J. metamorphic Geol., 2013, 31, 571–583
doi:10.1111/jmg.12035
Preservation of subduction-related prograde deformation in
lawsonite pseudomorph-bearing rocks
M. PHILIPPON,1 F. GUEYDAN,2 P.PITRA3 AND J-P. BRUN3
1
Utrecht University, Faculty of Earth Science, Budapestlaan 4, 3584, CD Utrecht, The Netherlands
([email protected])
2
Geosciences Montpellier, Universit
e de Montpellier 2, UMR CNRS 5243, 34095, Montpellier CEDEX 5, France
3
G
e osciences Rennes, Universit
e Rennes 1, UMR CNRS 6118, 35042, Rennes CEDEX, France
ABSTRACT
Lawsonite pseudomorphs are used to identify and distinguish the kinematic records of subduction
and exhumation in blueschist-facies rocks from Syros (Cyclades; Greece). Lawsonite is a hydrous
mineral that crystallizes at high-pressure and low-temperature conditions. During decompression,
lawsonite is typically pseudomorphed by an aggregate dominated by epidote and paragonite. Such
aggregates are easily deformable and if deformation occurs after the lawsonite breakdown, the
pseudomorphs are difficult to distinguish from the matrix. The preservation of the lawsonite crystal
shape, despite complete retrogression, indicates therefore that the host blueschist rock has not been
affected by penetrative deformation during exhumation, thus providing indication of strain-free conditions. Therefore, tracking the lawsonite growth and destabilization along the P–T path followed by
the rocks during a subduction/exhumation cycle provides information about the subduction/exhumation-related deformation. Using microstructural observations and P–T pseudosections calculated with
THERMOCALC, it is inferred that top-to-the-south sense of shear preserved in lawsonite pseudomorphbearing blueschists on Syros occurred during the prograde metamorphic path within the lawsonite
stability field, and is therefore associated with subduction. On the contrary, the deformation with a
top-to-the-north sense of shear is observed in surrounding rocks, where lawsonite pseudomorphs are
deformed or apparently lacking. This deformation occurred after the lawsonite breakdown during
exhumation. At the regional scale, exhumation-related deformation is heterogeneous, allowing the
preservation of lawsonite pseudomorphs in significant volumes of blueschists of the central and
southern Cyclades. It is argued that such successive shearing deformation events with opposite senses
more likely correspond to an exhumation process driven by slab rollback, in which subduction and
exhumation are not synchronous.
Key words: Aegean; exhumation; lawsonite blueschists; slab rollback; subduction.
Abbreviations: ab, albite; chl, chlorite; di, diopsidic clinopyroxene; ep, epidote; g, garnet; gl,
glaucophane; hb, hornblende; law, lawsonite; mu, muscovite; o, omphacitic clinopyroxene; pa,
paragonite; q, quartz; ttn, titanite (sphene); ru, rutile.
INTRODUCTION
High-pressure/low-temperature (HP/LT) metamorphic rocks are crustal material that was buried in
subduction zones and then exhumed. Ductile deformation can be attributed either to the subduction of
the rocks to mantle depths or to their exhumation. In
the Cyclades, ductile deformation observed in HP/LT
metamorphic rocks is commonly attributed to exhumation up to the brittle/ductile transition (Gautier &
Brun, 1994; Jolivet & Goffe, 2000; Mehl et al., 2005).
In the Aegean region, several models have been proposed to explain the exhumation of HP/LT rocks
(Jolivet et al., 2003; Ring & Layer, 2003; Brun &
Faccenna, 2008; Roda et al., 2010). To identify the
most appropriate exhumation model, it is essential to
© 2013 John Wiley & Sons Ltd
discern between deformation events related to subduction and exhumation, respectively.
Lawsonite is a hydrous mineral that crystallizes
under high-pressure and low-temperature conditions
characteristic of subduction zones (e.g. Ernst, 1971;
Deer et al., 1986 and references therein; Poli &
Schmidt, 1995). The lawsonite stability field can be
attained either on the prograde or the retrograde part
of the P–T path, associated with the burial or exhumation of the subducted rocks, respectively (Pognante et al., 1980; Ballevre et al., 2003; Zucali et al.,
2004; Schumacher et al., 2008; Angiboust et al.,
2009; Ravna et al., 2010). Metamorphic reactions of
lawsonite breakdown (e.g. lawsonite + glaucophane garnet = epidote + paragonite + chlorite +
H2O) occur when the rock undergoes decompression
571
572 M. PHILI PPON ET AL .
and/or heating (e.g. Ballevre et al., 2003; Zucali &
Spalla, 2011). Tracking lawsonite growth and destabilization along the P–T path during the subduction/
exhumation cycle should provide information about
subduction/exhumation-related deformation (Philippon et al., 2009).
The present study investigates lawsonite growth
along the P–T path followed by lawsonite blueschists from Syros Island (Cyclades, Greece), in
order to monitor the deformation associated with
lawsonite growth and identify deformation related
to subduction and exhumation. In the Cyclades, in
spite of superposed deformation events related to
subduction and exhumation processes, HP units
have surprisingly preserved (i) their initial sedimentary or magmatic geometry and even their stratigraphy (Philippon et al., 2012), and (ii) evidence of
HP/LT metamorphism – e.g. calcite pseudomorphs
after aragonite and pseudomorphs of epidote + paragonite + chlorite after lawsonite (hereafter called
‘lawsonite pseudormorphs’) (Trotet et al., 2001;
Brady et al., 2004; Keiter et al., 2004). The exceptional preservation of lawsonite pseudomorphs in
Syros rocks, and in many other high pressure terranes, provides a powerful tool to identify deformation that occurred in the lawsonite stability field,
located in the HP/LT part of the P–T path, and
that are at least partly, if not entirely, related to
subduction.
REGIONAL GEOLOGY
The Cycladic blueschist unit (CBU) outcrops in the
Cyclades, Attica and Evvia (Bonneau & Kienast,
1982) (Fig. 1). It comprises (i) an oceanic unit that
mainly consists of a serpentinite matrix containing
Triassic to Cretaceous metagabbro and metabasalt
knockers (Lagos et al., 2007; Bulle et al., 2010),
thrust on top of (ii) a basement-cover sequence
belonging to the continental margin of the Adria microplate (Bonneau et al., 1980a,b; Ridley, 1984). The
sedimentary cover of the basement comprises marbles
and schist sequences (Fig. 3). The whole unit underwent blueschist- to eclogite-facies metamorphism during the Eocene (Bonneau & Kienast, 1982; Putlitz
et al., 2005; Lagos et al., 2007) (Fig. 1a) and its
exhumation occurred at c. 35 Ma, the age of the
greenschist facies retrogression (Altherr et al., 1979;
Maluski et al., 1987; Wijbrans et al., 1990; Parra
et al., 2002).
At the regional scale, the CBU dominantly displays
N- to NE-trending stretching lineations, associated
with either top-to-the-N to NE, or top-to-the-S
to SW senses of shear (Fig. 1a). The N to NE
25°
24°
Rhodopia
ne
Vardar/Izmir-Ankara Suture Zone
o
eZ
r
tu
or
th
N
s
do
Su
Evvia
n
Pi
Pelagonia
Pindos Suture Zone
Adria
C yc
la d
i
Hellenic Subduction zone
t
De
38°
c
ac
hm
e
Andros
Kea
Attica
Fig. 2
n
t
Tinos
N
Syros Island
Grammata
Neogene Rocks
Fig. 3
Intrusive Granite
Volcanics rocks
Sediments
Syros
Pelagonian
Kythnos
Sedimentary cover
Mykonos
37°
Serifos
Naxos
Pindos Ocean
Meta-Ophiolite
Sifnos
Paros
Milos
Ios
Top-to-NE shearing
Fig. 4
Sikinos
Adria
Sedimentary cover
Basement
Finikas
lawsonite Occurence
Top-to-SW shearing
50 km
Thira
(a)
(b)
Fig. 1. (a) Simplified geological map of the Cyclades archipelago showing the three constitutive units of the Cycladic Blueschists
Unit (CBU): oceanic (black), sedimentary sequence (light grey) and the underlying basement (dark grey). The black and white
arrows show the two opposite senses of shear observed in the CBU top-to-S and top-to-N, respectively. The location of the
Cyclades is shown on the simplified structural map of the Aegean region (inset, upper-right corner). (b) Simplified lithological map
of the Syros island showing the continental margin units (light and dark grey) and the oceanic unit (black), as well as the location
of Figs 2, 3 and 4 (after Philippon et al., 2011).
© 2013 John Wiley & Sons Ltd
PRESER VATION OF L AWSONITE P SEUDOMORPH-B EAR ING ROCKS 573
dispersion of the stretching lineations is due to their
passive reorientation by subsequent (i) folding with
N-S to NE-SW trending axes, (ii) strike-slip deformation and (iii) brittle faulting (Philippon et al., 2012).
A few E-W trending stretching lineations occur only
in the central Cyclades, close to the Myrthes-Ikaria
strike slip fault, where the reorientation has been at a
maximum. This post exhumation deformation event
resulted in the late segmentation of the Cyclades
archipelago (Philippon et al., 2012). In this study we
consider the syn-metamorphic deformation of the
CBU, and therefore use the orientation of the
stretching lineation before the late segmentation (fig.
17 in Philippon et al., 2012). In order to clarify our
purpose, top-to-the-N–NE and S–SW sense of shear
are simplified in top-to-the-N and S.
In the Cyclades, these two opposite senses of shear
are considered to be synchronous and interpreted as
the result of either coaxial flattening at the crustal
scale (Rosenbaum et al., 2002; Bond et al., 2007) or
core complex-type extension (Gautier & Brun, 1994;
Tirel et al., 2009). However, detailed mapping of the
CBU shows that top-to-the-S sense of shear is
systematically observed where the lawsonite pseudomorphs are preserved (Fig. 1a).
Mapping shows that Syros displays the largest outcropping area of well-preserved lawsonite pseudomorphs in the Cyclades (Keiter et al., 2004;
Philippon et al., 2011; Fig. 2). The CBU lithological
pile is integrally preserved on Syros. From the base
to the top, it comprises two main tectonic units of:
(i) a 240 to 80 Ma oceanic crust (Lagos et al., 2007;
Bulle et al., 2010; Fig. 1) and (ii) a continental margin unit made of a 315 3 Ma gneissic basement
intruded by a 243–240 Ma pluton (U/Pb zircon
SHRIMP analysis; Keay, 1998; Tomaschek & Ballhaus, 1999; Tomaschek et al., 2003, 2008) and its
overlying sedimentary cover sequence ranging in age
from early Carboniferous to lower Triassic (Keay,
1998; Pohl, 1999; Tomaschek et al., 2003). The metamorphic conditions are estimated at 8–17 kbar and
350–500°C for the continental unit (Dixon & Ridley,
1987; Okrusch & Br€
ocker, 1990; Trotet et al., 2001;
Schumacher et al., 2008) and 19–24 kbar and 500–
580°C for the oceanic unit (Trotet et al., 2001; Gitahi, 2004; Holly et al., 2004). Geochronological data
give an Eocene age for the metamorphic peak
(52.2 0.3 Ma, Lu/Hf on eclogite facies garnet, Lagos et al., 2007) and an Oligocene age for the blueschist to greenschist facies transition (30.3 0.9 Ma,
24.88°E
37.49°N
Fig. 3
165
Lawsonite Occurences
Top to South
sense of shear
Thrust
Top to North East
sense of shear
Low angle Normal Fault
N
Foliation
Strike Slip Fault
High angle Normal Fault
Kambos thrust
231
Upper Marbles
Lower Metavolcanites
Metagabro/Metabasite
24.97°E
37.49°N
Pindus Ocean
Upper Metavolcanites
Kastri Marbles
Kastri Schists
Pyrgos Marbles
Pyrgos schists
Sedimentary cover
of Adria
Serpentine
434
1 km
Fig. 2. Detailed structural and lithological map of northern Syros. The main tectonic feature is the top-to-the-S Kambos thrust that puts
oceanic rocks on top of the volcano-sedimentary cover of the Adria continental margin. Top-to-the-S sense of shear (black arrows) is
only observed in oceanic rocks that contain lawsonite pseudomorphs (white lozenges). Top-to-the N sense of shear occurs above and
below the thrust but is not expressed in the lawsonite-bearing rocks.
© 2013 John Wiley & Sons Ltd
574 M. PHILI PPON ET AL .
40
Ar/39Ar dating of greenschist facies white mica,
Maluski et al., 1987).
CONTRASTING PRESERVATION OF LAWSONITE
PSEUDOMORPHS
In Syros, lawsonite pseudomorphs are observed only
in the oceanic unit, which comprises metavolcanic
blueschists (which are the lawsonite pseudomorphbearing rocks) along with metagabbro, serpentinite
and marble. In the north of the island (Fig. 2), where
the lawsonite pseudomorphs are especially well
exposed, the base of the oceanic unit comprises metagabbro and highly deformed serpentine schist thrust
on top of the Kastri marble along the so-called Kambos thrust (Figs 2 & 3; Bonneau et al., 1980a,b; Ridley, 1982; Keiter et al., 2004; Bond et al., 2007;
Keiter et al., 2011; Philippon et al., 2011). At the
island scale, top-to-the-S shearing is observed mainly
in the oceanic unit, situated in the hangingwall of this
large-scale thrust (Fig. 2). Top-to-the-N shearing is
observed in the hangingwall of the Kambos thrust
only locally, whereas it represents the dominant feature of the footwall. It is interpreted as coeval with
the retrogression from the blueschist/eclogite to
greenschist facies conditions (Trotet et al., 2001).
The lawsonite pseudomorph-bearing rocks of
Grammata Bay (37°29′57.11″N 24°53′37.29″E) occur
in the hangingwall of the Kambos thrust, just below
a tectonic melange of metavolcanic rocks and marble
displaying a layer-parallel foliation. The occurrence
of a tectonic melange at the interface between the
metavolcanic rocks and the upper marble unit indicates that minor thrusting also occurred in the hangingwall of the Kambos thrust (Fig. 3). These rocks
mainly display a N-S-trending stretching lineation
(b)
(a)
(c)
(d)
Fig. 3. (a) Map-scale structure of the tectonic melange, at the interface between the metavolcanic rocks and the upper marble unit,
above the Kambos thrust in the Grammata bay area (see Fig. 1 for location). Black arrows indicate the direction of the stretching
lineation and the associated sense of shear. (b) Outcrop photograph of a typical lawsonite pseudomorph-bearing rock (sampled
rock): a fine-grained metabasite showing (i) a foliation marked by glaucophane crystals, (ii) centimetric lawsonite pseudomorph
and (iii) garnet crystals in the matrix and as inclusions in the lawsonite pseudomorph. (c) Close-up photograph of the thin section
(sample I095b, white star on the map), showing the relationship between glaucophane-titanite foliation and porphyroblasts of
lawsonite and garnet and (d) its mineralogic content.
© 2013 John Wiley & Sons Ltd
PRESER VATION OF L AWSONITE P SEUDOMORPH-B EAR ING ROCKS 575
associated with top-to-the-S sense of shear. The highpressure foliation of the tectonic melange is cut by
N-S trending normal faults (Fig. 3a). At the outcrop
scale, the lawsonite-bearing rock is a light blue finegrained metabasic rock containing millimetric garnet
and centimetric lawsonite pseudomorphs that have
preserved the typical euhedral lozenge shape of fresh
lawsonite (Fig. 3b). The foliation wraps around the
lawsonite pseudomorphs. However, the pseudomorphs also contain stretched clusters of titanite
that define a curved internal foliation, typically oblique to but continuous with the matrix foliation
(Fig. 3c,d). These features suggest that the lawsonite
crystals are synkinematic, since they overgrew a foliated matrix and rotated during foliation development
that outlasted the crystallization of lawsonite. Despite
the deformation, the lawsonite pseudomorphs have
preserved the euhedral character of the original crystals. This suggests that lawsonite was relatively rigid,
and hence probably fresh at the time of the deformation. Therefore, the top-to-the-S sense of shear, associated with this foliation, must have occurred when
the rock was in the lawsonite stability field.
Conversely, lawsonite-bearing blueschists deformed
under greenschist facies conditions show lawsonite
pseudomorphs that are deformed and do not preserve their original lozenge shape. The destruction of
lawsonite pseudomorphs by shear strain is exemplified in rocks that have been affected by greenschist
facies shear zones in the Finikas area (37°23′51.20″
N, 24°52′40.49″E), on the south-western coast of
Syros. These shear zones trend parallel to large
NE–SW trending brittle-ductile faults (Keiter et al.,
2011; Philippon et al., 2011), and therefore are likely
related to this late deformation event. There, lawsonite blueschists display a sub-vertical N30° glaucophane-bearing foliation affected by upright folds and
late vertical shear zones trending N50° with a dextral
sense of shear (Fig. 4; Philippon et al., 2011). These
shear zones strongly overprinted the lawsonite blueschists, and glaucophane disappears close to the shear
zones (in a distance of up to ~ 30 cm from the
border of the shear zone), which display a greenschist-facies assemblage - actinolite, epidote, albite,
chlorite, calcite (Fig. 5). At the outcrop scale,
abundant cm-scale lawsonite pseudomorphs with
well-preserved shapes are present between the shear
zones, whereas they are completely absent inside the
shear zones (Fig. 4). Between the shear zones, the
matrix foliation is defined by the preferred orientation of glaucophane and epidote and smoothly wraps
around white lozenge-shaped lawsonite pseudomorphs composed of epidote, white mica and albite
(Fig. 5). Weakly oriented to unoriented crystals of
actinolite, chlorite, calcite and albite are also present
in the matrix. Albite calcite-bearing pressure shadows around the lawsonite pseudomorphs suggest that
lawsonite first acted as a rigid body during earlier
stages of the ductile deformation. Inside the shear
zones, the calcite-bearing matrix is a possible evidence for the introduction of a CO2-bearing fluid.
The matrix contains stretched and folded aggregates
of epidote, white mica and albite that can be interpreted as strongly deformed lawsonite pseudomorphs
only by comparison to the neighbouring rocks.
Hence, we infer that with increasing strain intensity,
lawsonite pseudomorphs were strongly deformed and
became progressively indistinguishable from the
matrix.
The above observations show that (i) top-to-the-S
sense of shear occurred within the lawsonite stability field, when lawsonite was fresh (before the lawsonite breakdown), and (ii) deformation that
occurred after the lawsonite breakdown led to the
Preserved
N
Lawsonite
pseudomorph
Destroyed
Fig. 5
87
Foliation trajectories
and mean dip
78
Shear zone and
sense of shear
59
80
82
85
78
30
Strike slip
42
84
54
79
46
72
64
70
60
57
25
82
43
62
51
66
72
Fig. 4. Outcrop-scale map of shear zones
affecting lawsonite pseudomorph-bearing
rocks, at Finikas (see location Fig. 1).
Foliation trajectories show a mean N30°
trend affected by upright folds and late
dextral N50° trending shear zones.
Location of the sampled shear zone
presented in Fig. 5 is also shown.
© 2013 John Wiley & Sons Ltd
84
59
69
80
80
44
81
68
75
42
79
83
83
36
61
58
5m
63
26
576 M. PHILI PPON ET AL .
Fig. 5. Lawsonite pseudomorph-bearing rock partly affected
by a shear zone after the lawsonite breakdown (see location
Fig. 4). Thin sections from outside and inside the shear zone
show the deformation of the lawsonite pseudomorph that
results from shearing.
destruction of the lawsonite pseudomorphs. Consequently, well-preserved lawsonite pseudomorphs
with their typical euhedral lozenge shape indicate
that the host rock has not undergone any significant penetrative deformation after lawsonite destabilization. Linking these observations to the P–T
conditions of lawsonite growth and breakdown
allows (i) to link the deformation events with particular parts of the P–T path and then (ii) to establish a correlation with the regional tectonic
evolution. Indeed, within subduction zones, burialrelated deformation is associated with the prograde
path whereas exhumation-related deformation is
related to the retrograde/decompressional part of
the P–T path. Therefore, lawsonite pseudomorphs
are a useful strain-free gauge providing clues about
the timing of deformation with respect to subduction and exhumation processes.
LAWSONITE PROGRADE GROWTH
Textural relations and mineral chemistry
In order to document lawsonite growth and destabilization, and attribute them to particular deformational events, a petrological analysis of a lawsonite-
bearing sample from the middle part of the oceanic
unit (sample I095b, Fig. 3a–c) has been performed.
Representative analyses of the main rock-forming
minerals are presented in Table 1. In the text, the
given composition ranges reflect variation among
grains within the sample.
The sample is a light blue, glaucophane-dominated
fine-grained (average grain size ~ 0.3 mm) metabasalt
containing euhedral porphyroblasts of garnet (up to
3 mm) and (pseudomorphed) lawsonite (up to
2 9 1 cm). The foliation is defined by the preferred
orientation of glaucophane (Si = 7.70–7.99 cpfu;
octahedral
Al = 1.57–1.84 cpfu;
XNa = Na/
(Na + Ca) = 0.79–0.96; XFe = Fe/(Fe + Mg) = 0.32–
0.37; recalculated Fe3+ = 0.02–0.10 cpfu), epidote
(XFe3 = Fe3+/(Fe3+ + Al–2) = 0.24–0.41), muscovite
(Si = 3.33–3.40;
XNa = Na/(Na + K) = 0.05–0.07),
chlorite (XFe = 0.33–0.34), and stretched clusters of
titanite (Fig. 6). Rutile and quartz are locally present.
This foliation wraps gently around mostly euhedral
crystals of garnet and lozenge-shaped aggregates containing epidote, chlorite, paragonite and locally
albite, interpreted as pseudomorphs after lawsonite
(cf. Ballevre et al., 2003).
Lawsonite pseudomorphs are euhedral and comprise unoriented small crystals of epidote, paragonite
(XNa = 0.81–0.89), chlorite and locally albite (An02).
They also contain larger crystals of glaucophane, epidote and stretched clusters of titanite, interpreted as
inclusions in the original lawsonite crystals. These
inclusions define a slightly curved internal foliation,
generally oblique to but continuous with the matrix
foliation (Figs 3 & 6b). Locally, garnet crystals are
also included in the pseudomorphs. The pseudomorphs display a hourglass-shaped distribution of
TiO2 (Fig. 6b), reminiscent of the sector zoning
observed in fresh lawsonite crystals (cf. Ueno, 1999).
Garnet is nearly euhedral and contains inclusions
of glaucophane, epidote, muscovite and titanite that
are mostly unoriented, or display rarely a weak preferred orientation, continuous with the matrix foliation. Crystallization tails of chlorite, parallel to the
foliation are present around matrix garnet, but are
absent around garnet included in lawsonite pseudomorphs (Figs 3 & 6). Garnet is almandine-rich (51–
59 mol%) and displays a chemical zoning distinct in
the core and in the rim, respectively (Fig. 6c,d). The
large garnet core is characterized by a rimward
decrease in spessartine (11?3 mol%) and XFe (=Fe/
(Fe + Mg) = 0.90 ? 0.87). The grossular varies
irregularly between 27 and 33 mol%. Conversely, the
thin garnet rim (~10% of the diameter) exhibits an
increase in spessartine (3?6 mol%), grossular (30?
33 mol%) and XFe (0.87?0.89). Garnet zoning is
identical in the matrix crystals and in the inclusions
in lawsonite pseudomorphs (Fig. 6c).
Microstructural features suggest that garnet crystallized before the main deformation, in a matrix
comprising unoriented crystals of glaucophane, epi© 2013 John Wiley & Sons Ltd
PRESER VATION OF L AWSONITE P SEUDOMORPH-B EAR ING ROCKS 577
Table 1. Representative mineral analyses
from sample I095b. The amount of ferric
iron was calculated from stoichiometric
constraints using the programme AX
(http://www.esc.cam.ac.uk/research/
research-groups/holland/ax). For
amphibole, the Fe3+ content corresponds
to the average from minimum and
maximum constraints. ir – inner rim, ig –
inclusion in garnet, ps – in lawsonite
pseudomorph and mx – matrix.
Mineral
Anal. #
Position
g
134
Core
g
178
ir
g
188
Rim
gl
54
ig
gl
39
mx
ep
29
ig
mu
32
mx
mu
69
ps
pa
73
ps
chl
13
mx
chl
22
Tail
SiO2
TiO2
A12O3
Cr2O3
FeO
MnO
MgO
CaO
Na2O
K2O
Total
No. O
Si
Ti
Al
Cr
Fe3+
Fe2+
Mn
Mg
Ca
Na
K
Total
XFe
XNa
Aim
Prp
Grs
Sps
37.88
0.14
21.42
0.05
25.43
4.84
1.75
9.84
0.04
0.00
101.50
12.00
2.97
0.01
1.98
0.00
0.06
1.62
0.32
0.21
0.83
0.01
0.00
8.00
0.89
0.01
0.59
0.06
0.28
0.07
38.25
0.10
21.16
0.00
27.91
1.75
2.28
10.12
0.01
0.00
101.69
12.00
2.99
0.01
1.95
0.00
0.06
1.76
0.12
0.27
0.85
0.00
0.00
8.00
0.87
0.00
0.59
0.09
0.28
0.04
38.39
0.00
21.35
0.00
26.56
2.76
1.83
11.34
0.01
0.01
101.85
12.00
2.99
0.00
1.96
0.00
0.05
1.65
0.18
0.21
0.95
0.00
0.00
8.00
0.89
0.00
0.55
0.07
0.32
0.06
57.89
0.00
12.21
0.03
10.54
0.10
10.06
1.16
6.66
0.01
98.67
23.00
7.87
0.00
1.96
0.00
0.03
1.17
0.01
2.04
0.17
1.76
0.00
15.02
0.36
0.91
56.70
0.00
11.13
0.06
10.24
0.05
11.17
2.88
6.18
0.07
98.47
23.00
7.77
0.00
1.80
0.01
0.10
1.07
0.01
2.28
0.42
1.64
0.01
15.14
0.32
0.79
38.70
0.16
28.34
0.01
6.22
0.14
0.02
23.86
0.01
0.00
97.46
12.50
3.00
0.01
2.59
0.00
0.37
0.04
0.01
0.00
1.98
0.00
0.00
8.01
51.10
0.12
28.86
0.07
2.00
0.02
3.89
0.04
0.53
10.44
97.10
11.00
3.33
0.01
2.22
0.00
0.06
0.05
0.00
0.38
0.00
0.07
0.87
6.99
50.29
0.08
27.45
0.07
1.95
0.00
3.45
0.02
0.52
10.68
94.52
11.00
3.39
0.00
2.18
0.00
0.00
0.11
0.00
0.35
0.00
0.07
0.92
7.01
46.90
0.00
39.43
0.04
0.27
0.01
0.17
0.19
6.65
1.40
95.07
11.00
3.01
0.00
2.98
0.00
0.00
0.01
0.00
0.02
0.01
0.83
0.12
6.97
28.25
0.00
20.05
0.09
18.37
0.13
20.76
0.00
0.00
0.01
87.67
14.00
2.86
0.00
2.39
0.01
0.00
1.55
0.01
3.13
0.00
0.00
0.00
9.95
0.33
27.67
0.00
20.09
0.07
18.83
0.10
20.33
0.01
0.03
0.02
87.14
14.00
2.83
0.00
2.42
0.01
0.00
1.61
0.01
3.09
0.00
0.01
0.00
9.97
0.34
0.07
0.07
0.87
dote, titanite and muscovite. The subsequent crystallization of lawsonite was contemporaneous with the
development of the foliation, which formed mostly
after garnet crystallization (Fig. 7). Accordingly,
crystallization tails of chlorite developed around garnet in the matrix, but not around the crystals
included in, and shielded by, the lawsonite crystals.
Lawsonite breakdown occurred after the deformation, as evidenced by the preservation of the euhedral
shape of the pseudomorphs and the sector zoning
(Figs 5 & 6). The compositional evolution of garnet
cores is characteristic of growth zoning (e.g. Tracy,
1982) and suggests it crystallized along a prograde
P–T path. The zoning of garnet rims is attributable
to incipient diffusional reequilibration of partly resorbed crystals (e.g. Tracy, 1982; Robinson, 1991),
possibly contemporaneous with the lawsonite breakdown. Identical zoning of garnet in the matrix and in
the pseudomorphs supports the microstructural
evidence and confirms that the crystallization of lawsonite is a relatively late feature.
P–T evolution
In order to determine the prograde P-T path and conditions of lawsonite growth and replacement, P-T
pseudosections were computed with THERMOCALC
v. 3.33i (Powell & Holland, 1988) and the internally
consistent thermodynamic dataset 5.5 (Holland &
Powell, 1998; November 2003 upgrade) in the model
© 2013 John Wiley & Sons Ltd
system NCKFMASHTO (Fig. 8). The solid-solutions
used in the modelling and the corresponding mixing
models are: clinoamphibole – Diener et al. (2007),
clinopyroxene – Green et al. (2007), garnet – White
et al. (2007) modified by Diener et al. (2008), paragonite-muscovite – Coggon & Holland (2002), chlorite –
Holland et al. (1998), epidote – Holland & Powell
(1998). Lawsonite, rutile, titanite and aqueous fluid
are pure end-member phases. Indeed, the studied sample is carbonate-free and despite local evidence for
CO2-rich fluids precipitating calcite, it has been shown
that XCO2 was lower than 0.01 in the glaucophanebearing marbles belonging to the oceanic unit
(Schumacher et al., 2008). Whole rock chemical composition was analysed by ICP-AES at the CRPG
Nancy. The FeO (v. Fe2O3) content has been determined by titration.
The P–T path followed by the Syros blueschists
proposed in this study (black arrow, Fig. 8) is
deduced from the textural relationships described
above. The evolution started in the stability field glchl-ep-mu-ru, as evidenced by the relict assemblage
preserved in garnet. The growth of garnet occurred
during an evolution dominated by burial (compression) and moderate heating, as evidenced by the preserved growth zoning (stage 1, Fig. 8). Then lawsonite
crystallized and reached the equilibration conditions
of the peak mineral assemblage, which corresponds
to the stability field g-gl-law-chl-ep-mu-ru (stage 2,
Fig. 8). The chemical composition of garnet, in par-
578 M. PHILI PPON ET AL .
(b)
(a)
(d)
(c)
Fig. 6. Thin section containing a lawsonite pseudomorph. (a) Photomicrograph (plane polarized light) of a lawsonite pseudomorph.
Note that garnet crystals included in the pseudomorph lack the chlorite crystallization tails, present around matrix garnet (arrows).
The black line indicates the location of the profile in d. X-ray maps showing the distribution of TiO2 (b) and MnO (c). (d) Electron
microprobe profile of garnet, with its composition in terms of the end-members almandine, spessartine, grossular and pyrope, as well
as the XFe ratio (= atomic Fe/(Fe + Mg).
ticular the grossular content, restricts these conditions
to ~550°C, 20 kbar. Subsequent destabilisation of
lawsonite, accompanied by limited dissolution of garnet, evidenced by the diffusion zoning in garnet rims,
suggests decompression and cooling (stage 3, Fig. 8).
The crystallization of lawsonite occurred therefore
just before the rock reached maximum pressure. The
contemporaneous main ductile deformation with topto-the-south shearing is consequently a feature
related to the prograde, subduction-related part of
the P–T evolution.
DISCUSSION & CONCLUSIONS
Blueschist-facies metabasic rocks in Syros preserve
spectacular euhedral pseudomorphs of lawsonite. Petrographic observations suggest that the crystallization
of lawsonite was contemporaneous with the main
deformation, characterised by top-to-the-S shearing.
In greenschist-facies deformation zones, which locally
affect the blueschists, lawsonite pseudomorphs are
strongly deformed or destroyed and undistinguishable
from the matrix. Petrological modelling suggests that
the growth of lawsonite – and the contemporaneous
deformation – occurred along the prograde part of the
P–T evolution, just before the pressure peak. By comparison with the greenschist-facies shear zones, the
preservation of the pseudomorphs indicates that no
significant later deformation affected the blueschists
and the preserved structures may therefore be attributed to the subduction processes.
Calculated phase diagram
The calculated phase diagram fails to reproduce some
of the petrographic observations. First, titanite is
© 2013 John Wiley & Sons Ltd
PRESER VATION OF L AWSONITE P SEUDOMORPH-B EAR ING ROCKS 579
Ductile
deformation
Mn-rich
g
present in the rock, but is not stable in the diagram
in the P–T interval of interest. This is probably
related to the fact that titanite was considered as a
pure end-member in the calculations, whereas it is
known to contain small but non-negligible amounts
of additional components like Al and F (e.g. Franz
& Spear, 1985; Enami et al., 1993; Harlov et al.,
2006), which can significantly extend its stability field.
Second, garnet growth zoning cannot be modelled
appropriately since Mn was not considered in the
calculations, because it is exclusively concentrated in
the garnet cores and has therefore a limited influence
on the matrix assemblages, equilibrating with the
Mn-poor garnet rims. However, including Mn would
have extended the stability domain of garnet towards
lower pressures and temperatures, well before the
rock entered the stability domain of lawsonite, in
agreement with the observations. Despite these
imperfections, the diagram reproduces the first-order
observations correctly. Furthermore, rather than estimating precisely the P–T conditions of the rocks, our
goal was to analyse qualitatively the crystallization
Lawsonite
breakdown
Mn-poor
law
gl
ep
chl
mu
ttn
ru
pa
ab
Fig. 7. Relationship between mineral growth and ductile
deformation in sample I095b. Ductile deformation ceased
before lawsonite breakdown. For the sake of clarity, minerals
depicted in the column “lawsonite breakdown” are exclusively
those that clearly pseudomorph lawsonite.
NCKFMASHTO
(+HO and Quartz)
g
SiO2 Al2O3 CaO MgO FeO K2O Na2O TiO2
O
53.124 9.914 9.684 12.768 8.632 0.220 3.235 0.761 1.662
ep
o g gl ep
mu ru law
18
g
ep
g
20
ep
law
hl
l c ru
g
o mu w
ep la
o
l
h
lc
o g mu ru
epdi o
di gl chl
ep mu ru
di
15
500
p
le
h
c w
gl l a
g ru o
o u
m
law
g
m gl c
u hl
ru e
la p
w
gl chl o mu
ru law
chl
o l ep w w
la a
gg
chl mu ru l g gl
ep mu
ru
Prograde growth
chl
16
g gl chl o
mu ru law
1
tion
20
28
26
24
gl chl ep
mu ru
g gl chl ep mu ru
g
hb
X(Grs)
550
Syndeformational
Lawsonite growth
in a
g, gl, chl, ep, mu, ru
matrix
Folia
22
30
chl
chl
2 3
Garnet growth
in a
gl, chl, ep, mu, ru
matrix
1
Law
30
gl
ep
hl
g c ru
u
g m hb
600
lws +
lws –
Retrograde
destabilization
P(kbar)
g gl o
mu ru law
chl
25
Undeformed
matrix
2
Lawsonite breakdown
chl
Lawsonite
breakdown
3
Fig. 8. Calculated P–T pseudosection for the selected sample with excess H2O. Quartz is present in all assemblages. Abbreviations
on field boundaries indicate the phase that is lost/gained on this transition. Field shading indicates the variance of the assemblage
(white – v = 2 and v = 3, light grey – v = 4, dark grey – v = 5). The P–T path is deduced from the textural relationship observed
in thin section and discussed in the text. Quartz is present in all fields, but has not been considered as an excess phase during the
calculations. Dashed lines are the isopleths of the grossular content in garnet (in %).
© 2013 John Wiley & Sons Ltd
580 M. PHILI PPON ET AL .
sequence and attribute the crystallization of lawsonite
to a specific part of the P–T evolution.
One of the pitfalls of using pseudosections is the
estimation of the rock volume at which equilibration
was efficient, and consequently the choice of the
appropriate bulk composition. This includes the estimation of the effective amount of H2O and Fe2O3
present in the rock (cf. Ballevre et al., 2003; L
opezCarmona et al., 2013). The presented pseudosection,
calculated for the analysed bulk composition of the
sample, successfully models the prograde evolution
and the peak pressure assemblage. This suggests
equilibration at the sample scale, and a posteriori
justifies the choice of the bulk composition. However,
the last stage of the P–T evolution, the destabilisation of lawsonite, cannot be accounted for appropriately in the framework of the diagram. Indeed,
the pseudomorphic replacement of lawsonite by the
epidote – paragonite – chlorite albite aggregates
reflects equilibration in volumes close to the size of
the lawsonite crystals, and hence an effective bulk
composition (e.g. Tracy, 1982; St€
uwe, 1997) significantly different from that of the entire sample. However, modelling accurately this process goes beyond
the scope of our work.
Tectonic implications for the Cyclades
Through the analysis of rocks with variable degrees
of lawsonite pseudomorph preservation combined
with new petrological and kinematic data and available geochronological data it is possible to identify
and distinguish the kinematics and timing of subduction- and exhumation-related deformation in the
CBU:
(1) Top-to-the-S shearing occurred in the lawsonite
stability field, on the prograde path of the CBU,
as indicated by the zoning of garnet and the preservation of lawsonite pseudomorphs (Fig. 9,
Stage 1). This deformation was therefore related
to subduction and occurred prior to 52 Ma,
which is the age of the high pressure metamorphic peak, obtained by Lu-Hf dating on garnetbearing HP/LT metamorphic rocks belonging to
the oceanic unit (Lagos et al., 2007).
(2) The retrograde overprint, which occurred during
the transition from the blueschist/eclogite to
greenschist facies conditions, was associated with
top-to-the-N shearing (Fig. 9, stage 2) and
affected large volumes of the CBU (Trotet et al.,
2001) during the exhumation up to the brittle/
ductile transition. In domains affected by this
exhumation-related deformation, lawsonite pseudomorphs were sheared and destroyed (cf. Philippon et al., 2009). Comparable behaviour of
aragonite crystals gives constraints on the retrograde path from their breakdown (estimated at 9
kbar from the retrograde path proposed by
Schumacher et al., 2008) to the surface. Aragonite is also only preserved in rocks that remained
unaffected by exhumation-related deformation
(Brady et al., 2004). The retrogression in the
greenschist facies is estimated at 35 Ma based on
K/Ar (Altherr et al., 1979) and 40Ar/39Ar (Maluski et al., 1987; Wijbrans et al., 1990; Parra et al.,
2002) dating of white mica belonging to greenschist facies rocks.
From a regional point of view and at variance with
previous studies (Gautier & Brun, 1994; Rosenbaum
et al., 2002; Tirel et al., 2009), the evidence provided
above shows that the two opposite senses of shear
affecting the whole CBU, top-to-the-S (mainly preserved in the oceanic unit) and top-to-the-N (mainly
expressed in the basement-cover unit), are not synchronous. They are associated with deformation
events that occur respectively before and after the
crystallization of lawsonite, which marks the pressure
peak and hence the maximum subduction depth of
+
w
La w –
La
1
20
52 Ma
Before 52 Ma
16
g+
g
–
Pressure (kbar)
Top-to-the-S-SW
12
2
8
1
Top-to-the-S-SW
prograde shearing
S
Lithospheric mantle
40-35 Ma
Top-to-the-NE
Trench retreat
4
200
300
400
500
Temperature (°C)
2
Top-to-the-NE
retrograde shearing
Pindos oceanic domain
Adria continental crust
Meditteranean oceanic domain
Fresh lawsonite
Preserved
Lawsonite pseudomorph
Destroyed
Asthenosphere
N
Fig. 9. P–T diagram showing the lawsonite
breakdown line and the inferred P–T path.
The prograde top-to-the-S sense of shear
occurred in the lawsonite stability field and the
retrograde top-to-the-N sense of shear
occurred after the lawsonite breakdown. These
two senses of shear are not synchronous and
are related respectively to the subduction and
exhumation part of the evolution. The
complete cycle is summarized in two
lithosphere scale cross sections of the Hellenic
subduction zone that show (i) the subduction
of the CBU with a top-to-the-S sense of shear
within the stability field of lawsonite and (ii)
the exhumation of the CBU with a top-to-theN sense of shear outside the lawsonite stability
field. Preservation of lawsonite pseudomorphs
results from the heterogeneity of retrograde
deformation during exhumation.
© 2013 John Wiley & Sons Ltd
PRESER VATION OF L AWSONITE P SEUDOMORPH-B EAR ING ROCKS 581
the rock units. These observations rule out the exhumation models of the CBU that involve two synchronous opposite senses of shear resulting from either
coaxial flattening at the crustal scale (Rosenbaum
et al., 2002; Bond et al., 2007) or core complex-type
extension (Gautier & Brun, 1994; Tirel et al., 2009).
Instead, they are in good agreement with a model of
exhumation driven by slab rollback in which deformations related to subduction and exhumation are not
synchronous within the thrust pile (Brun & Faccenna,
2008). The CBU was first buried with a top-to-the-S
sense of shear, underwent prograde metamorphism
and entered the stability field of lawsonite (Fig. 9,
stage 1). Following burial, the CBU was exhumed
with an opposite top-to-the-N sense of shear (Fig. 9,
stage 2), crossing the lawsonite breakdown reactions
and forming lawsonite pseudomorphs.
The ductile deformation related to the exhumation
was heterogeneous, which allowed the preservation of
lawsonite pseudomorphs, indicating that significant
volumes of lawsonite-bearing rocks were not affected
by the retrograde ductile deformation during exhumation. As exemplified in the Syros blueschists as
well as at the scale of the whole Cyclades, the subduction-related deformation that is sealed by the
lawsonite crystals, subsequently pseudomorphed,
occurred with a sense of shear top-to-the-S, prior to
52 Ma. The subsequent exhumation of the Cycladic
Blueschist Unit must be re-investigated in this light.
Ductile deformation and preservation of HP minerals
This study illustrates that lawsonite growth occurred
at a mantle depth during the southward thrusting of
the oceanic unit onto the continental sequence within
the subduction zone (Fig. 9). The occurrence of lawsonite-bearing rocks is rather restricted and the two
following hypotheses explain this scarcity (for a
worldwide synthesis of lawsonite-bearing eclogites,
see Tsujimori et al., 2006). First, some authors (e.g.
Zack et al., 2004; Teyssier et al., 2010; Zucali & Spalla, 2011) suggested that lawsonite-bearing rocks
required a very specific thermal exhumation path in
subduction zones in order to be preserved. Second,
our study confirms that the exceptional preservation
of lawsonite pseudomorphs is the result of a lack of
penetrative deformation during exhumation (Fig. 5),
as already suggested by others (Brady et al., 2004;
Keiter et al., 2004; Whitney & Davis, 2006; Philippon
et al., 2009).
Because lawsonite pseudomorphs are composed of
epidote + chlorite + white mica albite, any penetrative deformation makes them indistinguishable
from the host rock matrix. Consequently, wellpreserved lawsonite pseudomorphs with their typical
euhedral lozenge shape indicate that the host rock has
not undergone any significant penetrative deformation
after lawsonite destabilization. For this reason, lawsonite pseudomorphs provide a useful strain-free
© 2013 John Wiley & Sons Ltd
gauge to identify deformation that occurred before
the destabilization of lawsonite. In blueschists, this
provides a very efficient tool to discern between subduction- and exhumation-related deformation events,
as lawsonite is a hydrous high-pressure mineral that
crystallized along the prograde P-T paths during
subduction and that underwent breakdown during
decompression and/or heating.
ACKNOWLEDGMENTS
The present work was financially supported by the
ANR-EGEO project. We are extremely grateful to L.
Jolivet for having introduced us to Syros geology.
We acknowledge the reviews of U. Ring and two
anonymous reviewers as well as the editorial handling
of D. Whitney. M. Philippon also acknowledges the
financial support from the Marie Curie Initial Training Network TOPOMOD.
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