DIACHRONOUS FLUID RELEASE AND VEINING ASSOCIATED WITH OUT-OF-SEQUENCE THRUSTS IN THE NORTH SCANDINAVIAN CALEDONIDES

The Ose Thrust of northern Norway represents a major late Caledonian out-of-sequence thrust which duplicates a previously assembled nappe stack with inverted metamorphic gradient. fluid flow along this thrust gave rise to retrogression of peak metamorphic porphyroblastic phases in both footwall and hangingwall. Chlorite pseudomorphs after garnet are not smeared out suggesting that retrogression largely post-dates thrust movement. Dominated by vein quartz, the Ose Thrust and adjacent footwall and hangingwall rocks record three stages of veining in a protracted history of fluid flow; schistosity-parallel, prograde and peak metamorphic veins (V-A-V-B), and retrogression-related (post-D-3) veins which cross-cut the regional S, schistosity (V-C). V-C veins contain similar highly saline fluid inclusions to those associated with early ductile thrusts from the region. This indicates that various thrusts in the north Scandinavian Caledonides acted passively as important fluid conduits for late-stage fluid flow and retrogression during uplift. The proportion and composition of different vein types varies considerably between footwall and hangingwall rocks, largely as a function of bulk rock composition and tectonometamorphic evolution. Contoured maps of veining intensity have been produced by image-processing and visual analysis of photographs from 80 localities along the Ose Thrust zone. Footwall gneisses typically contain 15-30% vein material (most of which is peak metamorphic), whereas hangingwall rocks generally contain < 1% veins. Integration of new data on veining and fluid production with existing radiometric dates and P-T data has shown that during Scandian collisional orogenesis fluid events occurred as discrete pulses. These pulses were diachronous between nappes, some units experiencing prograde devolatilization coeval with others experiencing retrogression.