| 1. |
- Patten, C. G. C., et al.
(author)
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Gold mobilization during metamorphic devolatilization of Archean and Paleoproterozoic metavolcanic rocks
- 2020
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In: Geology. - 0091-7613 .- 1943-2682. ; 48:11, s. 1110-1114
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Journal article (peer-reviewed)abstract
- 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.
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| 2. |
- Peillod, Alexandre, et al.
(author)
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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
- 2017
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In: Journal of Metamorphic Geology. - : Wiley. - 0263-4929 .- 1525-1314. ; 35:7, s. 805-830
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Journal article (peer-reviewed)abstract
- 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.
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| 3. |
- Peillod, Alexandre, et al.
(author)
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Differences in decompression of a high-pressure unit : A case study from the Cycladic Blueschist Unit on Naxos Island, Greece
- 2021
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In: Lithos. - : Elsevier BV. - 0024-4937 .- 1872-6143. ; 386-387
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Journal article (peer-reviewed)abstract
- 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 °C to 572 ± 7 °C (2σ uncertainty), equilibration during decompression at 8.3 ± 1.5 kbar and 519 ± 12 °C followed by near-isobaric heating to 9.2 ± 0.8 kbar and 550 ± 10 °C (or even 584 ± 19 °C), and a final greenschist-facies equilibration stage at 3.8 ± 0.3 kbar and 520 ± 4 °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 ~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 °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.
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| 4. |
- Peillod, Alexandre, et al.
(author)
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Disruption of a high‐pressure unit during exhumation: Example of the Cycladic Blueschist unit (Thera, Ios and Naxos islands, Greece)
- 2023
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In: Journal of Metamorphic Geology. - : John Wiley & Sons. - 0263-4929 .- 1525-1314. ; 42:2, s. 225-255
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Journal article (peer-reviewed)abstract
- Reconstructing the original geometry of a high-pressure tectonic unit is challenging but important to understand the mechanisms of mountain building. While a single nappe is subducted and exhumed, nappe-internal thrusts may disrupt it into several subunits. The Middle-CBU nappe of the Cycladic Blueschist Unit (Hellenide subduction orogen, Greece) shows evidence of such disruption along a Trans-Cycladic-Thrust (TCT), however, the timing of this thrusting is unknown. Here, we report multi-petrological and geochronological data from the Middle-CBU nappe from the Thera and Ios islands (Greece). Using Zr-in-rutile thermometry coupled with quartz-in-garnet elastic barometry, average P–T and phase equilibrium thermodynamic modelling, we show that garnet growth in Ios occurred during prograde metamorphism at 6.7 ± 1.4 kbar to 13.0 ± 1.6 kbar and 326 ± 20°C to 506 ± 13°C (2σ uncertainty) followed by early exhumation to 10.1 ± 0.6 kbar and 484 ± 14°C and a greenschist facies overprint at 5.7 ± 1.2 kbar and 416 ± 14°C. For Thera, we constrain peak HP conditions of 7.6 ± 1.8 kbar and 331 ± 18°C, followed by exhumation and equilibration at ~2 kbar and ~275°C using average P–T and phase equilibrium thermodynamic modelling. For Ios, Uranium-Pb garnet geochronology provides ages of 55.7 ± 5.0 Ma (2σ uncertainties) for prograde and 40.1 ± 1.4 Ma for peak HP metamorphism. Combining our new P–T–t data from Thera and Ios islands with existing data from Naxos island, we conclude that the studied nappe segments represent remnants of a former coherent nappe. The P–T–t data define an Eocene subduction rate of 2.1 ± 1.0 km/Ma, which is distinctly slower than the current subduction rate of 40–45 km/Ma. After subduction, the exhumation of the Middle-CBU nappe occurred during the Oligocene at different rates for different localities. The Middle-CBU nappe of Naxos was exhumed at a rate of ~6 km/Ma, contrasting with the exhumation rate of ~3 km/Ma calculated for Ios. This result suggests that the Middle-CBU nappe of Naxos rocks was thrust on the Ios one during the Oligocene. Using P–T–t data and assuming realistic subduction angles during the Eocene and the Oligocene, we present a 2D structural reconstruction of the Middle-CBU nappe of these islands. This reconstruction helps to understand the mechanisms of subduction of a continental margin and its disruption during exhumation.
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| 5. |
- Peillod, Alexandre, 1998-
(author)
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How were high-pressure rocks exhumed in Naxos, Greece?
- 2016
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Licentiate thesis (other academic/artistic)abstract
- 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.
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| 6. |
- Peillod, Alexandre, et al.
(author)
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Quo vadis Zeus : is there a Zas shear zone on Naxos Island, Aegean Sea, Greece? A review of metamorphic history and new kinematic data
- 2021
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In: Journal of the Geological Society. - : Geological Society of London. - 0016-7649 .- 2041-479X. ; 178:5
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Research review (peer-reviewed)abstract
- 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.
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| 7. |
- Peillod, Alexandre, 1988-
(author)
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The metamorphic history of Naxos (central Cyclades, Greece) : Deciphering the Oligocene and Miocene exhumation events
- 2018
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Doctoral thesis (other academic/artistic)abstract
- 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.
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| 8. |
- Peillod, Alexandre, et al.
(author)
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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)
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Journal article (peer-reviewed)abstract
- Geothermobarometric and structural work indicates that metamorphism in the footwall of the migmatitic Naxos core complex is strongly coupled to heat advection in the lower crust. Using the average pressure-temperature (P-T) method of THERMOCALC for geothermobarometric calculations coupled with detailed textural work, we constructed P-T paths for nine samples that have different spatial relationships to the Naxos extensional fault system and a migmatite dome in the center of the Naxos core complex. All nine samples show early near isothermal decompression. The late, cooling segment of the P-T paths shows systematic spatial differences in temperature gradients relative to the distance of the samples below the Naxos extensional detachment (representing the top of the Naxos extensional fault system). The differences in late thermal gradients correlate with finite strain, a proxy for the intensity of ductile flow. High finite strain correlates with high thermal gradients that do not change with distance below the Naxos detachment, whereas low finite strain correlates with downward decreasing thermal gradients. The difference in late thermal gradients and finite strain define a thermal asymmetry controlled by ductile flow, which we interpret to show that ductile flow controlled advection of heat and thus high-grade metamorphism on Naxos. We conclude that advective heating is an important driver of metamorphism in extensional settings.
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| 9. |
- Ring, Uwe, et al.
(author)
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The timing of high-temperature conditions an d ductile shearing in the footwall of the Naxos metamorphic core complex, Aegean Sea, Greece
-
Journal article (peer-reviewed)abstract
- 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 metamorphic core complex lasted until about 14-12 Ma. One migmatite sample yielded an age of 14.34 ± 0.20 Ma (2σ uncertainties) 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 2σ uncertainties). 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 presence of melt in the Naxos migmatite dome lasted for at least 7 Ma (from ~18 to ~11 Ma). This demonstrates that the thermal anomaly 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.
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| 10. |
- Ring, Uwe, et al.
(author)
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The timing of high-temperature conditions and ductile shearing in the footwall of the Naxos extensional fault system, Aegean Sea, Greece
- 2018
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In: Tectonophysics. - : Elsevier BV. - 0040-1951 .- 1879-3266. ; 745, s. 366-381
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Journal article (peer-reviewed)abstract
- 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.
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