THE PRESSURE-TEMPERATURE-TIME HISTORY OF CALEDONIAN KYA-KFS-MUS ASSEMBLAGES FROM LOFOTEN, NORWAY, BASED ON MODELING OF ELEMENTAL ZONING IN GARNET AND 40AR/39AR AGE GRADIENTS IN MUSCOVITE

Prograde major and trace element zoning and mineral inclusions preserved in garnet enable reconstructions of the path a rock experienced to peak metamorphic conditions. However, in cases where zoning (chemical and isotopic) has been extensively modified by intracrystalline diffusion during protracted high-temperature conditions, following peak metamorphism, an interpretation of the overall history is uncertain as many combinations of inferred crustal depth, temperature and time could produce similar diffusion profiles. Pelitic gneisses and schists from the district of Røst (the farthest southeast islands of the Lofoten archipelago, Norway) contain the assemblage Kyanite+K-feldspar+Muscovite, that formed thorough deep partial subduction of the Baltic Margin during the Silurian (Scandian) phase of Caledonian orogeny. Millimeter-radius garnets in these rocks typically show little to no zoning with respect to Fe, Mg, Mn and Ca. In contrast, the garnets have conspicuous core-rim patterns of zoning for P, Y, and Cr, consistent with preserved prograde zoning of these trace elements. Phase equilibria indicate minimum conditions of ca. 720°C and 8.5 kbar for the assemblage, though the present mineral chemical compositions and classic thermobarometry yield temperatures and pressures far lower. Muscovite from these rocks has intracrystalline zoning of ⁴⁰Ar/³⁹Ar ages that are consistent with a path that remained at ca. 500-480°C from ~420-320 Ma, followed by more rapid cooling through final ⁴⁰Ar retention in muscovite by ~270 Ma. We suggest that prograde concentration gradients for major elements in garnet in these rocks were homogenized by lattice diffusion at conditions above or near 500°C, whereas millimeter-radius muscovite crystals were partially retentive of radiogenic 40Ar from ~ 500-400°C. Combining the diffusion data of these systems with phase equilibria constraints on peak P-T conditions leads to a reconstructed pressure-temperature-time path that is more fully constrained. Overall, the metamorphic history of sequences in Røst are consistent with ~35 km subduction, with an ensuing exhumation history characterized by episodes of rapid cooling separated by spans of very slowly changing temperature that lasted for tens to hundreds of millions of years.