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
Abstrakt
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 similar to 730-775 degrees C and similar to 6.3-9.8kbar, above the wet solidus in the sillimanite and kyanite stability fields. The event is dated at 702 +/- 5Ma by high-U zircon in a leucosome.
The S1 fabric was overprinted by a subhorizontal S2 fabric, which formed at similar to 600-660 degrees C and similar to 10-12kbar. 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 degrees C and Caledonian ages of 440-420Ma. Titanite grew at the expense of rutile at slightly lower temperatures of similar to 550 degrees C during ongoing S2 deformation; U-Pb ages of c. 440-430Ma 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.