A geochemical and structrual study of the gneisses and eclogites on the Molde Peninsula, west Norway.

The Molde Peninsula lies at approximately 64oN, in the Basal Gneiss Complex of W Norway, which represents a large area of basement rocks, mostly of Precambrian age and contains a large number of eclogites (i.e. gnt + omph. cpx rocks). To the NE of the Molde Peninsula lie several allochthonous units: the 'cover'. Previous mapping of the area had been of a reconnaissance nature and no systematic geochemical work had been done. Two major problems prevail in this area of Norway: the identification of and discrimination between true basement and true 'cover' rocks and the mode of formation of the eclogites. This study has been based on field mapping, petrography, geochemistry, mineral chemistry and some Rb-Sr isotope dating. Four major lithological units have been discerned. Basement rocks occur as two distinct types. The oldest is a calcalkaline suite formed at ~ 2000 m.y. ago in an Andean type collision margin. These were subsequently intruded by the second type of basement rock at 1477 ± 21 m.y. ago (confirmed by Rb-Sr isotope dating) as a granitic pluton, related to the emplacement of the anorthosite-rapakivi suite, in a tensional crustal regime. A swarm of tholeiite dykes with a layered basic intrusion, was intruded into both types of basement rock at ~ 1200-850 m.y. ago in the waning stages of this crustal extension. The subsequent Caledonian orogeny between the Baltic and Greenland 'plates' involved considerable crustal thickening and nappe translation. One thrust unit of postulated Palaeozoic-aged marginal basin rocks occurs on the Peninsula. All four of these lithological units suffered eclogite facies metamorphism at ~ 750oC and 18-20 kbar during orogenesis and the dykes were disrupted into the pods of eclogite now seen. Subsequent rapid uplift instigated partial melting in the pelitic rocks, shearing and a variety of retrograde textures; eclogite facies assemblages are only sporadically preserved. Tight folding in the S of the area assisted in the extensive hydration of the rocks there.