Geochemical evidence in support of sedimentary precursors to Proterozoic sillimanite-bearing rocks, Vest-Agder, South Norway

Abstract

Sillimanite-bearing alumina-rich schists and gneisses from three different areas within the Agder region in southern Norway are interpreted as having sedimentary precursors, transformed during deep-seated Sveconorwegian deformation and high-grade metamorphism which led to complete recrystallization, so that their precursors are now beyond recognition. The layers and lenses of rather rare occurrences of sillimanite-bearing schists and gneisses belong to the oldest recognisable layered formation of the Proterozoic rock sequence and are interlayered with quartzites and marbles in the easternmost localities. Major element analyses support the conclusion that the precursors were clay-dominated sediments derived from mafic rocks and to a varying degree mixed with more felsic rocks developed during extensive weathering in a relatively warm, humid climate. Trace element data also indicate the presence of an original clay fraction in the precursors and imply a high degree of continental source rocks. Distribution of the individual elements may be explained by physico-chemical parameters known to control element behaviour in sedimentary processes in connection with clay deposition.