Experimental study of the Ultra High T metamorphism of a restitic metapelitic granulite: role of a previous partial melting event on the UHT metamorphism and influence of the redox state

International audienceThe origin of UHT metamorphism prevailing in the continental crust is still matter of debate. Partial melting probably plays a key role. UHT metamorphism occurs at temperatures above the fluid-absent melting of most crustal rocks. Partial melting is an endothermic process that consumes heat and buffers the temperature at around 750-850°C. Thus, partial melting prevents a fertile crust to attain UHT. Conversely, UHT conditions can be more easily reached upon refractory / restitic rocks and occur preferentially in terranes that underwent previous partial melting event and melt loss. The Gruf complex (Lepontine Alps) is a field area that confirms this scenario; it is one of the rare occurrences of Eocene UHT metamorphism in the world. This complex previously suffered the post Hercynian high-thermal regime responsible for the widespread formation of granulites in the Austro-alpine domain and Southern Alps. We propose that the typical UHT parageneses of the Gruf complex crystallized from refractory/restitic lithologies. The refractory character was acquired through fluid-absent melting reactions during the post Hercynian metamorphic event, while UHT conditions were reached during the Alpine cycle. The typical mineral assemblage diagnostic of UHT metamorphism Spr + Qtz is sometimes replaced by the Spl + Qz assemblage under similar P-T conditions. The reason for this dichotomy is not yet understood. In order to bring new elements to this discussion, we conducted experiments in an internally-heated pressure vessel on a metapelitic granulite from the Ivrea zone composed of Q-Grt-Sil-Kf±Pl±Bt±Rt. This rock is representative of a component of the lower crust after melt extraction. It would represent a restite associated with the process of differentiation of the crust. The P and T intervals considered during these experiments were 0.3-0.8GPa and 950-1050°C; their duration lasted 7 or 13 days. Between 0.8-0.6 GPa, the paragenesis is Spr-Opx-Sil-Qz-TiMag-Melt (and Grt at T 900°C, whatever the duration of the experiment. In conclusion, our experiments show that UHT conditions applied to a restite from the lower crust produce typical parageneses of UHT metamorphism. They also suggest that Spl-Qz assemblages are indicative of reducing conditions while Spr-Qz assemblages prevail under more oxidised conditions