The high-grade meta-plutonic rocks of this study lie entirely within the Jotun Nappe of the southern Norwegian Caledonides. They are divisible on the basis of metamorphic grade and petrographic character into three units, the Storidalen Complex (So<), the Svartdalen Gneiss (SG), and the Mjnlkedda Purple Gabbro (MPG). The S C X is a differentiated series of ultrabasic to intermediate rocks now showing only tectonite fabrics. It has been metamorphosed to spinellherzolite granulite facies grade. The broadly monzonitic SG is weakly tectonized and internally differentiated. Its metamorphic grade does not exceed plagioclase-lherzolite granulite facies grade. The mis-named M P G is also broadly of momonitic composition but it retains a coarse ophitic texture, and is of amphibolite facies grade. A gradational boundary exists between the M PG and SG, but the contact between these two units and the SCX is the steeply dipping Tyin-Gjende Fault. The three units represent a comagmatic body of mid-Proterozoic age, metamorphosed during a Sveconorwegian event and finally dismembered and upthrust during the Caledonian Orogeny.The new trace element analyses reported here show that the three rock units have remarkably similar trace element abundances and trends. K-Rb covariation shows increasing K/Rb ratios with increasing K. These patterns were produced by magmatic fractionation processes acting at deep crustal levels, possibly in the presence of a non-aqueous fluid phase. With the exception of K and Sr, close similarities exist between the rocks of this study and present-day calc-alkaline basalts and andesites from island arcs. The high K content is regarded as a primary magmatic feature, but the available data are insufficient to indicate its origin. The Sr contents are abnormally high and are ascribed to metasomatism which occurred during either high-grade metamorphism or postclimactic cooling. There are no systematic geochemical variations with metamorphic grade or degree of deformation.
Coronas containing Ca-amphibole with aluminous minerals have been characterised optically and by scanning electron microscopy, analytical transmission electron microscopy and electron-probe microanalysis. The layers nearest to plagioclase are amphibole + epidote + kyanite, followed by amphibole + epidote + staurolite + spinel. These assemblages are consistent with waterundersaturated conditions, possibly at lower metamorphic grade than the commoner assemblage amphibole + spinel. Observed mineral proportions and compositions were used in a seven-layer model of steady-state, diffusion-controlled growth with local equilibrium. This model is not fully realistic, because the observed amphibole is strongly zoned from tschermakitic to actinolitic away from plagioclase, suggesting disequilibrium. However, the four-mineral layer has been successfully modelled assuming local equilibrium, with diffusion coefficients Lii larger for i = FeO and MgO than for SiO2, AlO3/2, CaO and FeO3/2. Retarded grain-boundary diffusion of the latter components is explicable by crystal-chemical effects. The number of minerals per layer is constrained by a modified form of the metasomatic phase rule of Korzhinskii, with the role of 'inert' components played by relatively immobile ones (having relatively small fluxes and relatively small diffusion coefficients).
A significant portion of the Caledonian nappe pile of south-central Norway is composed of tectonic flakes of continental crust older than the arkosic sediments of the Upper Precambrian-Lower Cambrian Hedmark and Valdres Groups. Models deriving these flakes from a suture zone S and SE of the Western Gneiss Region have found less favour than those deriving them from a root zone lying off the present-day coast line to the NW. Recent models propose that the largest flake, the Jotun-Valdres Nappe Complex, is a microcontinental suspect terrane structurally bounded above and below by ophiolitic sutures. The pre-Caledonian geological evolution of the Jotun-Valdres Nappe Complex and the Bergen Arcs is shown to be comparable to that of adjacent parts of the autochthonous Fennoscandian Shield, and to a putative root zone along strike from the Lofoten-Vesterhlen Province. It is concluded that the Jotun-Valdres Nappe Complex and, by implication, related units in the Bergen Arcs and elsewhere, are not 'suspect' and are portions of the Mid to Late Proterozoic Baltoscandian crust, detached and thrust into position during the c. 390 Ma Scandian (= late-Caledonian) orogenic event.
ABSTRACT. The petrography and geochemistry of a suite of dolerite dykes emplaced into o rthogneisses of the Jotun Nappe, central southern Norway, are described. The dolerites, which appear to be tholeiitic in character, show well-developed ophitic and doleritic textures and are not foliated. They contain corona structures of six types which represent the various stages of reaction between olivine or clinopyroxene and plagioclase, and oxide grains and plagioclase. Corona growth always proceeds after the plagioclase has become clouded with numerous extremely fine opaque particles, and reaches its fullest development in rocks with the most heavily clouded plagioclase. Garnet grows in coronas only in rocks with Fe203/ (Fe203+FeO) = 0.20 to 0.27. The ratio MgO/MgO+ Fe203 + FeO) is of lesser importance in controlling the growth of this mineral. The role of water in coronaforming reactions, and the preservation of original igneous textures indicates that the coronas are a product of post-solidification deuterism, possibly active at high pressures and temperatures, rather than of prograde metamorphism.
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