Monazite Behaviour during Metamorphic Evolution of a Diamond-bearing Gneiss

<p>We studied monazite behaviour in UHP diamond-bearing gneiss from Saxn&#228;s in the Seve Nappe Complex of the Scandinavian Caledonides (Petr&#237;k et al., 2019). Although the rock has been re-equilibrated under&#160; 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 &#160;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.</p><p>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.</p><p>U-Th-Pb chemical age dating of monazites yields an isochron centroid age of 472 &#177;3 Ma. We interpret this age as monazite growth under UHP conditions related to subduction of the Baltican continental margin in Early Ordovician time.</p><p>This work was supported by the projects APVV-14-0278 and APVV-18-0107, National Science Center &#8220;CALSUB&#8221; 2014/14/E/ST1/00321</p><p>Reference: Petr&#237;k, I., Jan&#225;k, M., Klonowska, I., Majka, J., Froitzheim, N., Yoshida, K., Sasinkov&#225;, V., Kone&#269;n&#253;, P., Vaculovi&#269;, T. 2019. Journal of Petrology doi: 10.1093/petrology/egz051</p>