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  • 1. Gasser, D.
    et al.
    Jeřábek, P.
    Faber, P.
    Stünitz, H.
    Menegon, L.
    Corfu, F.
    Erambert, M.
    Whitehouse, Martin J.
    Naturhistoriska riksmuseet, Enheten för geovetenskap.
    Behaviour of geochronometers and timing of metamorphic reactions during deformation at lower crustal conditions: phase equilibrium modelling and U–Pb dating of zircon, monazite, rutile and titanite from the Kalak Nappe Complex, northern Norway.2015Ingår i: Journal of Metamorphic Geology, ISSN 0263-4929, E-ISSN 1525-1314, Vol. 33, s. 513-534Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    This study investigates the behaviour of the geochronometers zircon, monazite, rutile and titanite in polyphase lower crustal rocks of the Kalak Nappe Complex, northern Norway. A pressure–temperature–time–deformation path is constructed by combining microstructural observations with P–T conditions derived from phase equilibrium modelling and U–Pb dating. The following tectonometamorphic evolution is deduced: A subvertical S1 fabric formed at ~730–775 °C and ~6.3–9.8 kbar, above the wet solidus in the sillimanite and kyanite stability fields. The event is dated at 702 ± 5 Ma by high-U zircon in a leucosome. Monazite grains that grew in the S1 fabric show surprisingly little variation in chemical composition compared to a large spread in (concordant) U–Pb dates from c. 800 to 600 Ma. This age spread could either represent protracted growth of monazite during high-grade metamorphism, or represent partially reset ages due to high-T diffusion. Both cases imply that elevated temperatures of >600 °C persisted for over c. 200 Ma, indicating relatively static conditions at lower crustal levels for most of the Neoproterozoic. The S1 fabric was overprinted by a subhorizontal S2 fabric, which formed at ~600–660 °C and ~10–12 kbar. Rutile that originally grew during the S1-forming event lost its Zr-in-rutile and U–Pb signatures during the S2-forming event. It records Zr-in-rutile temperatures of 550–660 °C and Caledonian ages of 440–420 Ma. Titanite grew at the expense of rutile at slightly lower temperatures of ~550 °C during ongoing S2 deformation; U–Pb ages of c. 440–430 Ma date its crystallization, giving a minimum estimate for the age of Caledonian metamorphism and the duration of Caledonian shearing. This study shows that (i) monazite can have a large spread in U–Pb dates despite a homogeneous composition; (ii) rutile may lose its Zr-in-rutile and U–Pb signature during an amphibolite facies overprint; and (iii) titanite may record crystallization ages during retrograde shearing. Therefore, in order to correctly interpret U–Pb ages from different geochronometers in a polyphase deformation and reaction history, they are ideally combined with microstructural observations and phase equilibrium modelling to derive a complete P–T–t–d path.

  • 2. Kotková, J.
    et al.
    Whitehouse, Martin J.
    Naturhistoriska riksmuseet, Enheten för geovetenskap.
    Schaltegger, U.
    D'Azbac, F.-X.
    The fate of zircon during UHT–UHP metamorphism: isotopic (U/Pb, δ18O, Hf) and trace element constraints.2016Ingår i: Journal of Metamorphic Geology, ISSN 0263-4929, E-ISSN 1525-1314, Vol. 34, s. 719-739Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Garnet–clinopyroxene ultra-high-pressure (UHP) rocks from the northern Bohemian Massif contain zircon with micro-diamond inclusions. Trace element concentrations, oxygen and hafnium isotopic composition and U–Pb age of distinct textural domains in zircon characterize their growth conditions and temporal evolution. Diamond-bearing zircon mantle domains with relicts of oscillatory zoning have uniform Th/U ratios (~0.1–0.2), high-Ti contents (110–190 ppm, corresponding to temperatures of at least 1100 °C), and some (two of 17 mantle analyses) preserve steep heavy rare earth element (HREE) patterns with YbN/GdN = 10–11, with a weak negative Eu anomaly. These signatures are consistent with crystallization from a melt under UHP/ultra-high-temperature (UHT) conditions. Some of the bright-cathodoluminscence (CL) rims preserve Th/U and Ti values characteristic of the zircon mantles, but others show elevated Th/U ratios of ~0.3–0.4 and lower Ti contents (20–40 ppm; only 13 ppm in a rare low-CL outer rim). As they feature flat HREE patterns and negative Eu anomalies and commonly make embayments and truncate the mantle zoning, we suggest that they have formed through recrystallization in the solid state during exhumation of the rock, when both garnet and plagioclase were stable. The three zircon domains, that is, cores, mantles and rims, yield U–Pb concordia ages of 340.9 ± 1.5, 340.3 ± 1.5 and 341.2 ± 3.4 Ma respectively. When linked to the previously reconstructed P–T path of the rock, the error limits of the zircon mantle and rim ages constrain the exhumation of the rocks from depth of ~140 km (UHP) to ~80 km (HP) to a minimum rate of 1.5 cm yr−1. The zircon cores are heterogeneous in terms of Th/U ratio (below 0.1 but also above 0.2) and REE characteristics, and their εHf values scatter between −15.7 and +4.8 with similar values for individual domains within a single zircon grain suggesting a very localized control on hafnium isotope composition on a grain scale. The non-equilibrated εHf values as well as a large range of the Hf-depleted mantle model ages possibly reflect the presence of a heterogeneous population of old zircon. Consequently, the uniform and young 238U/206Pb ages may represent (near-)complete resetting of the U–Pb geochronometer during the UHP–UHT event at c. 340 Ma through dissolution–reprecipitation process. In contrast to Hf, the oxygen isotope composition of zircon is homogeneous, ranging between 7.8‰ and 9.6‰ VSMOW, reflecting a source containing upper crustal material and homogenization at UHP–UHT conditions. Our study documents that continental crust was subducted to mantle depths at c. 340 Ma during the Variscan orogeny and was subsequently very rapidly exhumed, implying that the sequence of events was faster than can be resolved by the secondary ion mass spectrometry technique.

  • 3. Majka, Jarosław
    et al.
    Mazur, Stanisław
    Młynarska, Maria
    Klonowska, Iwona
    Tual, Lorraine
    Naturhistoriska riksmuseet, Enheten för geovetenskap.
    Kośmińska, Karolina
    Tarasiuk, Jacek
    Wroński, Sebastian
    Integrating X‐ray mapping and microtomography of garnet with thermobarometry to define the P–T evolution of the (near) UHP Międzygórze eclogite, Sudetes, SW Poland2018Ingår i: Journal of Metamorphic Geology, ISSN 0263-4929, E-ISSN 1525-1314Artikel i tidskrift (Refereegranskat)
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