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 highgrade 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 of 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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A B S T R A C TThe Saglek Block of coastal Labrador forms the western margin of the North Atlantic Craton, where Archean gneisses and granulites have been reworked during the Paleoproterozoic. Previous work has established that the block is a composite of Eoarchean to Mesoarchean protoliths metamorphosed to upper amphibolite and granulite facies at around 2.8-2.7 Ga. New in-situ microbeam dating of accessory minerals in granoblastic gneisses reveals a complex peak to post-peak thermal history. Zircon growth at ca. 3.7-3.6 Ga provides the age of formation of the tonalitic protoliths to the gneisses. Further zircon growth in syn-tectonic granitic gneiss and monazite growth in a variety of orthogneisses confirm peak metamorphic conditions at ca. 2.7 Ga, but also reveal high-temperature conditions at ca. 2.6 Ga and 2.5 Ga. The former is interpreted as the waning stages of the 2.7 Ga granulite event, whereas the latter is associated with a younger phase of granitic magmatism. In addition, apatite ages of ca. 2.2 Ga may represent either cooling associated with the 2.5 Ga event or a previously unrecognized greenschist-facies metamorphism event that predates the Torngat Orogeny.

Monazite is a common mineral in metapelitic rocks including those which underwent ultra-high pressure (UHP) metamorphism. During metamorphic evolution monazite adapts its composition to the changing mineral assemblage, especially in its heavy rare earth element contents. We studied this process in diamond-bearing gneiss containing monazite, from Saxnäs in the Seve Nappe Complex of the Scandinavian Caledonides. Although the rock has been re-equilibrated under granulite facies and partial melting conditions, it still preserves minerals from the UHP stage: garnet, kyanite, rutile, and especially diamond. Microdiamonds occur in situ as inclusions in garnet, kyanite and zircon, either as single-crystals or polyphase inclusions with Fe-Mg carbonates, rutile and CO2. Both monazite and diamond occur in the rims of garnet showing the highest pyrope content and a secondary peak of yttrium. Such a position indicates thermally activated diffusion under high temperature at the end of prograde metamorphism. Monazite compositions show negative Eu anomalies, which we interpret to be inherited from the source rock, not reflecting the coexistence with plagioclase and/or K-feldspar which are unstable at UHP conditions. Our results suggest that the effect of whole-rock composition may be more important than that of coexisting phases. The UHP monazite was most likely formed from allanite during subduction and prograde metamorphism. The monazites included in garnet and kyanite are mostly unaltered, whereas those in the matrix show breakdown coronas consisting of apatite, REE-epidote/allanite and REE carbonate, likely formed due to pressure decrease and cooling. U-Th-Pb chemical age dating of monazites yields an isochron centroid age of 472 ±3 Ma. We interpret this age as monazite growth under UHP conditions related to subduction of the Baltica continental margin in Early Ordovician time.