J. metamorphic Geol., 2005, 23, 795–812
doi:10.1111/j.1525-1314.2005.00610.x
Ocean-floor hydrothermal metamorphism in the Limousin
ophiolites (western French Massif Central): evidence of a rare
preserved Variscan oceanic marker
J. BERGER,1,2 O. FEMENIAS,2 J. C. C. MERCIER3 AND D. DEMAIFFE2
1
Département de Géologie, Section de Géologie Isotopique, Africa Museum, B-3080 Tervuren, Belgium
([email protected])
2
Laboratoire de Géochimie Isotopique et Géodynamique Chimique, DSTE, Université Libre de Bruxelles (CP 160/02),
50 Avenue Roosevelt, B-1050 Brussels, Belgium
3
CLDG, Université de La Rochelle, av. M. Crépeau, 17042 La Rochelle Cedex 1, France
ABSTRACT
The Limousin ophiolite is located at the suture zone between two major thrust sheets in the western
French Massif Central. This ophiolitic section comprises mantle-harzburgite, mantle-dunite, wehrlites,
troctolites and layered gabbros. It has recorded a static metamorphic event transforming the gabbros
into undeformed amphibolites and the magmatic ultramafites into serpentinites and/or pargasite-bearing
chloritites. With various thermobarometric methods, it is possible to show that the different varieties of
amphibole have registered low-P (c. 0.2 GPa) conditions with temperature ranging from high-T, latemagmatic conditions to greenschist–zeolite metamorphic facies. The abundance of undeformed
metamorphic rocks (which is typical of the lower oceanic crust), the occurrence of Ca–Al (–Mg)
metasomatism illustrated by the growth of Ca–Al silicates in veins or replacing the primary magmatic
minerals, the P–T conditions of the metamorphism and the numerous similarities with oceanic crustal
rocks from Ocean Drilling Program and worldwide ophiolites are the main arguments for an ocean-floor
hydrothermal metamorphism in the vicinity of a palaeo-ridge. Among the West-European Variscan
ophiolites, the Limousin ophiolites constitute an extremely rare occurrence that has not been involved in
any HP (subduction-related) or MP (orogenic) metamorphism as observed in other ophiolite
occurrences (i.e. France, Spain and Germany).
Key words: gabbros; hydrothermal metamorphism; ophiolite; thermobarometry; Variscan.
INTRODUCTION
Obducted ophiolites are important markers of orogenic processes. Fragments of oceanic lithosphere (and
more rarely continental lithosphere) can also be subducted and subsequently transformed into high-P or
even ultra-high-P eclogites. Relicts of a high-P
event have been identified in the well-known Ôleptynite–
amphibolite complexÕ in the French Massif Central
(Santallier et al., 1988). The amphibolite lenses often
contain relicts of eclogite facies parageneses suggesting
a pressure up to 2 GPa. They occur in the uppermost
unit (upper gneiss unit or upper allochton) of the
Modanubian-Arverne zone. By contrast, they are
totally absent from the central Armorican zone and
from the Saxothuringian zone (Fig. 1). The eclogites
are probably related to one (or several) subduction
episode of oceanic and continental crust (continental
HP granulites have indeed also been described; Pin &
Vielzeuf, 1988) during the Silurian (440–400 Ma, Pin &
Peucat, 1986; Franke, 1992).
However, there is no definite consensus on the
origin and the geodynamic implications of these
2005 Blackwell Publishing Ltd
eclogites. The vergence of the subduction is still
debated and there are no radiometric data for the
numerous eclogite occurrences, such as the Limousin
area (Faure et al., 1997). Moreover, there is no
consensus on the mono- or polycyclic character of
the Variscan orogen in the French Massif Central.
Some authors (Ledru et al., 1989; Matte, 2001) suggest that it is a one-stage orogen, with Silurian
subduction directly followed by an Upper Devonian
collision and related lithospheric delamination. Others (Pin, 1990; Faure et al., 1997; Pin & Paquette,
2002) proposed that it is a polycyclic orogen with a
Lower Palaeozoic eo-Variscan belt reworked during
a Variscan subduction (or subductions, see Pin &
Paquette, 2002) during Upper Devonian and a collision during the Early Carboniferous. In this model,
the first eo-Variscan subduction event was probably
responsible for the formation of most Silurian eclogites throughout the French Massif Central. During
the second subduction event, discrete calc-alkaline
intrusions were generated in the north of Massif
Central (Pin & Paquette, 2002) and eclogites were
formed in the Ligurian zone (U–Pb on zircon,
795
796 J. BERGER ET AL.
(b)
NEXON
La Plagne
La Flotte
Ladignac
La Porcherie
N
0
5
10 km
UZERCHE
Carboniferous leucogranite
(a)
Carboniferous quartz diorite
Variscan Nappes
CB
SAZ
CAZ
NAZ
b
MZ
b
Limousin Ophiolite
a
Upper allochton (a- eclogite, b-plagioclasic gneiss)
Intermediate allochton
c a (a-ophiolite, b-leucocratic gneiss, c- mica-gneiss)
Basal and lower allochton
Relative autochton
N
100 km
Variscan ophiolites
(eclogites included)
CAZ
Central Armorica Zone
CB
Cadomian Block
SAZ
South Armorica Zone
MZ
Moldanubian Zone NAZ
North Aquitania Zone
Fig. 1. (a) Location of the Limousin ophiolites in the Variscan Belt of Western Europe (modified after Féménias et al., 2003).
(b) Schematic geological map of the studied area in Limousin (adapted from Chenevoy et al., 1984).
Sm–Nd mineral isochron and Ar–Ar on phengite,
360–350 Ma, Bosse et al., 2000).
The absence of consensus on mono- or polycyclic
character of the Variscan orogen in the French Massif
Central is enhanced by the lack of important orogenic
markers such as preserved ophiolites that were not
metamorphosed under eclogite or MP amphibolite
facies during their tectonic emplacement, such as the
Semail ophiolite in Oman or the Troodos in Cyprus.
However, preserved ophiolites resulting from intraoceanic thrusting of the lithosphere and obduction
onto the continent (Michard et al., 1991) are unusual
and little known in the Variscan belt of Europe and
especially in the French Massif Central. Those ophiolites only recorded ocean-floor hydrothermal metamorphism and localized low-P amphibolite to
granulite facies metamorphism in their metamorphic
sole during the obduction of the oceanic lithosphere
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OCEAN-FLOOR HYDROTHERMAL METAMORPHISM IN THE LIMOUSIN OPHIOLITES 797
onto the continent. The only known occurrences of
ophiolites devoid of HP recrystallization in the
Variscan belt of France are the Chamrousse ophiolite
(Menot et al., 1988) and the Limousin ophiolite
(Dubuisson et al., 1989). The other Variscan
ophiolitic remnants, for example: Cabo Ortegal in
Spain (Gil Ibarguchi et al., 1990), Plankogel and Speik
in Austria (Neubauer et al., 1989) and Muenberg in
Germany (Stosch & Lugmair, 1990), have undergone
HP metamorphism related to subduction.
The aim of this paper is to determine whether the
Limousin ophiolite has undergone a low-P oceanic
metamorphism or an ÔorogenicÕ Variscan metamorphism. This would shed some light on the metamorphic evolution of ophiolites and on the
tectono-metamorphic history of the Variscan belt.
GEOLOGICAL CONTEXT OF THE LIMOUSIN
OPHIOLITE MASSIFS
Four major lithotectonic units have been recognized in
the Limousin area, in the western French Massif
Central (Dubuisson et al., 1988; Fig. 1): (1) a relative
autochthon consisting of micaschists with minor
leucocratic gneisses (ÔleptynitesÕ in local terminology),
(2) a basal and a lower allochthon made of leucocratic
gneisses, (3) an intermediate allochthon comprising
micaceous gneisses with micaschists and leucocratic
gneisses (units 2 and 3 are the lower gneiss unit of
Ledru et al., 1989) and (4) an upper allochthon (upper
gneiss unit of Ledru et al., 1989) of plagioclasic
gneisses containing numerous eclogite and amphibolite
bodies (leptynite–amphibolite complex) not considered
in the present study. Following Pin & Peucat (1986),
the main Variscan tectono-metamorphic events that
took place in the French Massif Central and, more
generally in Western Europe are: (1) an HP metamorphism at 440–400 Ma; (2) an MP amphibolite
facies metamorphism and related crustal anatexis at
380–340 Ma; and (3) late events linked to the granulitization of the lower crust at 320–280 Ma and locally
up to 260 Ma (Féménias et al., 2003).
The ophiolite massifs of the Limousin area occur as
thrust sheets along the suture zone between the upper
and intermediate allochton units (Fig. 2). The massifs
have been dismembered by the Variscan thrust faults
and by Late Carboniferous strike–slip faults. The size
of these massifs now embedded in a large-scale tectonic
mélange ranges from 1 to 5 km in length with a thickness of a few hundred metres up to 1 km. Major shear
zones outlining the ophiolitic complexes are difficult to
observe in the field. They are represented by chloriterich mylonites that are deeply altered into clayey
material. The metamorphic sole (upon which ophiolites
were obducted) of some ophiolitic massifs has not yet
been discovered. The lithological sequence within these
ophiolite massifs consists from bottom to top (Dubuisson et al., 1989) of diopside-bearing harzburgite (with
rare lherzolites), harzburgite, dunite with local pods of
2005 Blackwell Publishing Ltd
SW
Upper allochton
Intermediate allochton
0
5
10 km
Lower and basal
allochton
Relative autochton
NE
Ophiolites
Fig. 2. Schematic cross-section in the Limousin area showing
the relation between the Limousin ophiolite and the other lithotectonic units. Modified after Dubuisson et al. (1988).
chromite, wehrlite with a few troctolites and, finally,
foliated and isotropic gabbros and amphibolites. Only
a few mafic dykes have been observed within the wehrlites and gabbros, but neither massive dolerites, nor
pillow-basalts have been discovered.
SAMPLE SELECTION AND ANALYTICAL
METHODS
This work is focused on selected gabbroic rocks and
plagioclase-bearing serpentinites as no detailed petrological study had yet been performed on these rocks.
Moreover, the ultramafic rocks (harzburgites and
dunites) are highly serpentinized with almost no relict
olivine, pyroxene or spinel. Four ophiolite massifs
were best suited for our investigation according to
the large number of outcrops they offered: La Flotte,
La Plagne, Ladignac-le-Long and La Porcherie
(Fig. 1). The thin alteration crust of all the samples has
been excluded from the petrographic study to ensure
that some of the low-T metamorphic features were not
linked to meteoric alteration.
The major elements of minerals were determined with
a Cameca SX50 electron microprobe at the CAMST
(University of Louvain-la-Neuve, Belgium). Operating
conditions were set as follows: accelerating voltage of
15 kV, beam current of 20 nA, electron beam of 2 lm
diameter and counting time of 10–16 s per element.
PETROGRAPHY
Most samples (few exceptions only) are undeformed to
slightly deformed. Primary magmatic mineral associations are frequently replaced by secondary hydrated
minerals such as serpentine, amphibole and chlorite
with minor amounts of prehnite, zoisite, mica and
zeolite. Many undeformed veins consisting of prehnite,
chlorite, grossular, calcite, zoisite, zeolite and green
hornblende crosscut the mafic and ultramafic rocks;
they are parallel or oblique to the general foliation.
Gabbros and amphibolites
Two main types of mafic rocks have been distinguished
on the basis of their textures; namely undeformed and
798 J. BERGER ET AL.
flaser gabbros and amphibolites (str.s.) and gneissic
amphibolites.
Undeformed and flaser gabbros and/or amphibolites
Undeformed and flaser (i.e. slightly deformed) gabbros
are medium-grained rocks with relict pyroxene, partly
or totally replaced by green or brown hornblende. The
primary magmatic mineralogy consists of clino- and
orthopyroxene, plagioclase, olivine and brown spinel,
whereas the secondary metamorphic mineralogy consists of green and brown amphibole, small recrystallized polygonal plagioclase, titanite, chlorite, prehnite,
with minor sulphides, zoisite, muscovite, pumpellyite
and calcite.
A crude magmatic layering is still visible in a few
samples, it consists of variations in modal proportions
of plagioclase and amphibole (former pyroxene).
Clino- and orthopyroxenes coexist in aggregates of
subpolygonal grains that sometimes display the euhedral morphology of primary pyroxene (Fig. 3a). Exsolutions of low-Ca pyroxene are present in the
clinopyroxene (but are too thin to be analysed by
microprobe). Plagioclase locally shows relict magmatic
features such as Carlsbad twinning but it generally has
a polygonal morphology with albite twins.
In partly amphibolitized gabbros (estimated degree
of recrystallization: 0–30%), clinopyroxene has
recrystallized at the periphery, with an inner rim of
brown amphibole and an outer rim of green amphibole
Fig. 3. Photomicrographs of typical mineral associations in the gabbros and amphibolites of the Limousin ophiolite. (a) Partially
amphibolitized gabbro showing amphibole–chlorite veins; the euhedral morphology of the pyroxene is more or less preserved. (b)
Plagioclase–amphibole symplectite at the contact between an amphibole aggregate and a primary plagioclase in an undeformed
amphibolite. (c) Clinopyroxene-bearing gneissic amphibolite. (d) Chlorite-rich amphibolite (Limo 48). Note the presence of elongated
laths of prehnite + chlorite replacing plagioclase. Pl, plagioclase; Cpx, clinopyroxene; Amph, amphibole; Prh, prehnite; Chl, chlorite.
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OCEAN-FLOOR HYDROTHERMAL METAMORPHISM IN THE LIMOUSIN OPHIOLITES 799
and calcic plagioclase. This pyroxene may also be
replaced by green amphibole along the cleavage planes.
In two samples (Limo 25 & 50), calcite grains have
been observed between plagioclase grains or in veins
crosscutting them. The chalcopyrite of sample Limo 25
is rimmed by a corona of pumpellyite, goethite and
chlorite.
The fully amphibolitized gabbros are characterized
by abundant green amphibole (with small titanite
inclusions), pseudomorphosing clinopyroxene. The
latter is now replaced by aggregates of prismatic
amphibole showing a decreasing grain size from core
to rim of the aggregate. A brown amphibole is also
locally found in the core of the aggregates. A few
grains of colourless chlorite partly replaces the green
amphibole in some samples. Magmatic plagioclase is
still preserved and shows only thin recrystallized rims
with either polygonal texture or subgrains in optical
continuity with the primary plagioclase. The boundary
between amphibole aggregates and plagioclase is often
marked by thin elongated prismatic amphibole included in small subpolygonal plagioclase (Fig. 3b). Finegrained associations of polygonal amphibole and
plagioclase have also been observed along the boundary. The degree of recrystallization is quite high,
between 30% and 80%.
Gneissic amphibolites
These samples have a grano-nematoblastic texture
and consist of prismatic green amphibole (with titanite inclusions) elongated along the metamorphic
foliation with coarse, deformed plagioclase porphyroclasts within a matrix of recrystallized small plagioclases and titanite (Fig. 3c). This gneissic texture is
locally mylonitic in highly deformed rocks near shear
zones.
Some grains of anhedral clinopyroxene (no exsolution of low-Ca pyroxene), probably of metamorphic
origin, have been observed. They are frequently rimmed by green amphibole. Sample Limo 11 contains
quartz that occurs as a mono-mineralogical layer
within the fined-grained plagioclase-rich matrix; it is
also characterized by a ÔblackÕ microcrystalline association of zoisite with minor albite, chlorite and muscovite that partly replaces plagioclase.
Pl
Pl
Cpx-sp
Cpx-sp
Amph
Opx
Srp
100 µm
Srp
Amph
Opx
Ol
Pl
100 µm
Fig. 4. Schematic drawing of the two types of corona between
olivine and primary plagioclase (see text). Ol, olivine; Srp, serpentine; Sp, spinel; Pl, plagioclase; Cpx, clinopyroxene; Opx,
orthopyroxene; Amph, amphibole.
and olivine. Two types of corona have been observed
(Fig. 4) from plagioclase:
amphibole ! clinopyroxene=spinel symplectites
! orthopyroxene;
amphiboleðchloriteÞ ! orthopyroxene:
Troctolites and plagioclase-bearing serpentinites
These rocks are characterized by the association of
partly serpentinized olivine, plagioclase, minor clinoand orthopyroxene and brown spinel. Olivine is
replaced by serpentine group minerals and sometimes
by colourless pargasite. Plagioclase is sometimes partly
or totally replaced by an intergrowth of prehnite and
chlorite. Clinopyroxene remains fresh and spinel is
surrounded by a rim of clinopyroxene and/or amphibole. Degree of recrystallization is between 20% and
80%. Coronas are well developed between plagioclase
2005 Blackwell Publishing Ltd
These rocks show no evidence of deformation and
the texture before serpentinization was probably
equigranular subpolygonal. Only one sample (plagioclase-bearing wehrlite Limo 8) shows some layering
marked by a change in modal proportions of olivine
and clinopyroxene.
Chlorite-rich amphibolites
These undeformed amphibolites are characterized by
the presence of numerous light green tabular chlorite
800 J. BERGER ET AL.
flakes. Chlorite is clearly replacing the light green
amphibole whereas plagioclase, when present, is transformed into a microcrystalline zoisite–albite–chlorite
mixture with minor K-feldspar and muscovite.
Samples Limo 16 and Limo 22 consist of tabular
chlorite with interstitial aggregates of anhedral
amphibole (colourless to light green) and relict grains
of brown chromiferous spinel. Locally, these aggregates are in fibrous association with chlorite and/or
contain some olivine grains nearly totally transformed
to iddingsite and pargasite. A faint layering results
from variations in modal proportions of chlorite,
amphibole and olivine. Sample Limo 22 displays some
altered plagioclase, some muscovite in the fibrous
aggregates of amphibole and no olivine observed.
Sample Limo 48 displays different texture (Fig. 3d).
Aggregates of amphibole (colourless to light green)
and tabular chlorite are less abundant than in the other
samples. The matrix is fine-grained and composed of
amphibole, chlorite, muscovite, prehnite, thompsonite
and analcite. A grain of hydrogrossular has been found
in a vein at the contact between amphibole and tabular
chlorite. The most striking feature is the presence of
thin elongated or tabular pseudomorphs filled by a
mixture of prehnite with minor chlorite. This association probably replaces a former plagioclase as has
been observed in two troctolite samples. These rocks
probably represent completely recrystallized plagioclase-bearing rocks (gabbros or troctolites).
MINERAL CHEMISTRY
Several silicates and oxides were analysed by electron
microprobe to constrain the igneous and metamorphic
evolutions of the ophiolite in terms of pressure and
temperature. Composition of amphibole, plagioclase
and clinopyroxene will be discussed in detail and other
minerals only described briefly.
In the absence of independent determination of the
Fe3+/Fe2+ ratio for amphibole, the structural formula
for these minerals have been calculated from microprobe analyses by bracketing their composition
through maximizing Fe3+ or Fe2+ according to the
recommendations of the I.M.A.C. Subcommittee on
Amphiboles (Leake et al., 1997). However, it should be
noted that the difference in NaM4 estimates using different models of Fe3+ calculation for an amphibole
analysis does not exceed 0.2 and most often 0.1.
Accordingly, uncertainties on pressures should be no
more than ±0.1 GPa at low pressure (Brown, 1977)
and would decrease significantly with pressure (±0.01
at 0.65 GPa).
Amphibole
Amphibole has been classified by taking into account
both the petrographic aspect and the composition.
Two main groups have been distinguished (Fig. 5,
Table 1).
The first group comprises low-Al amphibole from
aggregates pseudomorphosing a pyroxene, the amphibole replacing clinopyroxene along fractures and the
amphibole from foliated and schistosed amphibolites.
They are essentially Al-poor magnesiohornblende and
subsidiary actinolite and tremolite. The Mg## varies
between 67 and 85. The Si content ranges from 6.5 to
7.8 a.p.f.u. whereas AlIV and Fe3+ display a range of
values between 0.2 and 1.2 and 0.09 and 0.16 a.p.f.u.
respectively. The Ti content is positively correlated
with the Al content but it is rather low
(<0.07 a.p.f.u.). (Na + K)A is also roughly positively
correlated with Al and Ti and varies between 0 and
0.4 a.p.f.u. The Cr content is rather high and varies
from 0.02 to 0.13 a.p.f.u. Amphibole of similar compositions is frequently observed in oceanic metagabbros and amphibolites sampled by Ocean Drilling
Program (ODP) Legs (Stakes & Vanko, 1986; Gillis,
1995; Manning & MacLeod, 1996; Maeda et al., 2002).
They are also typical of low- to medium-pressure
metabasites (Hynes, 1982).
The second group consists of Al-rich magnesiohornblende and pargasite with minor tschermakite.
Several subgroups can be defined on the basis of the
textural setting of the amphibole:
1. the amphibole from symplectites surrounding
Al-poor amphibole aggregates and some amphibole
from these aggregates (occurring either as individual
grains or as zoned parts of amphibole from the first
group) are characterized by a higher AlIV and Fe3+
contents (from 0.8 to 1.8 a.p.f.u. and from 0.15 to
0.27 respectively), low Cr content (<0.02 a.p.f.u)
and a Mg# varying from 65 to 88; the Ti content is
low and not correlated with the Al content;
2. the amphibole from the coronas between plagioclase
and olivine has a pargasitic composition with high
Fe3+ and AlIV contents (from 0.15 to 0.50 and from
1.8 to 2.3 a.p.f.u. respectively) as well as (Na + K)A
content (from 0.69 to 0.94 a.p.f.u.); the Mg# ranges
from 86 to 99;
3. the amphibole from chlorite-rich amphibolite has
compositions intermediate between those of the two
former subgroups; the (Na + K)A content is rather
high in one sample (Limo 16: 0.86–0.93) relative to
the other two (Limo 22 and 48: 0.40–0.62); the Mg#
ranges from 83 to 90;
4. the coronitic amphibole around clinopyroxene
comprises a brown Ti-pargasite (also observed
around olivine) in the inner corona and a light
green low-Ti pargasite in the outer corona (Mg#:
57–59 and 67–68 respectively).
The amphibole from subgroups (1)–(3) are common in medium- to high-P metabasites (Abd El
Naby et al., 2000; Franceschelli et al., 2002), but
they are also known from highly altered oceanic
samples (Stakes & Vanko, 1986; Gillis, 1995; Manning & MacLeod, 1996; Fletcher et al., 1997; Maeda
et al., 2002) and in ophiolites affected by hydrothermal metamorphism (Girardeau & Mével, 1982;
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OCEAN-FLOOR HYDROTHERMAL METAMORPHISM IN THE LIMOUSIN OPHIOLITES 801
-A-
-B-
1
1
0.9
0.9
Pargasite
0.8
Pargasite
0.7
0.7
0.6 Actinolite
Actinolite
Magnesiohornblende
0.5
8
Mg#
Mg#
0.8
0.6
Magnesiohornblende
0.5
7.5
7
6.5
6
5.5/8
7.5
7
Si (a.p.f.u.)
6.5
6
5.5
Si (a.p.f.u.)
2.5
2.5
ODP Leg 153
ODP Leg 153
2
2
ODP Leg 147
ODP Leg 147
Amphibolites
0.5
Undeformed
Foliated
Schistose
0
1
Al
Aggregates
Chlorite amphibolites
Symplectites rimming aggregates
Ol-Pl coronas
Clinopyroxene rim
Gabbros
1
1.5
1
IV
IV
Al
1.5
0.5
0
1
ODP Leg 153
0.75
ODP Leg 153
0.5
0.5
0.25
0
0
0.05
0.1
0.25
ODP Leg 147
ODP Leg 147
0
0.05
Ti (a.p.f.u.)
0.1
0.15
0.2
(Na+K)A
(Na+K)A
0.75
0.25
0.3
0.35
0.4
0
Ti (a.p.f.u.)
Fig. 5. Compositional variations for the two groups of amphibole of the Limousin ophiolites (classification diagram after Leake et al.,
1997). (a) Low-Al amphibole showing Ti correlated with Al. (b) Al-rich amphibole with no Al–Ti correlation. The amphibole
compositional domains for ODP legs 147 and 153 (Manning & MacLeod, 1996; Fletcher et al., 1997) are shown for comparison.
Huot et al., 2002). The Ti-pargasite (subgroup 4)
is common in oceanic gabbros metamorphosed
during a high-T late-magmatic or hydrothermal
event (Gaggero & Cortesogno, 1997a; Talbi et al.,
1999).
2005 Blackwell Publishing Ltd
Plagioclase
Plagioclase can be subdivided into four groups on the
basis of their composition and petrographic occurrences (Fig. 6, Table 2):
802 J. BERGER ET AL.
Table 1. Representative microprobe analyses of amphibole.
Type:
Sample:
Name:
Rock type:
Al-poor
Limo 28
Act
Und-am.
Al-poor
Limo 25
Act
Und-am.
Al-poor
Limo 51
Mg-Hbl
Gn-am.
Al-poor
Limo 50
Mg-Hbl
Gn-am.
Al-poor
Limo 14
Mg-Hbl
Und-am.
Al-rich
Limo 9
Mg-Hbl
Group 1
Al-rich
Limo 9
Mg-Hbl
Group 1
Al-rich
Limo 24
Ts
Group 1
Al-rich
Limo 13
Ts
Group 1
Al-rich
Limo 22
Prg
Group 2
Al-rich
Limo 30
Prg
Group 2
Al-rich
Limo 48
Ts
Group 2
Al-rich
Limo 8
Prg
Group 3
Al-rich
Limo 1
Prg
Group 3
Al-rich
Limo 28
Ti-Prg
Group 4
SiO2 (wt%)
TiO2
Al2O3
Cr2O3
FeO
MnO
MgO
CaO
Na2O
K2O
Total
53.90
0.17
2.93
0.31
8.18
0.19
17.93
13.01
0.37
0.06
97.05
53.47
0.06
3.01
0.18
8.21
0.18
17.96
12.67
0.36
0.05
96.15
52.11
0.23
6.18
0.35
7.33
0.05
17.54
12.87
0.94
0.10
97.70
48.90
0.45
7.56
0.13
10.33
0.22
14.96
12.67
0.90
0.16
96.28
48.78
0.43
8.01
0.30
9.52
0.16
15.25
12.92
1.06
0.22
96.66
49.04
0.38
9.36
0.00
7.11
0.15
16.72
12.07
1.06
0.10
95.99
47.20
0.29
11.58
0.02
7.89
0.14
15.48
12.07
1.27
0.13
96.07
44.72
0.08
15.13
0.04
7.51
0.11
14.78
11.60
1.83
0.14
95.93
44.03
0.06
16.18
0.00
8.77
0.22
13.65
11.97
1.75
0.17
96.80
45.21
0.11
15.46
0.03
5.33
0.10
15.74
12.48
2.35
0.15
96.96
45.17
0.06
14.38
0.79
6.44
0.13
15.31
11.59
2.15
0.04
96.05
46.70
0.05
15.63
0.11
5.06
0.17
16.15
12.50
1.66
0.06
98.07
42.65
0.04
17.66
0.00
5.01
0.05
17.01
11.44
3.43
0.18
97.47
40.42
0.00
20.53
0.01
4.81
0.00
15.99
11.88
2.90
0.05
96.57
40.01
3.15
14.47
0.39
12.29
0.21
10.10
12.09
1.75
1.16
95.63
7.67
0.33
0.16
0.02
0.05
0.01
0.96
0.02
3.80
1.98
0.10
0.01
0.80
7.65
0.35
0.16
0.01
0.03
0.10
0.88
0.02
3.83
1.94
0.10
0.01
0.81
7.36
0.64
0.39
0.02
0.06
0.00
0.87
0.01
3.70
1.95
0.26
0.02
0.81
7.12
0.88
0.42
0.05
0.02
0.09
1.16
0.03
3.25
1.98
0.25
0.03
0.73
7.07
0.93
0.44
0.05
0.05
0.01
1.15
0.02
3.30
2.01
0.30
0.04
0.74
7.02
0.98
0.59
0.04
0.00
0.16
0.70
0.02
3.57
1.85
0.29
0.02
0.84
6.79
1.21
0.75
0.03
0.00
0.17
0.78
0.02
3.32
1.86
0.35
0.02
0.81
6.43
1.57
1.00
0.01
0.01
0.24
0.67
0.01
3.17
1.79
0.51
0.02
0.83
6.32
1.68
1.06
0.01
0.00
0.25
0.81
0.03
2.92
1.84
0.49
0.03
0.78
6.44
1.56
1.04
0.01
0.01
0.00
0.64
0.01
3.34
1.91
0.65
0.03
0.84
6.50
1.50
0.93
0.01
0.13
0.09
0.68
0.02
3.28
1.79
0.60
0.01
0.83
6.51
1.49
1.08
0.00
0.02
0.07
0.52
0.02
3.36
1.87
0.45
0.01
0.87
6.01
1.99
0.95
0.00
0.00
0.35
0.25
0.01
3.58
1.73
0.94
0.03
0.94
5.72
2.28
1.15
0.00
0.00
0.53
0.03
0.00
3.37
1.80
0.80
0.01
0.99
6.11
1.89
0.71
0.36
0.07
0.00
1.57
0.03
2.30
1.98
0.52
0.23
0.59
Based on 23 O
Si (a.p.f.u.)
AlIV
AlVI
Ti
Cr
Fe3+
Fe2+
Mn
Mg
Ca
Na
K
XMg
Or
100
90
80
70
60
50
40
30
20
10
100
90
80
70
60
50
40
30
20
10
100
90
80
70
60
50
40
30
20
10
100
90
80
70
60
50
40
Ab
0
10
Secondary sodic
An
Pyroxene
0
10
Secondary intermediate
An
Ab
0
10
Igneous
An
Ab
10
Secondary calcic
An
4. intermediate to sodic plagioclase (An5)50) associated
with zoisite, chlorite and muscovite in microcrystalline aggregates but also an alteration of any of the
three former types.
Or
30
20
10
Ab
0
Fig. 6. Compositional ranges of the four plagioclase groups of
the Limousin ophiolites.
1. igneous (primary) plagioclase remnants found in
undeformed amphibolites, in some gneissic amphibolites (core of grains), in troctolites and in one
chlorite-rich amphibolite: the An content is between
An56 and An80;
2. secondary calcic plagioclase (An78)92) found in symplectites surrounding amphibole aggregates, in the
amphibole-bearing coronas around clinopyroxene
and as rims around relict magmatic plagioclase;
3. secondary polygonal plagioclase grains of intermediate composition (An35)65) in gneissic amphibolites; they sometimes rim the igneous plagioclase in
undeformed amphibolites;
Three types of pyroxene have been recognized: (1)
igneous clino- and orthopyroxene in three gabbros and
two plagioclase-bearing serpentinites; (2) anhedral
metamorphic clinopyroxene in gneissic amphibolites
and (3) coronitic pyroxene between olivine and plagioclase (Fig. 7a, Table 3).
The relict primary igneous clinopyroxene is diopside,
with relatively high Al (0.09–0.19 a.p.f.u.), Ti (0.01–
0.03 a.p.f.u.), Cr (0.01–0.02 a.p.f.u.) and Na (0.03–
0.06 a.p.f.u.) contents, but a relatively low Si content
(1.90–1.94 a.p.f.u.). The Mg## is high, ranging from 80
to 86 in the gabbros and from 89 to 93 in plagioclasebearing serpentinites. Rare igneous orthopyroxene (two
samples) has a Mg## of 76 in the gabbro and of 86 in the
troctolite. It also shows high Al contents (0.09–
0.11 a.p.f.u.). Metamorphic anhedral clinopyroxene
consists of diopside and augite (Fig. 7b), with significantly lower Al (0.04–0.08 a.p.f.u.), Cr (<0.01 a.p.f.u.),
Ti (<0.005 a.p.f.u.) and Na (0.02–0.03 a.p.f.u.) contents, and a higher Si content (1.96–1.99 a.p.f.u.). The
Mg## varies from 75 to 89. Coronitic clinopyroxene is
also a diopside, with generally higher Mg##s (91–94)
than ÔmetamorphicÕ clinopyroxene, while coexisting,
coronitic orthopyroxene has Mg## ranging from 84 to
88.
2005 Blackwell Publishing Ltd
OCEAN-FLOOR HYDROTHERMAL METAMORPHISM IN THE LIMOUSIN OPHIOLITES 803
Table 2. Representative microprobe analyses of plagioclase.
Sample:
Mineral:
Type:
Rock type:
Limo 3
Pl
Igneous
Troct.
Limo 51
Pl
Igneous
Und-am.
Limo 31
Pl
Calcic
Und-am.
Limo 13
Pl
Calcic
Und-am.
Limo 44
Pl
Calcic
Und-am.
Limo 6
Pl
Int
Gn-am.
Limo 51
Pl
Int
Und-am.
Limo 51
Pl
Int
Und-am.
Limo10
Pl
Sodic
Und-am.
Limo 14
Pl
Sodic
Und-am.
SiO2 (wt%)
Al2O3
CaO
Na2O
K2 O
Total
49.49
31.93
15.87
2.74
0.02
100.05
54.01
29.01
11.92
4.78
0.04
99.76
46.36
34.42
18.36
1.21
0.00
100.34
46.58
33.42
17.97
1.61
0.03
99.61
48.62
32.90
16.37
2.27
0.03
100.18
52.81
29.75
13.03
4.23
0.05
99.86
56.08
27.74
10.47
5.59
0.04
99.93
60.34
25.31
7.38
7.39
0.06
100.47
65.95
21.15
2.28
10.34
0.04
99.75
60.70
24.07
6.09
8.01
0.12
98.99
2.26
1.72
0.78
0.24
0.00
0.76
0.00
0.24
2.44
1.55
0.58
0.42
0.00
0.58
0.00
0.42
2.12
1.86
0.90
0.11
0.00
0.89
0.00
0.11
2.15
1.82
0.89
0.14
0.00
0.86
0.00
0.14
2.22
1.77
0.80
0.20
0.00
0.80
0.00
0.20
2.40
1.59
0.63
0.37
0.00
0.63
0.00
0.37
2.52
1.47
0.50
0.49
0.00
0.51
0.00
0.49
2.67
1.32
0.35
0.63
0.00
0.36
0.00
0.64
2.90
1.10
0.11
0.88
0.00
0.11
0.00
0.89
2.72
1.27
0.29
0.70
0.01
0.29
0.01
0.70
Based on 8 O
Si (a.p.f.u.)
Al
Ca
Na
K
XAn
XOr
XAb
Wo
(a)
Di
Spinel and titanite
30
Igneous
Ol-Pl coronas
20
Anhedral metamorphic
80
90
(b)
0
100 Fs
40
70
10
10
50 Hed
60
100
En 0
(0.01–0.07 a.p.f.u.) chlorite pseudomorphs plagioclase
and (3) high-Si chlorite (3.15–3.20 a.p.f.u.) is found
only in sample Limo 16. This high-Si chlorite is known
to replace olivine (Agrinier et al., 1996) in serpentinized ultramafic rocks. The other chlorite types are
very common in altered oceanic gabbros and serpentinites (Vanko & Stakes, 1991; Cornen et al.,
1996a; Mével & Stamoudi, 1996).
Wo
50
20
30
40
50
60
70
80
90
0.25
Al (a.p.f.u.)
0.2
0.15
Igneous
0.1
Metamorphic
0.05
0
0
0.02
0.04
0.06
0.08
0.1
0.12
Ti + Cr + Na (a.p.f.u.)
Fig. 7. Compositional spectrum of pyroxene from the Limousin
ophiolites: (a) quadrilateral plot; (b) Al v. (Ti + Cr + Na) for
clinopyroxenes. Note the compositional gap between metamorphic and magmatic pyroxenes.
Olivine
Olivine is always serpentinized but some relicts have
been observed in five troctolites and in a gabbro
(Table 4). The Mg## ranges from 81 to 87 in the
troctolites, for a value of 72 in the gabbro.
Chlorite
Three types of chlorite have been observed (Table 5):
(1) low-Ca (<0.01 a.p.f.u.) chlorite replaces amphibole and forms tabular crystals; (2) high-Ca
2005 Blackwell Publishing Ltd
Brown spinel is present as a relict primary phase in a
troctolite, in a plagioclase-bearing wehrlite and in a
chlorite-rich amphibolite (i.e. Limo 16). Crystals are
homogeneous with Cr## ranging from 12 to 22 for a
Mg## ranging from 48 to 70 (Table 6). An Al-chromite
has also been found in one sample (Limo 16), with a
Cr## of 49 with a Mg## of 20. These two types of spinel
are present in ultramafic rocks from ODP sites 897–
899 (Cornen et al., 1996b).
Titanite is ubiquitous in deformed and undeformed
amphibolites. It is always associated with green
amphibole. It has significant amounts of Al (0.14–
0.24 a.p.f.u., Table 6) and Fe (0.01–0.05 a.p.f.u.).
Ca–Al silicates
Several Ca–Al silicates have been identified: prehnite,
zoisite, pumpellyite, grossular and hydrogrossular
(Table 7). Prehnite (with minor chlorite) partly replaces plagioclase in the plagioclase-bearing serpentinites
and in a chlorite-rich amphibolite (Limo 48). It has
also been found in veins crosscutting the metamorphic
foliation in some amphibolites. The Fe3+ content of
prehnite is generally low, below 0.02 a.p.f.u. Zoisite
has only been observed as part of a microcrystalline
association with muscovite, albite and minor chlorite.
It is always associated with the alteration of plagioclase. It has generally low Fe3+ contents (below
0.01 a.p.f.u.) and coexists with rare grains of a Fe3+bearing epidote (0.25 a.p.f.u.). Pumpellyite was
804 J. BERGER ET AL.
Table 3. Representative microprobe analyses of pyroxene.
Sample:
Mineral:
Type:
Rock type:
Limo 25
Cpx
Igneous
Gb
Limo 28
Cpx
Igneous
Gb
Limo 3
Cpx
Igneous
Troct.
Limo 3
Opx
Igneous
Troct.
Limo 28
Opx
Igneous
Gb
Limo 11
Cpx
Metam
Gn-am.
Limo 6
Cpx
Metam
Gn-am.
Limo 6
Cpx
Metam
Gn-am.
Limo 47
Cpx
Ol-plag
Troct.
Limo 8
Cpx
Ol-plag
Troct.
Limo 8
Opx
Ol-plag
Troct.
Limo 3
Opx
Ol-plag
Troct.
SiO2 (wt%)
TiO2
Al2O3
Cr2O3
FeO
MnO
MgO
CaO
Na2O
Total
51.89
0.43
2.46
0.42
5.84
0.31
13.91
24.00
0.40
99.66
51.47
0.58
3.03
0.35
5.69
0.13
14.88
22.82
0.53
99.49
52.04
0.70
4.22
0.53
3.50
0.08
15.32
23.11
0.70
100.20
55.45
0.18
2.57
0.13
9.46
0.24
31.61
0.45
0.00
100.09
54.01
0.16
2.34
0.08
15.34
0.40
27.20
0.54
0.00
100.07
53.42
0.04
1.02
0.06
4.37
0.19
15.19
24.89
0.35
99.51
52.87
0.19
1.75
0.04
6.60
0.35
13.70
24.58
0.37
100.45
52.77
0.07
1.06
0.05
7.47
0.17
13.33
24.64
0.30
99.86
52.41
0.32
3.66
0.31
2.71
0.10
15.95
23.20
0.72
99.38
53.00
0.46
3.52
0.42
2.92
0.08
15.58
23.43
0.79
100.20
55.81
0.07
2.21
0.19
9.72
0.28
31.65
0.26
0.00
100.19
56.16
0.01
1.22
0.04
10.08
0.23
31.70
0.20
0.00
99.64
1.93
0.07
0.04
0.01
0.01
0.18
0.01
0.77
0.96
0.03
0.44
0.47
0.09
0.83
1.91
0.09
0.04
0.02
0.01
0.18
0.00
0.82
0.91
0.04
0.47
0.45
0.08
0.86
1.90
0.10
0.08
0.02
0.02
0.11
0.00
0.83
0.90
0.05
0.52
0.43
0.05
0.90
1.94
0.06
0.04
0.00
0.00
0.28
0.01
1.65
0.02
0.00
0.85
0.01
0.14
0.86
1.94
0.06
0.04
0.00
0.00
0.46
0.01
1.46
0.02
0.00
0.75
0.01
0.24
0.76
1.98
0.02
0.02
0.00
0.00
0.14
0.01
0.84
0.99
0.02
0.45
0.49
0.06
0.88
1.96
0.04
0.03
0.01
0.00
0.20
0.01
0.76
0.97
0.03
0.42
0.48
0.10
0.81
1.97
0.03
0.02
0.00
0.00
0.23
0.01
0.74
0.99
0.02
0.39
0.50
0.11
0.78
1.92
0.08
0.08
0.01
0.01
0.08
0.00
0.87
0.91
0.05
0.53
0.44
0.03
0.94
1.93
0.07
0.08
0.01
0.01
0.09
0.00
0.84
0.91
0.06
0.51
0.44
0.05
0.92
1.95
0.05
0.04
0.00
0.01
0.28
0.01
1.65
0.01
0.00
0.85
0.01
0.14
0.85
1.97
0.03
0.03
0.00
0.00
0.30
0.01
1.66
0.01
0.00
0.85
0.00
0.15
0.85
Based on 6 O
Si (a.p.f.u.)
AlIV
AlVI
Ti
Cr
Fe
Mn
Mg
Ca
Na
XEn
XWo
XFs
XMg
Table 4. Representative microprobe analyses of olivine.
Sample:
Mineral:
Rock type:
Limo 28
Ol
Gb
Limo 19
Ol
Troct.
Limo 3
Ol
Troct.
Limo 8
Ol
Troct.
Limo 47
Ol
Troct.
SiO2 (wt%)
FeO
MnO
MgO
Total
37.96
25.49
0.34
36.28
100.07
39.07
18.14
0.29
42.41
99.91
39.82
14.74
0.15
45.09
99.80
40.02
14.66
0.24
44.96
99.88
39.93
11.82
0.11
47.43
99.29
1.00
0.56
0.01
1.43
0.72
1.00
0.39
0.01
1.61
0.81
1.00
0.31
0.00
1.69
0.85
1.00
0.31
0.01
1.68
0.85
0.99
0.25
0.00
1.76
0.88
Based on 4 O
Si (a.p.f.u.)
Fe
Mn
Mg
XMg
observed in only one calcite-bearing sample, forming a
corona with chlorite and goethite around chalcopyrite.
Two distinct compositions were obtained, one Mgrich (0.49–0.59 a.p.f.u.) and the other Mg-poor
(<0.01 a.p.f.u.) but Fe3+-rich (0.88–0.98 a.p.f.u.).
Monomineral veins crosscutting the metamorphic
foliation essentially consist of grossular (Prp0)0.2
Alm3Grs96Sps0.3)0.6) with rare heterogeneous hydrogrossular (Prp6)12Alm1)2Grs86)92Sps0.3)0.4). These
compositions are similar to those observed in serpentinized peridotites (e.g. ODP leg 920B; Gaggero &
Cortesogno, 1997b).
Mica and zeolite
Muscovite is commonly associated with zoisite in
ÔblackÕ microcrystalline associations found in many
samples (Table 5). Analcite (sodic zeolite) and
thomsonite (sodi-calcic zeolite) are found in chloriterich amphibolites and one grano-nematoblastic amphibolite (Table 5). These zeolites are also commonly
observed in ultramafic and mafic ODP samples (Stakes
et al., 1991; Fluh-Green et al., 1996; Alt & Bach,
2001). They are a low-temperature product of the
alteration of plagioclase.
THERMOBAROMETRIC INVESTIGATIONS
Many rock samples contain different generations of
plagioclase and amphibole, which suggests that the
whole rock did not reach a global metamorphic state
of equilibrium. The difficulty is then to determine
which plagioclase is in equilibrium with which
amphibole. From petrographic observations, it
appears that the primary magmatic calcic plagioclase
should not be in equilibrium with any amphibole
because it is always separated from amphibole by a
secondary plagioclase of calcic or intermediate composition. By contrast, the secondary recrystallized Anrich plagioclase should most probably be in near
equilibrium with the Al-rich (and Ti-poor) amphibole
because they form symplectitic intergrowths around
magnesiohornblende aggregates. The recrystallized
polygonal intermediate plagioclase forms rims around
the magmatic plagioclase and is always in textural
equilibrium with low-Al amphibole.
The general absence of mineral pairs suitable to
determine pressure (such as garnet-pyroxene, garnetamphibole, etc.) is an additional difficulty. Hence, the
first step will be to estimate pressure(s) of formation,
this parameter being a prerequisite to evaluate
2005 Blackwell Publishing Ltd
OCEAN-FLOOR HYDROTHERMAL METAMORPHISM IN THE LIMOUSIN OPHIOLITES 805
Table 5. Representative microprobe analyses of phyllosilicate.
Sample:
Mineral:
Type:
Rock type:
Limo 22
Chl
Low-Ca
Chl-am.
Limo 30
Chl
Low-Ca
Chl-am.
Limo 48
Chl
High-Ca
Chl-am.
Limo 30
Chl
High-Ca
Chl-am.
Limo 16
Chl
High-Si
Chl-am.
Limo 22
Ms
Plag alt
Chl-am.
Limo 30
Ms
Plag alt
Chl-am.
Limo 48
Anl
Plag alt
Chl-am.
Limo 30
Tmp
Plag alt
Chl-am.
SiO2 (wt%)
Al2O3
FeO
MnO
MgO
CaO
Na2O
K2 O
Total
28.29
22.62
6.57
0.03
27.52
0.03
–
–
85.10
27.80
22.02
7.43
0.02
27.36
0.01
–
–
84.75
28.95
21.16
8.91
0.15
25.59
0.38
–
–
85.27
29.18
20.82
9.53
0.08
25.51
0.17
–
–
85.63
31.92
15.95
9.74
0.13
27.75
0.24
–
–
85.76
45.85
35.15
0.40
0.07
0.93
0.01
0.11
11.33
93.85
45.25
37.25
0.03
0.00
0.23
0.26
0.10
11.63
94.75
53.61
26.37
0.02
0.00
0.04
1.21
10.68
0.18
92.11
38.54
29.33
0.02
0.05
0.03
12.21
4.04
0.04
84.28
3.15
1.85
0.80
0.01
4.08
0.02
–
–
6.19
5.59
0.05
0.01
0.19
0.00
0.03
1.95
6.05
5.87
0.00
0.00
0.05
0.04
0.03
1.98
7.77
4.50
0.00
0.00
0.01
0.19
3.00
0.03
5.28
4.73
0.00
0.01
0.01
1.79
1.07
0.01
Based on 14 O for chlorite; 22 O for muscovite; 24 O for analcite and 20 O for thomsonite
Si (a.p.f.u.)
2.77
2.75
2.86
2.88
Al
2.61
2.56
2.47
2.42
Fe
0.54
0.61
0.74
0.79
Mn
0.00
0.00
0.01
0.01
Mg
4.01
4.03
3.77
3.76
Ca
0.00
0.00
0.04
0.02
Na
–
–
–
–
K
–
–
–
–
Table 6. Representative microprobe analyses of spinel and
titanite.
Sample:
Mineral:
Rock type:
Limo 47
Spl
Troct.
Limo 1
Spl
Troct.
Limo 16
Al-Chr
Chl-am.
Limo 31
Ttn
Gn-am.
Limo 51
Ttn
Und-am.
SiO2 (wt%)
TiO2
Al2O3
Cr2O3
FeO
MnO
MgO
CaO
Total
0.04
0.01
55.20
11.57
13.29
0.08
17.63
–
97.82
0.14
0.05
54.15
11.06
15.71
0.16
16.23
–
97.50
0.58
0.04
23.79
33.85
32.46
0.77
3.94
–
95.42
30.79
39.06
1.03
0.15
0.13
–
–
29.08
100.23
30.69
38.17
1.52
0.19
0.13
–
–
28.99
99.69
0.02
0.00
0.96
0.91
0.13
0.80
0.02
0.20
–
0.20
0.49
4.00
3.82
0.16
0.02
–
–
0.01
–
4.05
4.00
3.74
0.23
0.02
–
–
0.01
–
4.05
Based of 4 O for spinel and 4 Si for titanite
Si (a.p.f.u.)
0.00
0.00
Ti
0.00
0.00
Al
1.75
1.74
Cr
0.25
0.24
3+
0.01
0.02
Fe
2+
0.29
0.34
Fe
Mn
0.00
0.00
Mg
0.71
0.66
Ca
–
–
0.71
0.66
XMg
0.12
0.12
XCr
metamorphic temperatures through plagioclase–hornblende and two-pyroxene thermometry.
Pressure estimates
The amphibolites from the Limousin ophiolites are
characterized by the general absence of minerals that
record high (garnet) or medium (epidote) pressures.
Hence, the pressure probably never exceeded 0.4 GPa
according to the phase-relation experiments of Apted
& Liou (1983).
The Na content in the M4 site of amphibole may be
a useful semi-quantitative geobarometer for metabasites in amphibolite and greenschist facies (Brown,
1977; Laird et al., 1984), provided the coexistence of
2005 Blackwell Publishing Ltd
plagioclase (Na buffer) and of a chlorite + Fe-oxide
assembly (buffer without which NaM4 will increase
with Fe3+). In the mafic/ultramafic rocks from the
Limousin ophiolite, plagioclase always coexists with
amphibole and, accordingly, the NaM4 contents of
amphibole are maximum values in the absence of a
Fe3+ buffer.
The NaM4 concentrations are very low for all the
analysed amphibole (Fig. 8), below 0.2 a.p.f.u. and
even below 0.1 a.p.f.u. for the majority of the samples.
According to the semi-quantitative geobarometer of
Brown (1977), this implies a metamorphic pressure of
no more than 0.4 GPa at any time, and most probably
c. 0.2 GPa for all the amphibole-bearing rocks
(gabbros, amphibolites, troctolites and chlorite-rich
amphibolites). This pressure estimate is in good
agreement with the compositions of all analysed
amphibole that plots in the low-pressure (LP) domain
of Laird et al. (1984) diagram (Fig. 8).
Plyusnina’s (1982) semi-quantitative thermobarometer based on the Ca content of plagioclase and the Al
content of coexisting hornblende (isotherms parallel to
the AlHb axis in CaPl–AlHb space and isobars slightly
oblique to the CaPl axis) was also used for a few
amphibole coexisting with plagioclase having
XAn < 0.6. The pressure estimates are consistently
below 0.4 GPa.
Multilayer coronas formed between olivine and
plagioclase have been considered as resulting either
from medium-pressure metamorphism under amphibolite or granulite facies conditions (Gardner & Robins,
1974; Grant, 1988), or from late-magmatic origin at
low pressure (Joetsen, 1986; Turner & Stuewe, 1992;
De Haas et al., 2002). Accordingly, these coronas
cannot be used to further constrain the pressure estimates, but the absence of medium- to high-P indicators
(garnet, epidote, etc.) in the Limousin ophiolites
strongly argues in favour of a late-magmatic origin for
806 J. BERGER ET AL.
Table 7. Representative microprobe analyses of Ca–Al silicates.
Sample:
Mineral:
Type:
Rock type:
Limo 31
Zo
Plag alt.
Gn-am.
Limo 30
Zo
Plag alt.
Chl-am.
Limo 51
Zo
Plag alt.
Und-am.
Limo 51
Prh
Plag alt.
Chl-am.
Limo 48
Prh
Plag alt.
Chl-am.
Limo 24
Prh
Vein
Und-am.
Limo 26
Grs
Vein
Und-am.
Limo 26
Grs
Vein
Und-am.
Limo 16
H-Grs
Vein
Chl-am.
Limo 16
H-Grs
Vein
Chl-am.
SiO2 (wt%)
Al2O3
FeO
MnO
MgO
CaO
Total
40.02
32.89
0.41
–
0.01
25.05
98.40
39.64
32.85
0.01
–
0.01
24.43
97.26
39.10
29.51
3.96
–
0.06
24.48
97.29
43.98
23.70
0.42
–
0.55
26.40
95.19
43.24
24.22
0.47
–
1.40
25.21
94.93
42.76
23.63
0.14
–
0.00
27.50
94.14
39.25
21.49
1.65
0.15
0.01
37.67
100.25
39.32
21.50
1.64
0.28
0.07
36.92
99.80
38.68
21.70
0.57
0.13
1.69
35.66
98.47
38.03
21.52
1.07
0.20
3.34
33.97
98.20
6.07
3.85
0.05
–
0.11
3.90
5.98
3.95
0.05
–
0.29
3.74
5.99
3.90
0.02
–
0.00
4.13
5.95
3.84
0.21
0.02
0.00
6.11
0.96
0.01
0.03
0.00
5.97
3.85
0.21
0.04
0.01
6.01
0.96
0.00
0.03
0.01
5.91
3.91
0.07
0.02
0.38
5.84
0.92
0.06
0.02
0.00
5.83
3.89
0.14
0.03
0.76
5.58
0.86
0.12
0.02
0.00
Based on 12.5 O for zoisite; 22 O for prehnite and 24 O for garnet
Si (a.p.f.u.)
3.02
3.02
3.05
Al
2.93
2.95
2.71
Fe
0.03
0.00
0.26
Mn
–
–
–
Mg
0.00
0.00
0.01
Ca
2.03
2.00
2.05
XGrs
XPrp
XAlm
XSps
0.4
HP
MP
NaM4 (a.p.f.u.)
0.3
0.2
LP
0.1
0
0
0.2
0.4
0.6
VI
0.8
1
1.2
1.4
1.6
1.8
3+
Al + Fe + 2Ti + Cr (a.p.f.u.)
Fig. 8. Amphibole from the Limousin ophiolites plotted in the
pressure discriminative diagram of Laird et al. (1984). Symbols
as in Fig. 5.
them. Indeed, they are observed in oceanic cumulates
collected by drill holes in the oceanic crust (see e.g.
Dick et al., 2002).
All the analysed amphiboles from the mafic and
plagioclase-bearing ultramafic rocks of the Limousin
ophiolites are low-pressure metamorphic minerals that
recrystallized in equilibrium with plagioclase at c. 0.2
(±0.1) GPa. Accordingly, the observed range of
composition largely overlaps that of metamorphic
amphibole from the ODP Legs (Manning & MacLeod,
1996; Maeda et al., 2002).
Temperature estimates
Two-pyroxene thermometry (Bertrand & Mercier,
1985) applied to a troctolite and a plagioclase-bearing
serpentinite yields magmatic temperatures between
1030 and 1090 C. The same thermometer applied to
pyroxene in coronas between olivine and plagioclase
(three samples) gives lower temperatures, between 944
and 991 C. The Lindsley (1983) thermometer applied
to both primary and coronitic clinopyroxene yields the
same broad temperature range (900–1100 C). By
contrast, secondary anhedral metamorphic clinopyroxene yields much lower temperatures, between
500 and 700 C.
Hornblende–plagioclase thermometry based on the
NaSi«CaAl exchange reaction (Holland & Blundy,
1994) has been applied to amphibole of all rock types.
Those in the coronas between olivine and plagioclase
and around clinopyroxene display temperatures in the
range of 674–811 C (mean: 742 C; r: 47 C). Using
the same thermometer, the high-Al (low-Ti) amphibole
with calcic plagioclase in symplectitic association and
in amphibole aggregates were respectively equilibrated
between 547 and 682 C (mean: 624 C; r: 55 C) and
between 572 and 749 C (mean: 637 C; r: 70 C). The
amphibole of both undeformed and gneissic amphibolites was equilibrated with secondary intermediate
plagioclase at 601–719 C (mean: 657 C; r: 37 C).
Because of the thermodynamic approach followed
by Holland & Blundy (1994), we consider the above
temperature estimates as more reliable than those derived from the Spear (1981) experiments (550–883 C;
0.1–0.5 GPa), which show an increase of the AlIV
content with temperature. When applied to the Limousin amphibole, this independent thermometer yields
high temperature of c. 810 C in agreement with the
previous estimates, but also values down to 450 C.
The chlorite-rich amphibolites characterized by the
absence of calcic plagioclase have much lower temperature of equilibration, probably below 550 C
according to Liou et al. (1974) phase equilibrium
studies and even below 400 C according to
the work of Maruyama et al. (1980) on the
2005 Blackwell Publishing Ltd
OCEAN-FLOOR HYDROTHERMAL METAMORPHISM IN THE LIMOUSIN OPHIOLITES 807
greenschist–amphibolite transition, as attested by the
presence of chlorite coexisting with albite and epidote
s.l. Moreover, the coexistence of Al–chlorite, prehnite,
zeolite and zoisite coupled to the absence of pumpellyite in two samples (Limo 48 & 30) is evidence for a
subsequent very low temperature of reequilibration,
between 200 and 400 C and a pressure below
0.22 GPa (Liou et al., 1983, 1985).
Temperature estimates for serpentinization processes are rather difficult to establish for such rocks, but
according to classical phase diagrams on serpentine
minerals in the system MgO–SiO2–H2O, this phenomenon probably occurred between 700 and 350 C
(Peacock, 1987). The absence of talc in these serpentinites further argues for a temperature of crystallization of serpentine minerals in a narrower
temperature range, below 500 C at 0.2 GPa (Peacock,
1987). However, it should be borne in mind that talc
crystallization may also be influenced by Si concentrations: high Si contents favour talc formation during
serpentinization (Janecky & Seyfried, 1986).
DISCUSSION
Origin and evolution of parageneses
The primary igneous rocks of the Limousin ophiolites
studied in this paper are gabbros and troctolites. The
magmatic parageneses consist of plagioclase, clinopyroxene, orthopyroxene and olivine. This association was re-equilibrated under near magmatic
subsolidus conditions, as attested by the polygonal
texture of primary pyroxene and plagioclase.
Plagioclase first reacted with olivine under low-P–
high-T, late-magmatic conditions, thereby yielding
orthopyroxene–clinopyroxene–spinel coronas. These
coronas were amphibolitized during interaction with
late-magmatic fluids or with hot hydrothermal fluids in
high-T amphibolite facies conditions. The magmatic
clinopyroxene also interacted with high-T fluids to
produce the Ti-pargasite that occurs in coronas around
clinopyroxene or along the cleavage planes at the
temperature of the low-pressure granulite facies. This
reaction is common in oceanic gabbros and is generally
interpreted as a late-magmatic deuteritic reaction
(Gaggero & Cortesogno, 1997a). The appearance of
secondary anhedral metamorphic clinopyroxene is due
to the recrystallization of igneous pyroxene under
high-T amphibolite facies conditions as suggested by
the experiments of Spear (1981) on metabasite.
Magmatic and metamorphic clinopyroxenes were
subsequently amphibolitized through hydrothermal
circulation of hot seawater-derived fluids. They were
transformed into green magnesiohornblende (and
actinotes) with low Al and Ti contents (group 1).
During this amphibolitization, the magmatic plagioclase recrystallized to more sodic composition as indicated in the following reaction after Spear (1981):
2005 Blackwell Publishing Ltd
Cpx1 + Pl(An56)80) + Fe–Ti oxide + fluid
fi Mg-Hbl + Pl(An35)63) + Ttn
This reaction is responsible for the transformation
of most gabbros into amphibolites (Mg-hornblende + intermediate plagioclase). Some of the latter
have not been deformed whereas the gneissic
amphibolites underwent shearing within the oceanic
crust as attested by the low-pressure conditions inferred for this metamorphism.
The high-Al (low-Ti) amphibole (group 2) associated
with calcic plagioclase as symplectites around amphibole aggregates, or as zoned parts of Al-poor amphibole,
probably results from interaction of preexisting
amphibole or clinopyroxene with hot Al- and Mg-rich
seawater-derived fluids, during a reactivation of
hydrothermal circulation at a temperature corresponding to the amphibolite-to-greenschist facies transition
(Girardeau & Mével, 1982; Stakes & Vanko, 1986;
Fletcher et al., 1997). Mével (1987) also observed the
same reaction along amphibole veins in an altered
metagabbro, which consisted of high-Al amphibole
(pargasite), while plagioclase at the contact of the vein
recrystallized to more calcic composition. The high-Al
(low-Ti) amphibole present in the chlorite-rich
amphibolites results either from reaction of the amphibole pseudomorphosing pyroxene with hydrothermal
fluids (Girardeau & Mével, 1982; Stakes & Vanko, 1986;
Fletcher et al., 1997) or from hydrothermal alteration of
olivine and/or plagioclase as proposed by Manning &
MacLeod (1996), Fletcher et al. (1997) and Maeda et al.
(2002). We propose that the high-Al amphibole in these
samples replaces low-Al amphibole (clinopyroxene
pseudomorphs) and some olivine, as it was observed in a
few undeformed gabbros and troctolites. The plagioclase from chlorite-rich amphibolites is not transformed
into amphibole but rather into prehnite and/or chlorite
or into zoisite, albite and muscovite.
The abundance of tabular chlorite in chlorite-rich
amphibolites contrasts with its paucity or even absence, in other amphibolites. This chlorite clearly
replaces the Al-rich amphibole because it has a composition similar to that of the chlorite replacing
amphibole in deformed and undeformed amphibolites.
These chlorite-rich samples probably represent the
most hydrothermally metamorphosed gabbros.
Finally, plagioclase is replaced by the association
zoisite + albite + muscovite (samples Limo 22 & 51)
or prehnite + chlorite (samples Limo 1 & 48) under
greenschist facies conditions:
Pl1 þ H2 O ! zoisite þ albite þ muscovite
and
Pl1 þ Mg-fluid ! prehnite þ Mg chlorite:
The fluid responsible for the transformation of
plagioclase to prehnite + chlorite was rich in Ca and
Mg, but it leached out the alkalis of the plagioclase
(Fig. 9). Prehnite, Mg–chlorite, amphibole and gros-
808 J. BERGER ET AL.
CaO
0
100
10
90
20
80
30
70
40
50
Prh
60
50
Fluid
60
40
70
30
Pl
80
20
90
10
Chl
100
0
10
20
30
40
50
60
70
80
90
MgO
0
100
Al2O3
Fig. 9. CaO–Al2O3–MgO ternary diagram illustrating the composition of the fluid necessary to transform plagioclase into
prehnite (90%) + chlorite (10%) association.
sular are also present in veins crosscutting all the
metamorphic minerals. These veins were formed during a late-stage, low-T, Ca–Al–Mg metasomatism.
Moreover, the local development of muscovite during
the alteration of plagioclase attests for a localized
K-enrichment during hydrothermal metamorphism.
The presence of pumpellyite (rather uncommon in
the oceanic environment; Mével, 1981), in corona with
chlorite and goethite around chalcopyrite in a calcitebearing gabbro also suggest a low-temperature, fluidcontrolled, hydrothermal reaction. The same feature
has been observed by Ishizuka (1999): the pumpellyite
and the associated chlorite and goethite are the products of the reaction between plagioclase and an iron
oxide with a fluid bringing Cu and S.
A summary of the various mineral parageneses
observed in the crustal section of the Limousin ophiolite in function of the metamorphic grade is presented
in Table 8.
Conditions of metamorphism
The plagioclase-bearing rocks (gabbros and troctolites)
of the Limousin ophiolites have undergone a low-P
(£0.2 GPa) metamorphism with temperatures ranging
from granulite to zeolite facies (Fig. 10). The detailed
petrographic observations argue for a general recrystallization of the oceanic crust from high-T magmatic
crystallization to low-T greenschist to zeolite facies
conditions. Indeed, low-T parageneses coexist with
high-T parageneses in most mafic rocks. Moreover, the
amphibole-bearing coronas around clinopyroxene
consist of a brown, Ti-rich amphibole on the inner side
and a green, Ti-poor amphibole outward; the latter
being partly replaced by chlorite. This is classically
interpreted as the result of metamorphism from latemagmatic conditions to greenschist facies temperature
(Girardeau & Mével, 1982; Mével, 1987; Dick et al.,
2000; Franceschelli et al., 2002). There was probably a
continuum of retrograde events rather than a single
retrograde event, because hydrothermal activity at
mid-ocean ridges is intermittent. It is related to tectonically controlled fluid penetration and to heat flow
brought by periodic magmatic activity (Manning &
MacLeod, 1996).
The high temperature (1030–1090 C) obtained from
two-pyroxene thermometry on primary igneous
pyroxene probably corresponds to magmatic
crystallization temperatures. Plagioclase and olivine
presumably begin to react to form orthopyroxene–
clinopyroxene–spinel coronas during a high-T (900–
950 C) late-magmatic event, as attested by
two-pyroxene thermometry on coronitic assemblages.
These coronas were amphibolitized at 770–810 C, as
shown by hornblende–plagioclase thermometry. The
gabbros were then extensively amphibolitized at
temperatures characteristic of the amphibolite facies
(550–700 C). This event was followed by a low-T
(greenschist facies) to very low-T (zeolite facies)
metamorphism, below 550 C and maybe as low as
200–400 C. Serpentinization occurred during the
same temperature range (300–500 C).
Ocean-floor hydrothermal metamorphism v. Variscan
nappe emplacement-related metamorphism
The metamorphic history recorded in the mafic part
(gabbros and troctolites) of the Limousin ophiolites
could hypothetically be related either to (1) hydrothermal metamorphism in the oceanic crust, prior to
obduction, or (2) regional Variscan medium-P nappestacking-related metamorphic overprint following
ophiolite obduction. An age of the metamorphic
recrystallization event would resolve this point but
none is yet available. However, there are four main
Table 8. Stable and metastable minerals according to rock type and metamorphic grade.
Magmatic
Rock type
Ol
Di
Pl
En
Und-am.
Gn-am.
Chl-am.
Troct.
+
X
X
+
X
X
X
+
Cr-Spl
X
Late magmatic
Amphibolite
Ilm
Ti-Prg
Di
En
Spl
Mg-Hbl
Pl
Ttn
X
+
+
+
+
X
X
X
X
X
X
X
X
X
X
X
X
X
Greenschist and lower grade
Di
Srp
Chl
Prh
Zo
Ms
Ab
+
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
H-Grs
Grs
Pmp
Anl
Tmp
Sph
Cal
Gt
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
X, major; +, accessory. Abbreviations after Kretz (1983) except Sph: sulphides and H-Grs: hydrogrossular.
2005 Blackwell Publishing Ltd
Ep i
arguments in favour of an oceanic hydrothermal
metamorphism:
1. the low-P static metamorphism, ranging from highT (late-magmatic) to low-T (greenschist to zeolite
facies conditions) is also commonly seen in other
ophiolites (Elthon & Stern, 1978; Mével et al., 1978;
Girardeau & Mével, 1982; Girardeau et al., 1985;
Agrinier et al., 1988) and in rocks dredged from the
oceanic crust (Ito & Anderson, 1983; Honnorez
et al., 1984; Stakes & Vanko, 1986; Stakes et al.,
1991; Gillis, 1995; Manning & MacLeod, 1996;
Talbi et al., 1999; Maeda et al., 2002);
2. the mineral association observed in the mafic rocks
of the Limousin ophiolites are comparable with
those found in rocks from oceanic crust (ODP
boreholes, see references in the previous sections);
3. static metamorphism is widespread in the oceanic
crust (Stakes et al., 1991; Cornen et al., 1996a;
Kelley & Malpas, 1996; Maeda et al., 2002) and in
preserved ophiolites (Elthon & Stern, 1978; Mével
et al., 1978; Girardeau & Mével, 1982; Girardeau
et al., 1985; Agrinier et al., 1988). This style of
metamorphism is commonly related to hydrothermal circulation within the oceanic crust;
4. evidence for Ca–Al (–Mg) metasomatism is provided
by the presence of Ca–(Al) silicates (zoisite, prehnite,
grossular, calcite and hydrogrossular) in veins
crosscutting the metamorphic foliation or in highly
altered rocks. Transformation of Al-poor amphibole
to Al- and Mg-rich amphibole is also evidence for
Al–Mg enrichment by seawater-derived fluids. Such
metasomatism is responsible for the rodingitization
and hydrothermal processes active within the oceanic crust (Manning & MacLeod, 1996; Fletcher
et al., 1997; Puga et al., 1999; Kitajima et al., 2001;
Huot et al., 2002). Local K-enrichment is also a
common feature of seawater–gabbro interactions
(Mével, 1987).
300
400
(1
)
(3)
200
Amph-in (1)
0.1
Sil
T
Cp itan
x-o ite
ut (1) -in
Antigorite (3)
-in
Chlorite (5)
-in
And
0.3
0.2
2005 Blackwell Publishing Ltd
Ky
qtz
Analcime+
O
Albite+H 2
P (GPa)
0.4
dot
Prehnite-in (4)
0.5
Fig. 10. Schematic P–T path illustrating
sea-floor hydrothermal metamorphism of
the Limousin ophiolites. Curves (1) from
Spear (1981); epidote-in curve (2) from
Apted & Liou (1983); curves (3) from Spear
(1995); prehnite-in curve (4) from Liou et al.
(1983) and chlorite-in curve (5) from Liou
et al. (1974).
e-in (2)
OCEAN-FLOOR HYDROTHERMAL METAMORPHISM IN THE LIMOUSIN OPHIOLITES 809
500
600
T (°C)
700
800
900
The high degrees of oceanic metamorphic recrystallization (20–80%) observed in the Limousin ophiolites
is quite uncommon when compared with other ophiolites (Oman and Troodos) which only show high degree
of alteration in the uppermost crust, i.e. the extrusive
and the dyke sequence. However, a high degree of
metamorphism (c. 50% of recrystallization and up to
100%) has been observed in the plutonic sequence of
slow-spreading ridges (Stakes et al., 1991). In these
ridges, characterized by a thin heterogeneous crust,
Cannat et al. (1995) showed that the serpentinization
front (i.e. the hydration front) can be deeper than the
Moho. The lowermost oceanic crust of slow-spreading
ocean can then undergo a pervasive hydrothermal
metamorphism. The extended ocean-floor metamorphism observed in the Limousin ophiolite would mean
that it represents a fossil slow-spreading ridge. Detailed
geochemical and isotope data are needed to better
constrain the exact nature of this ocean.
Implications for the Variscan orogeny
The Limousin ophiolites, located along a major suture
zone between two major units, have not been affected
by a Variscan orogenic metamorphism. The country
rocks, above and below the ophiolitic remnants, have
registered a Variscan MP amphibolite event that is
generally related to nappe stacking (Pin & Peucat,
1986; Ledru et al., 1989). This interpretation has to be
re-evaluated because the ophiolite (mainly hydrated
mafic and ultramafic rocks that are very sensitive to
change in P–T conditions) did not register the MP
amphibolite facies event. The presence of altered
chloritic mylonites along the major Variscan shear
zones dismembering the ophiolitic massifs could suggest that the nappe emplacement has only produced a
localized metamorphism along these shear zones, as
observed in other Variscan (i.e. Chamrousse; Menot
810 J. BERGER ET AL.
et al., 1988; Guillot et al., 1992) and Alpine (i.e. Chenaillet massif; Mével et al., 1978) ophiolites. In these
ophiolites, most of the metamorphism has been related
to oceanic hydrothermalism with localized greenschist
and rare amphibolite facies conditions because of
nappe emplacement (even for the Chamrousse ophiolite which has undergone both Variscan and Alpine
metamorphisms). Moreover, the last low-grade metamorphism observed in the ophiolitic massifs is represented by veins of Ca–Al silicates crosscutting all the
metamorphic textures. Such veins are a common feature of oceanic metamorphism (see previous sections).
In fact, most of the ophiolite massifs did not register
the Variscan metamorphism, except locally along or
near the Variscan shear zones.
We feel that the regional MP amphibolite facies
metamorphism and related anatexis, which is well
documented in the French Massif Central, developed
before nappe emplacement and can probably be related to the subduction and subsequent exhumation of
subducted rocks.
CONCLUSION
The gabbros and troctolites belonging to the Limousin
ophiolite sequence have been overprinted by a low
pressure (c. 0.2 GPa) metamorphism with temperature
decreasing from high-T late-magmatic conditions to
late-stage low-T greenschist to zeolite facies conditions. Several generations of amphibole were formed
during this metamorphic stage. The first amphibole, a
brown Ti-pargasite, results from the reaction of the
primary pyroxene with late-magmatic fluids. The
widespread Al-poor Mg-hornblende was produced
later, during the general amphibolitization of the
gabbros which represents the first steps of hydrothermal metamorphism. High-Al amphibole crystallized
during the reaction of Ca–Al (–Mg)-rich fluids with
former low-Al amphibole. In the most altered samples,
the different high- to medium-T parageneses have been
partly transformed to low-T assemblages consisting of
amphibole, chlorite, prehnite, zoisite, mica, albite and
zeolite.
The development of a static low-P metamorphism,
of a Ca–Al (–Mg) metasomatism and the numerous
similarities (parageneses, mineral chemistry, grade of
metamorphism) with hydrothermally altered rocks
from ophiolites and from the ocean crust (ODP data)
is strong evidence for an ocean-floor hydrothermal
metamorphic event in the Limousin ophiolites.
There is no evidence for a regional (Variscan)
metamorphic overprinting. The Limousin ophiolitic
remnants are thus exceptional because they were not
involved in any HP and/or MP metamorphism, especially when they have been involved in the Variscan
nappe-stacking episode. The only explanation we see is
that the nappe stacking occurred after the main MP
amphibolite facies event, that is also responsible for the
development of crustal anatexis in the lower unit
(below the ophiolite) and amphibolitization in the
upper unit, the Ôleptynite–amphibolite complexÕ (above
the ophiolite). Nappe emplacement has probably only
produced greenschist facies conditions along major
shear zones.
ACKNOWLEDGMENTS
This work has been partly supported by a BRGM
grant (Bureau de Recherches Géologiques et Minières,
Orléans, France). The constructive and helpful reviews
of J.C. Alt and M. Terabayashi and the editorial
management of D. Robinson are gratefully appreciated.
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Received 9 March 2005; revision accepted 19 August 2005.
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