Proterozoic Metasediments from the Deep Crust of Rogaland, SW Norway: Chemistry of metabasites and granofelses

Abstract

Metamorphic basites and alumina-rich granofelses from the supracrustalFauref jell Formation and dark layers (amphibolites and metabasites) from the Basic-Acid Migmatite Sequence from Rogaland/Vest-Agder, SW Norway, have been studied chemically (figs. 1 and 2). Major elements, trace elements and REE confirm the magmaticprovenance of the metabasites and the continuous chemical trends strongly suggest a common progenitor for the granofelses (fig.3). Whencompared to metabasites, the granofelses are enriched in immobile elements and depleted in silica and calcium. Sapphirine-bearing granofelsesdiffer chemically from the other granofelses by a higher Mg and lower P, Zr, Hf, Y and REE content. Spinel-bearing metabasites and granofelses from one specific layer show the most pronounced enrichment trends of immobile elements. During the main metamorphism most rocks were variably affected by K-metasomatism (fig.4). The chemical data have been plotted in discriminant immobile element diagrams in order to charactarize the metabasites in terms of magma series and to reveal a possible tectonic setting. Metabasites, alumina-rich granofelses and amphibolites yield sub-alkaline, tholeiitic affinities on most diagrams. This is also supported by rather flat REE-patterns. The diagrams are not conclusive for the exact tectonic setting but from the coexistence of the metabasites together with quartzites and metadolomites in the Fauref jell Formation (Hermans et al,; 1975, Sauter, 1961) a back-arc basin position within the Proterozoic orogeny (> 1200 Ma) is tentatively suggested.