ECLOGITE FACIES METAMORPHISM, EIKSUND AREA, SUNNMORE, WESTERN NORWAY

Field and petrographic studies of eclogites and associated rocks in the Gursköy-Eksund-Hariedland portion of the Basal Gneiss region of Western Norway illustrate a large variety of cases of in situ eclogite formation under dynamic high pressure and high temperature metamorphism. These cases include the following protoliths:

1. Layered noritic intrusive (Eiksundal Eclogite Complex)

2. Basaltic units in meta-sedimentary host strata.

3. Narrow contact zones between ultramafic sills and felsic meta-sediments.

4. Narrow contact zones between amphibolite sills and felsic meta-sediments.

5. Basaltic ash layers in clay-rich meta-evaporites (?).

These inter-relationships of eclogite and other rock types permit the identification of eclogite facies mineral assemblages in non-basaltic compositional environments. In addition to omphacite and garnet, such eclogite assemblages include one or more of the following: Al-hypersthene, Na-biotite, Na-hornblende, quartz, and kyanite. Additional eclogite facies assemblages include Al-hypershene:Mg-garnet, garnet:diopside:olivine:quartz, Mg-garnet:Mg-olivine, diopside:Mg-olivine, and muscovite.

The exposure of eclogite facies mineral assembleges to subsequent dynamic and lower pressure sillimanite facies metamorphism developed definitive corona assemblages characteristic of very low water activity in sillimanite facies environments. Quartz, corundum, kyanite and sillimanite coexisting within a few square millimeters of each other in a plagioclase matrix highlights the extraordinary dry conditions of the area’s sillimanite facies metamorphism.

Structures, fractures and mineral orientations show that the eclogite and sillimanite facies metamorphism occurred during separate and distinct dynamic deformations.

The lithologic column of the layered noritic protolith (Eiksundal Eclogite Complex) consists of at least 21 distinct fractional crystallization layers, the oldest of which are alkali- and water-rich and rhythmically layered. These 21 layers appear to be the consequence of eight separate ultramafic and mafic magma intrusions in to the original magma chamber. Dunite and pyroxene-dunite sills in the surrounding gneisses also indicate existence of ultramafic magma intrusions prior to eclogite facies metamorphism and deformation.