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K-Rich Calc-Alkaline Augen Gneisses of Grenvillian Age in SW Norway: Mingling of Mantle-Derived and Crustal Components

Bernard Bingen, Daniel Demaiffe, Jan Hertogen, Dominique Weis and Jean Michot
The Journal of Geology
Vol. 101, No. 6 (Nov., 1993), pp. 763-778
Stable URL: http://www.jstor.org/stable/30065155
Page Count: 16
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
K-Rich Calc-Alkaline Augen Gneisses of Grenvillian Age in SW Norway: Mingling of Mantle-Derived and Crustal Components
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Abstract

Augen gneisses were emplaced as phenocryst-bearing granodiorites at $1040 \pm 40 Ma$ to the west of the Sveconorwegian belt (Rogaland-Vest-Agder, SW Norway) just before the main Sveconorwegian (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 $(^{206}Pb/^{204}Pb)_{i} = 17.06$ and $(^{207}Pb/^{204}Pb)_{i} = 15.46$. 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 $I_{Sr} (0.7036 to 0.709)$, of $FeO_{tot}/MgO$ 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.

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