P-T AND CHEMICAL EVOLUTION OF THE MONT-BLANC GRANITE
DURING THE ALPINE OROGENESIS.
Magali ROSSI(1), Yann ROLLAND(2) and Olivier VIDAL(1)
(1) Laboratoire de Géodynamique des Chaînes Alpines, Grenoble, France; (2) Géosciences Azur, Nice, France
Contact: Magali Rossi, mrossi@ujf-grenoble.fr
AAR
Quaternery formations
Gottard
Lausanne
Préalpes
External Alps:
Simplo-Tessin nappes
Genève
Prealpine Nappes ( Romand &
prealps )
Mont Rose
A.R.
Annecy
Rhô
ne
Lyon
Dent-Blanche
Grand
Paradis
lle
Grenoble
re
Be
Isè
Pennic Front
do
nn
e
Chambéry
French
Massif
Central
Austro-Alpine nappes
Dauphinois / Helvetic zone
Mt-B.
Valence
External crystalline massifs
Sesia
Internal Alps :
Ambin
Turin
Briançon Dora
Exotic Flysch (Helminthoïds, Antola
Pô
Maira
Pelvoux
Briançonnais Zone
e
nc
ra
Du
Gênes
Schistes Lustrés complex
Var
Digne
Rhône
Internal crystalline massifs
Arg
en
ter
a
Austro-Alpine Units of Occidental Alps
Avignon
Castellanne
Nice
a
err
dit
é
M
née
The Mont-Blanc massif is one of the Variscan external crystalline massifs of the western Alps. It is
composed of gneisses and a granitic intrusion of 35x10 km striking NNE-SSW. This batholith, which has
been dated 300 Ma was exhumed during the alpine orogenesis. The Mont-Blanc massif does not appear to
have been intensely deformed during the Hercynian tectonics events, but has been notably (althougth
heterogeneously) deformed and altered during the Alpine orogenesis. The Alpine deformation is featured
by:
1. shear zones : zones of intense vertical flattening. Avalable geochronological data point out to a main
tectonic event between 30 and 18 Ma (eg. Letewin and al., 1974; Marshall and al., 1998) and preliminar PT estimates obtained on shear zones record relatively high pressure of 5-6 kbar at 400±50°C.
2. zones of less penetrative flattening (boudins) that are locally altered into episyenites (zones of
dequartzified granite), developed as haloes surrounding open horizontal quartz veins.
Even though several magmatic facies are identified in the granitic intrusion depending on mineral grainsize, the Mont-Blanc massif chemistry is very homogeneous at a regional scale. This chemical
homogeneity has allowed us to study chemical composition changes from deformed or altered granite to
bulk granite.
Thin section of unaltered Montblanc granite.
Marseille
Structural relationships between episyenites and shear zones
Deformation in the Mont Blanc massif globally reflects strong NW-SE
compression and vertical extension. In cross-section (see rigth, A), the
shape of the Mont-Blanc defines a pop-up structure, with divergent NWand SE-directed thrusting of the crystalline basement over its Helvetic
cover.
Within the Mont Blanc crystalline basement, deformation is partitioned
into (see rigth, A):
1. Intensely schistosed granite = Shear Zones (due to intense vertical
flattening),
2. Zones of relatively undeformed granite = « granitic boudins ». The
width of these zones is of the order of several 100 m, and their length of
about 1000 m. They form the proper Mont Blanc massif spires, while the
shear zones form the depressions between them (see picture shear zone).
Fault data analysis in Shear Zones demonstrates that they have formed
perpendicularly to the main stress axis (s1, see left, B). Alternatively,
deformation in the granitic boudins is accommodated by vein opening,
parallel to s1 and perpendicular to s3 (see left, B).
M ont B lanc M assif
N
s1
Shear zones
4000
( m)
Boudins of
relatively
undeformed granite
S
The transition between shear zones and granitic boudins (see left, C) is
rather progressive, from intense flattening in shear zones to the core of
the boudins, where the granite is nearly unschistosed. In parallel to the
progressive attenuation of the flattening, veins width widen
progressively. Episyenitic alteration develops as haloes surrounding the
veins. Its width directly relates to that of the vein.
s1
3000
2000
Mt Blan c
t un nel
1000
0
A
1
2
3
4
5
6
7
8
9
10
( km)
11
Unalter ed granite
s3
vein
Episyenites
s1
C
Quartz-adularia
s1
Shear Zones
Episyenitic alteration (haloes)
NE
SW
veins
Late chlorite fillings
B
s3
Legend:
Helvetic schists
Mont Blanc granite
Mont Blanc gneisses
The deformation of the Mont-Blanc massif is mainly accomodated by a
network of major shear zones striking mostly NNE-SSW (partly dextral)
and N-S (partly sinistral). These two shear zone directions form a complex
anastomosing netwotk that crosscuts the whole massif. A progressive
transition from undeformed to highly deformed granite is observed in the
shear zones from rim to core.
Episyenites are horizontal alteration features within undeformed granite.
These features outcrop as highly porous metric lenses or as haloes
surrounding open veins that main contain authigenic quartz.
N
Mineralogy
Mineralogy
Cha monix
PF
Norma l
Fa ult
Thrus t
Ge olo gic a l form a tions :
The high episyenite porosity results from
the dissolution of quartz and biotite
grains. Secondary albite grows in the
continuity of ancient plagioclase and Kfeldspar.
Bria nçonna is Units
(Inte rnal Alps )
Mon t Bla nc
PF
P e nnic Front (P F )
ma rls a nd
lime s tone s
Ca rbonife rous He lve tic Units
Gra nite
PF
Episyenitization
Mass transfer within episyenites
Qtz II
4
55
D
50
45
3
40
35
2
30
25
Fe2O3
Episyenitization is an alteration process that results from strong fluidrock interactions. Three main stages are be differenciated during the
episyenitization of the Mont-Blanc granite:
1. vugs formation via quartz and biotite leaching (coupled with a
decrease of Si, Fe and Mg whole-rock contents)
2. Recrystallization of quartz and albite (coupled with an incresase of the
Al, and Na whole-rock contents)
3. vug filling by crystallization of vermicular chlorite (coupled with
increasing Al)
The dissolution and the precipitation of various minerals for each stage
implies that the fluid's chemical composition evolves through time.
20
15
1
0
10
0
0.1
0.2
0.3
0.4
Distances (m)
0
0.1
0.2
0.3
0.4
Distances (m)
Geochemical analysis: comparison of episyenites and shear zones
REE and trace compositions
Major element compositions
Possible range for isocon position
6
5
8
4
C
f
6
3
4
2
R
2
1
0
0
50
60
70
80
S iO2
90
50
(wt%)
8
60
70
80
90
(wt%)
S iO2
(wt%) 2 5
7
A l2 O 3
Na 2 O
(wt%)
R
C
C
6
20
C
15
5
4
R
10
Other major element contrasts between shear zones and episyenites are:
1°-The overall K2O enrichment in episyenites and K2O depletion in most
shear zones,
2°-The overall MgO (and FeO, not shown here) depletions in episyenites
and respective enrichments in the shear zones.
5
2
0
1
50
60
70
80
90
50
60
70
80
Al 2 O3
(wt%)
modelled variations of Al2 O3
concentrations due to SiO2 input /
output from the average unaltered
granite composition
90
S i O 2 (wt%)
S i O 2 (wt%)
20
Al enrichment
18
trend of episyenites
16
Legend:
Episyenites (Poty, 1974; this study)
One transect from unaltered granite to the episyenite core
( R : rim ---------> C : core)
Shear zones (Rolland et al., in prep.)
Central part of the massif
SE part of the massif
Unaltered granite compositions
(Marro, 1986; Bussy, 1990;
this study)
14
Al depletion
12
63
68
7 0 Mg
90
B a/ 1 0
S i
1 0 Cs
1 0 Cu
R2 = 0 . 9 9
Zn
1 0 Hf
1 0 Na
1 0U
40
V
R2 = 0 . 9 9
10 0 Ti
10 0P
maj or elements
( cat %)
Y
Th
trac e elements
( ppm)
4 0 Ca
6 0 0 Mn
Depletions
0
73
78
S iO2
(wt%)
Episyenites and shear zones have contrasted, and for most oxides -even-,
opposite behaviours in terms of major element compositions.
This observation agrees with processes of elemental diffusion and
advection between the altered granite (episyenites), and veins + shear
zones.At the scale of the massif, episyenites may provide a source of
silica, Mg and Fe for both veins and shear zones. In parallel to that, K
(and sometimes Al) which have been dissolved in shear zones are
provided to feldspar growth in episyenites and veins.
70
20
40
60
10K
MB148B
Episyenite,
80
1 0 Na
40
?
Ba/10
5Ni
60
Si
Ce
R2 = 0.99
40
600Mn
100Ti
Th
Nd
10 U
70Mg
10Hf
40Ca
Zn
5Gd
10Cs
Y
10 100P
Pr Pb
Nb
5Dy 10Yb
10Eu
20Ho
Sm 10Tb 10Ta
0
0
20
40
Zr/4
50Fe
R2= 0.98
60
6 0 0 Mn
10Ta
V
Possible range for isocon position
Nb
Th
10 0 Ti
10
100
DV < 0
10Cu
7 0 Mg
Zn
30
1 0 Cs
Pb
0
10
1 0 Na
Enrichments
R b/ 5 1 0 K
30
40
50
60
70
80
90
1 0 Hf
40
5 0 Fe3 +
V
30
Y
6 0 0 Mn
1 0 0T i
20
S r/ 2
Th
Nb
Ca
R b/ 5
1 00 P
1 0 Cs
Pb
100
1 0 Cu 5 N i
Zn
50
0
10
Depletions
10K
B a/ 1 0
1 0 Ta
0
20
5 Al
Si
60
10
Depletions
1 0 Sc
70
B a/ 1 0
Undeformed granite average
10Na
20
5 0 Fe3 +
5Al
m y l o ni t e
80
20
30
40
50
60
70
80
90
100Lu
80
Compositions of representative shear zones and
episyenite samples from across the massif have
been plotted. For each rock type, specific
elements are enriched and depleted. These
enrichments and depletions are directly related
to the nature of the recrystallizing assemblages
and possible substi-tutions in the mineral
lattice.
100
Undeformed granite average
In cases of muscovite recrystallisation, enrichments in K, Mg, Ba, Rb, Sr are
explained by the chemical composition of white mica (of phengite type). Similar
interpretations can be done for chlorite and epidote. In episyenite, enirchment of
Na, Sr and Ba indicates the crystallization of albite. Furthermore, most elements
of the episyenite are depleted, which is compatible with strong leaching. As for
episyeniye, depletions are explained by the alteration of previous (unstable)
magmatic minerals such as plagioclase (Ca, Sr) and K-feldspar (K, Ba, Rb, Cs) for
shear zone samples. REE compositions are variable because of the alteration of
magmatic REE-bearing minerals (mainly allanite) and precipitation of
metamorphic REE-minerals (aeschynite, bastnäsite, monazite….).
Conclusion
1. Shear zones networks and episyenites are Alpine structures metamorphosed in green schist facies conditions
(epidote, muscovite and chlorite-bearing assemblages). The chemical compositions allows the calculation of P-T
equilibria during deformation. Preliminar estimates provide temperatures of 400±50°C and pressures of 5-6
kbars. Timing of deformation is still poorly constrained, but it is suggested that it occured mainly at c. 20 Ma
from available radiometric datings. This major episode of deformation would have been the onset of the massif's
exhumation. Since 20 Ma, the Mont-Blanc massif would thus have been exhumed at a constant rate of ~1
mm.yr-1, as also reflected by fission tracks datings.
2. At the P-T conditions estimated for shear zones deformation, intense fluid-rock interactions are refelected by
chemical changes in the shear zones and episyenites. Subsequent integrated fluid fluxes estimated from mass
balance of the shear zone rocks are of the order of 106 m3.m-2. Within shear zones, elements are transported by
a circulating fluid phase while within episyenites, they are transported by diffusion through an almost stagnant
fluid. Both features are nevertheless connected to each other, and chemical exchange must consequently occured
between them. The comprehension of the inter-relationships of the shear zones and episyenites may provide new
insigths in the processes of fluid migration at mid-crustal levels.
100
Undeformed granite average
5Al
Sr /2
Rb /5
R2 = 0 .9 9 2 4
1 0 Cu
100P
Col du Geant
80
5 Ni
Y
REE ( ppm)
Undeformed granite average
100
Zr / 4
50
0
0
R2 = 0 .9 9 8 5
60
c h l o r it e - r i c h
Zr / 4
DV >
ou = 0
S i
Enrichments
20
Nb
Pb
C3 4
90
10 U
1 0Sc
80
5 Ni
60
5 0 Fe2 +
S r/ 2
DV = 0
7 0 Mg ( 4 8 2 )
5 0 Fe2 + ( 1 5 5 ) 1 0 U
4 0 Ca
c a t a c la s it e
5Al
1 0Sc
5 0 Fe
Enrichments
20
100
C7
E p i do t e - r i c h
S r/ 2
1 0K
10Ta
157 138
10 0
Zr / 4
R b/ 5
La
R
3
MB 6 4
Mus c ovit e -r ic h
m y lo nit e
80
MB64 muscovite-rich shear zone4
Mg O
K2O
10
7
MB148B episyenite
(wt%)
(wt%) 1 2
Both major elements composition fields of shear zones and episyenites
spread considerably with respect to initial values of unaltered granite
compositions.
Episyenites and shear zones do behave differently even though both are
Na2O enriched. While a majority of shear zones are SiO2 enriched, all
the analysed episyenites show a noticeable SiO2 depletion (between 0.1
to 7 wt%) which is directly related quartz dissolution. Concerning Al2O3
compositions, most shear zones follow the trend defined by modelled
variations of Al2O3 concentrations due to SiO2 input / output from the
average unaltered granite composition. This is in agreement with the
hypothesis of a relative Al immobility in shear zones (except some cases
of Al depletion). In contrast, episyenites plot on a line of higher Al2O3
concentrations at lower SiO2 values on the diagram. This is consistent
with the observed Al2O3 enrichment on episyenite isocon diagrams (see
left), subsequent to feldspar recrystallisation in granite vugs.
DV < 0
(10 1)
10 0
C34 chlorite-rich shear zone
C
B
5
60
Al2O3
Pl
Pl
Pl
Pl
A
70
65
Vug
6
From fresh granite to the core of episyenite lenses, the chemical composition evolves in
relation to the leaching and the precipitation of minerals. The observed trends have been fit
with curves obtained from the diffusion equation for semi-infini cases:
C=d+(a-d)*erf((x-b)/c).
a: composition of unaltered granite
b: distance from the most altered zone
c: diffusion paramter that depends on diffusivity and time
d: composition of the core of the episyenite
Diffusion curves fits well with Si and Al evolution when going farther from the core of the
episyenite (from the open vein if any). For these elements, diffusion thus appears to be the
main process of transport. For other elements, such as FE, Na and K a diffusion trend exists,
but it does not fit so well than for Si and Al. From the correlation between chemical evolution
and fits, episyenitization appears to be an alteration process which matter is mainly
transported by diffusion through an almost stagnant fluid-phase.
C7 epidote-rich shear zone
Ab
Vug
Qtz I
Na2O
75
Chl
SiO2
K-fs
K-fs
K2O
80
Bt
K-fs
(Exte rna l Alps )
Gne is s
Secondary quartz recrystallize as automorphous crystals within vugs and
open veins. Vermicular chlorite may fill vugs or grow as plating on
secondary quartz grains. Other minerals such as fluorite and calcite may
grow in association with albite, quartz and chlorite..
K-fs
Three main metamorphic assemblages are distinguished in the massif: mainly
epidote-bearing assemblages in the NW part of the massif, chlorite-bearing in
the centre and muscovite-bearing in the SW part.
In cataclasites, deformation is mostly accommodated by the fracturation of
magmatic minerlas and the polygonization of quartz grains. C-S bands are
formed in ultramylonites and mylonites by the elongation of K-feldspar clasts
and crystallization of metamorphic minerals (muscovite, epidote, chlorite) at
the expanse of plagioclase, biotite and K-feldspar. Quartz grains and
plagioclases are crushed in a fine matrix.
References
Bussy F. 1990. Pétrogenèse des enclaves microgrenues associées aux granitoïdes calco-alcalins: exemple des massifs varisque du
Mont-Blanc (Alpes occidentales) et miocène du Monte Capanne (Ile d'Elbe, Italie). Mem. Géologie (Lausanne), 7, 309p.
Leutwein, F., Poty, B;, Sonet, J., Zimmerman, J.L., 1970. Age des cavités à cristaux du granite du Mont-Blanc. C. R. Acad. Sci. Paris.,
271, 156-158.
Marro C. 1986. Les granitoïdes du Mont-Blanc en Suisse. Thèse, Fibourg University, 145 pp.
Marshall, D., Pfeifer, H;R., Hunziker, J.C., Kirschner, D., 1998. A pressure-temperature-time path for the NE Mont-Blanc massif:
fluid-inclusion, isotopic and thermobarometric evidence. Eur. J. Mineral., 10, 12227-12240.
Poty B. 1969. 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. Thèse 3e cycle, Mem. 17, Université de Nancy (France), 162p.
Rolland Y., Cox S.F., Boullier A.M., Pennachioni G., Mancktelow N. 2002. Fluid flow and element mobility in middle-crust shear
zones of collisional orogens: insights from the Mont Blanc Massif shear zone network. Goldschmidt conference, Davos.
Rolland Y., Cox S.F., Boullier A.M., Pennachioni G., Mancktelow N. in prep. Fluid flow and element transfer in mid-crustal shear
zones: insights from the Mont Blanc Massif (Western Alps).