Discovery of coesite-eclogite from the Nordøyane UHP domain, Western Gneiss Region, Norway: field relations, metamorphic history, and tectonic significance

Ultrahigh-pressure (UHP) rocks from the Western Gneiss Region (WGR) of Norway record subduction of Baltican continental crust during the Silurian to Devonian Scandian continental collision. Here, we report a new coesite locality from the island of Harøya in the Nordøyane UHP domain, the most northerly yet documented in the WGR, and reconstruct the P-T history of the host eclogite. The coesite-eclogite lies within migmatitic orthogneiss, interpreted as Baltica basement, that underwent multiple stages of deformation and partial melting during exhumation. Two stages of metamorphism have been deduced from petrography and mineral chemistry. The early (M1) assemblage comprises garnet (Pyr_38-41Alm_35-37Grs_23-26Spss₁) and omphacite (Na_0.35-0.40Ca_0.57-0.60Fe²⁺ _0.08-0.10Mg_0.53Fe³⁺ _0.01Al^VI _0.40-0.42)₂(Al^IV _0.03-0.06Si_1.94-1.97)₂O₆, with subordinate phengite, kyanite, rutile, coesite and apatite, all present as inclusions in garnet. The later (M2) assemblage comprises retrograde rims on garnet (Pyr_38-40Alm_40-44Grs_16-21Spss₁), diopside rims on omphacite (Na_0.04-0.06Ca_0.88-0.91Fe²⁺ _0.09-0.13Mg_0.81-83Fe³⁺ _0.08Al^VI _0.03)₂(Al^IV _0.07-0.08Si_1.92-1.93)₂O₆, plagioclase, biotite, pargasite, orthopyroxene and ilmenite. Metamorphic P-T conditions estimated using thermocalc are ∼3GPa and 760°C for M1, consistent with the presence of coesite, and ∼1GPa and 813°C for M2, consistent with possible phengite dehydration melting during decompression. Comparison with other WGR eclogites containing the same assemblage shows a broad similarity in peak (M1) P-T conditions, confirming suggestions that large portions of the WGR were buried to depths of ∼100km during Scandian subduction. Field relations suggest that exhumation, accompanied by widespread partial melting, involved an early phase of top-northwest shearing, followed by subhorizontal sinistral shearing along northwest-dipping foliations, related to regional transtension. The present results add to the growing body of data on the distribution, maximum P-T conditions, and exhumation paths of WGR coesite-eclogites and their host rocks that is required to constrain quantitative models for the formation and exhumation of UHP metamorphic rocks during the Scandian collision.