Rare-earth element distribution and K/Rb ratios in granulites, mangerites and anorthosites, Lofoten-Vesteraalen, Norway

Neutron activation analysis for La, Ce, Nd, Sm, Eu, Gd, Tb, Yb and Lu and X-ray fluorescence determination of K/Rb ratios have been carried out on a suite of anorthosites, gabbros, mangerites, granulite facies gneisses and amphibolite facies gneisses, selected from three separate anorthosite intrusions and their associated country rocks in the Precambrian Lofoten-Versteraalen high-grade metamorphic terrain. All samples except one show an enriched, linear fractionated rare-earth distribution relative to chondrites. The anorthosites, gabbros and mangorites have a conspicuous positive Eu anomaly but most of the granulite facies gneisses have normal Eu values. The K/Rb ratios are >2000 for the anorthosites and gneisses in the immediate contact zone and are between 500 and 1400 for the mangerites, compared with 200-500 for dioritic-monzonitic granulite facies gneisses distant from the anorthosite. Separate, small gabbro intrusions have K/Rb ratios of 200-800. The REE and K/Rb data suggest that the anorthosites and the major mangerite intrusions are not genetically related. The small Moskenesöy anorthosite may have been produced by melting in the dioritic gneisses, yielding monzonitic veins and dykes and an anorthositic residuum. The presence of a Eu anomaly in the K-feldspar-rich mangerites and the high K/Rb ratios for these rocks supports a two-stage melting mechanism for their generation in the deep crust. Later retrogression and complete recrystallization of the mangerites has not affected the rareearth element distribution or the K/Rb ratios. Also, the amphibolite facies gneisses have similar rare-earth element distribution to granulite facies gneisses.