NJG69-3-03
1989
Geochemistry of Sveconorwegian augen gneisses from SW Norway at the amphibolite-granulite facies transition
69
3
177-189

The augen gneisses of Rogaland-Vest-Agder were emplaced as amphibole-biotite granodiorites rich in K-feldspar phenocrysts. They were probably pre- or syntectonically emplaced and were metamorphosed under lower amphibolite to granulite facies conditions after their emplacement. Three metamorphic zones defined by two isograds (Cpx-in and Opx-in) can be established in the augen gneisses using ferromagnesian minerals: zone 1 = Bt ± Am zone; zone 2 = Bt ± Am ± Cpx zone and zone 3, close to the Rogaland anorthosite complex, in granulite facies = Bt ± Am ± Cpx ± Opx zone. Pyroxene forming reactions occur in two stages in the augen gneisses (first producing Cpx and then Opx); amphibole appears to be an important reactant in the pyroxene-forming reactions while biotite is not. Sphene and allanite, present in zone l, disappear as Cpx appears (zone 2). Trace amounts of monazite also appear at this isograd. The augen gneisses are granodioritic to granitic in composition. They are of K-rich calc-alkaline affinity and they are enriched in incompatible elements (Ba, Rb, K, Sr). They contain various kinds of enclaves, including some of calc-alkaline lamprophyric composition. The augen gneisses of the three metamorphic zones define a single geochemical trend, implying that they represent a single magmatic episode. The augen gneisses of granulite facies (zone 3) are not depleted in Rb, K nor, presumably, in other LIL elements, relative to their amphibolite facies equivalents (zones 1 and 2). The presence of K-rich minerals (K-feldspar and biotite) in the augen gneisses seems to inhibit the LILE depletion process.

0029-196X
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