Monazite Behaviour during Metamorphic Evolution of a Diamond-bearing Gneiss: a Case Study from the Seve Nappe Complex, Scandinavian Caledonides

Abstract

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 reequilibrated 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 CO 2 . 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.