Coesite-bearing garnet xenocrysts from diatexite in the Nordøyane domain, Western Gneiss Region, Norway: implications for eclogite-melt interaction at ultra-high pressure

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Ultra-high-pressure (UHP) metamorphic rocks in the Western Gneiss Region (WGR) of Norway formed in subducted Baltican crust during the Scandian phase of the Caledonian orogeny. In the Nordøyane UHP domain, eclogites locally preserve coesite and microdiamond. Along the north coasts of Haramsøya and Flemsøya, in situ eclogite bodies are surrounded by dioritic “diatexite selvages” containing abundant eclogitic enclaves and xenocrysts. In one location eclogite and diatexite are associated with a quartz diorite dyke that is highly contaminated with eclogitic material. The diatexite matrix and dyke have similar plagioclase + quartz + biotite assemblages with abundant embayed and fragmented garnet and clinopyroxene xenocrysts. Coesite is present in some garnet xenocrysts but has not been found in the adjacent eclogite bodies. Pressure–temperature ( P– T) estimates from in situ eclogites range from 2.2–3.2 GPa and 810–840 °C, spanning the quartz–coesite transition, with P– T estimates from xenocrysts spanning a wider range. Diorites and retrogressed eclogites record amphibolite-facies conditions, ca. 1.0–1.5 GPa and 700–800 °C. The wide range and large uncertainties in P– T estimates reflect high-temperature re-equilibration and a relative lack of P-sensitive assemblages. Differences in texture, composition, and included assemblages indicate that the xenocrysts were not derived from the nearby eclogite bodies. By implication, their dioritic hosts were not locally derived but must have originated from a different, probably deeper, source. The results are integrated with a numerical model for WGR tectonic evolution to assess the sources of the melts and their possible effects on the exhumation of UHP rocks in western Norway.