Sample description
Samples involved in the average P–T thermobarometric calculations are all described
in details addressing briefly outcrop conditions, mineral habits, phase relationships and
mineral compositions. Mineral abbreviations used in this work are after Whitney and Evans
(2010). Nomenclature of amphibole is after Leake et al. [1997]; pyroxene after Morimoto et
al. [1988] and epidote after Armbruster et al. [2006]. Average P–T calculations were focussed
on metapelite, metabasite rock samples and rocks of intermediate compositions such as
glaucophane-(chloritoid) micaschists.
Samples Sik09006 (glaucophane-chloritoid-micaschists)
Garnets are generally abundant, reaching locally 6-8 mm and Mn rich (Fig.S1a, S1b
and
S1e).
Garnet
cores
(Alm52Grs23Sps20Prp05)
are
relatively enriched
when
compared
in
with
Mg
and
mantle
depleted
in
Fe
compositions
(Alm57Grs18Sps21Prp4; Fig. S1e). The chemical composition corresponding to the last
garnet generation (Alm33Grs18Sps47Prp2) has often been erased as garnet outer rims present
Mn retrodiffusion fringes (Fig. S1a) [Butcher and Frey, 2002; Angiboust et al., 2011].
Inclusions trapped into the garnet include large chloritoid, non-zoned blue amphibole,
epidote, paragonite, phengite and rutile (Fig. S1a and S1b). Garnets are wrapped in a strongly
foliated matrix consisting of quartz, blue-amphibole, phengite, paragonite, epidote, chlorite
and rutile (Fig. S1a and S1b). Chloritoid shows homogeneous values of XMg between 0.23
and 0.25 (Fig. S1f). Interestingly, blue-amphiboles from the matrix are clearly zoned with
pale-blue cores and darker rims in natural light (Fig. S1h). Chemical compositions thus show
two distinct compositions with a glaucophane core composition (XFe between 0.59 and 0.68)
consistent with the composition of blue-amphibole inclusions within the garnet and
ferroglaucophane rim compositions (XFe between 0.39 and 0.52; Fig. S1i). The white mica is
mainly phengitic in composition and shows variable silica-content mainly depending on the
microtextural domain (Fig. S1c). The latter ranges from 3.32 to 3.14 per formula unit
(hereafter noted p.f.u.) in the foliation and in late shear bands; it increases continuously
towards the core of the largest crystals and in the inclusions trapped in the garnet, where it
locally reaches 3.29 to 3.43 p.f.u. (XMg between 0.38 and 0.59). Chlorite is abundant along
the main foliation where it exhibits clearly secondary microstructures, growing at the expense
of garnet and blue amphiboles (Fig. S1a). Presumed primary chlorites are however present as
small-scale inclusions within the garnet and in the most proximal parts of the garnet pressure
shadows. As for white mica, most of the chlorite compositions plot along a clear trend visible
in the XMg vs Si contents, which indicates progressive composition changes during the
retrograde evolution (Fig. S1d). Epidote, that shows small composition variations
(Ca1.99(Fe0.73Mn0.03Al2.34)Si3.09O12(OH) and 93 to 97% of the iron is ferric. and occurs either
in inclusion in garnets or in their adjacent pressure shadows and scarcely preserved in the
matrix (Fig. S1a and S1b). Rutile is rare and partly replaced by titanite. Besides, ca. 5 mm
long lozenge-shaped porphyroblasts observed on the same outcrop that correspond patchy
aggregates of phengite, epidote and quartz are be interpreted as pseudomorphs after lawsonite
in line with the interpretation of Gupta [1995].
Samples Sik09002 (glaucophane -micaschists)
Garnets are present as large ca. 8-12 mm blasts and are weakly zoned with an average
mantle
composition
of
Alm74Grs16Py10
and
an
average
rim
composition
of
Alm71Grs14Py14Sps01. Blue-amphibole compositions show two distinct compositions with
glaucophane core compositions (XFe between 0.56 and 0.68) and ferroglaucophane rim
compositions (XFe between 0.40 and 0.50; Fig. S1i). A first generation of white mica (Si
content 3.47 and 3.36 p.f.u.; Fig. S1) is frequently included within the garnet and in lenseshaped domains while texturally secondary white micas (3.37 and 3.24 p.f.u.) occur along the
main foliation, shear bands or even in-between truncated blue-amphiboles. In these
microstructures, Si content of phengite increases continuously towards the core of the largest
crystals, where it locally reaches 3.43 to 3.29 p.f.u (XMg between 0.38 and 0.59). Chlorite
mostly exhibits secondary microstructures while small-scale presumed primary chlorites are
however present as inclusions within the garnet with generally higher XMg and Si content
(Fig. S1d). Epidote is frequent and displays low composition variations. Rutile, which
completes the mineral association, is locally abundant.
Sample Sik11 295 (metabasite)
Metabasites are a rather rare component of the metamorphic series of Sikinos and
Folegandros (Figure 6) and best outcrop conditions are found to the North of Sikinos. There,
metabasites occur as rather continuous layer embedded within metasediments, as
dismembered variably sized “rounded” pods from a few tens of centimeters to ca. 3 m in
length or as genuine blocks in metaconglomerate layers as on Syros [e.g. Bonneau et al.,
1980]. Characteristic assemblages observed in the field are omp + gln + rt + ph supplemented
by various amounts of iron-copper sulfides. Garnet is generally rare and concentrated to
stripes that may correspond to initial chemical layering of the rock.
Away from deformed and retrograded areas, omphacite appears as 3-5 mm weakly-zoned
porphyroblasts
with
compositions
varying
from
Di40Hed3Jd42Acm15
to
Di47Hed5Jd31Acm17 (Fig. S1g). White mica is a highly substituted phengite with silica
content ranging between 3.53 and 3.46 p.f.u., and consistent XMg values of 0.69-0.77 (Fig.
S1c). Garnet (Fig. S1e) is rare, generally small (i.e. 2-3mm), Mn-rich and poorly zoned
(Alm51Grs17Sps27Py5, on average). Retrograde blue-amphiboles are glaucophane
characterised by rather homogeneous compositions (i.e. XMg = 0.65-0.69; Fig. S1i). Quartz
occurs as scarce xenomorphic, small crystals in the matrix. Conversely, rutile is locally
abundant.
Sample Sik11266 (metapelite)
Metapelites are widespread on Sikinos and Folegandros. They occur as meter to
several tens of meters-thick continuous layers and more occasionally as isolated lenses
embedded into marble layers (Fig. 6). Being often strongly deformed, metapelites are
generally also severely reequilibrated in the greenschist-facies conditions and characterised by
synkinematic chlorite, albite and low-silica content white micas. Higher metamorphic
conditions assemblages are, in turn, preserved in low-strain domains and in more quartzitic
lenses (Fig. 7d).
Chloritoid appear as corroded 1-3 mm blue-green porphyroclasts surrounded by
secondary green chlorite in natural light. It shows low XMg values that cluster in the narrow
0.06-0.10 range (Fig. S1f). Garnet is moderately abundant (> 10 vol.%), 5-7 mm wide and
very rich in inclusions of quartz, chloritoid, white micas, chlorite and calcite. Garnet is
weakly zoned with an average composition of Alm64Grs24Spss7Py5 (Fig. S1e). White mica
is mainly phengitic in composition and shows variable silica content (Fig. S1c). In phengitic
inclusions in the garnet, silica content reaches 3.38 to 3.33 p.f.u. while it presents lower and
more scattered values in the matrix (3.35 to 3.26 p.f.u.). Chlorite is abundant and texturally
secondary in most cases. It is particularly present along the main foliation or along top-to-thenorth shear bands and growing at the expense of chloritoid. A former generation is trapped in
the outer part of garnet as (primary?) inclusions and in their adjacent pressure shadows.
Despite this textural control, chlorite shows continuous variations of silica content (from 2.58
to 2.79 p.f.u.) correlated with an increase of XMg values ranging from 0.44 to 0.64. Quartz is
abundant and calcite is observed in minor amount (< 5 vol.%) either in the matrix or in
inclusion in garnet. Aluminosilicates are absent. Carbonaceous material (CM) is locally
abundant and provides a typical dark-grey color to the rock.
REFERENCES
Angiboust, S., P. Agard, H. Raimbourg, P. Yamato, and B. Huet (2011), Subduction interface
processes recorded by eclogite-facies shear zones (Monviso, W. Alps), Lithos, 127(1),
222 – 238.
Armbruster, T., P. Bonazzi, M. Akasaka, V. Bermanec, C. Chopin, R. Gieré, R. and M.
Pasero (2006), Recommended nomenclature of epidote-group minerals, European
Journal of Mineralogy, 18(5), 551 – 567.
Bonneau, M., J. R. Kienast, C. Lepvrier, and H. Maluski (1980), Tectonique et
métamorphisme haute pression d'âge Eocène dans les Hellénides: exemple de l'île de
Syros (Cyclades, Grèce), Comptes rendus de l’Académie des Sciences de Paris, Série
2, 291, 171 – 174.
Butcher, K., and M. Frey (2002), Petrogenesis of Metamorphic Rocks, 7th ed., Springer,
Berlin, Germany, 341 pp.
Gupta, S. (1995), Structure and metamorphism of Sikinos, Cyclades, Greece, Ph.D. thesis,
248 pp., University of Cambridge, Cambridge, United Kingdom.
Leake, E., A. Woolley, C. Arps, W. Birch, C. Gilbert, J. Grice, F. Hawthorne, A. Kato, H.
Kisch, V. Krivovichev, K. Linthout, J. Laird, J. Mandarino, W. Maresch, E. Nickel,
N. Rock, J. Schumacher, D. Smith, N. Stephenson, L. Ungaretti, E. Whittaker, and Y.
Guo (1997), Nomenclature of
amphiboles; Report of the Subcommittee on
Amphiboles of the International Mineralogical Association, Commission on New
Minerals and Mineral Names, Canadian Mineralogist, 35, 219 – 246.
Morimoto, N. (1988), Nomenclature of pyroxenes, Mineralogy and Petrology, 39(1), 55 – 76.
Whitney, D. L., and B. W. Evans (2010), Abbreviations for names of rock-forming
minerals. American Mineralogist, 95(1), 185 – 186.