Metamorphic style and development of the blueschist- to eclogite-facies rocks, Cyclades, Greece

Abstract

Abstract. 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. IntroductionConstraining the P, T and deformation histories of Syros and Sifnos is a work in progress with many contributors who have applied traditional empirical geothermobarometry, phase equilibria and the pseudosection approach using various mineral assemblages (e.g. [9], 10]). Differences among these studies, some major and some minor, can be partly attributed to variation in methods and data that were used, but also to the authors' interpretation of the timing of formation of mineral assemblages, state of compositional preservation of the minerals and coexisting equilibrium assemblage. In spite of the range of published P-T estimates, we believe that a few common and widespread mineral assemblages give good limits on the P-T path that was followed during the subduction and exhumation of the interlayered marbles and schists.For the marbles and schist on Syros, peak metamorphic conditions are limited to just under 16 kbar at about 480°C and just above 500°C at 15 kbar, which fits well with estimates (15-16 kbar and 500°C) most workers have taken as the peak of metamorphism on both Syros and Sifnos. However, some recent work suggests higher P-T conditions on both Sifnos and Syros. Maximum P-T conditions on Sifnos have been estimated at about 19 kbar and 580 °C ([1]), and [2] got similar results of about 19 kbar and 570 °C for Syros. For Syros, higher peak-metamorphic conditions for the mafic-ultramafic unit proposed by [1] are at about 19 kbar and 525°C, and [3], who applied the geothermobarometer of [11], obtained conditions as high as 19-24 kbar (± 2 kbar) at temperatures of 500-580°C (± 65°). On Syros these higher conditions are problematic, because they exceed P-T limits imposed by assemblages in the marbles. A possible scenario to reconcile these observations is that the mafic and ultramafic rocks, already metamorphosed at higher P-T eclogite-facies conditions, became juxtaposed with slices of the schist and marble units at about 15 kbar and 500°C, which are the peak metamorphic conditions seen in the schist and marble units. The fabrics of the main penetrative deformation would have formed at these condi-

Preservation of Blueschist-facies Minerals along a Shear Zone by Coupled Metasomatism and Fast-flowing CO2-bearing Fluids

“…Ring et al, 2010), occurred over a long period of time from the Oligocene into the Miocene (Altherr et al, 1982;Wijbrans et al, 1990;Bro« cker et al, 1993;Ring & Layer, 2003), probably at 0Á8^1Á0 GPa and 400^5008C (e.g. Schumacher et al, 2008a).…”

Η Τεκτονική Εξέλιξη Του Κεντρικού Τμήματος Της Αττικο-Κυκλαδικής Μάζας

“…Επίσης, οιNeubauer & Genser (2004), σε μια προκαταρκτική εργασία, κατέγραψαν ένα γραμμικό ιστολογικό στοιχείο με ΑΝΑ-ική διεύθυνση σε μαζώδεις εκλογίτες στο ακρωτήρι Ζιλομύτης (Εικ. 3.2) Schumacher et al 2008β)…”

“…άξονες των κρυστάλλων ασβεστίτη αναπτύσσονται σε μια γωνία περίπου 50°-60° ως Εικόνα 4.2. Απλοποιημένο P-T διάγραμμα που δείχνει τις συνθήκες κατά (α) την τεκτονική τοποθέτηση των μεταβασικών/μετα-ηφαιστειακών πετρωμάτων επάνω στα μάρμαρα/σχιστόλιθους και (β) την αντικατάστασης αραγωνίτη από ασβεστίτη (απόSchumacher et al 2008β). προς 4.2α).…”

“…Οι λεπτομέρειες δείχνουν τις μικροδομές χαλαζία και ασβεστίτη σε κάθε στάδιο. P-T διάγραμμα απόSchumacher et al 2008β. Δες κείμενο για λεπτομέρειες.…”

“…4.19 και 4.21β). Σύμφωνα με τουςSchumacher et al (2008β), η ανάπτυξη των 150 και 400°C. Η ανάδρομη αυτή φάση μεταμόρφωσης συνέβη κατά τη διάρκεια του εκταφιασμού των πετρωμάτων και έχει χρονολογηθεί στα30-21 Ma (π.χ.…”