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Τετάρτη, 2 Ιανουαρίου 2019

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

  Fig.1 (a) Simplified geological map of Penteliarea.The numbers correspond to the estimated temperatures based on graphite thermometry.(b) Inset shows the location of Penteli,as part of the NW extension of the HP Cyclades belt. (c,d) Geological cross-sections showing the large-scale structure and lithological relations in LTU (adapted from Lozios, 1994)
Αρθρο για τη μεταμορφική εξέλιξη της περιοχής της Πεντέλης ως Δυτική επέκταση της ενότητας των Κυκλάδων. Χρησιμοποιώντας σύγχρονες θερμοβαρομετρικές μεθόδους σε ορυκτά, η ομάδα Ελλήνων επιστημόνων κατέδειξε ότι τα πετρώματα της εν λόγω περιοχής έφθασαν σε ένα βάθος ~40 χιλιομέτρων (στους περίπου 300 βαθμούς κελσίου) κι εν συνεχεία, ανέβηκαν σε μικρότερο βάθος (περίπου 25 χιλιόμετρα) θερμαινόμενα (μέχρι τους 480 βαθμούς κελσίου) πριν τελικώς ισορροπήσουν στα περίπου 13 χιλιόμετρα (σε θερμοκρασία μικρότερη των 400 βαθμών κελσίου).
Fig. 2. (a,b) Optical microscope and (c-f) scanning electron microscope images of the LTU schists. (a,b) Optical microscope and (c-f) scanning electron microscope images of the LTU metabasites. The main and oblique foliation is given as straight dashed white and black lines, respectively.

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 lawsonitebearing).
Fig.3  (a,b) Optical microscope and (c-f) scanning electron microscope images of the LTU schists. (a,b) Optical microscope and (c-f) scanning electron microscope images of the LTUmetabasites
Our new integrated thermodynamic data indicate that prograde metamorphism was different in the two units, which were juxtaposed relatively late, at P b 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.

Fig.4. Ternary plots for Ms-Cel-Prl showing the tschermakitic and pyrophylliticsubstitutions of LTU (a) and UTU (b) micas from Penteli. Mineral abbreviations:Ms.: muscovite, Cel: celadonite, and Prl: pyrophyllite

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[Επιμέλεια: Π. Τζεφέρης]