MINERAL FLUID MELT INTERACTIONS IN HIGH-PRESSURE SHEAR ZONES IN THE BERGEN ARCS NAPPE COMPLEX, CALEDONIDES OF W NORWAY - IMPLICATIONS FOR THE FLUID REGIME IN CALEDONIAN ECLOGITE-FACIES METAMORPHISM

Felsic extension veins consisting of plagioclase (Ab91), phengite, quartz, clinozoisite and minor/accessory calcite occur in the central parts of Caledonian eclogite-facies shear zones at Holsnoy island in the Bergen Arcs nappe complex. The veins have the field and textural characteristics of syntectonic intrusions and have crystallized at pressures and temperatures compatible with the peak of eclogite-facies metamorphism (ca. 700-degrees-C, 16-19 kbar). The quartz contains early ("primary") fluid inclusions of (a) aqueous brine (31-34 wt.% NaCl) and (b) a CO2-N2 mixture with X(N2) greater-than-or-equal-to 5 mole percent. The two fluids were present simultaneously and are interpreted as immiscible at the PT conditions of vein formation. Later generations of fluid inclusions comprise nitrogen-rich CO2-N2 mixtures and aqueous fluids with low salt contents. The latter were trapped at a late stage of the cooling and uplift history of the nappe complex. At the temperature of the peak of eclogite-facies metamorphism at Holsnoy (ca. 700-degrees-C), introduction of a fluid phase with high water activity caused partial melting of the surrounding mafic to intermediate lithologies; the muscovite-granite extension veins represent the crystallization products of anatectic melts migrating through the shear zones at eclogite-facies conditions.