K-RICH CALC-ALKALINE AUGEN GNEISSES OF GRENVILLIAN AGE IN SW NORWAY - MINGLING OF MANTLE-DERIVED AND CRUSTAL COMPONENTS

Augen gneisses were emplaced as phenocryst-bearing granodiorites at 1040 +/- 40 Ma to the west of the Sveconorwegian belt (Rogaland-Vest-Agder, SW Norway) just before the main Sveconowegian (Grenvillian) metamorphic event. They define a typical K-rich calc-alkaline trend. Initial Sr and Nd isotopic ratios are close to Bulk Earth values at 1040 Ma: (I-sr = 0.7036 and average epsilon(Nd) = + 0.3). Average initial Pb isotopic values of 11 K-feldspar phenocrysts are (Pb-206/Pb-204)(i) = 17.06 and (Pb-207/Pb-204)(i) = 15.46. The augen gneisses contain ultrapotassic gabbroic inclusions of calc-alkaline lamprophyric composition. The geochemical evolution of the augen gneiss series can be accounted for by fractional crystallization, without contamination by surrounding banded gneisses, of a magma resulting from the mixing between ultrapotassic, mantle-derived mafic magma (20-25%) with a granodioritic magma generated in the lower crust. Accessory minerals (apatite, zircon, titanite, allanite) control the behavior of most trace elements. In Rogaland-Vest-Agder, the Sveconorwegian granitoids (1040 to 930 Ma) show a systematic geochemical evolution with time. They display an increase of Is, (0.7036 to 0.709), of FeOtot/MgO ratio (2.2 to 18) and Zr content (360 to 900 ppm). This evolution corresponds to an increase in the partial melting temperature together with an increase of the proportion of upper crustal component. The calc-alkaline affinity of the augen gneisses implies that a subduction-related geodynamic regime prevailed just before the main Sveconorwegian event. The Sveconorwegian cycle ends by the emplacement of A-type charnockitic granites.