Cycladic Blueschist Unit Research Articles

Geometry and Kinematics of Bivergent Extension in the Southern Cycladic Archipelago: Constraining an Extensional Hinge Zone on Sikinos Island, Aegean Sea, Greece

AbstractWe report the results of a field study on Sikinos Island in the Aegean extensional province of Greece and propose a hinge zone controlling incipient bivergent extension in the southern Cyclades. A first deformation event led to top‐S thrusting of the Cycladic Blueschist Unit (CBU) onto the Cycladic basement in the Oligocene. The mean kinematic vorticity number (Wm) during this event is between 0.56 and 0.63 in the CBU, and 0.72 to 0.84 in the basement, indicating general‐shear deformation with about equal components of pure and simple shear. The strain geometry was close to plane strain. Subsequent lower‐greenschist‐facies extensional shearing was also by general‐shear deformation; however, the pure‐shear component was distinctly greater (Wm = 0.3–0.41). The degree of subvertical pure‐shear flattening increases structurally upward and explains alternating top‐N and top‐S shear senses over large parts of the island. Along with an increased coaxial deformation component, the strain geometry became oblate. Published quantitative data from nearby Ios Island are similar and both data sets define an extensional hinge zone between top‐N extensional deformation across large parts of the central and northern Cyclades and top‐S extensional deformation at the southern and western fringe of the archipelago. This extensional hinge zone is an important large‐scale structure forming early in the history of lithospheric extension due to southward retreat of the Hellenic slab.

Quo vadis Zeus : is there a Zas shear zone on Naxos Island, Aegean Sea, Greece? A review of metamorphic history and new kinematic data

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°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.

The Late Cretaceous magmatic arc of the south Aegean: Geodynamic implications from petrological and geochemical studies of granitoids from Anafi island (Cyclades – Greece)

ABSTRACT In the Aegean region (Cyclades – Greece), a large part of the Island of Anafi consists of exhumed high-grade metamorphic sequences (amphibolites, serpentinites and metasediments) that have been intruded by Late Cretaceous intermediate and felsic granitoids. These correspond to I-type arc-related rocks, displaying calc-alkaline geochemical affinities. Variability in their petrographic features, mineral composition and geochemistry is assigned to differentiation processes that mostly involved plagioclase and/or K-feldspar, and to a lesser extent amphibole and biotite. Mineral chemistry and geochemical results suggest that the Anafi granitoids are highly comparable with the Late Cretaceous granitoid intrusive rocks from East Crete and Donousa island. The amphibole and zircon saturation thermometry yields relatively moderate temperature crystallization conditions (~790°C) for the intermediate granitoids and lower for the felsic granitoids (~630°C). Geobarometric calculations based upon the chemistry of magnesiohornblende, as well as the normative (Qz), (Ab) and (Or) contents clearly point to shallow intrusion conditions (~2.0–6.5 kbar), corresponding to an estimated depth of crystallization of ~12 ± 4 km. The thrust sheets that overly the Palaeogene flysch in Anafi (metasediments with serpentinized peridotites, amphibolites and metabasites), constitute a subducted and metamorphosed oceanic sequence. These metamorphic units likely represent a part of the Pindos – Cycladic Blueschist Unit domain that was subducted at an earlier pre-Campanian stage. In the hydrated mantle wedge, incorporation of shallow level granitoids within metamorphic units was likely facilitated via corner flow intrusion mechanisms. Following the intrusion, the granitoid rocks were exhumed in a syn-convergent setting, along with the metamorphic thrust sheets, by continuous underplating of more external units, thus, escaping penetrative ductile deformation.

Differences in decompression of a high-pressure unit: A case study from the Cycladic Blueschist Unit on Naxos Island, Greece

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.

Lateral Termination of a Cycladic‐Style Detachment System (Hymittos, Greece)

AbstractThe bedrock of Hymittos, Attic peninsula, Greece, exposes a pair of low‐angle crustal‐scale ductile‐then‐brittle detachment faults. The uppermost detachment fault separates sub‐greenschist facies phyllite and marble of a Pelagonian Zone hanging wall, from greenschist facies metasedimentary schist, calc‐schist, and marble correlated to the Cycladic Blueschist Unit. A second, structurally lower detachment fault subdivides the metamorphic rocks of the Cycladic blueschist unit footwall into middle and lower units. There is a marked step in metamorphic grade between the sub‐greenschist facies uppermost package, and the middle‐to‐upper greenschist facies middle and lower packages. A suite of new white mica40Ar/39Ar and zircon (U‐Th)/He dates indicates accommodation of deformation along these faults occurred from the late Oligocene to the late Miocene with both faults active during the middle Miocene. The structures have clear top‐S/SSW kinematics determined from flanking folds, sigmoids, shear bands, stair‐stepping of strain shadows on porphyroclasts, and SCC' fabrics. The ductile‐to‐brittle deformation of the structures, morphology of the massif, and the increase in metamorphic grade suggest these low‐angle structures are part of a major, crustal‐scale extensional complex, located at the northwest end of the West Cycladic Detachment System, that accommodated Miocene bivergent exhumation of Attic‐Cycladic metamorphic core complexes in the central Aegean. Taken together, the above data suggest that multiple coeval detachment branches may form in areas with high strain gradients to accommodate the mechanically necessary termination of Cycladic‐style detachment systems.

The Cycladic Blueschist Unit on Tinos, Greece: Cold NE Subduction and SW Directed Extrusion of the Cycladic Continental Margin Under the Tsiknias Ophiolite

AbstractHigh pressure‐low temperature (HP‐LT) metamorphic rocks structurally beneath the Tsiknias Ophiolite make up the interior of Tinos Island, Greece, but their relationship with the overlying ophiolite is poorly understood. Here, new field observations are integrated with petrological modeling of eclogite and blueschists to provide new insight into their tectonothermal evolution. Pseudomorphed lawsonite‐, garnet‐, and glaucophane‐bearing schists exposed at the highest structural levels of Tinos (Kionnia and Pyrgos Subunits) reached ~22–26 kbar and 490–520°C under water‐saturated conditions, whereas pseudomorphed lawsonite‐ and aegirine‐omphacite bearing eclogite reached ~20–23 kbar and 530–570°C. These rocks are separated from rocks at deeper structural levels (Sostis Subunit) by a top‐to‐SW thrust. The Sostis Subunit records P‐T conditions of ~18.5 kbar and 480–510°C and is overprinted by pervasive top‐to‐NE shearing that developed during exhumation from (M1) blueschist to (M2) greenschist facies conditions of ~7.3 ± 0.7 kbar and 536 ± 16°C. These P‐T‐D relationships suggest that the Cycladic Blueschist Unit represents a discrete series of tectonometamorphic subunits that each experienced different tectonic and thermal histories. These subunits were buried to variable depths and sequentially extruded toward the SW from a NE dipping subduction zone. The difference in age and P‐T conditions between the HP‐LT rocks and the overlying metamorphic sole of the Tsiknias Ophiolite suggests that this NE dipping subduction zone was active between circa 74 and 46 Ma and cooled at a minimum rate of ~1.2–1.5°C/km/Myr prior to continent‐continent collision between Eurasia and Adria/Cyclades.

The Kallianos Au-Ag-Te mineralization, Evia Island, Greece: a detachment-related distal hydrothermal deposit of the Attico-Cycladic Metallogenetic Massif

The Kallianos Au-Ag-Te deposit in Evia Island (Greece) comprises sixteen syntaxial veins that intersect the schists and marbles of the Cycladic Blueschist Unit. The veins are filled with quartz-I and quartz-II, contain two hypogene ore stages (stages I and II), and are surrounded by infrequent muscovite-chlorite alteration zones. A magmatic origin is deduced for the Kallianos Au-Ag-Te mineralization, based on stable (S, C, Si, O, and H) and radiogenic (Pb, Sr, and Rb) isotopes, and fluid chemistry. The veins were deposited under pressures of ~ 220 to ~ 145 bars and temperatures of ~ 260 to 120 °C, from almost neutral H2Te-bearing hydrothermal fluids with a salinity of 2.2 to 15.8 wt.% NaCl equivalent. The telluride mineralization related to stage II was deposited at ~ 180 °C and evolved from Au- through Bi- to Ag-bearing tellurides. The vein minerals precipitated by simple cooling of almost non-reactive fluids that retained their initial magmatic signature. Taking into account the magmatic origin of the Kallianos deposit, we examine two proximal areas with granodioritic and leucogranitic intrusions similar to those associated with base and precious metal mineralization at Lavrion (~ 8.3 to ~ 7.1 Ma) and Tinos Island (~ 15 to ~ 13 Ma), as potential sources for the Kallianos fluids. Rb-Sr geochronology dating of the distal Kallianos Au-Ag-Te veins gave an age range of 8.1 to 7.6 Ma, implying that an overlapping and channelized hydrothermal system was active at the northern part of the Attico-Cycladic Metallogenetic Massif, utilizing the CBU detachment zones as fluid pathways.

P–T–tevolution of the Cycladic Blueschist Unit in Western Anatolia/Turkey: Geodynamic implications for the Aegean region

AbstractEclogite and blueschist facies rocks occurring as a tectonic unit between the underlying Menderes Massif (MM) and the overlying Afyon Zone/Lycian Nappes and the Bornova Flysch Zone in western Anatolia represent the eastward continuation of the Cycladic Blueschist Unit (CBU) in Turkey. This high‐Punit is attributed to the closure of the Pindos Ocean and consists of (a) a Triassic to Upper Cretaceous coherent series derived from passive continental margin sediments and (b) the tectonically overlying Upper Cretaceous Selçuk mélange with eclogite blocks embedded in a pelitic epidote‐blueschist matrix. The coherent series has experienced epidote‐blueschist facies metamorphism (490 ± 25°C/11.5 ± 1.5 kbar; 38 km depth).40Ar/39Ar white mica and206Pb/238U monazite dating of quartz metaconglomerate from coherent series yielded middle Eocene ages of 44 ± 0.3 and 40.1 ± 3.1 Ma for epidote‐blueschist facies metamorphism, respectively. The epidote‐blueschist facies metamorphism of the matrix of the Selçuk mélange culminates at 520 ± 15°C/13 ± 1.5 kbar, 43 km depth, and is dated at 57.5 ± 0.3–54.5 ± 0.1 Ma (40Ar/39Ar phengite). Eclogite facies metamorphism of the blocks (570 ± 30°C/18 ± 2 kbar, 60 km depth) is early Eocene and dated at 56.2 ± 1.5 Ma by206Pb/238U zircon. Eclogites experienced a nearly isothermal retrogression (490 ± 40°C/~6 to 7 kbar) during their incorporation into the Selçuk mélange. The retrograde overprints of the coherent series (410 ± 15°C/7 ± 1.5 kbar from Dilek Peninsula and 485 ± 33°C/~6 to 7 kbar from Selçuk–Tire area) and the Selçuk mélange (510 ± 15°C/6 ± 1 kbar) are dated at 35.8 ± 0.5–34.3 ± 0.1 Ma by40Ar/39Ar white mica and 31.6 ± 6.6 Ma by206Pb/238U allanite dating methods, respectively. Regional geological constrains reveal that the contact between the MM and the CBU originally formed a lithosphere‐scale transform fault zone.40Ar/39Ar white mica age from the contact indicates that the CBU and the MM were tectonically juxtaposed under greenschist facies conditions during late Eocene, 35.1 ± 0.3 Ma.

Forced Return Flow Deep in the Subduction Channel, Syros, Greece

AbstractWe present the results of a detailed structural study in the Cycladic Blueschist Unit at Fabrika on Syros Island, Greece, and discuss their significance for tectonic processes at the subduction interface. Some samples record top‐to‐the‐west shear reflecting prograde (burial), peak high‐pressure (HP) and initial decompression (exhumation) conditions. Other nearby samples record top‐to‐the‐east shear during HP metamorphism and exhumation. Some rocks re‐equilibrated at greenschist‐facies conditions and record top‐to‐the‐west shear. Greenschist‐facies top‐to‐the‐west shear is also found at the base of non‐HP upper units above the Fabrika HP sequence. We interpret the HP structures to reflect forced return flow and incipient formation of an extrusion wedge in the subduction channel. The HP top‐to‐the‐west structures resulted from thrusting along the base of the wedge and started to form during burial before the rocks reached their deepest point. The HP top‐to‐the‐east structures reflect deformation near the top of the developing extrusion wedge. After considerable exhumation during ongoing subduction, out‐of‐sequence, top‐to‐the‐west thrusts emplaced the non‐HP upper units above the exhuming extrusion wedge ~10 Myr after the wedge initially formed. Our work suggests that the HP rocks were considerably exhumed during sustained lithospheric shortening in the subduction channel by forced return flow. Because return flow is controlled by the velocity of the subducting slab, it may explain why HP rocks can be exhumed at subduction rates. On the regional scale we find that four distinct HP belts were sequentially accreted and exhumed between ~50 and 20 Ma suggesting continuous subduction‐channel return flow in the Hellenic subduction zone.

Zircon U‐Pb Chronostratigraphy and Provenance of the Cycladic Blueschist Unit and the Nature of the Contact With the Cycladic Basement on Sikinos and Ios Islands, Greece

AbstractSikinos and Ios Islands, located in the Southern Cyclades, represent part of a Cenozoic metamorphic core complex system that exposes subduction‐related metamorphic rocks in the highly extended back‐arc region of the Hellenic subduction zone. These exhumed HP‐LT metamorphic units are composed of Mesozoic metasedimentary rocks of the Cycladic Blueschist Unit (CBU) and the Paleozoic Cycladic Basement (CB). The magmatic and stratigraphic evolution of these units, as well as the nature of the contact between the CBU and CB, have remained poorly understood. We used zircon U‐Pb dating to determine crystallization ages of the CB on Sikinos and the maximum deposition ages and detrital provenance of the metasedimentary units to reconstruct the Mesozoic to early Cenozoic stratigraphic and tectonic evolution of the CBU on both islands. The results reveal that the CB in Sikinos is composed of Cambrian‐Silurian metasedimentary rocks intruded by Carboniferous granites and is overlain by metasedimentary rocks of the CBU with depositional ages spanning from Permo‐Triassic to Late Cretaceous. The provenance data from the CBU records a long‐lived tectonic evolution from Paleo‐Tethys subduction and rifting, to passive margin formation, and to subduction of the Neo‐Tethyan Pindos basin. The continuous stratigraphic record and provenance evolution from the CB into the CBU imply a para‐autochthonous relationship. On NE Sikinos and Ios, stratigraphic constraints suggest older‐over‐younger relationships along cryptic‐thrusts, supporting premetamorphic or synmetamorphic structural repetition of the CBU by imbrication, likely during subduction underplating.

Tectono‐magmatic and Stratigraphic Evolution of the Cycladic Basement, Ios Island, Greece

AbstractThe Cycladic Basement (CB) and the overlying Cycladic Blueschist Unit (CBU) are part of the Paleogene Cycladic subduction complex exposed in Miocene metamorphic core complexes in the distended back‐arc of the retreating Hellenic subduction zone of the southern Aegean. While the Cenozoic tectono‐metamorphic evolutions of the CB and the CBU have been the foci of numerous studies, this study presents new laser ablation inductively coupled plasma mass spectrometry bedrock and detrital zircon (DZ) U‐Pb ages that place robust constraints on the presubduction tectonic, magmatic, and paleogeographic evolution of the CB. Zircon U‐Pb ages of crystalline CB are ~306‐330 Ma, demonstrating local plutonism associated with regional voluminous, protracted Carboniferous magmatism related to Paleo‐Tethys subduction. The plutons intruded the CB metasedimentary host‐rock sequence, characterized by distinct Gondwanan DZ U‐Pb provenance, Neoproterozoic to early Paleozoic maximum depositional ages, and synmagmatic, contact metamorphic zircon rims (~300‐330 Ma). DZ U‐Pb dating revealed postmagmatic Permian metasedimentary rocks (~270‐295 Ma) that unconformably overlie the CB and have unimodal DZ spectra that indicate exhumation of the CB prior to Permian deposition within extensional basins, as well as mark the onset of CBU deposition prior to formation of the Pindos rift domain. These U‐Pb results clarify the late Paleozoic‐early Mesozoic evolution of the CB as a peri‐Gondwanan terrane composed of Neoproterozoic and early Paleozoic metasedimentary rocks, intruded by voluminous Carboniferous arc magmatism, and exhumed in the Permian, prior to Triassic rifting and CBU deposition. Additionally, these data provide a chronostratigraphic framework and illuminate subduction‐related juxtaposition within the CB metasedimentary sequence.

Along‐strike consistency of an extensional detachment system, West Cyclades, Greece

AbstractThe Lavrion district of the Attica Peninsula, Greece, exposes the West Cycladic Detachment System (WCDS), a low‐angle crustal‐scale extensional fault system separating tectonostratigraphic units of the Cycladic Blueschist Unit. New multiple single‐grain fusion and step‐heated white mica 40Ar/39Ar ages integrated with existing (U–Th)/He ages and independent paleothermometry resolves a syn‐ to post‐orogenic deformation history. A structurally higher unit records Oligocene greenschist facies deformation that evolved into brittle conditions by the middle Miocene, and shares a similar history to Serifos at the southern end of the detachment system. The structurally lowest unit remained ductile until the late Miocene, preserving pervasive post‐orogenic structures, similar to along‐strike structures at the centre, deepest part of the fault. The similarities of structural styles and the timing of deformation across > 150 km of strike length of the detachment system indicates tens of kilometers of offset and extraordinary potential for correlating observations along Cycladic‐style detachment systems.

Deformation within the Cycladic subduction–exhumation channel: new insights from the enigmatic Makrotantalo nappe (Andros, Aegean)

New detailed geological/structural mapping coupled with microstructural investigations and amphibole chemistry analyses are used to elucidate the enigmatic tectonic evolution of the Makrotantalo nappe on northern Andros (Aegean region). On the basis of the new geological map, we re-evaluate the structural position of samples with previously published metamorphic ages to show that the Makrotantalo nappe has been affected by the early Eocene high-pressure (HP) metamorphism. Therefore, the Makrotantalo nappe is an integral part of the Cycladic Blueschist Unit (CBU). The early, main ductile deformation phase recognized in the mapped area, is associated with the emplacement of the Makrotantalo nappe over the North Cyclades nappe at deep subduction levels during early Eocene. The subsequent deformation phase occurred at the middle Eocene–Oligocene synchronous with the decompression of blueschist rocks from the stability field of glaucophane to that of actinolite. During this phase, the nappe pile was folded by map-scale folds, which were locally transposed by ductile shear zones. These shear zones were related with NE-directed oblique thrusting and, locally, either restacked the nappe pile or reworked the early nappe contact. Our results in combination with previous studies in the northern Cyclades show that the top-to-the-NE shearing was dominant at a large part of the CBU during exhumation. Opposite, SW-directed shearing was restricted at the upper structural levels of the CBU. We suggest that the observed spatial variation in shear sense is compatible with NE- rather than SW-directed ductile extrusion of the CBU.

Exhumation of the high-pressure northwestern Cyclades, Aegean: New P-T constraints, and geodynamic evolution

The Cycladic Blueschist Unit in the southern Aegean of Greece comprises Lower and Upper Tectonic Units (LTU, UTU), both of which were affected by high-pressure, low-temperature metamorphism. However, in the Penteli area the LTU contains predominant greenschists with only traces of blueschist facies minerals (glaucophane- and phengite-bearing). In contrast, the UTU contains well-preserved blueschists (glaucophane- and lawsonite-bearing). Our new integrated thermodynamic data indicate that prograde metamorphism was different in the two units, which were juxtaposed relatively late, at P < 0.6 GPa. New P-T data from the LTU show that peak P (~1.3–1.2 GPa) was reached at ~300 °C, followed by exhumation, first with a significant temperature increase to ~480 °C at pressures of ~0.7–0.8 GPa, and subsequent drastic cooling. For the UTU, the maximum P was at ~0.9 GPa at a T of ~310–330 °C, followed by gentle cooling during exhumation.

Geodynamic significance of the Santorini Detachment System (Cyclades, Greece)

AbstractWe present field and geochronologic evidence for a Miocene top‐to‐SSE detachment system on Santorini, which exhumed the Cycladic Blueschist Unit (CBU) below a package of low‐grade Upper Triassic marbles that contain well‐preserved Megalodon fossils forming a syncline with metaflysch in the core. White mica bundles from the mylonitic foliation in the CBU yield 40Ar/39Ar dates of 25–19 Ma and zircon (U–Th)/He dates are 11–8 Ma. A weakly foliated granitic intrusion that cross‐cuts the CBU is 8.5 Ma based on zircon U–Pb geochronology. Detrital white mica from the metaflysch yields Jurassic to early Palaeogene single grain 40Ar/39Ar dates, with a dominant Palaeocene signature. Zircon (U–Th)/He dating similarly reveals dispersed ages between 36 and 15 Ma suggesting Miocene metamorphic temperatures were insufficient (&lt;200°C) to completely reset all of the zircon cooling ages. The low‐grade rocks of the hangingwall above the newly discovered Miocene low‐angle Santorini Detachment System most likely belong to the Pelagonian zone with a Triassic carbonate platform discordantly transgressed by an Eocene flysch deposit.

Exhumation of eclogite and blueschist (Cyclades, Greece): Pressure–temperature evolution determined by thermobarometry and garnet equilibrium modelling

AbstractHigh‐P rocks such as eclogite and blueschist are metamorphic markers of palaeo‐subduction zones, and their formation at high‐P and low‐T (HP–LT) conditions is relatively well understood since it has been the focus of numerous petrological investigations in the past 40 years. The tectonic mechanisms controlling their exhumation back to the surface are, however, diverse, complex and still actively debated. Although the Cycladic Blueschist Unit (CBU, Greece) is among the best worldwide examples for the preservation of eclogite and blueschist, the proposed P–T evolution followed by this unit within the Hellenic subduction zone is quite different from one study to another, hindering the comprehension of exhumation processes. In this study, we present an extensive petrological data set that permits refinement of the shape of the P–T trajectory for different subunits of the CBU on Syros. High‐resolution quantitative compositional mapping has been applied to support the thermobarometric investigations, which involve semi‐empirical thermobarometry, garnet equilibrium modelling and P–T isochemical phase diagrams. The thermodynamic models highlight the powerful use of reactive bulk compositions approximated from local bulk compositions. The results are also combined with Raman spectrometry of carbonaceous material (RSCM) to retrieve the metamorphic peak temperature distribution at the scale of the island. A major result of this study is the good agreement between all the independent thermobarometric methods, permitting reconstruction of the prograde and retrograde P–T trajectories. Garnet compositional zoning was used to retrieve prograde, peak and retrograde growth stages in line with the results of the P–T isochemical phase diagrams, RSCM temperature and peak‐pressure crystallization of the garnet–omphacite–phengite assemblage. Our results are consistent with previous thermobarometric estimates from other occurrences of CBU rocks (Tinos, Andros), suggesting a multistage exhumation process with (1) early syn‐orogenic exhumation within the subduction channel, (2) isobaric heating at mid‐crustal depths (~10–12 kbar) following thermal re‐equilibration of the lithosphere from a cold syn‐orogenic regime in the subduction zone to a warmer post‐orogenic regime in the back‐arc domain and (3) exhumation and cooling related to a post‐orogenic phase of extension following slab retreat. Expanding to the general aspects of subduction zones, we suggest that such metamorphic evolution of HP–LT units should be regarded as a characteristic feature of exhumation driven by slab rollback.

Understanding phengite argon closure using single grain fusion age distributions in the Cycladic Blueschist Unit on Syros, Greece

The preservation of 40Ar/39Ar ages of high pressure (HP) metamorphic white mica reflects an interplay of processes that mobilise 40Ar, either through mica recrystallisation or by diffusive 40Ar loss. The applicability of resulting ages for dating tectonic processes is critically dependent on whether either of these processes can be proven to be efficient and exclusively active in removing 40Ar from mica. If not, preservation of an inherited or mixed age signal in a sample must be considered for interpretation. The Cycladic Blueschist Unit on Syros has become a new focal area in the discussion of the geological significance of argon age results from multi-grain step heating experiments. While some argue that age results can directly be linked to deformation or metamorphic growth events, others interpret age results to reflect the interplay of protracted recrystallisation and partial resetting, preserving a mixed age signal. Here, we demonstrate the potential of a new approach of multiple single grain fusion dating. Using the distribution of ages at the sample, section and regional scale, we show that in Northern Syros mica ages display systematic trends that can be understood as the result of three competing processes: 1) crystallisation along the prograde to peak metamorphic path, 2) a southward trend of increasing 40Ar loss by diffusion and 3) localised and rock type dependent deformation or metamorphic reactions leading to an observed age spread typically limited to ∼10 Myr at the section scale. None of the sections yielded the anomalously old age results that would be diagnostic for significant excess 40Ar. The recorded trends in ages for each of the studied sections reflect a range of P–T conditions and duration of metamorphism. Diffusion modelling shows that in a typical subduction metamorphic loop, subtle variations in P–T–t history can explain that age contrasts occur on a regional scale but are limited on the outcrop scale. Our new approach provides a comprehensive inventory of the range of ages present in different rocks and at different scales, which results in a more refined understanding of argon retention and isotopic closure of phengite and the geological significance of the ages. We verify the added value of our new approach by comparison with multi-grain step heating experiments on selected samples from the same sections.

Physicochemical controls on bismuth mineralization: An example from Moutoulas, Serifos Island, Cyclades, Greece

The 11.6 to 9.5 Ma Serifos pluton intruded schists and marbles of the Cycladic Blueschist unit, causing thermal metamorphism, the development of magnetite Ca-exo- and endo-skarns and the formation of low-temperature vein and carbonate-replacement ores. Potentially, the most important ores occur in the Moutoulas prospect where the mineralization in retrograde skarn and quartz veins culminated with the deposition of native bismuth. A combination of fluid inclusion microthermometry and isotope geothermometry suggests that the Moutoulas mineralization formed at a hydrostatic pressure of ~100 bars, from moderate-to-low temperature (~190–250 °C), and low-salinity (1.3–5.6 wt% NaCl equivalent) fluids. The calculated δ 34 S H 2 S compositions are consistent with the ore fluids having been derived from the Serifos pluton. Bismuth mineralization is interpreted to have occurred as a result of wall-rock interaction and mixing of a Bi-bearing ore fluid with meteoric waters. Native bismuth and bismuthinite deposited at ~200 °C, near neutral pH (6.5), low f S 2 ( f O 2 (

THE TECTONOSTRATIGRAPHY OF CYCLADIC BLUESCHIST UNIT ON SIKINOS AND SIFNOS: IMPLICATION FOR THE MESOZOIC TECTONIC SETTING

This study presents new results for the tectonostratigraphic configuration of the Cycladic Blueschist unit (CBU) in the islands of Sikinos and Sifnos. These results show that the observed tectonostratigraphy in CBU results from tectonic repetitions of a thinner sequence. Tectonic repetitions have been achieved by a series of largescale ductile thrusts that operated during burial and exhumation of CBU. Restoration of the tectonostratigraphy in both islands implies an original, pre-metamorphic sequence, which was made up by a volcano sedimentary complex at the base and an overlying carbonate-rich sedimentary succession. This Mesozoic protolithic sequence was possibly formed in a continental terrane or in a transitional domain between a continental terrane and an oceanic basin. We suggest that the Mesozoic protolith of the CBU was formed in an incipient oceanic basin rather than a mature ocean like Pindos Ocean. This incipient oceanic basin was developed either along south part or along the north margin of Pelagonian microcontinent.

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

AbstractGeothermobarometric 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°C and 15.5 ± 0.5 to 16.3 ± 0.9 kbar from a subsequent greenschist facies overprint at 384 ± 30°C and 3.8 ± 1.1 kbar. Multi‐mineral Rb–Sr isochron dating yields crystallization ages for near peak‐pressure blueschist facies assemblages between 40.5 ± 1.0 and 38.3 ± 0.5 Ma. 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. 32 Ma, with one sample showing rejuvenation at c. 27 Ma. 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 ± 4.6 km/Ma, completing a full blueschist‐/greenschist facies metamorphic cycle soon after subduction within c. 8 Ma. 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.