Zircon ages in granulite facies rocks: decoupling from geochemistry above 850 °C?

Granulite facies rocks frequently show a large spread in their zircon ages, the interpretation of which raises questions: Has the isotopic system been disturbed? By what process(es) and conditions did the alteration occur? Can the dates be regarded as real ages, reflecting several growth episodes? Furthermore, under some circumstances of (ultra-)high-temperature metamorphism, decoupling of zircon U–Pb dates from their trace element geochemistry has been reported. Understanding these processes is crucial to help interpret such dates in the context of the P–T history. Our study presents evidence for decoupling in zircon from the highest grade metapelites (> 850 °C) taken along a continuous high-temperature metamorphic field gradient in the Ivrea Zone (NW Italy). These rocks represent a well-characterised segment of Permian lower continental crust with a protracted high-temperature history. Cathodoluminescence images reveal that zircons in the mid-amphibolite facies preserve mainly detrital cores with narrow overgrowths. In the upper amphibolite and granulite facies, preserved detrital cores decrease and metamorphic zircon increases in quantity. Across all samples we document a sequence of four rim generations based on textures. U–Pb dates, Th/U ratios and Ti-in-zircon concentrations show an essentially continuous evolution with increasing metamorphic grade, except in the samples from the granulite facies, which display significant scatter in age and chemistry. We associate the observed decoupling of zircon systematics in high-grade non-metamict zircon with disturbance processes related to differences in behaviour of non-formula elements (i.e. Pb, Th, U, Ti) at high-temperature conditions, notably differences in compatibility within the crystal structure

Permian high-temperature metamorphism in the Western Alps (NW Italy)

During the late Palaeozoic, lithospheric thinning in part of the Alpine realm caused high-temperature low-to-medium pressure metamorphism and partial melting in the lower crust. Permian metamorphism and magmatism has extensively been recorded and dated in the Central, Eastern, and Southern Alps. However, Permian metamorphic ages in the Western Alps so far are constrained by very few and sparsely distributed data. The present study fills this gap. We present U/Pb ages of metamorphic zircon from several Adria-derived continental units now situated in the Western Alps, defining a range between 286 and 266 Ma. Trace element thermometry yields temperatures of 580-890°C from Ti-in-zircon and 630-850°C from Zr-in-rutile for Permian metamorphic rims. These temperature estimates, together with preserved mineral assemblages (garnet-prismatic sillimanite-biotite-plagioclase-quartz-K-feldspar-rutile), define pervasive upper-amphibolite to granulite facies conditions for Permian metamorphism. U/Pb ages from this study are similar to Permian ages reported for the Ivrea Zone in the Southern Alps and Austroalpine units in the Central and Eastern Alps. Regional comparison across the former Adriatic and European margin reveals a complex pattern of ages reported from late Palaeozoic magmatic and metamorphic rocks (and relics thereof): two late Variscan age groups (~330 and ~300 Ma) are followed seamlessly by a broad range of Permian ages (300-250 Ma). The former are associated with late-orogenic collapse; in samples from this study these are weakly represented. Clearly, dominant is the Permian group, which is related to crustal thinning, hinting to a possible initiation of continental rifting along a passive margin

U-Pb and Sm-Nd geochronology of the KIzIldag (Hatay, Turkey) ophiolite: Implications for the timing and duration of suprasubduction zone type oceanic crust formation in the southern Neotethys

The KIzIldag (Hatay) ophiolite in Turkey represents remnants of the southern Neotethyan ocean and is characterized by a complete ocean lithospheric section. It formed in a fore-arc setting above a N-dipping intraoceanic subduction zone, and represents the undeformed, more northerly part of the same thrust sheet that also forms the Baer-Bassit ophiolite to the south. The ophiolite was emplaced southwards from the southerly Neotethyan ocean in Maastrichtian time. U-Pb and Sm-Nd dates are used to constrain the crystallization age and duration of magmatic activity of the KIzIldag ophiolite. U-Pb dating yielded ages of 91.7 ± 1.9 Ma for a plagiogranite and 91.6 ± 3.8 Ma for a cumulate gabbro. The cumulate gabbro also yielded a Sm-Nd isochron age of 95.3 ± 6.9 Ma. The measured ages suggest that the oceanic crust of the KIzIldag ophiolite formed in a maximum time period of 6 Ma, and that the plagiogranite may have formed later than the gabbroic section. The U-Pb zircon ages from the KIzIldag ophiolite and the cooling age of a metamorphic sole beneath the Baer-Bassit ophiolite are indistinguishable within the analytical uncertainties. This indicates the presence of young and hot oceanic lithosphere at the time of intraoceanic subduction/thrusting in the southern Neotethys. The U-Pb zircon ages from the KIzIldag, the Troodos and the Semail ophiolites overlap within analytical uncertainties, suggesting that these ophiolites are contemporaneous and genetically and tectonically related within the same Late Cretaceous southern Neotethyan ocean. ©2012 Cambridge University Press

Age and duration of intra-oceanic arc volcanism built on a suprasubduction zone type oceanic crust in southern Neotethys, SE Anatolia

The southeastern Anatolia comprises numbers of tectono-magmatic/ stratigraphic units such as the metamorphic massifs, the ophiolites, the volcanic arc units and the granitoid rocks. All of them play important role for the late Cretaceous evolution of the southern Neotethys. The spatial and temporal relations of these units suggest the progressive development of coeval magmatism and thrusting during the late Cretaceous northward subduction/accretion. Our new U-Pb zircon data from the rhyolitic rocks of the wide-spread volcanic arc unit show ages of (83.1 ± 2.2)-(74.6 ± 4.4) Ma. Comparison of the ophiolites, the volcanic arc units and the granitoids suggest following late Cretaceous geological evolution. The ophiolites formed in a suprasubduction zone (SSZ) setting as a result of northward intra-oceanic subduction. A wide-spread island-arc tholeiitic volcanic unit developed on the top of the SSZ-type crust during 83-75 Ma. Related to regional plate convergence, northward under-thrusting of SSZ-type ophiolites and volcanic arc units was initiated beneath the Tauride platform (Malatya-Keban) and followed by the intrusion of I-type calc-alkaline volcanic arc granitoids during 84-82 Ma. New U-Pb ages from the arc-related volcanic-sedimentary unit and granitoids indicate that under-thrusting of ophiolites together with the arc-related units beneath the Malatya-Keban platform took place soon after the initiation of the volcanic arc on the top of the SSZ-type crust. Then the arc-related volcanic-sedimentary unit continued its development and lasted at ~75 Ma until the deposition of the late Campanian-Maastrichtian shallow marine limestone. The subduction trench eventually collided with the Bitlis-Pütürge massif giving rise to HP-LT metamorphism of the Bitlis massif. Although the development of the volcanic arc units and the granitoids were coeval at the initial stage of the subduction/accretion both tectono-magmatic units were genetically different from each other. © Elsevier B.V. All rights reserved.106Y231This work is a part of Ph.D study of Fatih Karaoğlan. Financial support from the TÜBİTAK (Project No. 106Y231 ) and Çukurova University Research Foundation (Project No. MMF2009D11 ) are gratefully acknowledged. The first author acknowledges International Research Fellowship Programme supported by TUBITAK–BİDEB. Ercan Aksoy and Yener Eyüboğlu are thanked for their valuable suggestions that improved the quality of the present paper

The temporal evolution of the active margin along the Southeast Anatolian Orogenic Belt (SE Turkey): Evidence from U-Pb, Ar-Ar and fission track chronology

The Southeast Anatolian Orogenic Belt (SAOB) resulted from the north-dipping subduction of the southern Neotethyan oceanic lithosphere in late Mesozoic and early Cenozoic. However, the timing and the rate of the continental collision are still under debate. Here, we present new U-Pb, Ar-Ar and Fission Track ages from the I-type calc-alkaline granitoids (Esence, Doğanşehir and Baskil) cutting the Göksun, Ispendere and Kömürhan ophiolites and also the Malatya-Keban metamorphics representing the Tauride active continental margin along the SAOB. In this study, high- to low-temperature thermochronological methods, applied to the granitoids, are used to understand the temporal and geodynamic evolution of the Tauride active continental margin and continental collision between Tauride and Arabian platforms in the frame of the Neotethyan convergence during Cretaceous and Miocene. The U-Pb zircon ages range from 83 to 88 Ma for the Baskil and from 81 to 83 Ma for the Esence granitoids. The 39Ar-40Ar ages indicate that these late Cretaceous granitoids cooled below 300 °C in 6-10 Ma. The formation ages and the timing of the gradual cooling of the late Cretaceous granitoids are similar to metamorphism age and the timing of the exhumation of the HP/LT Bitlis metamorphics during this stage. The combined field, geochemistry and geo-thermochronological data suggest that the first continental collision event occurred between Bitlis-Pütürge micro-continents to the south and Tauride platform to the north in an oblique subduction zone between 84 and 74 Ma. The granitoids continued uplifting during early-middle Eocene together with the exhumation of HP/UHT Berit metaophiolite in an extensional regime related to the opening of the Maden back-arc basin. This period also let the intrusion of the Doğanşehir arc magmatism, intruding the Pütürge and Malatya-Keban metamorphics, Berit metaophiolite, and Maden Complex. The Eocene Doğanşehir granitoid has similar U-Pb and 39Ar-40Ar ages, suggesting a fast cooling. The apatite fission track (AFT) ages for all granitoid bodies suggest that they were mainly cooled or uplifted in two episodes, where the first one is in the Eocene in an extensional setting. The AFT data marked the final continental collision between the Taurides and the Arabian platform first in Oligocene and the break-off of the subducted slab and the delamination process caused fast uplift of the Eastern Anatolia during middle to late Miocene. © 2016 International Association for Gondwana Research.106Y231This study is a part of the PhD thesis of the first author and was financially supported by TUBİTAK (project no: 106Y231 ). We are grateful to two anonymous reviewers for their invaluable suggestions that improved the quality of the paper. The editorial handling by Dr. Yener Eyüboğlu is greatly acknowledged. Appendix

U-Pb and Sm-Nd geochronology of the ophiolites from the SE Turkey: Implications for the Neotethyan evolution

The ophiolites in southeast Turkey crop out along two distinct belts. The ophiolites in the north are attached to Tauride active margin and represented by Göksun, Berit, İspendere, Kömürhan and Guleman ophiolites. Whereas the ophiolites in the south are observed as tectonically overlying the Arabian continental margin and characterized mainly by Kizildag (Hatay) and Koçali ophiolites. In this paper, new U-Pb and Sm-Nd isotopic ages are presented. The zircons extracted from the gabbroic cumulates of the Kömürhan ophiolite yielded a concordia age of 87.2 ± 3.1 Ma. The zircons in the gabbroic cumulates of the İspendere ophiolite yielded a Concordia age of 84.5 ± 3.9 Ma. Moreover, the Sm-Nd age of the gabbroic cumulates of the İspendere ophiolite yielded 85.1 ± 7.1 Ma (?Nd = + 7.8). The gabbroic rocks of the Kizildag (Hatay) ophiolite yielded 110 ± 11 Ma (?Nd = + 7.3) Sm-Nd isochron age. The new and already published U-Pb and Sm-Nd ages from the Kizildag ophiolite suggest that the time span between the melt generation in a subduction zone setting and SSZ-type oceanic crust crystallization was ?3 my. All the ages from the Southeast Anatolian ophiolites suggest that the ophiolites between the Bitlis-Pütürge continent and the Arabian platform formed around 99-102 Ma whereas the ophiolites between the Bitlis-Pütürge continent and the Tauride platform formed around 84-90 Ma, suggesting that the peri-Arabic belt ophiolites are 10 My older than the ophiolite attached to the Malatya-Keban platform in the north. Detailed comparison suggests that there are number of differences between the ophiolites to the north and south of the Bitlis-Pütürge continental unit based on the geological, geochronological, petrological, internal stratigraphy of the ophiolites as well as their relationships with the continental fragments during the late Cretaceous. Therefore, the ophiolites were rooted from two different oceanic basins, one to the north and other to the south of the Bitlis-Pütürge continent. © 2014 Taylor & Francis.Türkiye Bilimsel ve Teknolojik Araştirma Kurumu MMF2009D11 106Y231This work is a part of PhD study of Fatih Karaoğlan. Financial support from the TÜBİTAK (106Y231) and Çukurova University Research Foundation (MMF2009D11) is gratefully acknowledged. We would like to thank Monika Horschinik for performing Sm–Nd analyses. The first author acknowledges International Research Fellowship Programme support by TÜBITAK–BİDEB. Erdinç Yiğitbaş and Yann Rolland are acknowledged for their critical reviews that improved the quality of the paper. Erdin Bozkurt is thanked for editorial handling