Publikationsdatum:
2015-04-01
Beschreibung:
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-temperature 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 (1) monazite can have a large spread in U-Pb dates despite a homogenous composition, (2) rutile may lose its Zr-in-rutile and U-Pb signature during an amphibolite facies overprint, and (3) 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. This article is protected by copyright. All rights reserved.
Print ISSN:
0263-4929
Digitale ISSN:
1525-1314
Thema:
Geologie und Paläontologie
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