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The metamorphic history of Naxos (centralCyclades, Greece)Deciphering the Oligocene and Miocene exhumation eventsAlexandre Peillod
Academic dissertation for the in Geology at Stockholm University to be publicly defended onMonday 4 June 2018 at 10.00 in De Geersalen, Geovetenskapens hus, Svante Arrhenius väg 14.
AbstractHigh pressure, low temperature (HP-LT) rocks observed at the surface of the Earth are evidence of past subduction zones.Understanding the tectonics processes that control the exhumation of HP-LT metamorphic rocks in these subduction zonesrequires full comprehension of the pressure-temperature-time (P–T–t) cycle that the rocks experienced. In the Cyclades,Greece, the Cycladic Blueschist Unit (CBU) hosts eclogite and blueschist facies rocks. However, the processes thatexhumed them are debated. The overall aim of this thesis is to understand how the Eocene HP-LT rocks were exhumed inthe central Cyclades based on a study of the metamorphic history of Naxos Island and nearby Syros Island. In this thesis,I carried out a systematic geothermobarometric and geochronological investigation on Naxos to better constrain the P–T–t paths that are recorded by the rocks. The data indicate that high-P metamorphism on Naxos occurred in the Eocene at c.40Ma and the HP-LT rocks were exhumed by two tectonic events. The first exhumation event occurred in the Oligocene.The HP-LT rocks were exhumed in a convergent setting by an extrusion wedge. The top of the sequence reached greenschistfacies conditions at c. 32 Ma, whereas the bottom of the sequence remained at greater depth (equating to pressures of 8–12kbar). Additionally rocks from southeastern Syros recorded a similar Eocene/Oligocene P–T–t history to that recorded bythe top of the sequence on Naxos, suggesting a common Eocene/Oligocene metamorphic history for the central Cyclades.The second exhumation event occurred in the Miocene. The rocks were further exhumed in an extensional setting from c.20 to 8 Ma. The top of the sequence on Naxos was already in the brittle crust at that time and therefore did not record thisMiocene metamorphism. The bottom of the sequence was first isothermally exhumed at high-T conditions and thereaftercooled rapidly.
Keywords: Cycladic Blueschist Unit, Exhumation, Fluid flow, Geothermobarometry, Hellenide orogen, Rb-Sr dating,Subduction-zone metamorphism, Heat flow, Lower crust, Extensional domain, Metamorphic core complex.
Stockholm 2018http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-155218
ISBN 978-91-7797-240-2ISBN 978-91-7797-241-9
Department of Geological Sciences
Stockholm University, 106 91 Stockholm
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THE METAMORPHIC HISTORY OF NAXOS (CENTRAL CYCLADES, GREECE)
Alexandre Jean Daniel Peillod
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The metamorphic history of Naxos (centralCyclades, Greece)
Deciphering the Oligocene and Miocene exhumation events
Alexandre Jean Daniel Peillod
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©Alexandre Jean Daniel Peillod, Stockholm University 2018 ISBN print 978-91-7797-240-2ISBN PDF 978-91-7797-241-9 Front page: "For my friend 'Hard or nuthin' Alexandre Peillod"Efstathios Reppas-Chrysovitsinos Back page: Photo from Etienne Pauthenet Printed in Sweden by Universitetsservice US-AB, Stockholm 2018Distributor: Department of Geological Sciences, Stockholm University
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For my grandfather,Jean Peillod A mon grand-père,Jean Peillod
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“For something to be interesting, one has to stare at it for a long time”
Gustave Flaubert
“Pour qu’une chose soit intéressante, il suffit de la regarder longtemps”
Gustave Flaubert
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Abstract
High pressure, low temperature (HP-LT) rocks observed at the surface of the Earth are evidence of
past subduction zones. Understanding the tectonics processes that control the exhumation of HP-LT
metamorphic rocks in these subduction zones requires full comprehension of the pressure-
temperature-time (P–T–t) cycle that the rocks experienced. In the Cyclades, Greece, the Cycladic
blueschist Unit (CBU) hosts eclogite and blueschist facies rocks. However, the processes that
exhumed them are debated. The overall aim of this thesis is to understand how the Eocene HP-LT
rocks were exhumed in the central Cyclades based on a study of the metamorphic history of Naxos
Island and nearby Syros Island.
In this thesis, I carried out a systematic geothermobarometric and geochronological
investigation on Naxos to better constrain the P–T–t paths that are recorded by the rocks. The data
indicate that high-P metamorphism on Naxos occurred in the Eocene at c. 40 Ma and the HP-LT rocks
were exhumed by two tectonic events. The first exhumation event occurred in the Oligocene. The HP-
LT rocks were exhumed in a convergent setting by an extrusion wedge. The top of the sequence
reached greenschist facies conditions at c.32 Ma, whereas the bottom of the sequence remained at
greater depth (equating to pressures of 8–12 kbar). Additionally rocks from southeastern Syros
recorded a similar Eocene/Oligocene P–T–t history to that recorded by the top of the sequence on
Naxos, suggesting a common Eocene/Oligocene metamorphic history for the central Cyclades. The
second exhumation event occurred in the Miocene. The rocks were further exhumed in an extensional
setting from c. 20 to 8 Ma. The top of the sequence on Naxos was already in the brittle crust at that
time and therefore did not record this Miocene metamorphism. The bottom of the sequence was first
isothermally exhumed at high-T conditions and thereafter cooled rapidly.
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Abstrakt
Högtryck och lågtemperatur (HP-LT) bergarter som observeras på jordytan är bevis på tidigare
subduktion zoner. För att förstå de tektoniska prosesser som kontrollerar exhumeringen av HP-LT
metamorfa bergarter i dessa subduktions zoner behövs en full förståelse av den tryck-temperatur –tids
(P–T–t) cykel som bergarten genomgått. I Cykladerna, Grekland hittas den Cykladiska blåskiffer
enheten (CBU) som består av både eklogit och blåskiffer facies. Processen som har exhumerat dessa
bergarter är omdebaterad. Målet med den här avhandlingen är att förstå hur de eocena HP-LT
bergarterna exhumerades i centrala Cykladerna baserat på studier av metamorfa bergarter från öarna
Naxos och Syros.
I den här avhandlingen har en systematisk geotermobarymetrisk och geokronologisk
undersökning gjorts på Naxos för att bättre avgränsa den P–T–t väg som bevarats i bergarten. Data
visar att högtryck metamorfism på Naxos skedde under eocen ca. 40 Ma och att HP-LT bergarterna
exhumerades under två tektoniska skeden. Den första exhumeringen skedde under oligocen. HP-LT
bergarterna exhumerades under konvergens av en extruderingskil. Den högre delen av sekvensen
nådde grönskiffer facies förhållanden ca. 32 Ma, däremot så förblev den nedre delen av sekvensen på
större djup (likställt med tryck 8–12 kbar). Bergarter från sydöstra Syros visar en liknande
eocen/oligocen P–T–t historik som den övre sekvensen på Naxos vilket antyder en gemensam
eocen/oligocen metamorf historik för centrala Cylkaderna. Den andra exhumeringen skedde under
miocen. Bergarterna exhumerades ytterligare under divergens från ca. 20 till 8 Ma. Den övre delen av
sekvensen på Naxos var redan i den spröda skorpan och har därför inga spår av den miocena
metamorfismen. Den nedre delen av sekvensen var först isotermiskt exhumerad under högtemperatur
förhållanden och därefter snabbt nedkyld.
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Résumé
Les roches de haute pression et basse température (HP-BT) qui sont observées à la surface de la Terre
sont les témoins des zones de subduction passées. Comprendre les processus tectoniques qui
contrôlent l’exhumation des roches métamorphiques de HP-BT dans ces zones de subduction
nécessite une bonne compréhension du cycle pression-température-temps (P–T–t) auquel ces roches
ont été soumises. L’unité des Schistes Bleus Cycladiques (SBC), dans les Cyclades en Grèce, contient
des roches en condition de paragenèse éclogitique et des schistes bleus. Cependant, les processus
associés à l’exhumation de ces roches sont discutés. L’objectif principal de cette thèse est de
comprendre comment les roches de HP-BT formées à l’Éocène ont été exhumées dans la partie
centrale des Cyclades, en étudiant l’histoire métamorphique de l’île de Naxos et de Syros.
Dans cette thèse, une étude géothermobarométrique et géochronologique des roches
métamorphiques à Naxos a permis de contraindre le trajet P–T–t. Les données indiquent que le
métamorphisme de haute pression s’est produit autour de 40 Ma à l’Éocène et que ces roches ont été
exhumées en deux évènements tectoniques. La première exhumation s’est effectuée pendant
l’Oligocène. Les roches de HP-BT ont été exhumées par prisme d’exhumation en contexte de
convergence tectonique. Le sommet de la séquence structurelle atteint les conditions de faciès schistes
verts à ~32 Ma, alors que le bas de la séquence structurelle reste à des pressions plus élevées (8–12
kbar). De plus, les roches au Sud Est de Syros ont enregistré une histoire P–T similaire à celle
enregistrée au sommet de la séquence structurale à Naxos. Ces résultats suggèrent une histoire
Éocène/Oligocène commune pour la partie centrale des Cyclades. La deuxième exhumation s’est
effectuée durant le Miocène. Les roches ont été exhumées en contexte extensif de ~20 à 8 Ma. Le
sommet de la séquence structurale à Naxos était déjà en condition de la croûte fragile et par
conséquent n’a pas enregistré l’histoire Miocène. Le bas de la séquence a été initialement exhumé de
façon isotherme à des conditions de haute température, puis a refroidi rapidement.
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List of papers and author contributions
This thesis is based on the following papers:
I Peillod, A., Ring, U., Glodny, J. and Skelton, A. (2017) An Eocene/Oligocene blueschist-
/greenschist facies P–T loop from the Cycladic Blueschist Unit on Naxos Island, Greece:
Deformation-related re-equilibration vs. thermal relaxation. Journal of Metamorphic Geology
35, 805–830
II Peillod, A., Ring, U., Skelton, A., Linnros. H. and Hansman, R. The role of ductile flow of
the lower crust in controlling heat advection in the footwall of the Naxos extensional fault
system (Aegean Sea, Greece). (manuscript in revision)
III Ring, U., Glodny, J., Peillod, A., and Skelton, A. The timing of high-temperature conditions
and ductile shearing in the footwall of the Naxos metamorphic core complex, Aegean Sea,
Greece. (submitted)
IV Skelton, A., Peillod, A., Ring, U., Glodny, J. Coupled replacement and preservation of high
pressure rocks caused by infiltration of mixed H2O-CO2 fluids. (manuscript)
Paper I: Peillod, A. and Glodny, J. carried out field sampling. Peillod, A. carried out rock cutting
microscope observations, electron microprobe analyses, P–T determination, interpretation of the data
and wrote the manuscript. Ring, U. provided structural data. He provided support for field sampling,
data interpretation and manuscript revisions. Glodny, J. provided age data and data interpretation. He
provided manuscript corrections. Skelton, A. provided support for field sampling, data interpretation
and manuscript corrections.
Paper II: Peillod, A. carried out field sampling, laboratory preparations, microscope observations,
electron microprobe analyses, P–T determination, data interpretation and wrote the manuscript. Ring,
U. provided support for field sampling, structural data, data interpretation and manuscript corrections.
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Skelton, A. provided support for field sampling, data interpretation and manuscript corrections.
Linnros. H. and Hansman, R. provided 3D model and manuscript corrections.
Paper III: Ring, U. provided data interpretation and wrote the manuscript. Glodny, J. provided field
sampling, age data, data interpretation and manuscript corrections. Peillod, A. provided P–T data and
support for field sampling data interpretation and manuscript corrections. Skelton, A. provided
support for data interpretation and manuscript corrections.
Paper IV: Skelton, A. and Ring, U. carried out field sampling and Skelton, A. wrote the manuscript.
Peillod, A. carried out laboratory preparations (rock cutting, powder for Rb/Sr age and whole rock
analyse powder), electron microprobe analysis, P–T determination, petrographic analysis and
interpretation. Glodny, J. provided age data and data interpretation.
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Contents
1. Introduction and aim ...................................................................................................................... 8
1.1. Exhumation in an extensional setting ................................................................................... 10
1.2. Extrusion wedge in a subduction zone setting ...................................................................... 11
1.3. Source of heat - conduction vs heat flow in extensional orogeny ......................................... 12
1.4. Problem in the Cyclades ....................................................................................................... 13
1.5. Aim and strategy of the thesis ............................................................................................... 14
2. Geological background ................................................................................................................ 15
2.1. The geodynamic setting - the Hellenic subduction zone ...................................................... 15
2.2. Geology of Naxos ................................................................................................................. 18
2.3. Metamorphism and timing recorded in the Cyclades Blueschist Unit .................................. 20
2.3.1. P–T estimates ................................................................................................................ 20
2.3.2. Timing of metamorphism .............................................................................................. 22
2.3.3. High-P metamorphism and wedge extrusion of the CBU in two stages ....................... 24
3. Methods ........................................................................................................................................ 25
3.1. Collection of Samples ........................................................................................................... 25
3.2. Constraining the P–T-t trajectory .......................................................................................... 27
3.2.1. Average Pressure-Temperature – A statistical problem ................................................ 27
3.2.2. Rb/Sr dating – The geochronological problem of incomplete resetting ....................... 30
4. Results .......................................................................................................................................... 33
4.1. Paper I - An Eocene/Oligocene blueschist-/greenschist facies P–T loop for the top of the
passive margin sequence on Naxos Island ........................................................................................ 33
4.2. Paper II - Characterising the P–T path for the Miocene exhumation related to the
extensional setting for the bottom of the passive margin sequence on Naxos .................................. 35
4.3. Paper III - The timing of the high-T metamorphism in the bottom of the passive margin
sequence in northern Naxos .............................................................................................................. 36
4.4. Paper IV - A P–T–t loop on SE Syros Island (Fabrika section): A common
Eocene/Oligocene metamorphic history for the central Cyclades .................................................... 38
5. Discussion .................................................................................................................................... 39
5.1. Exhumation of high-P rocks by extrusion wedge in a subduction zone setting during the
Eocene and Oligocene ...................................................................................................................... 39
5.2. Relation between the Eocene/Oligocene and the Miocene metamorphic events .................. 41
6. Conclusion and future work ......................................................................................................... 43
7. Acknowledgement ........................................................................................................................ 44
8. References .................................................................................................................................... 47
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Abbreviations
P–T–t Pressure-Temperature-time
high-P; high-T High-pressure; High-temperature
HP-LT high-pressure low-temperature
CBU Cyclades Blueschist Unit
Mineral abbreviations See Whitney and Evans (2010)
ZFT Zircon Fission Track dating
AFT Apatite Fission Track dating
avP–T Average Pressure Temperature
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1. Introduction and aim
In convergent plate settings, rocks from the subducting plate are buried deep in the crust and the
mantle and experience high-pressure low-temperature (HP-LT) metamorphism. In the orogenic belt
that arises, HP-LT metamorphic rocks can be observed at the surface which implies that geodynamic
processes exhumed the rocks. Understanding processes that control the exhumation of HP-LT
metamorphic rocks requires comprehension of the complete burial-exhumation pressure-temperature-
time (P–T–t) cycle that the rocks experienced. Two types of metamorphic associations that commonly
characterise orogenic belts are (1) the HP-LT metamorphic series and (2) the Barrovian series.
(1) In the HP-LT metamorphic series, the slow thermal conductivities of the rocks combine
with a fast subduction rate of the descending slab, 1 to 10 cm/yr for modern subduction zones (Stern,
2002), modifies the thermal structure in the subduction zone. The subducting plate is cold and as it
descends into hot mantle, HP-LT metamorphism with a geotherm between 5 and 15 °C/km (perhaps
more) arises (e.g. Dumitru, 1991; Penniston-Dorland et al., 2015). At the new P–T conditions that the
rocks experience, metamorphic mineral assemblages form. These assemblages define a metamorphic
facies (Eskola, 1915). From thermodynamic databases, Evans (1990) and Spear (1995) constrained
the P–T stability of the metamorphic mineral assemblages defining the metamorphic facies. The
boundary between the facies depends on rock, mineral and fluid compositions. During the HP-LT
metamorphic stage, rocks can successively experience prograde metamorphism under zeolite,
prehnite-pumpellyite, lawsonite blueschist, epidote blueschist and eclogite facies metamorphism (Fig.
1a and b).
(2) Barrovian metamorphism is defined by medium pressure and low- to high-temperature
metamorphism (15–35°C/km). The Barrovian series is often seen in the field as a sequence of
metamorphic facies along a transect across a metamorphic belt. The typical Barrovian facies series is
greenschist, epidote-amphibolite, amphibolite and granulite facies metamorphism which follows the
index mineral sequence (chlorite, biotite, garnet, staurolite, kyanite, sillimanite) in pelitic rocks laid
out by Barrow (1912) (Fig. 1a).
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Figure 1: P–T diagram showing the main metamorphic facies a) from Spear (1995), b) from Evans (1990).
Thermal gradient divided into the subduction zone and Barrovian metamorphism.
The tectonic processes that control the exhumation of the HP-LT metamorphic series and the
Barrovian series to the surface are, however, not well known and are actively debated (e.g. Avigad et
al., 1997; Jolivet et al., 2003; Ring & Glodny, 2010; Thompson & England, 1984). The Cycladic
Blueschist Unit (CBU), in the Aegean Sea (Greece), is one of the best worldwide examples of the
preservation of eclogite and blueschist facies rocks. The P–T–t paths that are suggested for the
exhumation of this unit differ between studies. Commonly two processes are used to explain the
exhumation of the CBU; exhumation related to extension and exhumation in an extrusion wedge that
formed during shortening. These processes are described below.
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1.1. Exhumation in an extensional setting
Exhumation in an extensional setting has been argued for based on P–T–t paths recording two
metamorphic events. The first event is prograde HP-LT metamorphism occurring in a subduction
zone setting. The second event, which records exhumation in an extensional setting is characterised
by a low- to high temperature metamorphism overprint (Barrovian type) (England & Thompson,
1984; Spear, 1995) (Fig. 2a).
Figure 2: Comparison between a) P–T–t path generated by regional metamorphic process (England &
Thompson, 1984) and b) P–T–t path generated by extrusion wedge c) Naxos P–T–t path (Avigad, 1998;
Duchêne et al., 2006; Wijbrans & McDougall, 1986).
During the first (HP-LT) metamorphic event, crustal thickening is thought to be rapid with the
result that the thermal steady state is perturbed within the crust (England & Thompson, 1984). The
thickened crust is isostatically unstable and denudation by normal faulting and erosion occurs. The
time frame for exhumation to the surface is considerably longer than for crustal thickening, and an
environment is created for thermal relaxation controlled by radiogenic decay within the rocks.
Consequently, temperature increases and pressure decreases, which is recorded by the second
(Barrovian) metamorphic event. The typical time frame for thermal relaxation driven by radioactive
decay within the crust is 20–30 Ma (Glazner & Bartley, 1985) (Fig. 2a). Heat transfer is largely
conductive and lies behind observed metamorphic reequilibration and its duration. Later in the
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exhumation process, when the rocks approach the cold upper crust they are cooled and reequilibrated
at greenschist-facies metamorphic conditions (Fig. 2a) (England & Thompson, 1984). The full P–T–t
loop records isobaric heating followed by exhumation (Fig. 2a). England and Thompson (1984)
calculated that the loop is completed in 50–70 Myr for 50–60 km of exhumation. Thermal relaxation
is needed for exhumation which implies that temperature mainly controls metamorphic reequilibration
and its duration in the extensional setting.
1.2. Extrusion wedge in a subduction zone setting
Although exhumation of rocks is often thought to take place in an extensional setting
(Thompson & England, 1984), exhumation of high-pressure rocks in convergent settings can also
occur in an extrusion wedge (Ring & Glodny, 2010) (Fig. 3). During this process, deformation at
high-P conditions enables the rocks to be exhumed laterally and upwards. The extrusion wedge is
defined by two main fault structures working in concert during overall horizontal shortening (Fig. 3).
At the bottom of the wedge a basal thrust fault enables one plate to slip beneath another one. At the
top of the structure a normal fault unroofs the wedge. In subduction zone extrusion wedges,
exhumation occurs rapidly at rates close to the rate of subduction (Ring & Reischmann, 2002; e.g.
Ring et al., 2007b). Therefore, thermal relaxation cannot play a significant role (Fig. 2b). Exhumation
in an extrusion wedge implies that deformation assisted by fluid controls metamorphic reequilibration
and its duration.
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Figure 3: An example of extrusion wedge in horizontal shortening (modified from Ring & Glodny, 2010).
Colour gradient show the reequilibration of the High-rocks in greenschist facies metamorphism.
1.3. Source of heat - conduction vs heat flow in extensional orogeny
In the case of exhumation related to extension, heat produced by radiogenic decay within the rock is
transferred by conduction. The degree of metamorphism is a function of the distance from the heat
source, forming symmetrical concentric metamorphic isograds around the heat source, which typically
record Barrovian metamorphism.
In the case of exhumation in an extrusion wedge, because this is faster, thermal relaxation
cannot occur. The heat transfer mechanism must therefore be different. It is envisaged that heat
transfer occurs by advection, i.e. hot material is physically transported from one location to another.
For example magma intrusions physically transport heat to a higher crustal level (e.g. Brune et al.,
2014; Zito et al., 2003). A number of numerical models indicate that heat advection might be
important in extensional settings (Brune et al., 2014; Gessner et al., 2007; Rey et al., 2009). In an
extensional setting, rapidly exhuming lower-crustal rocks can physically transport deep, hot and
ductile material to higher levels (Axen et al., 1998; Block & Royden, 1990; Brune et al., 2014;
Gessner et al., 2007). Numerical models show that advection of lower crustal material results in an
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asymmetric thermal architecture of the resultant isograds. It should be possible to observe this
asymmetry in the field.
1.4. Problem in the Cyclades
Exhumation related to extension is generally envisaged for the exhumation of the CBU in the Aegean
Sea (Jacobshagen et al., 1986; Okrusch & Bröcker, 1990). This has been argued on the basis of P–T–t
loops from the Cyclades (example of P–T–t path in Naxos in Fig. 2c). These show blueschist facies
metamorphism in the Eocene (50–40 Ma) followed by a greenschist to amphibolite facies overprint
with local partial melting in the Miocene (25–20 Ma) (Jacobshagen et al., 1986; Okrusch & Bröcker,
1990).
Over the last decade, a few studies show that the regional metamorphism scheme does not hold
true for the eastern and western edges of the Cyclades (Evia and Samos) where P–T–t paths suggest
that rocks were exhumed at greenschist facies metamorphic conditions in a subduction-related
extrusion wedge in the Eocene and Oligocene (Ring et al., 2007a, 2007b). These results imply two
stages of exhumation of the CBU: an earlier extrusion stage in a subduction zone setting and a later
extensional stage with associated core complex formation during the Miocene. Furthermore, other
workers have speculated that the CBU might have been exhumed in two stages on Tinos, Syros, Ios
and Sifnos (e.g. Huet et al., 2009; Laurent et al., 2018; Parra et al., 2002; Schmädicke & Will, 2003;
Trotet et al., 2001a). However uncertain linkages between age determinations, P–T data and/or
deformational stages have led to active debate.
For Naxos, in the central part of the CBU, exhumation related to extension is generally
envisaged (Andriessen et al., 1979; Avigad, 1998). The greenschist/amphibole facies overprint of the
Eocene high-P metamorphic assemblage is considered the consequence of the Miocene extensional
deformation. However, this exhumation mechanism does not work for the upper part of the structural
sequence in Naxos. Zircon fission-track (ZFT) ages that constrain rock cooling at ~240°C are
estimated at 20.5 ± 2.4 Ma and 25.2 ± 3.8 Ma (2σ errors) (Seward et al., 2009). These ages indicate
that the greenschist overprint could not have occurred in the Miocene. Thus an exhumation of the
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high-P rocks between the Eocene and the Miocene must also have occurred on Naxos. Construction of
P–T–t paths for Naxos would allow for timing of exhumation stages in the central CBU to be
determined.
1.5. Aim and strategy of the thesis
The overall aim of this thesis is to understand how the Eocene HP-LT rocks were exhumed in the
central Cyclades based on a study of the metamorphic history of Naxos Island. Naxos is selected
because the HP-LT and greenschist facies metamorphic rocks in the upper part of the sequence are
expected to give information about the pre-Miocene exhumation, whereas the amphibolite facies
metamorphic rocks in the lower part of the sequence are expected to give information about the
Miocene exhumation. A second aim of this thesis is to show how the Eocene HP-LT metamorphism
and the Miocene high-T metamorphism are related to each other. In order to fulfil the aims of this
thesis, both the P–T conditions and timing of each metamorphic stage were determined as follows:
(1) The P–T–t conditions for the peak high-P metamorphism and the greenschist facies overprint
were determined by analysing adjacent blueschist and greenschist metamorphic rocks in the top of the
structural sequence in southern Naxos (Paper I). This allowed us to hypothesise that the rocks
completed a full blueschist-/greenschist facies metamorphic loop before the Miocene as indicated by
the zircon fission track ages. If so, the exhumation of the high-P rocks cannot be the consequence of
large scale extension in the Miocene. Therefore, another tectonic process that exhumed the high-P
rocks can be envisaged.
(2) The P–T (Paper II) and the timing (Paper III) of Miocene high-T metamorphism were determined
by analysing amphibolite facies rocks that occur in the bottom of the structural sequence. This
contributed to the understanding of the thermal history of Naxos rocks during their exhumation in an
extensional setting.
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(3) The P–T–t path for nearby Syros Island was determined by analysing HP-LT and greenschist
facies rocks (Paper IV). This allowed comparison between exhumation of the high-P rocks to
greenschist facies conditions on Syros and Naxos in the central Cyclades.
2. Geological background
2.1. The geodynamic setting - the Hellenic subduction zone
The Hellenides subduction system in the eastern Mediterranean is an orogen resulting from the NNE-
ward subduction of the African plate beneath Eurasia (Aubouin & Dercourt, 1965; Brunn et al., 1976;
Jacobshagen et al., 1978). The Aegean Sea in the Hellenides shows large-scale continental extension
above the subduction zone where high-pressure rocks were exhumed. Five tectonic zones define the
Hellenides (Fig. 4). From the top (north) to bottom (south) these zones are: (1) the Internal zone, (2)
the Vardar-Izmir-Ankara suture zone, (3) the Pelagonian zone, (4) the Cycladic zone and (5) the
External Hellenides (Dürr et al., 1978; Robertson et al., 1991).
Internal zone
The Internal zone is a continental fragment which was amalgamated with Eurasia in the early Jurassic
and Cretaceous (Krohe & Mposkos, 2002; Mposkos & Kostopoulos, 2001; Robertson et al., 1991).
These continental fragments reached HP-LT and ultra-high pressure metamorphic conditions (Liati &
Mposkos, 1990; Liati & Seidel, 1996; Mposkos & Kostopoulos, 2001) in the Jurassic and were
exhumed during the Cenozoic (Liati & Gebauer, 1999; Reischmann & Kostopoulos, 2002;
Wawrzenitz & Krohe, 1998).
Vadar-Izmir-Ankara suture zone
The Vardar-Izmir-Ankara suture zone is a result of the convergence of the Internal zone with the
Pelagonian zone. This unit comprises Jurassic ophiolitic rocks from a magmatic arc (Ricou et al.,
1998). The Vardar-Izmir-Ankara oceanic unit was subducted, reaching blueschist facies conditions,
beneath the Internal zone during the Cretaceous (Sherlock et al., 1999).
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Figure 4: a) Tectonic map of Aegean domain with major tectonic units, location of present-day retreating
Hellenic subduction zone and major Miocene extensional detachments from Paper I (modified from Dürr et al.,
1978; Jacobshagen et al., 1986; Ring & Layer, 2003). Inset shows position of main map in Mediterranean
region. (b) Simplified N–S cross-section showing extensional faults overprinting earlier top-S thrust between
passive-margin sequence and Carboniferous basement (shown by crosses).
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Pelagonian zone
The Pelagonian zone is composed of continental basement and an ophiolitic nappe which were
obducted in the late Jurassic (Bonneau & Kienast, 1982; Franz & Okrusch, 1992). This zone was
metamorphosed at blueschist-facies conditions in the Late Cretaceous (Ring & Layer, 2003; Sherlock
et al., 1999).
Cycladic zone
The Cycladic zone consists of fragments of the northern passive margin of the Adriatic plate and
ophiolites from the remnants of the Pindos Oceanic Unit. The Cycladic zone can be subdivided into
three tectonic units (Ring et al., 1999). (1) The top unit is the non- to weakly metamorphosed
Cycladic ophiolite nappe. (2) The Cycladic Blueschist Unit (CBU) which records the deepest
exhumed part of the Hellenides that is composed of blueschist and eclogite facies metamorphic rocks.
This unit is subdivided from top to bottom into three separate members: (a) an ophiolitic mélange, (b)
a Permo-Carboniferous to latest Cretaceous passive-margin sequence and (c) a Carboniferous
basement nappe made of schist, granite and orthogneiss. (3) The Basal Unit is exposed in windows
and is considered a part of the External Hellenides (Avigad & Garfunkel, 1989; Avigad et al., 1997).
This unit shows metamorphism at high pressure as well as a greenschist facies overprint (Ring et al.,
1999).
External Hellenides
The External Hellenides are made up of the Pindos Oceanic Unit and Tripolitza block. The Pindos
Oceanic Unit is a heterogeneous paleogeographic domain made of oceanic crust formed in the Late
Cretaceous (Keay, 1998; Tomaschek et al., 2003) and continental margin (basement-cover)
sequences. This unit was accreted in the Eocene (Brunn, 1956; Jones & Robertson, 1991; Robertson
et al., 1991; Stampfli et al., 2003). The Tripolitza block is a continental platform unit and flysch dated
from the Early Triassic to the Eocene (Jacobshagen et al., 1986). This block was underthrust during
the Oligocene (Sotiropoulos et al., 2003; Thomson et al., 1998).
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2.2. Geology of Naxos
Naxos is located in the central part of the Aegean Sea in the CBU of the Cycladic zone. The CBU on
Naxos is structurally subdivided into three units which are from top to bottom: (1) non-
metamorphosed sedimentary and ophiolitic rocks, (2) the Permo-Carboniferous to latest Cretaceous
passive-margin sequence and (3) the Carboniferous basement (Fig. 7).
The non-metamorphosed sedimentary succession has been deposited on an ophiolite nappe
(Kuhlemann et al., 2004). The passive margin sequence is mainly composed of alternating layers of
marble, metapelite and metabasalt. The basement consists of a N-S elongated migmatitic gneiss and
marble dome located in the centre of the island. The migmatite dome is dominated by diatexite
surrounded by metatexite (Vanderhaeghe, 2004). Lenses of ultramafic rocks are found between the
basement and the passive margin sequence.
The major structure is the Miocene Naxos extensional fault system, which exhumed the
Lower and Middle units (Buick, 1991; Gautier & Brun, 1993). This extension caused crustal anatexis
forming the migmatite dome, the small leucogranite intrusions in northwest Naxos and the intrusion
of a granodiorite body in the western part of the island (Buick & Holland, 1989; Jansen & Schuiling,
1976).
Two metamorphic events affected the passive-margin sequence and the basement. A high-P
metamorphism which affected the top of the passive margin sequence (southern part of Naxos) at >12
kbar and 470 ± 50°C (Avigad, 1998) occurred in the Eocene 45 ± 5 Ma (Andriessen et al., 1979). For
the bottom of the passive margin sequence (northern part of Naxos) high-P metamorphism was dated
at 69–42 Ma (Martin et al., 2006) and c. 40 Ma (Bolhar et al., 2017a). This high-P metamorphism is
followed by Barrovian-type greenschist to upper amphibolite facies overprinting related to large-scale
continental extension (Buick & Holland, 1989). The P–T conditions at which this occurred were
estimated for the passive margin sequence at 470°C and 5 kbar in southern Naxos to 700–760°C and 8
kbar close to the migmatite dome (Buick & Holland, 1989; Duchêne et al., 2006; Jansen & Schuiling,
1976; Katzir et al., 2000). The P–T conditions of metamorphism in the migmatite dome have been
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calculated to be 6–11.7 kbar and 550–650°C followed by 3.5–10.5 kbar and 620–720°C (Buick &
Holland, 1989).
Figure 5: a) Geological map of Naxos modified from Paper I showing the principal units and the Miocene
isograds around the migmatite dome (Jansen & Schuiling, 1976; Kruckenberg et al., 2010; Vanderhaeghe,
2004). b) North-south cross-section showing the domal architecture of Naxos. Note that the detachment fault is
not shown in the cross section but usually is considered following the northern coast of Naxos and should be
located near the question mark.
High-T metamorphism was dated at ~20–14 Ma (Andriessen et al., 1979; Bolhar et al.,
2017b; Keay et al., 2001; Wijbrans & McDougall, 1988). The age for the greenschist facies
metamorphism is not constrained. However, zircon fission tracks (ZFT) data show that rocks from
south Naxos were already at temperatures above ~240°C in the late Oligocene (20–25 Ma). For the
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northern part of Naxos, ZFT data indicate that the rocks were in the brittle crust at 9–11 Ma.
Geothermobarometry and geochronology are detailed in the papers I, II and III.
2.3. Metamorphism and timing recorded in the Cyclades Blueschist Unit
2.3.1. P–T estimates
The Cyclades Blueschist Unit records HP-LT metamorphism during the Hellenide mountain building.
Figure 5 is an updated compilation of the P–T paths for the Cyclades (modified from Ring et al.,
2010). For the passive margin sequence, prograde metamorphic conditions are constrained by the
transition lawsonite-blueschist to epidote-blueschist metamorphism on the islands of Syros, Sifnos,
Naxos and Ios. This transition is estimated to have occurred at ~12 kbar ~470°C on Naxos (Avigad,
1998); 8–11 kbar 320–380°C on Ios (Grütter, 1993); 15–20 kbar 500–550°C on Sifnos (Groppo et al.,
2009; Schmädicke & Will, 2003; Trotet et al., 2001a) and probably ~14 kbar 300–400°C on Syros
(Schumacher et al., 2008).
The peak high-P metamorphic conditions range from upper epidote-blueschist to eclogite
facies. Peak high-P metamorphism for the passive margin sequence ranges between ≥11.5 kbar and
~450°C for the western part of the Cyclades (Evia) (Katzir et al., 2000; Shaked et al., 2000) and 19
kbar and 520°C for the eastern part (Samos) (Will et al., 1998). For the central part of the Cyclades,
high-P conditions are reported for Sifnos, Syros and Tinos and P–T conditions are 15–24 kbar and
500–575°C (Bröcker, 1990; Dixon, 1976; Dragovic et al., 2015; Groppo et al., 2009; Laurent et al.,
2018; Matthews & Schliestedt, 1984; Schmädicke & Will, 2003; Schumacher et al., 2008; Trotet et
al., 2001b). Relatively lower P–T conditions: 9–12 kbar 350–500 for Ios (Grütter, 1993; Thomson et
al., 2009; van der Maar & Jansen, 1983) and >12 kbar ~470°C for Naxos (Avigad, 1998) are
reported.
For the Cycladic Basement Unit, high-P metamorphism is poorly described and no clear
indications show that this unit underwent P–T conditions similar to those experienced by the passive
margin sequence. High-P metamorphism at blueschist facies conditions is described only for Ios and
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Sikinos. For Ios, P–T is estimated at 9–11 kbar and 350–400°C (van der Maar, 1981; van der Maar &
Jansen, 1983) and at 8–11 kbar and 530–600°C (Thomson et al., 2009). For Sikinos, P–T conditions
of 11 kbar and 475°C are estimated (Gupta & Bickle, 2004).
Figure 6: Compilations of P–T–t paths from the Cyclades show peak high-P metamorphism and retrograde
metamorphism during the Eocene (modified from Ring et al., 2010). P–T data are from: Evia Katzir et al.
(Katzir et al., 2000); Samos Will et al. (1998); Naxos Avigad, (1998); Duchêne et al. (2006); Ios Grütter,
(1993); Thomson et al. (2009); van der Maar and Jansen (1983); Sifnos (1) Matthews and Schliestedt (1984);
(1) Schliestedt and Matthews, (1987); (1) Schmädicke and Will, (2003); (2) Trotet et al. (2001a, 2001b); (3)
Ashley et al. (2014); (3) Dragovic et al. (2015, 2012); (3) Groppo et al. (2009); Syros (1) Ridley, (1984); (1)
Schumacher et al. (2008); (1) Trotet et al. (2001a, 2001b); (2) Laurent et al. (2018); Tinos (1) Bröcker, (1990);
(2) Parra et al. (2002). For geochronological data see Figure 6.
The HP-LT metamorphism of the passive margin sequence is followed by retrogression at
greenschist facies conditions. In the western part of the Cyclades (Evia), retrogression is estimated to
have occurred at 6–8 kbar and 290–350°C (Katzir et al., 2000). In the central Aegean, the retrograde
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path is more complex. For Syros and Sifnos, post high-P metamorphism is characterised by a near
isothermal decompression under blueschist facies conditions at 8–17 kbar and 460–550°C followed
by a retrogression under greenschist facies conditions at 3–6 kbar and 300–400°C (Groppo et al.,
2009; Trotet et al., 2001b). A thermal anomaly is reported on Tinos, Naxos, Ios and in some part of
Syros. For Tinos, a first retrogression at blueschist facies conditions (8–9 kbar and ~370°C) is
followed by isobaric heating reaching ~570°C (Parra et al., 2002). The greenschist facies
retrogression is estimated to have occurred at ~2 kbar and 420–500°C implying an unusually high
near-surface geotherm of 60-70°C/km and a brittle-ductile transition at about 5 km depth. On the
islands of Naxos and Ios, high-T metamorphism of the passive-margin sequence occurred at
amphibolite facies conditions and is associated with partial anatexis of the Cycladic basement. The P–
T conditions are estimated at 6–10 kbar and 500–760°C (Buick & Holland, 1989; Duchêne et al.,
2006; Katzir et al., 1999). Recently on Syros, Laurent et al. (2018) showed a first retrogression of the
high-P rocks at 10–12 kbar and ~500°C followed by isobaric heating reaching ~570°C.
2.3.2. Timing of metamorphism
The ages for peak high-P metamorphism (blueschist/eclogite facies) and for retrogression in the upper
crust (greenschist facies) are compiled in Figure 6 and are plotted on the P–T path in Figure 5. For
references see the figure caption.
The peak high-P metamorphism is estimated between 50 and 40 Ma for most of the islands
and 30 Ma for Evia and southern Sifnos. This peak high-P metamorphism is estimated by different
geochronological methods and is consequently well constrained for most islands. The Cyclades were
affected by two post peak high-P metamorphic stages. The first stage was at blueschist facies
conditions and lasted until ~27 Ma for Evia and about ~37–30 Ma in Sifnos, Syros and Tinos. The
second stage is at greenschist facies conditions. This stage is less well constrained and based on only a
few analyses/samples. This stage may have begun at ~37–30 Ma on Sifnos, Syros and Tinos, and
continued until 20 Ma. This large age variability is interpreted as a continuous retrogression of the
high-P rocks. For the islands of Naxos, Ios and Evia, the greenschist overprint is dated at ~20 Ma.
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Note that for Naxos and Ios, greenschist and the amphibolite facies metamorphism are dated to have
occurred simultaneously
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Figure 7: a) Age of blueschist/eclogite and greenschist facies metamorphism for studies of principal islands
(modified from Philippon et al., 2012). Evia (1) Ring et al. (2007a); (2) Klein-Helmkamp et al. (1995); (3)
Maluski et al. (1987); Sifnos (4) Dragovic et al, (2015); (5) Altherr et al. (1979); Syros (6) Tomaschek et al.
(2003); (7) Putlitz et al. (2005); (8) Bröcker and Enders (2001); (9) Baldwin (1996); (10) Maluski et al. (1987);
(11) Lagos et al. (2002); (23) Cliff et al. (2017); Tinos (12) Bröcker and Franz (1998); (13) Bröcker et al.
(1993); Naxos (14) Andriessen et al. (1979); (15) Wijbrans and McDougall (1986); Ios (16) Henjes-Kunst and
Kreuzer (Henjes-Kunst & Kreuzer, 1982)(Henjes-Kunst & Kreuzer, 1982)(Henjes-Kunst & Kreuzer,
1982)(1982); (17) Baldwin and Lister (1998); (18) Andriessen et al. (1987); (19) Kreuzer et al. (1978); (20)
Thomson et al. (2009); (21) Baldwin et al. (1998); Samos (22) Ring and Layer (2003); (23) Bröcker et al
(2013). b) Geological map of the Cyclades with the major tectonic units modified from Paper I (modified after
Dürr et al. (1978); Jacobshagen, (1986); Ring and Layer, (2003).
2.3.3. High-P metamorphism and wedge extrusion of the CBU in two stages
The peak high-P metamorphism at eclogite/blueschist facies conditions occurred at ~53–30 Ma
reflecting different high-P ages, probably due to repeated underplating and nappe formation (Ring et
al., 2010).
Early exhumation of the high-P rocks within the CBU is inferred in this thesis to have
occurred in an extrusion wedge. The timing of extrusion is dated at 33–21 Ma for Evia and 42–32 Ma
for Samos on the edge of the Cyclades (Ring et al., 2007a, 2007b). The extrusion wedge formed at
different periods, and thus the high-P overprint also occurred at different periods. In the eastern part
of the CBU (Samos), the extrusion wedge started to extrude at c. 40 Ma when high-P metamorphism
was occurring in parts of the central CBU. The extrusion wedge on Samos ended at about 30 Ma
when high-P metamorphism was occurring in the lower crust on southern Sifnos. This suggests a
close relationship between downward propagation of high-P metamorphism in the CBU and the
subsequent formation of extrusion wedges.
A second exhumation is related to large scale extension creating sedimentary basin and dated
at ~23 Ma. The rocks were exhumed into the near surface. In this stage the Cycladic basement was
exhumed and formed a migmatite dome in Naxos and Paros.
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3. Methods
3.1. Collection of Samples
To constrain the P–T path for the top of the sequence, three samples were collected in the
southernmost part of Naxos (Paper I; Fig. 8). These were a metabasalt and a metasediment with well-
preserved blueschist facies assemblages, which were used to constrain peak high-P metamorphic
conditions and a calcschist with a greenschist facies assemblage, which was used to constrain the P–T
conditions of the overprint. Eight more samples were collected from various localities in the southern
half of Naxos to better constrain the age of the greenschist overprint. The above mentioned zircon
fission track ages indicate that blue- and greenschist-facies metamorphism must have occurred before
the late Oligocene and that the rocks of south Naxos where already in the brittle crust during the
Miocene and formed a structural lid during the Miocene high-T history.
To characterise the high-T metamorphism in the passive margin sequence related to large
scale extension, we collected nine samples around the migmatite dome. They were used to constrain
how the P–T path is spatially related to the Naxos extensional fault system and the migmatite core
(Paper II; Fig. 8). Because the top of the fault is eroded away, a three-dimensional reconstruction of
the detachment surface at the top of the extensional fault system and metamorphic isograds was used.
The three-dimensional reconstruction was constructed by Linnros (2016).
We selected nine samples associated with high-temperature metamorphism for age dating
(Paper III; Fig. 8). The samples were collected nearby to those used in Paper II to build a P–T–t path
of the high-temperature metamorphism.
To constrain the P–T–t path in Syros we collected seven samples of blueschist, eclogite and
greenschist rocks (Paper IV). Samples of transitional rocks (blueschist-eclogite) were also collected to
understand the metamorphic texture reequilibration.
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Figure 8: a) Geological map of Naxos from paper III (Jansen & Schuiling, 1976; Kruckenberg et al., 2011;
Vanderhaeghe, 2004) showing the sample localities of paper I, III and IV. b) Cross-section showing domal
architecture of Naxos and sample localities projected for paper III into the cross-section plane.
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3.2. Constraining the P–T-t trajectory
Average P–T (avP–T) was used to calculate P–T metamorphic conditions in Naxos and Syros, based
on chemical reactions assuming mineral assemblage equilibrium (Paper I, II and IV). The mineral
compositions used in avP–T were determined by electron-probe microanalysis (EPMA). Rb/Sr age
dating was used to determine the metamorphic ages for Naxos and Syros (Paper I, II and IV). Detailed
information about specific instruments and run conditions used for EMPA and Rb/Sr isotope analyses
can be found in the papers. The avP–T and Rb/Sr methods used to construct the P–T–t path for Naxos
and Syros are discussed below.
3.2.1. Average Pressure-Temperature – A statistical problem
The P–T conditions of metamorphism can be calculated if the chemistry of co-existing
minerals and thermodynamic properties of these minerals (e.g. enthalpies, entropy, activity) are
known. The basic assumption is that co-existing minerals are in thermodynamic equilibrium. If this is
true, P-T conditions can be calculated from the positions of reaction equilibria between these minerals
in P-T space. The P–T condition of metamorphism is given by the intersection of two or more
reaction equilibria in P-T space (Fig.9a). Early applications of geothermobarometry used the
intersection of two reaction equilibria constrained from the activities of mineral end members and
their activities which were calibrated by direct experimental investigations. This method, however,
depends on single reaction thermometers and barometers that can give large uncertainties. Having
more than two reaction lines might reduce the uncertainties. Calculation with more than two reaction
equilibria (Fig. 9b) was possible when all available experimental information was combined in an
internally consistent thermodynamic data set.
However, a problem arises when more than two reaction equilibria are used to constrain P–T
conditions. This is that the calculation is overdetermined and further statistical methods must
therefore be applied (Fig. 9b). One method uses the least-squares method for averaging all the
available equilibrium reactions in the rock, incorporating uncertainties and their inter-correlations
(Powell, 1985). In other words, the method works best when the activity of a single mineral is used in
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several equations and the uncertainties of the respective reaction constants are therefore correlated
(cor). This implies that the uncertainties in the calculations can be better constrained. This approach is
applied by the avP–T mode of the software THERMOCALC (Powell & Holland, 1994, 1988, 2008).
Other methods (not used in our research) try to optimise P–T estimations of overdetermined systems
include WEBINVEQ software (Gordon, 1992, 1998) and TWEEQU software (Berman, 1991).
Figure 9: Geothermobarometry calculation using a) intersection of two reaction lines and b) intersection of three
reaction lines. Red dot is the optimal thermobarometry estimation. The ellipse represents the correlated
uncertainly (cor) and fits in the uncertainly box (σPT). Note that for the three reaction-line calculations, the
correlated uncertainly ellipse is large due to large temperature errors. Modified from Powell and Holland
(Powell & Holland, 1994).
An example of how avP–T works is shown here (Fig. 9). In the case of two reaction
equilibria, their intersection generates a P–T point and a P–T uncertainty ellipse can be derived from
the uncertainties of the equilibria (Fig 9a). Adding more reaction lines with their uncertainties
provides more than one P–T intersection. In Figure 9b three reaction lines give three intersections (A,
B, C). As mentioned above, to resolve this problem AvP–T uses a least-squares method to determine
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the optimal P-T estimate. The positions of the equilibria are varied in proportion to their uncertainties
and correlations, so that they intersect at one optimal point (𝑃–𝑇̅̅ ̅̅ ̅̅ in Fig. 9b). This optimal point is
located in an area where all three uncertainties overlap and generate a correlated uncertainty ellipse
within an uncertainly box (σP and σT).
The avP–T method provides several diagnostic tools to evaluate the quality of the calculation
and the influence of the input data on the optimal P–T. The diagnostics can be illustrated by referring
to Table 1 which shows the P–T estimate of the peak high-P in the sample 15SY03. The mineral end
members, their activities and respective uncertainties are calculated using the A-X program
(http://www.esc.cam.ac.uk/research/research-groups/research-projects/timhollands-software-
pages/ax). Using avP–T, nine independent reaction equilibria were calculated for this sample and an
optimal P–T estimate with uncertainties (T; sd(T) = σT; P; sd(P) = σT) and a table of diagnostics (fit,
χ2, e* and hat) are generated. The first diagnostic information is the σfit (the scatter of the enthalpies
and activities normalised by their uncertainties). This value should be close to 1; however, larger
values need to be tested to determine that the average is statistically consistent with the input data (χ2
= 95% confident). The calculation reveals σfit = 1.65, which is larger than allowed from the χ2 test.
The second diagnostic information is the hat value, which measures the degree of influence of the end
member on the least-squares result (ranges between 0 and 1). In the present example, the final result is
very sensitive to the activity of clinozoisite (= 0.42) but not to that of acmite (= 0). A change in
clinozoisite activity would provide a significant change to the P–T estimate. The last diagnostic test is
the activity residual normalised e* which is the difference between the measure of the activity of one
end member and the calculated activity required for all the equilibria to intersect at the optimal P and
T. A large value for e* shows that this residual activity fits poorly with respect to the other values.
This is the case for pyrope (=2.85) in the example (Tab. 1). Additionally, this large e* is associated
with a significant hat value and consequently pyrope may be an outlier in this avP–T estimate.
The next step is to remove the outlier end member. The result is shown in the second
diagnostic table. Here σfit passes the χ2 test (1.10/1.45). No more anomalies in the e* values are found
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and the avP–T estimate is considered acceptable. The P–T results and their uncertainties (σP and σT)
mean that σP and σT have the same σfit. In other words, the P–T estimates of 16.8 ± 0.6 kbar and 479
± 11°C (± is 2σ) means that the rock had experienced conditions anywhere within the ranges 16.2–
17.4 kbar and 468–490°C.
Table 1: Thermocalc output for the peak high-P assemblage in 15SY03. Abbreviation from the THERMOCALC
documentation.
3.2.2. Rb/Sr dating – The geochronological problem of incomplete resetting
Rubidium–Strontium isotopic mineral dating is a method using the isotopic ratio of 87
Sr and 86
Sr on
the isotopic ratio of 87
Rb and 86
Sr (Fig. 10). In these ratios, 87
Sr is radiogenic and 87
Rb decays to 87
Sr
over time. At the time the metamorphic assemblage is formed, all the minerals in the sample are
Activities and their uncertainties
gl fgl py gr alm pa di hed jd acm cz ep ilm geik q H2O ru law
a 0.16 0.078 0.003 0.021 0.22 0.79 0.23 0.14 0.44 0.18 0.36 0.58 0.58 0.007 1 1 1 1
sd(a)/a 0.216 0.2952 0.653 0.472 0.15 0.05 0.1779 0.2571 0.094 0.24 0.1229 0.1 0.05 1.538 0 0 0
Independent set of reactions
1) 11gr + 4alm + 9q + 3law = 12hed + 12cz
2) 4jd + 4cz + law = 3gr + 4pa + q
3) alm + 3hed + 6ru = gr + 6ilm + 6q
4) pa + 3di + jd = gl + gr
5) alm + 10jd + 6cz + 3law = fgl + 5gr + 8pa
6) alm + 15hed + 12jd + 6law = 6fgl + 7gr + 6H2O
7) 2alm + 3di + 6ru = py + gr + 6ilm + 6q
8) alm + 16jd + 6ep + 3law = fgl + 5gr + 8pa + 6acm
9) 2fgl + 2pa + 9geik + 8q = 3gl + 2alm + H2O + 9ru
Single end-member diagnostic information Single end-member diagnostic information
For 95% confidence, fit (=σfit) should be < 1.42 (χ2) For 95% confidence, fit (=σfit) should be < 1.45 (χ
2)
e* < 2.5 e* < 2.5
hat > 0.5 hat > 0.47
P sd(P) T sd(T) cor fit e* hat P sd(P) T sd(T) cor fit e* hat
gl 17.33 0.88 490 15 0.817 1.56 -1 0.03 gl 16.78 0.63 479 11 0.831 1.09 -0.25 0.01
fgl 17.53 0.82 496 14 0.817 1.46 1.6 0.02 fgl 16.9 0.58 482 10 0.836 0.93 1.13 0.03
py 16.98 0.73 483 13 0.831 1.26 2.85 0.13 gr 16.78 0.63 479 11 0.829 1.1 0.06 0.04
gr 17.42 0.92 493 16 0.812 1.65 0.17 0.04 alm 16.61 0.87 476 15 0.913 1.09 0.2 0.36
alm 17.96 1.1 502 18 0.881 1.58 -0.87 0.27 pa 16.77 0.67 479 11 0.822 1.1 -0.05 0.08
pa 17.41 0.99 493 16 0.806 1.65 -0.02 0.08 di 16.78 0.61 478 11 0.828 1.07 0.66 0.04
di 17.43 0.92 493 16 0.811 1.65 0.2 0.03 hed 16.73 0.58 479 10 0.834 1.01 -1.14 0.01
hed 17.39 0.91 493 15 0.816 1.62 -0.84 0.01 jd 16.92 0.77 481 12 0.859 1.09 -0.23 0.25
jd 17.38 1.17 492 18 0.861 1.65 0.07 0.29 acm 16.8 0.61 479 11 0.832 1.07 -0.45 0
acm 17.44 0.92 493 15 0.815 1.64 -0.42 0 cz 16.78 0.63 479 12 0.744 1.1 -0.05 0.42
cz 17.4 0.92 495 17 0.718 1.64 -0.34 0.42 ep 16.79 0.63 479 11 0.833 1.09 0.19 0
ep 17.43 0.92 493 16 0.816 1.65 0.18 0 ilm 16.45 0.75 474 13 0.889 1.05 -0.58 0.3
ilm 17.53 1.08 495 18 0.864 1.65 0.24 0.24 geik 16.79 0.63 479 11 0.833 1.09 -0.32 0
geik 17.42 0.92 493 16 0.816 1.64 -0.45 0 q 16.78 0.63 479 11 0.833 1.1 0 0
q 17.42 0.92 493 16 0.816 1.65 0 0 H2O 16.78 0.63 479 11 0.833 1.1 0 0
H2O 17.42 0.92 493 16 0.816 1.65 0 0 ru 16.78 0.63 479 11 0.833 1.1 0 0
ru 17.42 0.92 493 16 0.816 1.65 0 0 law 16.78 0.63 479 11 0.833 1.1 0 0
law 17.42 0.92 493 16 0.816 1.65 0 0
T = 479¡C, sd = 11,
T = 493¡C, sd = 16, P = 16.8 kbars, sd = 0.6, cor = 0.833, sigfit = 1.10/1.45
P = 17.4 kbars, sd = 0.9, cor = 0.816, sigfit = 1.65/1.42
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assumed to have the same 87
Sr/86
Sr ratio which defines the initial isotopic equilibrium among a set of
paragenetic minerals (Fig. 10). The 87
Rb/86
Sr ratio is different for the different minerals. The minerals
containing low amounts of Rb (and K which can be replaced by Rb in crystal lattices) (e.g.
paragonite, apatite, plagioclase and calcite) have low 87
Rb/86
Sr ratios whereas minerals with high
amounts of Rb (and K) (e.g. phengite) plot at high 87
Rb/86
Sr ratios (Fig. 10). When the system remains
closed after formation of the minerals, the 87
Sr/86
Sr ratio increases with time whereas 87
Rb/86
Sr ratio
decreases (Fig. 10). The final result is a line called an isochron defined a linear array representing
three or more minerals. An isochron in a metamorphic context is commonly considered to directly
date a geological process. As an example our sample NAX15-26 shows an isochron of six minerals
which yields an age of 40.5 ± 1 Ma (Fig. 11a).
Figure 10: Evolution of the 87
Sr/86
Sr and 87
Rb/86
Sr ratio over time.
This method works only if the metamorphic overprint fully resets any preexisting contrasts in
87Sr/
86Sr ratios in the rock (Cliff & Meffan-Main, 2003; Freeman et al., 1998; Glodny et al., 2003). A
mixed age is obtained if the resetting is not completed. This is frequently the case in the Cyclades
where different generations of white mica can be identified (Bröcker et al., 2013; Cliff et al., 2017).
Single multi-grain fractions of white micas will provide mixed ages without direct geochronological
meaning as shown in the sample Na07-10 (Fig. 11b). In this sample three populations of white micas
have different Rb/Sr signatures. The age of 36.1 ± 2.6 Ma resulting from this Sr disequilibria gives a
high MSWD (Mean Square Weighted Deviation).
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Figure 11: Examples of multimineral isochrons obtained for Naxos (Paper I). a) NAX15-26 b) Na07-10.
A solution to the problem is based on the separation of different grain-size fractions to
provide different age signatures in accord with the petrological observations (cf. Müller et al., 1999).
Commonly a correlation between white mica grain-size fraction and apparent Rb–Sr ages can be
observed in the samples, with higher apparent ages for the larger grain-size fractions (Fig. 11b). These
ages are consistent with petrological textures showing that the larger grains correspond to the early
assemblages and chemical compositions. In Naxos the larger grains of white mica correspond to the
early metamorphic stage with high Si content. In contrast the smaller grains are characteristic of the
later stage. Smaller grains tend to react more and faster to fluid-induced reequilibration (Straume &
Austrheim, 1999).
The ages obtained by this method cannot be termed isochron ages because there is no certainty
that the two minerals were formed isotopically in equilibrium during the same petrological process.
Ages calculated from “scattered data” are termed ‘apparent ages’. The geological meaning of apparent
ages obtained from Rb/Sr disequilibrium patterns needs to be reinforced by other arguments such as
petrological observations. In most cases, the interpretation of the apparent age is a minimum age for
the formation of the relict assemblage (like a large, relict flake of white mica combined with a
feldspar porphyroclast) and a maximum age for the crystallization of small grains during the last
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increments of deformation in a rock. For sample Na07-10, the apparent age of 37 ± 0.6 Ma is defined
by the larger white mica and feldspar. It is interpreted as the minimum age for blueschist facies
metamorphism. The apparent age of 32.5 ± 0.5 Ma is defined by the smaller white mica combined
with feldspar and is interpreted as the maximum age for the end of the greenschist facies
metamorphism (Fig. 11b).
4. Results
4.1. Paper I - An Eocene/Oligocene blueschist-/greenschist facies P–T loop for the
top of the passive margin sequence on Naxos Island
We revisited the known locality of high-P rocks at Kalados Bay in south Naxos (top of the passive
margin sequence) to constrain P–T conditions of high pressure metamorphism and the greenschist
facies overprint (Fig. 12). The high-P samples record blueschist facies metamorphism and preserve
the prograde path in garnet and glaucophane cores. We estimated that prograde high-P metamorphism
occurred at 12–13 kbar and 450–470°C in the metapelite sample and 14.5–15 kbar and 540–570°C in
the metabasite sample. Peak high pressure metamorphism was estimated from the metapelite and the
metabasite sample at 15–17.1 kbar and 550–651°C and dated at 41.5–38 Ma. The greenschist facies
overprint was estimated to have occurred at 2.7–4.9 kbar and 350–410°C and dated at 29–35 Ma. The
exhumation of the high-P rocks occurred at a rate of 7.4 ± 4.6 km/Ma. The P–T loop for the top of the
sequence passed though the temperature of ~240°C in the late Oligocene and early Miocene (20.5 ±
2.4 Ma and 25.2 ± 3.8 Ma) (Seward et al., 2009). The final exhumation from greenschist facies
conditions through fission track cooling occurred at distinctly smaller rates of 0.49 ± 0.24 km/Ma.
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Figure 12: P–T–t path of the top of the passive margin sequence in south Naxos (modified from Peillod et al.,
2017; Paper I). Pressure converted to depth assuming average rock density of 2700 kg/m3. Metamorphic facies
are is from Evans (1990): PA, pumpellyite–actinolite; LBS, lawsonite blueschist; EBS, epidote blueschist; E,
eclogite; AEA, albite–epidote–amphibolite; A, amphibolite). zircon (ZFT) and apatite fission-track (AFT) ages
(Seward et al., 2009).
The most important finding of Paper I is that the rocks at the top of passive margin sequence of
the CBU record a full Eocene/early Oligocene blueschist/greenschist facies P–T loop. The rocks
reached upper crustal levels about 10 Ma before the Miocene high-T metamorphism overprint during
the large scale extension.
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4.2. Paper II - Characterising the P–T path for the Miocene exhumation related to
the extensional setting for the bottom of the passive margin sequence on Naxos
We collected nine samples at different distances from the projected trace of the Naxos extensional
fault system and the migmatite dome. We constrained the P–T paths for the high grade
metamorphism. The P–T paths of all samples are characterised by two distinct P–T path segments.
The early segment is characterised by near isothermal decompression between 8–12 kbar and 650–
750°C and 5.5–7.1 kbar and 550–630°C. The second segment is characterised by pronounced cooling
(cooling path) from 5.5–7.1 kbar and 550–630°C to the surface (Fig 13).
Figure 13: Pressure-temperature (P–T) paths (grey shading) derived from P–T estimates of the nine samples in
Paper II.
The main finding of this study is that there is not one single path for the high-T rocks but that
rocks from different locations have different paths. The results show that the cooling path has
systematic differences in metamorphic field temperature gradient relative to the distance from the
Naxos extensional fault system (Fig. 14). Samples from north of the migmatite dome show a constant
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P–T gradient with distance from the fault system, whereas rocks from further south show a decreasing
P–T gradient downwards from the fault system. We correlated these differences of the late thermal
gradients with finite strain data, which represents a proxy for the intensity of ductile flow. High finite
strain (d = 3.56–10.86) is associated with high thermal gradients (northern sample group, red line in
Fig. 14). In contrast low finite (d = 0.15–0.51) is associated with a downward decreasing thermal
gradient (southern sample group, green line in Fig. 14).
Figure 14: Vertical distance from detachment vs temperature gradient and Naxos map that shows the samples
locations (Modified from Paper II).
4.3. Paper III - The timing of the high-T metamorphism in the bottom of the passive
margin sequence in northern Naxos
In this paper we reported eight Rb-Sr multi-mineral isochron ages which constrain the timing of high-
T metamorphism and partial melting associated with the ductile deformation in the footwall of Naxos
metamorphic core complex (bottom of the passive margin sequence). A migmatite sample provided
an age of ~14 Ma for crystallisation of the migmatisation-related melt. Pegmatite samples provided
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ages of 12–14 Ma that are in part associated with the partial melting of the surrounding host rocks.
Schist samples metamorphosed at amphibolite facies conditions yielded ages of about 14 Ma.
Apparent ages of 9.5–13 Ma for fluid- and/or deformation-assisted white mica and biotite reworking
are interpreted as a late stage of extensional shearing at greenschist facies conditions. The main
finding is that high-T metamorphic conditions and partial melting lasted until 14–12 Ma.
Figure 15: Simple P–T–t path for the bottom of the passive margin sequence during the Miocene high-T
metamorphism.
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We combined the reported age data and the P–T estimates from Paper II with published U-Pb
zircon ages of migmatite and granite crystallisation (Bolhar et al., 2017b; Keay et al., 2001; Martin et
al., 2006) and Ar-Ar hornblende ages (Wijbrans & McDougall, 1988) to construct a generalised P–T–
t path for the bottom of the passive margin sequence (Fig. 15). The early near isothermal segment
started at 20 and lasted until 14–12 Ma. The final decompression and cooling segment occurred after
12–11 Ma. The results show that melt was present in central and northern Naxos for at least 7 Ma and
that the melt-weakened crust considerably aided large-magnitude displacement at the Naxos
extensional fault system.
4.4. Paper IV - A P–T–t loop on SE Syros Island (Fabrika section): A common
Eocene/Oligocene metamorphic history for the central Cyclades
The finding that the greenschist facies overprint in southern Naxos occurred already in the Oligocene
is an important and new finding. Because the Oligocene age for the greenschist-facies overprint has
been demonstrated for the first time in the central Cyclades, we decided to study another, similar rock
sequence on a nearby island. The Fabrika coastal section in southeastern Syros provides a perfect
locality for this. In paper IV we constrained a P–T–t path for the Fabrika section (Fig. 16).
Garnet cores in blueschist and eclogite facies rocks preserve the prograde path which was
estimated at 10.7–14.4 kbar and 380–460°C. The peak high-P metamorphism was estimated for
blueschist and eclogite rocks at 15.4–17.7 kbar 470–500°C and dated at 42.9–39.5 Ma. The
greenschist facies overprint was estimated at 1.3–5.5 kbar, 388–524°C and dated at 33–26 Ma. These
results demonstrate a common Eocene/Oligocene P–T metamorphic history for southeastern Syros
and southern Naxos, suggesting that Eocene/Oligocene exhumation of the CBU might have been
important for the Cyclades.
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Figure 16: P–T–t path of blueschist-eclogite overprinted by greenschist facies rocks in Syros.
5. Discussion
5.1. Exhumation of high-P rocks by extrusion wedge in a subduction zone setting
during the Eocene and Oligocene
In this thesis we constrain a P–T–t loop for the top of the passive margin sequence in Naxos
(Paper I) and calculated peak high-P conditions at c. 40 Ma. The data show rapid exhumation soon
after peak high-P metamorphism from a depth of 60 km to a depth of 14 km until c. 32 Ma
(Oligocene). This is followed by slower exhumation across the brittle-ductile transition in the
Miocene (25–20 Ma). The full P–T–t loop shows that the top of the passive margin sequence in
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southern Naxos was rapidly exhumed into the brittle upper crust by the Oligocene/Miocene boundary.
This finding contrasts with previous conclusions that the greenschist facies overprint of the blueschist
facies rocks is related to large scale extension and the formation of the migmatite dome in the
Miocene. An important consequence is that the top of the passive margin sequence in southern Naxos
does not record the later Miocene high-T metamorphism and was already above crustal levels where
metamorphism can occur. We have corroborated our findings through a study in southeastern Syros
(Paper IV).
Figure 17: extrusion wedge of the passive margin sequence for Naxos during the Eocene/Oligocene.
The process that exhumed the passive margin sequence is likely to be an extrusion wedge similar
to those demonstrated for the eastern and western edges of the Cyclades in Evia and Samos (Ring et
al., 2007a, 2007b) and suspected in the central Cyclades (Huet et al., 2009; Ring et al., 2011). In
Naxos, the serpentine lenses (Fig. 5) at the boundary between the basement and the passive margin
sequence are interpreted as remnants of a lithospheric-scale thrust that emplaced the passive margin
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sequence onto the Cycladic Basement (Jansen & Schuiling, 1976; Katzir et al., 2007). The thrust
should cause high-P metamorphism in the basement. However, no relics of high-P assemblages have
been reported from the Cycladic Basement. In Paper I we interpret top-to-the-N shear sense indicators
in the passive margin sequence to be related to normal faulting at the top of the passive margin
sequence at the top of the extrusion wedge (Fig. 17). This normal fault working in concert with the
thrust at the bottom of the passive margin sequence bound the extrusion wedge and caused rapid
exhumation of the passive margin sequence onto the Cycladic Basement and thereby allowed the
basement to escape high-P metamorphism (Fig. 17). The rapidity of this exhumation gives no chance
for thermal relaxation to play a significant role in the reequilibration of the Naxos blueschists.
5.2. Relation between the Eocene/Oligocene and the Miocene metamorphic events
Our data demonstrate three distinct metamorphic events for Naxos. The top of the sequence
shows high-P metamorphism at 41–38 Ma and a greenschist facies overprint at 37–29 Ma. The
bottom of the sequence shows an extension-related high-T metamorphism at 20–12 Ma. This result
raises the question: what is the relation between the Eocene/Oligocene and the Miocene metamorphic
events?
At the top the passive margin sequence in Naxos blueschist facies metamorphism is observed
to the south of the biotite isograd (Fig. 7). Additionally, the Eocene/Oligocene metamorphic history is
retained in the Rb-Sr isotopic signature (samples NAX15-13 and NAX15-14 in paper I) and in garnet
cores (sample NP39 in paper II). At the bottom of the passive margin sequence in central and northern
Naxos, zircon rims preserve a geochronological memory of the high-P metamorphism (Bolhar et al.,
2017b; Martin et al., 2006). Consequently, we argue that the entire passive margin sequence
experienced the Eocene high-P metamorphism and was subsequently exhumed within an extrusion
wedge. The rocks of the lower passive margin sequence in central and northern Naxos were later
metamorphosed again at amphibolite facies conditions in the Miocene. Consequently, the bottom of
the sequence experienced three metamorphic stages. However, P–T conditions for the bottom of the
passive margin sequence during the Eocene and Oligocene are unknown.
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In Paper I we demonstrate that the top of the passive margin sequence in southern Naxos
reached ~4 kbar ~400°C in the Oligocene. In Paper II and III, we constrain the maximum pressure of
the high-T metamorphism at 8–12 kbar 750°C. How do the ~4 kbar (equalling about 15 km depth) in
the Oligocene related to the 8-12 km (about 28-40 km depth) in the Miocene? Because the Miocene
high-T metamorphism occurs in an extensional setting, any thickening/reburial between the Oligocene
and Miocene can be ruled out. Therefore the bottom of the passive margin sequence needs to be have
been at greater depth than the top of the sequence in the Oligocene. The current structural thickness of
the passive margin sequence exposed in Naxos is ~8 km. The baric difference between the top and
bottom of the sequence is 4–8 kbar (equalling 14–29 km). This estimate implies that the ~8 km of
current structural thickness was 14–29 km in the Oligocene. The pressure of 8–12 kbar for the bottom
of the sequence in the Oligocene and that about 50% vertical thinning of the passive margin sequence
took place due to extensional deformation. This reasoning suggests that rocks from the base of the
passive margin sequence were at a pressure of 8-12 kbar between the Oligocene and the high-T
overprint in the Miocene.
To determine the P–T conditions of the bottom of the passive margin sequence during the
Eocene, Avigad (1998) used the current structural thickness (~8 km) and a geothermal gradient of
13°C/km to extrapolate the P–T condition at the bottom of the sequence and obtained >14 kbar and
~550°C. However, we have shown that the structural thickness was 14–29 km in the Oligocene,
which may reflect a better estimate of the structural thickness of the passive margin sequence at that
time. Additionally, we calculated a prograde P–T path segment from 12–13 kbar ~450°C to 15–17
kbar 550–643°C (in Paper I,) which gives a geothermal gradient of 12–14°C. Using the structural
thickness of 14–29 km, the geothermal gradient of 12–14°C/km and our peak high-P conditions for
blueschist facies metamorphism at the top of the sequence, we provide an extrapolation similar to that
of Avigad (1998). Our extrapolation results in 20–24 kbar and 750–950°C for the bottom of the
sequence. The pressure range is similar to the highest pressures obtained for nearby islands (e.g.
Syros, Tinos and Sifnos; see Fig. 5). The temperatures, however, are surprisingly high, distinctly
higher than temperatures recorded for nearby islands. The estimate of 750–950°C is nevertheless
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corroborated by Ti-in-zircon thermometry providing a temperature of 650–850°C for Eocene zircon
rims (Bolhar et al., 2017b). Additionally Katzir et al. 1999 estimated 1057–1070°C for relict crystals
of orthopyroxene, olivine and spinel in metaperidotites which were juxtaposed with the bottom of the
passive margin sequence at the base of the orogenic wedge.
6. Conclusion and future work
The main conclusion of this thesis is that the passive margin sequence in Naxos experienced two
exhumation events: an Eocene/Oligocene exhumation phase in the subduction zone setting within an
extrusion wedge, and a Miocene exhumation related to the extensional deformation. During the
Oligocene exhumation, the top of the passive margin sequence reached upper crustal levels whereas
the bottom of the passive margin sequence was at greater depth (8–12 kbar). During the Miocene the
top of the passive margin sequence was already in the brittle crust and therefore did not record the
Miocene metamorphism. The bottom of the sequence was first isothermally exhumed at high-T and
subsequently cooled rapidly. Another conclusion is that rocks from southeastern Syros record a
similar Eocene/Oligocene P–T metamorphic history than the south Naxos rocks suggesting a common
Eocene/Oligocene metamorphic history for the central Cyclades.
Although we characterised the Oligocene exhumation of the Eocene high-P rocks on Naxos,
several avenues for future research emerge from this thesis. There is a gap of 10–15 Ma between the
metamorphic stages. During this time, the bottom of the passive margin sequence apparently stayed at
the same pressure (8–12 kbar). The temperature, however, could not be constrained. It is possible that
during the 10–15 Ma time gap, rocks at the bottom of the passive margin sequence experienced
isobaric heating by thermal relaxation. On Tinos, Parra et al. (2002) show isobaric heating of 200–
300°C at a depth comparable to about 9 kbar. Unfortunately, the timing of this heating stage is
unconstrained and the drastic geological consequences (extreme thermal gradient, see above)
unexplored. Recently Laurent et al. (2018) also suggest isobaric heating of ~70 °C at ~10 kbar for
Syros. An area for further investigation would be to better characterise the P–T conditions during the
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10–15 Ma time gap in Naxos, so as to understand the metamorphic relationships during the apparent
tectonic lull between the Oligocene and Miocene.
Another point that this thesis raises is the lack of P–T data for high-P metamorphism at the
bottom of the passive margin sequence. The P–T of 20–24 kbar and 750–950°C estimated from the
extrapolation of peak high-P from the top of the sequence is surprising but maybe not impossible
given the thermometric estimates provided by Bolhar et al. (2017b) and Katzir et al. (1999). A
petrological study aiming at better understanding high-P conditions for the bottom of the passive
margin sequence on Naxos might be warranted.
7. Acknowledgement
The root of this thesis is from my fourteenth year when I decided to study geoscience. From now to
back to this time, the contributions and support of many people were infinitely precious in my eyes.
First I would like to thank Uwe Ring for sharing his great experience of the geology of the Aegean
domain. These past years have been challenging and you were always available to provide good
guidance and help at the good time. You have clearly improved my scientific rigour and critical
thinking. Scientific criticism was coupled with free space to develop my own ideas.
This supervision was completed by my co-supervisor Alasdair Skelton. Thanks you for your
continuous support and knowledge in metamorphic petrology. I will be available for the Scottish
dance you will organise in the department. I also thank Johannes Glodny for his awesome age dating
work. I really appreciate the discussions during the samples safari we had in Naxos to get the best
ages. Thanks Henrik Linnros for your 3D model of Naxos and for the nice time we had in the field. I
am looking forward for the next time you will pay me a glass of ouzo.
Undying thanks to Hildred Crill. You believe in me and give all your time to improve my
scientific writing. I really appreciated our discussion about politics and music. You are clearly the
type of teacher that every university needs.
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The EPMA data in this thesis were carried out in Uppsala. I met their nice geologist team and
made friends. Thanks Iwona Klonowska, Jarek Majka and Abigail Barker for strong support. I had a
difficult relationship with the emotionally disturbed microprobe machine but it is true love. I thank
Jan-Olov Persson greatly for his help with statistical problems.
I am very grateful to Runa Jacobsson, Dan Zetterberg, Marianne Ahlbom, Eve Arnorld, Arne
lif, Inês Jakobsson, Krister Junghahn, Malin Andersson and Draupnir Einarsson for sample
preparations, technical supports and administrative tasks.
All my love goes to my family who believe in my study project. They support all my projects
and give encouragement during the difficult moments.
I thank also all the people I met during this Swedish adventure. Barbara Kleine thanks for all
your knowledge about glaucophane. I hope I was a good office mate. Reuben Hansman, Clifford
Patten and Hagen Bender are my “bros” in the department. Kiwi, beer and cheese are definitely a
good mix. Special thanks to Stefano Bonaglia, Statis Repas, Christophe Sturm and Etienne Pauthenet
for this great music band we have! Not bad at all! A special thanks to Charlotta Högberg for all this
good time we had together. You were a great support. Lot of thanks and kram to Helene Petit,
Melanie Schmitt, Ross Kielman, Robert Graham, Mélissa Glodny, Charlotte Depin, Jan Jan Fidom,
Fiona Thiessen, Lara Schultes, Sandra Gdaniec, Elin Tollefsen, Henrik Swärd, Susanne Sjöberg,
Natalia Barrientos, Jenny Sjöström, Frans Weis, Pedro Petro, Alexander Lewerentz, Lisa Bröder,
Patrick Winiger, Francesco and Pilvi Muschitiello, Arman Boskabadi, Catherine Hirst, Christophe
Dupraz, Johanna Holmberg, Kweku Afrifa Yamoah, Kerstin Winkens, Tobias, Gabrielė Kniukštaitė,
Xiaojing Zhang, Bo, Berit Gewert and all people I forgot to name!
I thank all the Strasbourg people that I met and who all contributed to the success of my
studying path. We had the best time there from the laughing to crying. Bastien Walter, Pierre Dietrich,
Leila Mezri, Benoit Deleplanc, Thibaud Denoyer, Maxime Mareschal, Fabien Humbert, Marie-Laure
Bachschmidt, Étienne Skrzypek, Francis Chopin, Véronique Adam, Geoffroy Mohn…
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I thank all my childhood friends from Lyon that support me, Florian and Olivia Migeot,
Gaëtan and Agnes Saubiez, Patrice and Laurence Marques, Jerome and Amandine Jamet,
Emmanuelle Blanc, Marie Guerard, Louise Charlotte Barut, Gaël Lester, Adrien and Florence
Demaison, Claire Veale, Fabien and Elsa Ludjer, Benjamin Gonin.
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lower crust. Tectonics, 9, 557–567.
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