Monazite Behaviour during Metamorphic Evolution of a Diamond-bearing Gneiss

We studied monazite behaviour in UHP diamond-bearing gneiss from Saxnäs in the Seve Nappe Complex of the Scandinavian Caledonides (Petrík et al., 2019). Although the rock has been re-equilibrated under  granulite facies and partial melting conditions, the UHP stage is recorded by the presence of diamond. Microdiamonds occur in situ as inclusions in garnet, kyanite and zircon, either as single-crystal or polyphase inclusions with Fe-Mg carbonates, rutile and CO₂. Two garnet types have been recognised: dominant Grt I  with inclusions of diamond found mostly in the garnet rims, which suggests that originally the bulk of Grt I grew at UHP conditions. Grt II, forming small crystals, overgrowths on, or domains within Grt I originated by dehydration melting reactions involving breakdown of phengite and clinopyroxene during decompression. Monazite occurs in the rims of Grt I close to microdiamond, where garnet shows 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. Based on such textural relations, we argue that monazite formed at UHP conditions.

Monazite composition shows negative Eu anomalies and moderate Y contents, which is not in agreement with common interpretation that UHP conditions necessarily lead to the absence of Eu anomaly and low Y content due to absence of plagioclase and high garnet content. We explain this by the effect of whole-rock composition. LA ICP MS analyses show that whole-rock budget is controlled by monazite, apatite and garnet, all having negative Eu anomalies. Whole rock composition is successfully modelled by (wt. %) garnet 16, apatite 3, monazite 0.06. We conclude that the Eu anomaly is inherited from the source rock, not reflecting the coexistence with plagioclase and/or K-feldspar, which are unstable at UHP conditions. Uniform garnet abundance (16 vol. %) above 20 kbars predicted by pseudo-section modelling explains the lack of Y decrease due to the increase of garnet content at UHP conditions. Our results suggest that the effect of the whole-rock composition may be more important than that of coexisting phases.

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 Baltican continental margin in Early Ordovician time.

This work was supported by the projects APVV-14-0278 and APVV-18-0107, National Science Center “CALSUB” 2014/14/E/ST1/00321

Reference: Petrík, I., Janák, M., Klonowska, I., Majka, J., Froitzheim, N., Yoshida, K., Sasinková, V., Konečný, P., Vaculovič, T. 2019. Journal of Petrology doi: 10.1093/petrology/egz051