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

Finite lattice distortion patterns in plastically deformed zircon grains

This study examines finite deformation patterns of zircon grains from high-temperature natural shear zones. Various zircon-bearing rocks were collected in the Western Tauern Window, eastern Alps, where they were deformed under amphibolite facies conditions, and in the Ivrea–Verbano Zone (IVZ), southern Alps, where deformation is related with granulite-facies metamorphism. Among the sampled rocks are granitic orthogneisses, metalamprophyres and paragneisses, all of which are strongly deformed. The investigated zircon grains ranging from 10 to 50 μm were studied in situ using a combination of scanning electron microscope (SEM) techniques, backscattered electron (BSE) imaging, forward-scattered electron (FSE) imaging, cathodoluminescence (CL) imaging, and crystallographic orientation mapping by electron backscatter diffraction (EBSD), as well as micro-Raman spectroscopy. Energy-dispersive X-ray spectrometry (EDS) was applied to host phases. Microstructural analysis of crystal-plastically deformed zircon grains was based on high-resolution EBSD maps. Three general types of finite lattice distortion patterns were detected: type (I) is defined by gradual bending of the zircon lattice with orientation changes of about 0.6–1.8° per micrometer without subgrain boundary formation. Cumulative grain-internal orientation variations range from 7 to 25° within single grains. Type (II) represents local gradual bending of the crystal lattice accompanied by the formation of subgrain boundaries that have concentric semicircular shapes in 2-D sections. Cumulative grain-internal orientation variations range from 15 to 40° within single grains. Type (III) is characterized by formation of subgrains separated by a well-defined subgrain boundary network, where subgrain boundaries show a characteristic angular closed contour. The cumulative orientation variation within a single grain ranges from 3 to 10°. Types (I) and (II) predominate in granulite facies rocks, whereas type (III) is restricted to the amphibolite facies rocks. The difference in distortion patterns is controlled by strain rate and by ratio between dislocation formation and dislocation motion rates, conditioned by the amount of differential stress. Investigated microstructures demonstrate that misorientation axes are usually parallel to the and crystallographic directions; dominant slips are {001}, {100} and {010}, whereas in some grains cross-slip takes place. This study demonstrates that activation of energetically preferable slip systems is facilitated if zircon grain is decoupled from the host matrix and/or hosted by a soft phase

Interpretation of zircon coronae textures from metapelitic granulites of the Ivrea–Verbano Zone, northern Italy: two-stage decomposition of Fe–Ti oxides

In this study, we report the occurrence of zircon coronae textures in metapelitic granulites of the Ivrea–Verbano Zone. Unusual zircon textures are spatially associated with Fe–Ti oxides and occur as (1) vermicular-shaped aggregates 50–200 µm long and 5–20 µm thick and as (2) zircon coronae and fine-grained chains, hundreds of micrometers long and ≤ 1 µm thick, spatially associated with the larger zircon grains. Formation of such textures is a result of zircon precipitation during cooling after peak metamorphic conditions, which involved: (1) decomposition of Zr-rich ilmenite to Zr-bearing rutile, and formation of the vermicular-shaped zircon during retrograde metamorphism and hydration; and (2) recrystallization of Zr-bearing rutile to Zr-depleted rutile intergrown with quartz, and precipitation of the submicron-thick zircon coronae during further exhumation and cooling. We also observed hat-shaped grains that are composed of preexisting zircon overgrown by zircon coronae during stage (2). Formation of vermicular zircon (1) preceded ductile and brittle deformation of the host rock, as vermicular zircon is found both plastically and cataclastically deformed. Formation of thin zircon coronae (2) was coeval with, or immediately after, brittle deformation as coronae are found to fill fractures in the host rock. The latter is evidence of local, fluid-aided mobility of Zr. This study demonstrates that metamorphic zircon can nucleate and grow as a result of hydration reactions and mineral breakdown during cooling after granulite-facies metamorphism. Zircon coronae textures indicate metamorphic reactions in the host rock and establish the direction of the reaction front

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

Mechanisms of strain accommodation in plastically-deformed zircon under simple shear deformation conditions during amphibolite-facies metamorphism

This study documents the strain accommodation mechanisms in zircon under amphibolite-facies metamorphic conditions in simple shear. Microstructural data from undeformed, fractured and crystal-plastically deformed zircon crystals are described in the context of the host shear zone, and evaluated in the light of zircon elastic anisotropy. Our work challenges the existing model of zircon evolution and shows previously undescribed rheological characteristics for this important accessory mineral. Crystal-plastically deformed zircon grains haveaxis oriented parallel to the foliation plane, with the majority of deformed grains havingaxis parallel to the lineation. Zircon accommodates strain by a network of stepped low-angle boundaries, formed by switching between tilt dislocations with the slip systems{010} and{110} and rotation axis [001], twist dislocations with the rotation axis [001], and tilt dislocations with the slip system{001} and rotation axis [010]. The slip system{110} is newly described for zircon. Most misorientation axes in plastically-deformed zircon grains are parallel to the XY plane of the sample and have [001] crystallographic direction. Such behaviour of strained zircon lattice is caused by elastic anisotropy that has a direct geometric control on the rheology, deformation mechanisms and dominant slip systems in zircon. Young's modulus and P wave velocity have highest values parallel to zircon [001] axis, indicating that zircon is elastically strong along this direction. Poisson ratio and Shear modulus demonstrate that zircon is also most resistant to shearing along [001]. Thus, [001] axis is the most common rotation axis in zircon. The described zircon behaviour is important to take into account during structural and geochronological investigations of (poly)metamorphic terrains. Geometry of dislocations in zircon may help reconstructing the geometry of the host shear zone(s), large-scale stresses in the crust, and, possibly, the timing of deformation, if the isotopic systems of deformed zircon were reset