Monazite Behaviour during Metamorphic Evolution of a Diamond-bearing Gneiss
Authors: Petrík Igor , Janák Marian , Klonowska Iwona , Majka Jaroslaw , Froitzheim Niko , Yoshida Kenta , Sasinková Vlasta , Konečný Patrik , Vaculovič Tomáš 
Published: March 2020

&lt;p&gt;We studied monazite behaviour in UHP diamond-bearing gneiss from Saxn&amp;#228;s in the Seve Nappe Complex of the Scandinavian Caledonides (Petr&amp;#237;k et al., 2019). Although the rock has been re-equilibrated under&amp;#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 CO&lt;sub&gt;2&lt;/sub&gt;. Two garnet types have been recognised: dominant Grt I &amp;#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.&lt;/p&gt;&lt;p&gt;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.&lt;/p&gt;&lt;p&gt;U-Th-Pb chemical age dating of monazites yields an isochron centroid age of 472 &amp;#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.&lt;/p&gt;&lt;p&gt;This work was supported by the projects APVV-14-0278 and APVV-18-0107, National Science Center &amp;#8220;CALSUB&amp;#8221; 2014/14/E/ST1/00321&lt;/p&gt;&lt;p&gt;Reference: Petr&amp;#237;k, I., Jan&amp;#225;k, M., Klonowska, I., Majka, J., Froitzheim, N., Yoshida, K., Sasinkov&amp;#225;, V., Kone&amp;#269;n&amp;#253;, P., Vaculovi&amp;#269;, T. 2019. Journal of Petrology doi: 10.1093/petrology/egz051&lt;/p&gt;