphic zone of Chamonix in the west (a former half-ditch) between the two basement blocks of Aiguilles Rouges and the Montblanc massif, these sediments have been folded, shifted laterally (steep fold axes) and sheared. The ultra-helvetic on the east side contains the Mont Chétif granite as a small inserted granite lens . This very thin band of ultrahelvetic is then crossed further east by the internal zone of the Alps.

Internal structure

View from the Val Ferret above Plampincieux to the southwest to the Mont-Blanc. In the middle distance there is a wedged gray ultrahelvetic.

The internal structure of the Montblanc massif is divided into two parts. On its north-western side, which is a maximum of 5 kilometers wide, there are steep orthogneiss , paragneiss and subordinate mica schists , which owe their formation to a high pressure metamorphosis during the end of the Ordovician . In the process, an east-northeast-striking foliation developed .

A subsequent regional metamorphosis under medium pressure (almandine amphibolite facies) led to the migmatization of the gneisses, whose steep foliation N 020 to N 025 deletes. This led to the formation of La Tour granite near Montenvers (also Montenvers granite on the Mer de Glace ), which is now an elongated, mylonitic orthogneiss. The rocks were then intensively mylonitized under green slate conditions and anatectically melted at high temperatures and relatively low pressure (with the formation of cordierite ). The vertical mylonites again follow the north-northeast direction and contain a steep lineation. Through the regional, penetrative mylonitic deformation, quartz was polygonized and other minerals were broken. Alkali feldspar porphyroclasts are eye-shaped. The following anatexis mainly affected the southern part of the Montblanc massif.

Only now did the actual Montblanc granite separate itself, which is connected to the metamorphic rocks in the southeast by means of a steep, northeast-trending fault , the Faille de l'Angle (or Faille du Midi ). It is a syenitic-monzonitic biotite granite with centimeter-sized alkali feldspar clasts , which, however, do not occur in the edge areas of the intrusion . The original intrusive contact of the granite, particularly exposed in the southwest of the massif, is clearly discordant and sends numerous apophyses and dykes into the adjacent rock.

The Montblanc granite was then mylonitized in the course of the Alpine orogeny to a very high degree of green slate facially and sheared like a lens in the deca and hectometer range, with the two directions N 050 to N 070 and N 000 predominating. N 120 is also of regional importance. The mylonite surfaces are almost vertical and form a three-dimensional network - the movements in the direction of N 050 to N 070 shifted to the right, those in the direction of N 000 and N 120 shifted to the left. Postponements usually favor the south and east sides. The individual shear ligaments vary in thickness from several millimeters to 50 meters. The mylonite surfaces contain the minerals muscovite , green biotite, albite , chlorite ± epidote ± sphene . However, later faults are mostly covered by fibrous chlorite or fibrous epidote.

Montblanc granite

View from Col de la Seigne (2516 m) to the northeast to Mont Blanc and the Grandes Jorasses . On the right in the foreground a steeply rising ultrahelvetic, in the middle distance dark paragneiss and in the background the Montblanc granite.

The Montblanc granite has several petrological facies. Its coarse-grained, usually white-black colored normal facies is porphyry with large, often regulated alkali feldspar crystals of up to 5 centimeters in length. The color scheme, however, is changeable and can take on purple, green and even orange tones. Towards the edge of the intrusion, the structure becomes finer-grained and also smaller-grained due to the size reduction of the alkali feldspars. On its eastern edge, the granite changes into a schisty quartz porphyry ( rhyolite ), which forms the boundary to the steeply plunging ultrahelvetic.

The granite partly contains streaks , amphibolite inclusions, micromonzodioritic sticks and microgranular mafic inclusions (English microgrenular mafic enclaves or MME ), which indicate a socialization (English mingling ) with a residual magma.

The preferred orientation of the biotites, the feldspars and the restitic inclusions forms a sub-vertical igneous foliation , which strikes in a north-northeast-south-southwest direction and is penetrated by aplites . In the horizontal plane, numerous fissures and crystallized quartz veins appear between the shear ligaments. The latter contain, in addition to spectacular automorphic quartz crystals (smoky quartz), epidote, adulara , fluorite , muscovite and calcite . Around the fissures the parent rock was metasomatically changed to so-called episyenites , where it was successively desilicated in the direction of the fissure opening (removal of quartz), but enriched with the elements Al, K and Na.

The following table provides information on the geochemical composition of the Montblanc granite, which corresponds to the average value of 17 analyzes, the average value of 7 analyzes and the average value of 5 analyzes on the Aiguille du Midi . The fine-grain facies (10 analyzes) and the porphyry are also given Facies (46 analyzes).

Oxide
wt.%		 Average (17) Average (7) Aiguille du Midi Fine-grained Coarse-grained
SiO 2 72.00 73.75 68.60 74.22 72.45
TiO 2 0.30 0.15 0.30 0.22 0.27
Al 2 O 3 13.96 13.10 13.52 12.99 13.83
Fe 2 O 3 2.38 2.35 1.77 1.93 2.25
MnO 0.04 0.05 0.04 0.05 0.05
MgO 0.68 0.25 0.51 0.45 0.48
CaO 1.09 1.04 1.13 1.23 1.31
Na 2 O 3.82 3.60 2.86 3.52 3.60
K 2 O 4.63 4.80 5.17 4.40 4.77
P 2 O 5 0.14 0.07 0.10 0.09
H 2 O + 0.82 0.54 1.74 0.52 0.59

The Montblanc granite is therefore an alkaline-calcareous, peraluminous to metaluminous Fe-K granite of the S type, which was derived from paragestones, but also contains a smaller earth mantle component, recognizable by the very frequent mafic inclusions. Its alkaline character is manifested in zirconia with A and T indices. It is very similar to other Fe-K granites such as the Aar granite or the Gotthard granite, which also appeared in the period 305 to 295 million years. It is characterized by high contents of potassium (2.5 - 6.0 percent by weight), rubidium (170 - 490 ppm), yttrium (30 - 70 ppm) and zirconium (40 - 400 ppm), and has a high Fe / Mg Ratio (low magnesium number ), but only a very low initial 87 Sr / 86 Sr ratio of 0.7050. The contents of uranium (7.4 - 19 ppm) and thorium (3 - 50 ppm) are also increased, so the rock is weakly radioactive.

The following table shows the content of trace elements and rare earths in Montblanc granite :

Trace element
ppm		 Average (7) Rare earths
ppm		 Average (7)
Ba 16-869 La 18.4-49
Be 2.2-6.0 Ce 40.9-102
Co 45 - 72 Pr 4.8-11.4
Cr 5 - 37 Nd 17.8-41.8
Cu 5-15 Sm 4.7-9.4
Ga 5-14 Eu 0.05-0.95
Nb 11-25 Gd 5 - 10.2
Ni 5 Tb 0.8 - 1.9
Rb 142-490 Dy 4.2-11.4
Sc 3.9 - 7.4 Ho 0.84-2.29
Sr 11-174 He 2.6 - 6.3
V 5 - 16 Tm 0.4-1.1
Y 20 - 70 Yb 2.7-6.9
Zn 22-82 Lu 0.45-1.0
Zr 40-400
Th 3 - 50
U 7.4-19

The main minerals are quartz (often violet colored), plagioclase , alkali feldspar and iron-rich biotite , and rarely hornblende. In addition, pyrite , violet-colored zircon , allanite , anatase , sphene , fluorite , beryl , molybdenite , magnetite , hematite , thorite , calcite and apatite are added .

Fe-K granites usually indicate the beginning of the postorogenic phase in the Wilson cycle - characteristic of the readjustment of a tectonically thickened crust. Their melts are likely to have been influenced by mantle springs, which were assimilated and contaminated by the orogenically thickened crust on their ascent. At the same time, biotite-feldspar fractionation in the middle crust may have taken place under high pressures. It can be assumed that the bimodal magma association of the Montblanc massif was created within a laterally shifting, transtensive regime in a pull-apart basin with dehydration and pressure relief.

deformation

The 3407 meter high Aiguille de Roc . The needle shows very nicely the vertical mylonitic shear bands - characteristic of Montblanc granite.

The alpine deformation of the Montblanc massif had different effects on the basement and the sedimentary shell on top of it. However, both units were initially tectonically stressed in the north-north-west and then in the west. Consequently, the Mont Blanc massif subject to a rotation counterclockwise, in the final analysis on a large scale took place rotation Africa in Neogen is expected to decline. Since the Chamonix zone was active as a major right-shifting shear zone, the current location of the Aiguilles Rouges in relation to the Montblanc massif is accidental and only temporary. It is assumed that the Montblanc massif was at least 20 kilometers further east before the onset of alpine orogenesis.

Alpine metamorphosis

In the course of the alpine metamorphosis, the green slate facies were generally reached in the Montblanc massif, and in places the beginning of the amphibolite facies was also realized. This corresponds to pressure-temperature conditions of 0.25 to 0.3 gigapascals and 400 to 420 degrees Celsius. With a geothermal gradient of 37 degrees per kilometer, an approximately 10 kilometer thick rock column should have supported the massif. These results are in good agreement with an erosion rate of 0.7 millimeters per year on Montblanc. Quartz fluid inclusions in open alpine fissures even resulted in an overburden of 13 kilometers. More recent studies go even further and suggest a pressure of 0.45 to 0.55 GPa and thus a load of 15 to 20 kilometers.

The pT conditions show a drop in both space and time, spatially from the massif towards the foreland and temporally from the beginning to the end of the metamorphosis.

During this metamorphosis, the minerals green biotite , chlorite , epidote , stilpnomelane and albite were formed in the granite . Blue-green hornblende , biotite, chlorite, phengite , actinolite , epidote, chloritoid and clinozoisite formed in the gneisses .

Structural interpretation

The spatial structure of the Montblanc massif has so far been interpreted very differently:

  • as a crystalline fold core of the back- compressed Morcles ceiling (Ramsay et al., 1983)
  • as an actively emerging pop-up structure , recognizable by the fan-like arrangement of the faults in the granite (Bertini et al., 1985)
  • as steep imbricate slice , the southeastern edge of which is viewed as an erect fault (Butler, 1985)
  • as a fan-like, positive flower structure (engl. positive flower structure ) within a fairly shifting corridor (Hubbard and Mancktelow, 1992)
  • as exposed Horst above a very young fault plane on the south side of the massif (Seward and Mancktelow, 1994)
  • as a bulge in a thrust ramp inclined to the northwest above the Alpine Sole Thrust , with subordinate back thrust (Leloup, 2005).

Age

Eye gneisses of the Montblanc massif (former granitoids of the S type) were dated to 453 ± 3 million years using the uranium-lead method on zircons (Upper Ordovician, Katium ). Using uranium lead at Monazite, migmatites revealed ages of 321 to 317 million years (Upper Carboniferous, Bashkirian ). The Montenvers granite, also an S-type, is 307 ± 1 million years old ( Kasimovian ). The radiometric dates of the Montblanc granite are very different ; a more recent date by Capuzzo and Bussy in 2000 showed an age of 303 ± 2 million years ( Gzhelium ).

Alpine overprinting

The biotite of the Mont Blanc granite have provided 30 to 18 million years ago (Oligocene to Miocene middle) very young Alpidic age and therefore are evidence of a partial alpidic rock alteration. The Alpid mylonite zones were also created during this period. However, the first thrusts on the Pennine nappe front had started somewhat earlier (35 million years ago at the beginning of the Oligocene ) and reached the area of ​​the Mont Blanc massif and the Pelvoux around 29.5 million years ago.

It is now assumed (Leloup et al., 2005) that the elevation of the Montblanc massif began around 22 million years ago at the beginning of the Miocene . Mylonitic shear zones formed between 18 and 13 million years. In the southeast of the massif they can be dated 16 million years ago, the movements here were pushed back to the southeast. However, since the other shearings mainly excavated to the northwest, a fan -shaped structure was created on Montblanc from the Langhium . In the opinion of Leloup and colleagues, the main fault zone on the north-western edge of the massif was only activated 12 million years ago towards the end of the Serravallium , with a vertical offset of 4 to 8 kilometers.

The dextral lateral shifts in the trapped zone of Chamonix then probably ended in the Zancleum 4 million years ago . The elevation of the massif continued, however, and is now mainly developing along the thrust back on the south-eastern edge.

literature

  • Francis Bussy, Jean Hernandez and Jürgen Von Raumer: Bimodal magmatism as a consequence of the post-collisional readjustment of the thickened Variscan continental lithosphere (Aiguilles Rouges-Mont Blanc Massifs, Western Alps) . In: Transactions of the Royal Society of Edinburgh: Earth Sciences . tape 91 , 2000, pp. 221-233 .
  • Pierre Gourlay: La déformation alpine des massifs cristallins externes (Mont-Blanc, Aiguilles Rouges, Belledonne) et celle de leur couverture mésozoïque (Alpes occidentales) - PhD thesis . Université Pierre et Marie Curie - Paris 6, 1984, p. 131 .
  • LE Ricou: Les Alpes occidentales: chaîne de décrochement . In: Bull. Soc. Géol. Fr. 7, t. XXVI, no.5, 1984, p. 861-874 .
  • Paul Tapponnier : Évolution tectonique du système alpin en Méditerranée: poinçonnement et écrasement rigid-plastique . In: Bull. Soc. Géol. Fr. (7) XIX, 1977, pp. 437-460 .

Individual evidence

  1. ^ G. Perrier and P. Vialon: Les connaissances géophysiques du Sud-Est de la France. Implications géodynamiques. In: Géologie Alpine . tape 56 , 1980, pp. 13-21 .
  2. ^ Émile Argand : Sur l'arc des Alpes occidentales . In: Eclogae geologicae Helveticae . tape 15 , 1916, pp. 145-192 .
  3. ^ J. Goguel: L'interprétation de l'arc des Alpes occidentales . In: Bull. Soc. Géol. Ms. Band (7) 5/1 , 1963, pp. 20-33 .
  4. ↑ J. Boudon, P. Vialon and JP Gratier: L'arc alpin occidental: réorientation de structures primitivement EW par glissement et étirement dans un système de compression global NS . In: Eclogae geologicae Helveticae . tape 69/2 , 1976, pp. 509-519 .
  5. ↑ LE Ricou: La zone subbriançonnaise des Alpes occidentales interprétées comme la trace d'un ample décrochement senestre subméridien . In: CR Acad. Sci. Paris . 290 (D), 1980, pp. 835-838 .
  6. ^ Y. Rolland, S. Cox, A.-M. Boullier, G. Pennacchioni and N. Mancktelow: Rare earth and trace element mobility in mid-crustal shear zones: Insights from the Mont Blanc Massif (western Alps) . In: Earth Planet. Sci. Lett. tape 214 , 2003, p. 203-219 .
  7. ^ François Bussy: Petrogenèse des enclaves microgrenues associees aux granitoïdes calc-alcalins: example des massifs varisque du Mont Blanc (Alpes occidentales) et miocène du Monte Capanne (Ile d'Elbe, Italie). PhD thesis . In: Mémoires de Géologie (Lausanne) . tape 7 , 1990.
  8. ↑ JF Von Raumer: On the metamorphosis of amphibolitical rocks in the old crystalline of the Mont-Blanc and Aiguilles Rouges massif . In: Switzerland. Mineral. Petrogr. Mitt. Band 54 , 1967, p. 471-488 .
  9. ↑ B. Poty: La croissance des cristaux de quartz dans les filons sur l'exemple du filon de la Gardette (Bourg d'Oisans) et des filons du massif du Mont-Blanc . In: Sciences de la Terre . Mem. 17, 1969, pp. 162 .
  10. ↑ Jürgen von Raumer and François Bussy: Mont Blanc and Aiguilles Rouges - Geology of their polymetamorphic basement (External Massifs, Western Alps, France-Switzerland) . In: Mémoires de Géologie (Lausanne) . No. 42, 2004, p. 203 .
  11. ↑ Ralph Böhlert et al: Comparison of Exposure Ages and Spectral Properties of Rock Surfaces in Steep, High Alpine Rock Walls of Aiguille du Midi, France . In: Ninth International Conference on Permafrost . 2008, p. 143-148 , doi : 10.5167 / uzh-2822 .
  12. ↑ Magali Rossi, Yann Rolland, O. Vidal and SF Cox: Geochemical variations and element transfer during shear zone development and related episyenitisation at middle crust depths: insights from the study of the Mont Blanc Granite (French Italian Alps) . In: Geological Society London Special Publications . tape 245 (1) , 2005, pp. 373-396 , doi : 10.1144 / GSL.SP.2005.245.01.18 .
  13. ↑ F. Debon, F. and M. Lemmet: Evolution of Mg / Fe ratios in late Variscan plutonic rocks from the External Crystalline Massifs of the Alps (France, Italy, Switzerland) . In: Journal of Petrology . tape 40 , 1999, pp. 1151-1185 .
  14. ^ Francis Bussy, Jean Hernandez and Jürgen Von Raumer: Bimodal magmatism as a consequence of the post-collisional readjustment of the thickened Variscan continental lithosphere (Aiguilles Rouges-Mont Blanc Massifs, Western Alps) . In: Transactions of the Royal Society of Edinburgh: Earth Sciences . tape 91 , 2000, pp. 221-233 .
  15. ^ JG Ramsay, M. Casey and R. Kligfield: Role of shear in development of the Helvetic fold-thrust belt of Switzerland . In: Geology . tape 11 , 1983, pp. 439-442 .
  16. ↑ G. Bertini, M. Marucci, R. Nevini, P. Passerini and G. Sguazzoni: Patterns of faulting in the Mont Blanc granite . In: Tectonophysics . tape 111 , 1985, pp. 65-106 .
  17. ↑ RWH Butler: The restoration of thrust systems and displacement continuity around the Mont Blanc massif, NW external Alpine thrust belt . In: Journal of Structural Geology . tape 7 , 1985, pp. 569-582 .
  18. ↑ M. Hubbard and NS Mancktelow: Lateral displacement during Neogene convergence in the western and central Alps . In: Geology . tape 20 , 1992, pp. 943-946 .
  19. ↑ D. Seward and NS Mancktelow: Neogene kinematics of the central and western Alps: evidence from fission-track data . In: Geology . tape 22 , 1994, pp. 803-806 .
  20. ^ A b P. H. Leloup, N. Arnaud, ER Sobel and R. Lacassin: Alpine thermal and structural evolution of the highest external crystalline massif: The Mont Blanc . In: Tectonics . tape 24 , 2005, doi : 10.1029 / 2004TC001676 .
  21. ^ Francis Bussy and Jürgen von Raumer: U – Pb geochronology of Palaeozoic magmatic events in the Mont Blanc crystalline massif, Western Alps . In: Swiss Mineralogical and Petrographic Communication . tape 74 , 1994, pp. 514-515 .
  22. ^ A b Francis Bussy and J. Hernandez: Short-lived bimodal magmatism at 307 Ma in the Mont Blanc / Aiguilles Rouges area: a combination of decompression melting, basaltic underplating and crustal fracturing. Abstract 3rd workshop on Alpine Geological Studies . In: Quad. Geodin. Alpina e Quaternaria . tape 4, 2 . Oròpa-Biella 1997.
  23. ↑ N. Capuzzo and F. Bussy: High-precision dating and origin of synsedimentary volcanism in the Late Carboniferous Salvan-Dorénaz basin (Aiguilles-Rouges Massif, Western Alps) . In: Switzerland. Mineral. Petrogr. Mitt. Band 80 , 2000, pp. 147-167 .
  24. ^ P. Baggio, G. Ferrara, R. Malarodo: Results of some Rb / Sr age determinations of the rocks of the Mont-Blanc tunnel . In: Bull. Soc. Geol. It. (Roma) . tape 86 , 1967, pp. 193-212 .
  25. ↑ Bénédicte Cenki-Tok, James R. Darling, Yann Rolland, Bruno Dhuime and Craig D. Storey: Direct dating of mid-crustal shear zones with synkinematic allanite: New in-situ U-Th-Pb geochronological approaches applied to the Mont Blanc massive . In: Terra Nova . tape 26 , 2014, p. 29-37 , doi : 10.1111 / ter.12066 .