Retrogression of eclogite-facies shear zones by short-lived fluid infiltration during the Caledonian orogeny, Lofoten islands, Norway
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Norwegian eclogite-facies shear zones in the Lofoten islands served as major pathways for short-lived pulses of both 40Ar-rich and 40Ar-poor hot fluids during eclogitization and retrogression events related to the Caledonian orogeny. The open system for Ar affected most of the minerals leading to old 40Ar/39Ar ages, particularly in trioctahedral micas. 40Ar concentration in the fluids appears to decrease during the amphibolite-facies retrogression and Ca-rich amphiboles yield 40Ar/39Ar ages of c. 415 Ma (Scandian event). Late- and post-Caledonian 40Ar/39Ar muscovite and K-feldspar ages and Rb/Sr biotite ages coincide with multiple extensional events, fluid infiltration and thermal activity during the final exhumation of the crustal rocks, potentially reflecting major tectonic episodes such as rifting of Pangaea and seafloor spreading between Europe and North America.