The trace element geochemistry of some high‐grade Bergen‐Jotun kindred gneisses from the Jotun Nappe around Gjendebu, central southern Norway

Abstract

Abstract 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 Storadalen Complex (SCX), the Svartdalen Gneiss (SG), and the Mjølkedøla Purple Gabbro (MPG). The SCX is a differentiated series of ultrabasic to intermediate rocks now showing only tectonite fabrics. It has been metamorphosed to spinel‐Iherzolite 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 MPG is also broadly of monzonitic composition but it retains a coarse ophitic texture, and is of amphibolite facies grade. A gradational boundary exists between the MPG 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 post‐climactic cooling. There are no systematic geochemical variations with metamorphic grade or degree of deformation.