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The occurrence of eclogitic rocks in northwest Spitsbergen, Arctic Caledonides, has been known for forty years. However, detailed petrology and documentation of true eclogites has been lacking prior to this study. The eclogites and related high-pressure rocks occur within the Richarddalen complex on Biscayarhalvøya. The eclogite-bearing complex, which is a very heterogeneous unit with a variety of high-grade orthogneisses and meta-intrusive rocks, constitutes the uppermost unit of a relatively simple stack of thrust sheets where the metamorphic grade is increasing structurally upward in the pile. Relics of eclogite facies metamorphism are preserved in mafic lenses, pods and layers within amphibolite facies orthogneiss. The eclogite facies assemblage consists of garnet, omphacite, phengite, quartz, clinozoisite, rutile and high-Al titanite (XAl up to 0.26). P-T conditions for the eclogite facies can be constrained at ca. 2.5 GPa and 700 °C based on garnet-clinopyroxene-phengite thermobarometry. The retrograde post-eclogite stages comprise amphibolite facies associations formed along a steep, near-isothermal decompression type P-T path. Metamorphic titanite from amphibolite and felsic gneiss yields U-Pb ages of c. 455 Ma, which most probably represent the amphibolite facies re-equilibration. The main part of the exhumation took place along a rather steep decompression path. Thrusting combined with erosion is the favored exhumation mechanism given the trust geometry of the area. Structural analyses have shown that thrusting of the Richarddalen complex onto the lower-grade nappe units was synchronous with constrictive north-south stretching. Late Silurian/Devonian conglomerates were deposited directly on the Richarddalen complex, and mark the final exhumation of the nappe stack.
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