Lllite crystallinity and prograde metamorphism in thrust zones of the Scandinavian Caledonides. La cristallinité de l'illite et le métamorphisme prograde dans les zones de chevauchement des Calédonides scandinaves

Abstract

During Late Silurian overthrustingof the Caledonian allochthon, the Rôde Sandstone underwent progressive deformation upwards towards the basal thrust of the overriding Offerdal Nappe (central Scandinavian Caledonides). Over a vertical distance of 80 m, the rock fabric changes from a discontinuous, irregular cleavage to a spaced, pressure diffusional cleavage which is in turn crenulated. Farther upwards towards the thrust, this crenulation cleavage becomes tighter and is eventually transposed into a myionitic schistosity defined by quartz blades, phengitic muscovite and biotite «fishes ». In the same direction, illite crystallinity values of the< 2�m fraction in the Rode Sandstone range from 0.31" 26 in the little deformed sandstone to 0.25" 26 (using the scale of Kisch, 1980) in the myionitic sandstone. Close to the thrust, I.C. values show a considerable dispersion due to the income of biotite. The (060-33 l)spacings show a parallel change from 1 .5074 À in the well preserved sandstone to 1 .5057 Д in the myionitic sandstone. Feldspathic sandstones occurring above the thrust underwent prograde recrystallization to garnetiferous mica schists downwards towards the thrust plane. Results obtained from illite crystallinity studies in the units underlying the Rôde Sandstone demonstrate that no heat flux from depth contributed to the recorded metamorphic change. Significant downwards directed heat flux through the overlying low-grade nappes must also be excluded. The results are discussed in the context of widespread evidence of prograde strain-induced metamorphism in zones of concentrated strain in the imbricate front of the Swedish Caledonides.