Tectonometamorphic Evolution of Cycladic Subduction Zone Rocks: The Syros Blueschist-Eclogite Terrane III (original) (raw)
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Metamorphic style and development of the blueschist- to eclogite-facies rocks, Cyclades, Greece
IOP Conference Series: Earth and Environmental Science, 2008
The island of Syros, Greece is part of the Attic-Cycladic blueschist belt, formed during Mesozoic Eurasia-Africa subduction. The rocks of Syros can be broadly divided into three tectono-stratigraphic units: (I) metamorphosed sedimentary and volcanic rocks (marble-schist sequence), (II) remnants of oceanic crust with fault-bounded packages of blueschist/eclogite-facies mafic rocks and serpentinite (mafic-ultramafic rocks) and (III) the Vari gneiss, which is a tectonic klippe. Low-temperature, high-pressure assemblages are found on several islands in the Cyclades. The best preserved of these rocks are on Syros and Sifnos islands. Mineral compositions and peak metamorphic assemblages are similar on both islands. Both islands are considered to share similar P-T histories with highest-pressure mineral assemblages reflecting conditions of at least 15 kbar and about 500°C.
Several distinct tectono-metamorphic slices in the Cycladic eclogite–blueschist belt, Greece
Contributions to Mineralogy and Petrology, 2005
Several relatively thin tectono-metamorphic slices have been recognized in the Cycladic eclogiteblueschist belt, through detailed studies on Ios, Sifnos, Syros, and Tinos. A sequence of distinct metamorphic mineral growth events has been documented. These recur in each tectonic slice, although individual slices are dominated by different events. To constrain the timing of these processes, the method of asymptotes and limits has been used to reanalyze published 40 Ar/ 39 Ar apparent age spectra. This reanalysis supports the concept that there were separate and quite distinct high-pressure metamorphic mineral growth events, and allows potential constraints as to the timing of some of these events to be developed. M 1B eclogite-facies metamorphism is estimated to have occurred at some time in the period 53-49 Ma, the M 1C blueschist-facies metamorphic event at some time in the period 44-38 Ma, and the M 1D transitional blueschist-facies metamorphic event is estimated to have occurred at some time in the period 35-30 Ma. A kinematic model is proposed to explain the geometry of a thinly sliced tectono-metamorphic stratigraphy, as observed, and the reason as to why individual tectonic slices in this 'tectono-metamorphic stratigraphy' should display distinctive patterns of fabrics and micro-structures, as well as characteristic temperature-time curves as inferred by 40 Ar/ 39 Ar geochronology.
Journal of Metamorphic Geology, 2002
Meta-peridotites outcropping at different structural levels within the Alpine metamorphic complex of the Cycladic island of Naxos were studied to reexamine their metamorphic evolution and possible tectonic mechanisms for emplacement of mantle material into the continental crust. The continental margin section exposed on Naxos, consisting of pre-Alpine basement and c. 7 km thick Mesozoic platform cover, has undergone intense metamorphism of Alpine age, comprising an Eocene (M1) blueschist event strongly overprinted by a Miocene Barrovian-type event (M2). Structural concordance with the country rocks and metasomatic zonation at the contact with the felsic host rocks indicate that the meta-peridotites have experienced the M2 metamorphism. This conclusion is supported by the similarity between metamorphic temperatures of the ultrabasic rocks and those of the host rocks. Maximum temperatures of 730-760°C were calculated for the upper-amphibolite facies meta-peridotites (Fo-En-Hbl-Chl-Spl), associated with sillimanite gneisses and migmatites. Relict phases in ultrabasics of different structural levels indicate two distinct pre-M2 histories: whereas the cover-associated horizons have been affected by lowgrade serpentinization prior to metamorphism, the basement-associated meta-peridotites show no signs of serpentinization and instead preserve some of their original mantle assemblage. The geochemical affinities of the two groups are also different. The basement-associated meta-peridotites retain their original composition indicating derivation by fractional partial melting of primitive lherzolite, whereas serpentinization has led to almost complete Ca-loss in the second group. The cover-associated ultrabasics are interpreted as remnants of an ophiolite sequence obducted on the adjacent continental shelf early in the Alpine orogenesis. In contrast, the basement-associated meta-peridotites were tectonically interleaved with the Naxos section at great depth during the Alpine collision and high P/T metamorphism. Their emplacement at the base of the orogenic wedge is inferred to have involved isobaric cooling from temperatures of c. 1050°C within the spinel lherzolite field to eclogite facies temperatures of c. 600°C.
Journal of Metamorphic Geology, 1988
U'Ar/"Ar age spectrum analysis of phengite separates from Naxos, part of the Attic Cycladic Metamorphic Belt in Greece, indicates that cooling following high-pressure, low-to medium-temperature metamorphism, M I , occurred about 50 Ma ago. Phengite has *"AT* gradients that suggest that part of the scatter observed in conventional K-Ar ages was caused by diffusion of radiogenic argon from the minerals during a younger metamorphism, M2. In central Naxos, this metamorphism (M 2) has overprinted the original mineral assemblages completely, and is associated with development of a thermal dome. Excellent "'Ar/'yAr plateaus at 15.0 2 0.1 Ma, 11.8-C 0.1 Ma, and 11.4 2 0.1 Ma, obtained on hornblende, muscovite and biotite, respectively, from the migmatite zone, indicate that relatively rapid cooling followed the M2 event, and that no significant thermal overprinting occurred subsequent to Mz. Toward lower M I metamorphic grade, Q'Ar/"9Ar plateau ages of hornblendes increase to 19.8 2 0.1 Ma; concomitantly the proportion of excess '"Ar in the spectra increases as well. We propose that the peak of M 2 metamorphism occurred beween 15.0 and 19.8 Ma ago. K-Ar ages of biotites from a granodiorite on the west coast are indistinguishable from those found in the metamorphic complex, and hornblende K-Ar ages from the same samples are in the range 12.1-13.6 Ma. As the latter ages are somewhat younger than most ages obtained from the metamorphic complex, intrusion of the granodiorite most likely followed the peak of the M 2 metamorphism. The metamorphic evolution of Naxos is consistent with rapid crustal thickening during the Cretaceous or early Tertiary, causing conditions at which supracrustal rocks experienced pressures in the range 900-1500 MPa. Transition to normal crustal thicknesses ended the 'Current address: Z. W.O. Laboratorium voor Isotopen Geologie, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands. M , metamorphism about 50 Ma ago. The M 2 metamorphism and granodiorite intrusion occurred during a period of heat input into the crust, possibly related to the migration of the Hellenic volcanic arc in a southerly direction through the area.
Geological Society, London, Special Publications, 2007
The Alpine orogen in the Cyclades, wherein both high-pressure metamorphic rocks and ultramafic rocks co-occur, is a key area in studying the emplacement of mantle rocks into the crust. Within the Cyclades three distinct ultramafic associations occur: (1) HP-LT ophiolitic mélanges of the Cycladic Blueschist Unit (CBU) on Evia and Syros; (2) meta-peridotites associated with migmatized leucogneisses on Naxos, which represent the deepest exposed levels of the CBU; (3) a greenschist-facies metamorphosed dismembered ophiolite juxtaposed on top of the CBU by an extensional detachment on Tinos. Most of the Cycladic ultramafic rocks were serpentinized prior to Alpine metamorphism, suggesting denudation prior to reburial. The Naxos metaperidotites preserve, however, relict mantle assemblage and mantle-like oxygen isotope ratios, and thus indicate direct emplacement from the mantle into an underthrust continent during collision and HP metamorphism (M 1). Thus conditions for M 1 in the Naxos leucogneiss core are constrained by ultramafic assemblages to 550-650 8C and 14 kbar. Mafic blocks of the ophiolitic mélanges in the NW Cyclades span a wide range of chemical compositions indicating derivation from variable oceanic settings and sequential events of alteration and metasomatism. Given the comparable geochemical heterogeneity in the Syros and Evian mélange intervals, the garnetbearing meta-basites of the Syros mélange record higher M 1 temperatures (450-500 8C) than the garnet-free epidote blueschists of the Evian mélanges (400-430 8C). It follows that going southeastwards from Evia progressively deeper (i.e. hotter) levels of the subducted plate are exposed. Correspondingly, temperatures of the M 2 overprint also increase from pumpellyitebearing assemblages on southern Evia, through greenschists on Syros to upper-amphibolite, sillimanite-bearing gneisses on Naxos. The diverse P-T paths of the CBU form an array wherein the deeper a rock sequence is buried, the 'hotter' is its exhumation path. Such a pattern is predicted by thermal modelling of tectonically thickened crust unroofed by either erosion or uniform extension.
The tectono-metamorphic evolution of a dismembered ophiolite (Tinos, Cyclades, Greece)
Geological Magazine, 1996
The six exposures of the Upper tectonic Unit of the Cycladic Massif occurring on the island of Tinos are shown to comprise a metamorphosed dismembered ophiolite complex. The common stratigraphic section consisting of tens-of-metres- thick tectonic slices of mafic phyllites overlain by serpentinites and gabbros is considered to have been derived by a combination of thrusting during obduction and subsequent attenuation by low-angle normal faults. All rock types show evidence of a phase of regional greenschist-facies metamorphism, which in the case of the phyllites is accompanied by penetrative deformation. The greenschist-facies metamorphism in gabbros is preceded by high temperature sea-floor amphibolite-facies alteration, whereas in the serpentinites, the antigorite + forsterite greenschist-facies assemblage overprinted an earlier low temperature lizardite serpentinite. Trace element patterns of the mafic phyllites and a harzburgitic origin of meta-serpentinites suggest a supra subd...