Bergen Arcs: the Secrets of the Deep Crust

If there is a place where rocks tell stories, then in Bergen area, western Norway they are talking like crazy. I just returned from a petrology field course there. It was amazing. I have never seen rocks like that. The outcrops are full of dynamics, everything is in a motion, different rock types changes into each other at the scale of centimeters and every step of this transformation is preserved and exposed in numerous outcrops.

Shear zone “drags in” the amphibolites producing banded gneiss

Geological setting

modified from Håkon Fossen: http://folk.uib.no/nglhe/GeolgyOfBergen.html

Bergen area often referred as Bergen Arcs consists of multiple nappes, thrusted onto each other and folded around the city of Bergen, resembling arcuate structures when watching from space. About 420 Ma ago two continents Laurentia and Baltica-Avalonia collided there resulting in a mountain range comparable with today’s Himalayas. But even mountains do not last forever. Eventually the forces driving collision died out and the phase of extension followed. Then, following millions of years of erosion, the ancient mountains are long gone. But the rocks once buried deep down in the Earth (70-100 km) are lifted up to the surface again. This makes the Bergen Arcs quite a special place. This is where to see how do the rocks beneath the Himalayas or other high mountain ranges could look like and what happens in the orogenic root zones.

What did we see?

Granulite facies anorthosite with corona structures

We spend most of the time on Lindås nappe. The original igneous complex here was metamorphosed to granulite facies about 930 Ma ago. These granulite facies rocks contain peculiar corona structures which consist of an olivine core surrounded by garnet rim. Often multiple rings of piroxenes and chlorite are present, too. All the white stuff is plagioclase dotted with small spinel inclusions.

Original granulites are mostly hydrated and metamorphosed into amphibolite or eclogite facies assemblages. After hydration the rocks often preserve the same corona pattern just the colors are different because of the mineralogical changes.  In the picture above you can see that granulite lenses usually are isolated and surrounded by amphibolites.

Amphibolite facies. The coronas here are altered by hydration reactions. The core is made of talc and the rim consists of amphibole. Plagioclase is whiter and finer grained.

The clast has undergone partial mineral replacement in amphibolite shear zone. Seems that the fracture on the right side opened the clast for fluids. The garnet rim is partially lost (preserved only on upper part) and pyroxene core are partially replaced by amphibole and talc. The sharp borders between different minerals suggest that the reaction was not a temperature-driven diffusion but fluid-induced dissolution-reprecipitation.

Floating and partially transformed corona structures in amphibolite shear zone. Plagioclase (white) shows completely ductile behavior. The clasts with garnet rims (red) stay strong, but the ones altered to amphiboles (black) are starting to behave in a ductile manner. Note that only the clasts far away from other clasts succeeded to preserve the garnet rim. The ones crushed together are the ones most altered.

Around 420 Ma ago Lindas nappe was subducted to the depth of 70 – 100 km. The granulite facies rocks were partially transformed to eclogites. The eclogite in picture has similar corona structures as the granulites but instead of plagioclase matrix coronas are floating in kyanite and omphacite. The garnet rims seem dissolving in matrix and transforming into green omphacite.

The strong blocks of granulite facies are surrounded by weak eclogite. Sometimes granulite blocks have a white rim of plagioclase (upper left block) and often their outer rims are deformed in a ductile manner (lower right block).

In conclusion

This area clearly shows that in nature the transition between different facies of rocks is not that simple as in this diagram:

Pressure–temperature diagram showing the approximate conditions in which the assemblages of the principal metamorphic facies are stable. (Modified from Yardley 1989 and Yardley et al. 1990). Hfls, hornfels; A–E, albite–epidote; Hbl, hornblende; Px, pyroxene, Preh–pump, prehnite–pumpellyite (source: http://science.jrank.org/pages/47852/metamorphism-metamorphic-facies-metamorphic-rocks.html)

The rocks characterized by different temperature-pressure conditions can coexist together and if the fluids are not introduced in the system, the reaction that should happen can be delyed so the mineral assemblages does not always correspond to the metamorphic conditions they are in.

Fascinating place, therefore I took it for my master thesis.

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