Island Of Sifnos Research Articles

Pressure–temperature evolution of blueschist facies rocks from Sifnos, Greece, and implications for the exhumation of high‐pressure rocks in the Central Aegean

AbstractThe blueschist and greenschist units on the island of Sifnos, Cyclades were affected by Eocene high‐pressure (HP) metamorphism. Using conventional geothermobarometry, the HP peak metamorphic stage was determined at 550–600 °C and 20 kbar, close to the blueschist and the eclogite facies transition. The retrograde P–T paths are inferred with phase diagrams. Pseudosections based on a quantitative petrogenetic grid in the model system Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O reveal coeval decompression and cooling for both the blueschist and the greenschist unit. The conditions of the metamorphic peak and those of the retrograde stages conform to a similar metamorphic gradient of 10–12 °C km−1 for both units. The retrograde overprint can be assigned to low‐pressure blueschist to HP greenschist facies conditions. This result cannot be reconciled with the (prograde) Barrovian‐type event, which affected parts of the Cyclades during the Oligocene to Miocene. Instead, the retrograde overprint is interpreted in terms of exhumation, directly after the HP stage, without a separate metamorphic event. Constraints on the exhumation mechanism are given by decompression‐cooling paths, which can be explained by exhumation in a fore‐arc setting during on‐going subduction and associated crustal shortening. Back‐arc extension is only responsible for the final stage of exhumation of the HP units.

The tectonic significance of a porphyroblastic blueschist facies overprint during Alpine orogenesis: Sifnos, Aegean Sea, Greece

The MacArgon program ∗ ∗ To obtain the most recent version of MacArgon go to WWW address: http://artemis.earth.monash.edu.au/MacArgon/. has been used to model published 40Ar 39Ar apparent age spectra for white micas from the island of Sifnos, Cyclades, Greece. These micas formed during a period of porphyroblastic mineral growth ( M 2) in the epidote-blueschist facies, at > ~ 460 °C and < ~ 14 kbar. M 2 marked the onset of a major period of deformation ( D 3) during which kilometre-scale ductile shear zones formed and widespread recumbent folding took place. The modelling experiments suggest that flat apparent age spectra observed in the eclogite-blueschist domain (EBD) can only be obtained if the rocks cooled rapidly below ~ 350 °C after M 2 at ~ 42 Ma. Cooling rates must be > 50 °C/m.y., and since M 2 is followed by D 3 we infer that these are the cooling rates during D 3. Modelling experiments for micas from the greenschist domain (GSD) structurally underneath the EBD suggest that the GSD cooled rapidly, but at ~ 32 Ma. One tectonic model that might explain the rapidity of cooling rates inferred for the EBD model supposes that ambient temperatures were at ~ 500 °C when M 2 took place, but thereafter the high pressure rocks were thrust to the north and rapidly cooled as the result of their juxtaposition against cooler, shallower levels of the crust.

Fluid and Geochemical Constraints on the Formation and Retrogression of High-Pressure Metamorphic Rocks from Sifnos (Greece)

The Alpine metamorphic evolution of the Cycladic Crystalline Complex of the Aegean aera of Greece is characterized by an Eocene eclogite-to-blueschist facies metamorphism and an Oligocene/ Miocene greenschist-to-amphibolite facies metamorphism. High pressure assemblages are well preserved in the north of Sifnos island (Blueschist Unit), whereas the rest of the island is dominated by greenschist-facies rocks (Greenschist Unit). However, rocks from Sifnos island preserve a complex record of both metamorphic events at all scales. In this paper we combine structural and petrological observations with major and trace geochemistry and stable-isotope data to characterize (a) the fluid involvement during highpressure and greenschist-facies metamorphism and (b) the nature of the retrogression from blueschist to greenschist facies rocks.

Tectonic juxtaposition of blueschists and greenschists in Sifnos Island (Aegean Sea)—implications for the structure of the Cycladic blueschist belt

In the Cyclades, kilometre-thick high-pressure rock sequences, displaying blueschist- and eclogite-facies mineral assemblages, overlie a rock sequence which was thoroughly overprinted in the greenschist and amphibolite facies during its exhumation. Both rock sequences, subducted and metamorphosed at high-pressures in Eocene times, have been considered by previous workers to have been exhumed as a coherent rock unit. In contrast, it is suggested here that the preserved high-pressure rock sequences were exhumed more rapidly and prior to the underlying greenschists. Significant metamorphic, structural and geochronological discontinuities exist across the blueschist-greenschist contact and field evidence suggests that the high-pressure metamorphic rocks in Sifnos are tectonically juxtaposed above the greenschists. These two rock sequences were juxtaposed by a low-angle fault subsequent to the Oligocene-Miocene greenschist-facies overprint. Published geochronological data and petrological criteria are used to show that the high-pressure sequence cooled below 350°C when the rocks now immediately underlying it suffered a greenschist-facies overprint at temperatures of ca 450°C. The section inferred to absorb this temperature difference is now missing and it is suggested that it has been cut out by the low-angle fault.

Cooling during the exhumation of a blueschist terrane: Sifnos (Cyclades), Greece

Alpine high-pressure metamorphic rocks exposed in the Cyclades were overpinted during their exhumation. We studied the metamorphic evolution of a pervasively overprinted rock sequence, the greenschist unit, exposed in the island of Sifnos. We show that the metamorphic evolution in the Greenschist unit is characterized by the progressive transformation of eclogite-facies rocks through albite-bearing epidote blueschists into greenschists. The evolution of the mineral assemblages during exhumation is shown by a series of chemographic diagrams which are referenced to specific P-T fields in the petrogenetic grid proposed by Evans (1990). P-T conditions corresponding to the major paragenetic groups are estimated also on the basis of mineral chemistry, Fe/Mg exchange thermometry and the use of GEOCALC-PTX and THERMOCALC softwares. A P-T path for the Greenschist unit is presented indicating eclogite-facies conditions of around 480-520 ° C and 12-15 kbar, followed by overprinting in the albite-bearing epidote-blueschist facies at P-T conditions of 8-10 kbar and ca. 480-500 ° C. Pressures of 5.5 kbar and temperatures around 450 ° C are estmated for the subsequent overprint in the greenschist facies. The decompression part of the P-T path is characterized by cooling with no indication of a temperature rise. Thus, the high-pressure metamorphic rocks of Sifnos appears not to follow the thermal evolution path predicted by models which assume whole crust uplift and erosion. On a regional scale the exhumation of the Cycladic blueschist belt is characterized by cooling in the west, but broad clockwise P-T loops with significant heating during decompression are found in the east. We note that cooling affected rocks situated at the deepest levels of the Alpine orogenic wedge (Sifnos, Syros, Tinos) whereas heating has affected rocks exhumed from shallower structural levels (Naxos, Ios)

Evolution of the blueschist and greenschist facies rocks of Sifnos, Cyclades, Greece

The metamorphism on the island of Sifnos is characterized by the Eocene development of a coherent highpressure blueschist terrane and an early Miocene greenschist facies overprint. This study documents the metamorphic evolution of the blueschist assemblages, still preserved in the northern parts of the island, and their subsequent transformation into greenschists in the central and southern parts. The oxygen isotope geothermometry is based on calibrations for quartz, pyroxenes and magnetite (Matthews et al. 1983a) augmented by revised calibrations for the minerals muscovite (Δ Qz−Mu=1.55×106 T −2), epidote (δ Qz−Ep= 1.56+1.92Δ ps)106 T −2), and rutile (Δ Qz−Ru=4.54×106 T −2). Oxygen isotope analyses of minerals from the Blueschist unit of northern Sifnos give consistent fractionations which are independent of rock type. An average temperature of 455° C was obtained, although the scatter in temperatures deduced from the various geothermometers suggests that equilibration occurs under slightly changing physicochemical conditions. Analyses of minerals and whole rocks shows that pervasive equilibration in the presence of a common metamorphic fluid has not occurred. The minerals and whole rocks of the greenschists of central Sifnos are systematically enriched in 18O relative to the blueschist assemblages. Chemical data indicate that the greenschist overprint was accompanied by a metasomatic enrichment of Ca2+ and CO2. The petrologic, isotopic and chemical evidence favour a metamorphism governed by the infiltration of 18O-CO2 enriched aqueous solutions. It is reasonable to assume that this is connected with the Miocene magmatic activity observed throughout the Cyclades. The marbles separating the Blueschist from the Greenschist unit probably acted as barriers to fluid infiltration into the blueschists and were responsible for their preservation. The pressure of the blueschist metamorphism is estimated at 14±2 kbar, corresponding to a depth of ca. 50 km. The structural style and stratigraphy of Sifnos are suggestive of the subduction of a continental margin sequence. It is clear that the considerable tectonic depression may be associated with continental collision and underthrusting.

Preliminary note on deerite from high-pressure metamorphic rocks of Sifnos, Greece

A new occurrence of deerite-bearing quartzites from blueschist metamorphic rocks of the Greek island of Sifnos is described, and analytical and X-ray data on deerite are presented.