A phase of isobaric heating during exhumation of high-pressure rocks is often reported, but the tectonic significance of isobaric heating remains uncertain. Constraining the timescales of isobaric heating is essential for connecting the heating to tectonic processes of lithospheric thickening and extension. An isobaric heating phase has been reported for the Cycladic Blueschist Unit in the Hellenide orogen of Greece. We present diffusion modeling of major elements in garnet using data from Naxos Island to provide new independent estimates of the duration of isobaric heating. We also present radioactive trace element analyses of the Cycladic basement and a heat conduction model to explore the heat production generated in the basement and its influence on isobaric heating. Our model indicates that isobaric heating occurred over ∼9.7 Myr, during which the Cycladic basement generated enough heat to explain the observed temperature increase. At the end of this heating phase, the temperature increase caused a significant drop in crustal strength, which controlled the style of crustal extensional deformation during subsequent rollback of the subducting slab. Our work implies that the underthrusting of radiogenic material in convergent settings produces sufficient heat to significantly increase temperature which weakens the crust and enables pervasive deformation.

The geometry of large-scale extensional deformation in the southwestern Cyclades archipelago is a controversially discussed issue. Recent studies suggest a bivergent (i.e., coeval top-to-the-S and top-to-the-N shear) geometry of early Miocene extensional deformation. On Sikinos Island, top-to-the-S shear structures in the Cycladic basement are overprinted by a prominent ductile-to-brittle top-to-the-N extensional shear zone in the Cycladic basement at its contact with the tectonically overlying passive-margin sequence of the Cycladic Blueschist Unit. Hitherto, the top-to-the-S shear structures have been related to thrusting of the passive-margin sequence onto the Cycladic basement. Rb–Sr multimineral dating constrains the age of retrograde, lower greenschist-facies top-to-the-S shearing at 23.20 ± 0.25 Ma. This age is identical to published ⁴⁰Ar/³⁹Ar ages of ~ 23 Ma for a lower greenschist-facies top-to-the-S extensional shear zone on nearby Folegandros Island. Therefore, an interpretation of the top-to-the-S shear zone in the Cycladic basement of Sikinos as resulting from extensional deformation is consistent with the regional scale context of crustal extension invoked for the Cyclades in the early Miocene. On Folegandros Island, top-to-the-S extension occurs at the top of the passive-margin sequence of the Cycladic Blueschist Unit, whereas on Sikinos  coeval top-to-the-S extension occurs near the top of the underlying Cycladic basement. Our new Rb–Sr age of ~ 23 Ma indicates that top-to-the-S fabrics reflect the earliest Miocene extensional structures. They are intimately related to the top-to-the-N extensional structures forming an early Miocene bivergent extensional hinge zone at the southern end of the Cyclades archipelago.

Determining the tectonic evolution and thermal structure of a tectonic unit that experiences a subduction-related pressure temperature (P-T) loop is challenging. Within a single unit, P-T conditions can vary from top to bottom which can only be revealed by detailed petrological work. We present micropetrological data from the middle section of the Cycladic Blueschist Unit (CBU) in Naxos, Greece, which indicates a different P-T loop than that for the top of the sequence. Using Zr-in-rutile and Ti-in-biotite thermometry coupled with quartz-in-garnet elastic barometry and phase equilibrium thermodynamic modeling, we identify a prograde path from 15.4 +/- 0.8 kbar to 19.9 = 0.6 kbar and from 496 +/- 16 degrees C to 572 +/- 7 degrees C (2a uncertainty), equilibration during decompression at 8.3 +/- 15 kbar and 519 +/- 12 degrees C followed by near-isobaric heating to 92 +/- 0.8 kbar and 550 +/- 10 degrees C (or even 584 +/- 19 degrees C), and a final greenschist-facies equilibration stage at 3.8 +/- 0.3 kbar and 520 +/- 4 degrees C. We compare these P-T estimates with published data from the top and also the bottom of the CBU section and find that the bottom half of the CBU on Naxos records higher peak high-pressure (HP) of about 4 kbar than the top of the unit, defining the thickness of the CBU section on Naxos to about 15 km in the Eocene. We determine that crustal thickening of up to similar to 15% occurs in the upper half of the CBU section during exhumation of the HP rocks in an extrusion wedge in a convergence setting. At about 30 Ma, the bottom half of the CBU was finally thrust onto the radiogenic Cycladic basement. Subsequently this bottom half of the CBU section underwent isobaric heating of 9-96 degrees C between c. 32-28 and 23-21 Ma. Isobaric heating occurred below the upper CBU section that thickened during decompression and commenced when HP metamorphism in the Cyclades ended. This suggests that thermal relaxation following tectonic accretion in the Cyclades controlled heating of the evolving Cycladic orogen during a tectonically quiescent period before lithospheric extension commenced by 23-20.5 Ma.

Detecting zones of considerable early-orogenic displacement in rather monotonous rock sequences which have undergone a late and pervasive tectonometamorphic overprint is challenging. It has been proposed that the alleged Zas shear zone in the passive margin sequence of the Cycladic Blueschist Unit on Naxos Island, Greece, separates amphibolite facies, non-high-pressure rocks (Koronos Unit) below the shear zone from Eocene high-pressure rocks (Zas Unit) above the shear zone. We review existing pressure-temperature data from the Koronos Unit and present new kinematic data from the anticipated Zas shear zone to evaluate the tectonic significance of this recently proposed structure. This has implications for unravelling the subduction history of rock units from tectonometamorphic datasets. Common to all pressure-temperature data from the Koronos Unit is a well-defined amphibolite facies equilibration stage at 8-11 kbar and 600-700 degrees C, followed by initial near-isothermal to slightly prograde decompression and subsequent pronounced cooling. This segment of the high-temperature pressure-temperature loop is associated with top-to-the-NNE extensional deformation in the footwall of the Miocene Naxos-Paros detachment. Little is known about the metamorphism preceding the amphibolite facies overprint in the Koronos Unit. Our review shows that it is likely that the rocks experienced a prior high-pressure metamorphic overprint that is typical of rocks of the Cycladic Blueschist Unit. Our kinematic data show that the Zas shear zone contains variably deformed rocks with dominantly top-to-the-NNE shear sense indicators that developed under greenschist facies metamorphism in the footwall of the Naxos-Paros detachment. No significant offset can be detected across the Zas shear zone and the geology on either side of it does not support large-scale movement across the shear zone. We discuss a model in which the Zas shear zone is considered to be a minor zone of deformed schist near the biotite-in isograd of Miocene high-temperature metamorphism. We conclude that there is no need to tectonically subdivide the passive margin sequence of the Cycladic Blueschist Unit on Naxos.

Volcanic rocks in Archean and Paleoproterozoic greenstone belts are abundant and have been suggested as a potential Au source for orogenic Au deposits. The behavior of Au during metamorphism of these rocks is, however, poorly known. We present ultra-low-detection-limit Au analyses from a suite of variably metamorphosed rocks from the Archean La Grande subprovince, Canada, and the Paleoproterozoic Central Lapland greenstone belt, Finland. Roth areas are well endowed in Au and have great potential for discovery of new orogenic Au deposits. The metavolcanic rocks in these belts are grouped into tholeiite and calc-alkaline magmatic series, for which the protolith Au contents are calculated using Au versus Zr/Y power-law regressions from greenschist facies samples. In the tholeiitic rocks, Au is compatible during magmatic processes and decreases with differentiation, whereas in the cafe-alkaline rocks,Au is incompatible and increases with differentiation. Mass-variation calculations show that as much as 77% and 59% of the initial Au content is lost during progressive metamorphism to upper amphibolite facies conditions (>550 degrees C) in La Grande and Central Lapland respectively. This study highlights, first, that metavolcanic rocks release Au during metamorphism in Archean and Paleoproterozoic greenstone belts and are thus a good potential source rocks for orogenic Au deposits; second, that the Au fertility of the metavolcanic rocks is controlled by their mantle source and magmatic evolution; and third, that the metamorphic devolatilization model can he applied to Archean and Paleoproterozoic orogenic Au deposits.

Eclogites, blueschists and greenschists are found in close proximity to one another along a 1-km coastal section where the Cyclades Blueschist Unit (CBU) is exposed on SE Syros, Greece. Here, we show that the eclogites and blueschists experienced the same metamorphic history: prograde lawsonite blueschist facies metamorphism at 1.2-1.9 GPa and 410-530 degrees C followed, at 43-38 Ma, by peak blueschist/eclogite facies metamorphism at 1.5-2.1 GPa and 520-580 degrees C. We explain co-existence of eclogites and blueschists by compositional variation probably reflecting original compositional layering. It is also shown that the greenschists record retrogression at 0.34 +/- 0.21 GPa and T = 456 +/- 68 degrees C. This was spatially associated with a shear zone on a scales of 10-100-m and veins on a scale of 1-10-cm. Greenschist facies metamorphism ended at (or shortly after) 27 Ma. We thus infer a period of metamorphic quiescence after eclogite/blueschist facies metamorphism and before greenschist facies retrogression which lasted up to 11-16 million years. We suggest that this reflects an absence of metamorphic fluid flow at that time and conclude that greenschist facies retrogression only occurred when and where metamorphic fluids were present. From a tectonic perspective, our findings are consistent with studies showing that the CBU is (a) a high-P nappe stack consisting of belts in which high-P metamorphism and exhumation occurred at different times and (b) affected by greenschist facies metamorphism during the Oligocene, prior to the onset of regional tectonic extension.

High pressure, low temperature (HP-LT) rocks observed at the surface of the Earth are evidence ofpast 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.

We present eight Rb-Sr multi-mineral isochron ages showing that high-temperature metamorphic conditions and partial melting during top-to-the-NNE extensional shearing in the footwall of the Naxos extensional fault system (i.e. Naxos metamorphic core complex) lasted until about 14-12 Ma. One migmatite sample yielded an age of 14.34 +/- 0.2 Ma (2 sigma uncertainty) for crystallization of migmatization-related melt pockets. Four pegmatite samples, which are in part associated with partial melting of their host rocks, provided overlapping ages ranging from 13.81 to 12.23 Ma (age range includes 2 sigma uncertainty). Additional three samples of amphibolite-facies schist supplied Rb-Sr ages of around 14 Ma. Samples showing fluid- and/or deformation-assisted white mica and biotite reworking gave Rb-Sr mineral apparent ages of 11.1 +/- 2.7, 10.16 +/- 0.24, 9.7 +/- 0.7 and 9.6 +/- 0.15 Ma. These ages are interpreted to be associated with late stages of extensional shearing under greenschist-facies metamorphic conditions. Together with published U-Pb zircon ages of migmatite, and S- and I-type granite crystallization, the data indicate that the presence of melt in the footwall of the Naxos extensional fault system lasted for at least 7 Ma (from similar to 18 to similar to 11 Ma). This demonstrates that high temperatures and crustal melting resulting from and aiding extensional deformation was a long-lived and not a transient event. We conclude that melt-assisted deformation facilitated large-scale displacement on the Naxos extensional fault system by drastically weakening the extending crust for long periods of time.

Geothermobarometric and geochronological work indicates a complete Eocene/early Oligocene blueschist/greenschist facies metamorphic cycle of the Cycladic Blueschist Unit on Naxos Island in the Aegean Sea region. Using the average pressure-temperature (P-T) method of thermocalc coupled with detailed textural work, we separate an early blueschist facies event at 576 +/- 16 to 619 +/- 32 degrees C and 15.5 +/- 0.5 to 16.3 +/- 0.9kbar from a subsequent greenschist facies overprint at 384 +/- 30 degrees C and 3.8 +/- 1.1kbar. Multi-mineral Rb-Sr isochron dating yields crystallization ages for near peak-pressure blueschist facies assemblages between 40.5 +/- 1.0 and 38.3 +/- 0.5Ma. The greenschist facies overprint commonly did not result in complete resetting of age signatures. Maximum ages for the end of greenschist facies reworking, obtained from disequilibrium patterns, cluster near c. 32Ma, with one sample showing rejuvenation at c. 27Ma. We conclude that the high-P rocks from south Naxos were exhumed to upper mid-crustal levels in the late Eocene and early Oligocene at rates of 7.4 +/- 4.6km/Ma, completing a full blueschist-/greenschist facies metamorphic cycle soon after subduction within c. 8Ma. The greenschist facies overprint of the blueschist facies rocks from south Naxos resulted from rapid exhumation and associated deformation/fluid-controlled metamorphic re-equilibration, and is unrelated to the strong high-T metamorphism associated with the Miocene formation of the Naxos migmatite dome. It follows that the Miocene thermal overprint had no impact on rock textures or Sr isotopic signatures, and that the rocks of south Naxos underwent three metamorphic events, one more than hitherto envisaged.

Combining petrological and geochronological data we were able to show that rocks in south Naxos completed a full blueschist-/greenschist-facies metamorphic loop in about 10 Myr, distinctly faster than hitherto believed. We show that the high-pressure rocks reached peak pressure (16.3±0.9 kbar - 619 ±32°C) between 40.5±1.0 and 38.3±0.5 Ma and were re-equilibrated in the middle crust (3.8±1.1 kbar - 384±30 °C) under greenschist-facies metamorphism during in the Oligocene ~32 Ma. Our data indicate that this exhumation occurred at rates of 7.4±4.6 km Ma-1. Therefore, the Oligocene greenschist-facies overprint of the blueschist-facies rocks from south Naxos is unrelated to the Miocene amphibolite-/greenschist-facies metamorphism observed in Naxos.

Genom att kombinera petrologiska och geokronologiska data kan vi visa att bergarter på södra Naxos fullföljt en fullständig blåskiffer-/grönskiffer-facies metamorfisk loop på 10 Ma. Detta är mycket snabbare änn vad man tidigare trott. Vi visar att högtrycks bergarter nådde maximalt tryck (16.3±0.9 kbar - 619 ±32°C) mellan 40.5±1.0 och 38.3±0.5 Ma och var åter i jämnvikt i mitten av jordskorpan (3.8±1.1 kbar - 384±30 °C) under grönskiffer-facies metamorfism under Oligocen vid ~32 Ma.Våra data visar att exhumeringen skedde med hastigheter av 7.4±4.6 km Ma-1. Därför är den Oligocena grönskiffer-facies ersättningen av blåskiffer-facies på södra Naxos ej relaterad till den Miocena amfibolit-/grönskiffer-facies som observerats på Naxos.

En combinant les données pétrologiques et géochronologiques, nous avons été en mesure de montrer que les roches du Sud Naxos ont effectué un trajet complet en métamorphisme schiste bleue/schiste vert en 10 Ma, distinctement plus rapide que ce qui était admis jusqu’alors. Nous montrons que les roches de haute pression ont atteint un pic de pression (16.3±0.9 kbar - 619 ±32°C) entre 40.5±1.0 et 38.3±0.5 Ma et ont été rééquilibrées en croûte moyenne (3.8±1.1 kbar - 384±30 °C) en métamorphisme de facies schiste vert pendant l’Oligocène ~32 Ma. Nos données indiquent que cette exhumation a une vitesse de 7.4±4.6 km Ma-1. Par conséquent, la surimpression des roches de haute pression en métamorphisme de facies schiste vert dans le sud de Naxos n’est pas à relier au métamorphisme de facies amphibolite/schiste vert du Miocène observé à Naxos.