THERMOBAROMETRY OF ECLOGITE-FACIES SHEAR ZONES IN THE LOFOTEN ISLANDS, NORWAY

The exhumed lower crustal rocks of the Lofoten Islands in northern Norway provide a rare opportunity to study deep crustal processes forming eclogite-facies shear zones (ESZ) during Caledonian continental collision. The 2-5 cm-wide ESZ found in Nusfjord and Flakstad, on the island of Flakstadøy, are within metastable gabbroic host rocks, specifically gabbroic gneiss and gabbronorite. These ESZ contain relict eclogite-facies omphacite along garnet grain boundaries but are generally overprinted by a retrograde amphibolite-facies metamorphic event. This is the first report of positively-identified relict omphacite for Flakstadøy shear zones. Peak metamorphic P-T conditions are not well-constrained, but will be determined using Fe-Mg exchange between omphacite and garnet pairs in the ESZ. Average omphacite chemistries are Aeg12.5Jd41Di46.5 and garnet chemistries adjacent to the omphacite are Alm47Pyr37Grs17 based on quantitative analyses from an EDS on an FE-SEM at San Francisco State University. Preliminary P-T estimates for the eclogite-facies metamorphism calculated for Fe-Mg exchange equilibria are 1.5 GPa and 820 ⁰C, somewhat higher temperature than previous estimates. EBSD mapping will be used in conjunction with mineral chemistries to assess the role ductile deformation may have played in eclogitization reactions.