High-P/T metamorphic belts were classified into types A and B according to their protoliths. The A-type (collision-type) blueschists possess passive-margin protoliths characterized by platform-type carbonates, bimodal volcanics, and peraluminous sediments. B-type (Cordilleran-type) blueschists consist of active continental-margin protoliths in an accretionary complex characterized by bedded chert, MORB and ocean-island basalts, reef limestones, and graywackes. The spatiotemporal distribution of blueschists and eclogites of the world was compiled; among 250 recognized high-P/T belts, about 20% belong to the A type and the rest to the B type. Most A-type zones lie in Europe and the Tethyan domain, include ultrahigh-pressure metamorphic terranes, and have metamorphic pressure up to 45 kbar. B-type zones occur mainly in the circum-Pacific orogenic belts and intracontinental orogens in Asia, and were recrystallized at P <12 kbar. Associated peridotites include garnet peridotite in the A type and strongly serpe...

Blueschists and Eclogites of the World and Their Exhumation

The Aegean region is a concentrate of the main geodynamic processes that shaped the Mediterranean region: oceanic and continental subduction, mountain building, high-pressure and low-temperature metamorphism, backarc extension, post-orogenic collapse, metamorphic core complexes, gneiss domes are the ingredients of a complex evolution that started at the end of the Cretaceous with the closure of the Tethyan ocean along the Vardar suture zone. Using available plate kinematic, geophysical, petrological and structural data, we present a synthetic tectonic map of the whole region encompassing the Balkans, Western Turkey, the Aegean Sea, the Hellenic Arc, the Mediterranean Ridge and continental Greece and we build a lithospheric-scale N-S cross-section from Crete to the Rhodope massif. We then describe the tectonic evolution of this cross-section with a series of reconstructions from ~70 Ma to the Present. We follow on the hypothesis that a single subduction has been active throughout most of the Mesozoic and the entire Cenozoic, and we show that the geological record is compatible with this hypothesis. The reconstructions show that continental subduction (Apulian and Pelagonian continental blocks) did not induce slab break-off in this case. Using this evolution, we discuss the mechanisms leading to the exhumation of metamorphic rocks and the subsequent formation of extensional metamorphic domes in the backarc region during slab retreat. The tectonic histories of the two regions showing large-scale extension, the Rhodope and the Cyclades are then compared. The respective contributions to slab retreat, post-orogenic extension and lower crust partial melting of changes in kinematic boundary conditions and in nature of subducting material, from continental to oceanic, are discussed.

/pdf/cenozoic-geodynamic-evolution-of-the-aegean-2w6i0upj7j.pdf

Cenozoic geodynamic evolution of the Aegean

http://perso.numericable.com/laetitia.le.pourhiet/publications_files/Jolivetetal2012.pdf

Aegean tectonics: Strain localisation, slab tearing and trench retreat

Zircons were studied from high-pressure/low-temperature metamorphosed meta-igneous lithologies from Syros. These rocks carry several zircon generations related to each other by dissolution---reprecipitation processes. One generation is pristine zircon that shows growth zoning, relatively elevated contents of trivalent cations and high Th/U ratios. The other end-member is a skeletal zircon generation with negligible trivalent cation contents and low Th/U ratios ( 0 1). Texturally between these two, there is a range of zircon crystals with complex inclusion populations of Y---HREE---Th phases and fluid inclusions, showing variable progress of replacement--recrystallization. Both end-members yield distinct sensitive high-resolution ion microprobe (SHRIMP) U---Pb ages. The pristine generation has an age of 80 2 1 6Ma from a metagabbro, and 76 4 2 1Ma from a meta-plagiogranite dyke. The skeletal, low-Th/U zircon generation yields an age of 52 4 0 8Ma. The older, Late Cretaceous, zircons are interpreted to date emplacement of the magmatic protoliths in a small segment of oceanic crust. The younger, Eocene, age, however, dates a zircon recrystallization event, which possibly coincides with high solubility and mobility of high field strength elements in a high-pressure aqueous fluid phase. Intergrowth relations between zircon and peak-metamorphic garnet, and excellent agreement of the U---Pb ages with white mica Ar---Ar ages for the same samples support the conclusion that Eocene is the true age of high-pressure metamorphism on Syros.

/pdf/zircons-from-syros-cyclades-greece-recrystallization-and-3xn3f4ac0s.pdf

Zircons from Syros, Cyclades, Greece—Recrystallization and Mobilization of Zircon During High-Pressure Metamorphism

Overprinting of white micas from high pressure, low to medium temperature (M
1) metamorphic assemblages in pelitic schists on Naxos during subsequent thermal dome (M
2) metamorphism ranges from minor in the southeast of the island to complete recrystallization in the amphibolite facies rocks near the migmatites in the centre of the dome. The original (M
1) minerals are phengites (Si4+=6.7–7.0) and the overprinting minerals are muscovites (Si4+=6.0–6.45). 40Ar/39Ar step heating analyses of white mica separates from rocks in the area where phengite and muscovite occur together yield complex age spectra, characterized by low apparent ages in the first and the last stages of gas release and high apparent ages in between. These upward-convex age spectra are shown to be caused by mixing of two generations of micas, each of which has a different age spectrum and argon release pattern. Seemingly good plateaus in some age spectra from white micas of the area must be interpreted as providing meaningless intermediate ages. Further, the upward-convex age spectra have been used to trace the isotopic signature of phengites toward increasing M
2 metamorphic grade, and suggest that as long as phengites can be observed in the rocks upward-convex age spectra occur. On Naxos, crystallization of muscovite at the expense of phengite appears to be the main mechanism of resetting argon isotopic ages in white micas. However, there is also good evidence for argon loss by volume diffusion from phengites. Simple diffusion calculations suggest that the M
2 metamorphism was caused by a shortlived heat source.

40 Ar/ 39 Ar dating of white micas from an Alpine high-pressure metamorphic belt on Naxos (Greece): the resetting of the argon isotopic system

Polymetamorphic rocks of Sifnos (Greece) have been investigated by Rb-Sr, K-Ar, and fission track methods. Critical mineral assemblages from the northern and southernmost parts of Sifnos include jadeite+quartz+3T phengite, and omphacite+garnet +3T phengite, whereas the central part is characterized by the assemblage albite+chlorite+epidote+2M
1 phengite. K-Ar and Rb-Sr dates on phengites (predominantly 3T) of the best preserved high P/itTmetamorphic rocks from northern Sifnos gave concordant ages around 42 m.y., indicating a Late Lutetian age for the high P/T metamorphism. Phengites (2M
1+3T) of less preserved high P/T assemblages yielded K-Ar dates between 48 and 41 m.y. but generally lower Rb-Sr dates. The higher K-Ar dates are interpreted as being elevated by excess argon. K-Ar and Rb-Sr ages on 2M
1 phengites from central Sifnos vary between 24 and 21 m.y. These ages date a second, greenschist-facies metamorphism which overprinted the earlier high-pressure metamorphic rocks.

Geochronology of high-pressure rocks on Sifnos (Cyclades, Greece)

Gesamtgesteins- und Mineraldatierungen des - Falkenberger Granits

Radiometric ages of pre-Mesozoic rocks from northern Victoria Land, Antarctica

Hornblende and biotite from an andesite sample from the top of a Lower Pliocene Marine Section on Aegina Island, Greece, and from a tuff sample from 7 m below the andesil:e yielded a discordant pattern of dates. However, it is suggested that the KI Ar date for the biotil:e in the andesite (3 .87 ± 0.05 m.y.) and the KI Ar date for the hornblende in the tuff (4.4 :t 0.2 m.y.) approximate the time of extrusion of the respective volcanics. Zusammenfassung. Radiometrische Datierung zweier Extrusiva eines unterpliozanen marinm Profils der Insel Kgina, Griechenland. Hornblende und Biotit aus dem Andesit, der das unter­ pliozane Profil der Insel Kgina, Griechenland, iiberlagert sowie aus einem Tuff des Profils, 7:m unter dem Andesit entnommen, ergaben diskordante Rb/Srund KI Ar-Modellalter. Es wird jedoch angenommen, daB das Extrusionsalter des Andesites durch das K/Ar-Biotitalter des And,�­ sites (3 .87 ± 0,05 Ma) und das Extrusionsalter des Tuffes durch das KI Ar-Hornblendealter des Tuffes (4,4 ± 0,2 Ma) angenahert werden. Description of the section A marine sequence of non-bedded, silty clays, marls, and fine-bedded diatomite out­ crops on the southern slope of Agios Thomas Hill on Aegina Island (30 km SW of Athens). It is classified as Lower Pliocene according to planktonic foraminifera of the Globorotalia puncticulata zone and nannoplankton of the Reticulofenestra pseudoum­ bilica and Ceratolithus acutus (amplificus) zones. A detailed description of the litholog;y and of the fossil content is given by BENDA et aL in this volume, who also provided the samples for radiometric dating. One sample was taken from the andesite at the top of the section, the other from the tuff which is intercalated with fossiliferous sediments 7 m below the andesite. A description of the samples is given in the appendix. \". Authors addresses: Dr. P. MUL ER, H. KREUZER, H. LENZ, and Dr. W. HARRE, Bundesanstalt fiir Geowissenschaften und Rohstoffe, Postfach 510153 , D-3000 Hannover 51. 0078-0421/79/0008-0070 $ 2.25 © 1979 Gebriider Borntraeger, D-l000 Berlin ' D·7000 Stuttgart (c) 2015 www.schweizerbart.com DOI:10.1127/nos/8/1979/70

Heinz Lenz

Papers

Geochronology of high-pressure rocks on Sifnos (Cyclades, Greece)

Gesamtgesteins- und Mineraldatierungen des - Falkenberger Granits

Radiometric ages of pre-Mesozoic rocks from northern Victoria Land, Antarctica

Radiometric dating of two extrusives from a Lower Pliocene Marine Section on Aegina Island, Greece