Journal of


  (Formerly Journal of the Czech Geological Society)

Original Paper

Jaroslaw Majka, Yaron Be’eri-Shlevin, David G. Gee, Anna Ladenberger, Stefan Claesson, Patrik Konečný, Iwona Klonowska

Multiple monazite growth in the Åreskutan migmatite: evidence for a polymetamorphic Late Ordovician to Late Silurian evolution in the Seve Nappe Complex of west-central Jämtland, Sweden

Journal of Geosciences, volume 57 (2012), issue 1, 3 - 23


Monazite from granulite-facies rocks of the Åreskutan Nappe in the Scandinavian Caledonides (Seve Nappe Complex, Sweden) was dated using in-situ U-Th-total Pb chemical geochronology (CHIME). Multi-spot analyses of a non-sheared migmatite neosome yielded an age of 439 ± 3 Ma, whereas a sheared migmatite gave 433 ± 3 Ma (2σ). Although the obtained dates are rather similar, a continuous array of single dates from c. 400 Ma to c. 500 Ma suggests possibly a more complex monazite age pattern in the studied rocks. The grouping and recalculation of the obtained results in respect to Y-Th-U systematics and microtextural context allowed distinguishing several different populations of monazite grains/growth zones.
In the migmatite neosome, low-Th and low-Y domains dated at 455 ± 11 Ma are considered to have grown under high-grade sub-solidus conditions, most likely during a progressive burial metamorphic event. The monazites with higher Th and lower Y yielded an age of 439 ± 4 Ma marking the subsequent partial melting event caused by decompression. The youngest (423 ± 13 Ma) Y-enriched monazite reveals features of fluid-assisted growth and is interpreted to date the emplacement of the Åreskutan onto the Lower Seve Nappe.
In the sheared migmatite, the high-Th and low-U (high Th/U) monazite with variable Y contents yielded an age of 438 ± 4 Ma, which is interpreted to date the partial melting event. Relatively U-rich rims on some of the monazite grains again reveal features of fluid-assisted growth, and thus their age of 424 ± 6 Ma is interpreted as timing the nappes emplacement. These results call, however, for further more precise, isotopic (preferably ion microprobe) dating of monazite in the studied rocks.

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