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. (29th April 2015)

• Record Type: Journal Article
• Authors: Gasser, D.; Jeřábek, P.; Faber, C.; Stünitz, H.; Menegon, L.; Corfu, F.; Erambert, M.; Whitehouse, M. J.
• Is Part Of: Journal of metamorphic geology. Volume 33:Number 5(2015:Jun.)
• Page Start: 513
• Page End: 534
• Publication Date: 2015-04-29
• Subjects: Metamorphism (Geology) -- Periodicals; 552.4
• DOI: 10.1111/jmg.12131
• Languages: English
• ISSNs: 0263-4929

Abstract

<abstract abstract-type="main" id="jmg12131-abs-0001"> <title>Abstract</title> <p>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 <italic>P–T</italic> 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 <italic>c</italic>. 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‐<italic>T</italic> diffusion. Both cases imply that elevated temperatures of >600 °C persisted for over <italic>c</italic>. 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 <italic>c</italic>. 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 <italic>P–T–t–d</italic> path.</p> </abstract>