Metamorphism and Partial Melting at UHP Conditions Revealed by Microdiamonds and Melt Inclusions in Metapelitic Gneiss from Heia, Arctic Caledonides, Norway.

This article focuses on the discovery and analysis of microdiamonds and associated multiphase inclusions in garnet from metasedimentary ultrahigh-pressure (UHP) rocks of the Nordmannvik Nappe at Heia, in the Arctic Caledonides of Norway. The study identifies two types of primary inclusions in garnet: Type I multiphase fluid inclusions containing microdiamonds, carbonates, Al-phyllosilicates, and CO₂ fluid, and Type II melt inclusions with minerals crystallized from anatectic melt. These findings provide evidence for partial melting and primary fluid-melt immiscibility at UHP conditions (~3.7–3.8 GPa and 840–870°C) during pre-Scandian metamorphism linked to arc–continent collision events preceding the main Caledonian Orogeny. The coexistence of diamond-bearing fluid inclusions with melt inclusions suggests complex fluid and melt interactions during deep subduction and exhumation of Baltican continental crust.