Mishmash in the Earth’s crust

Although metamorphic rocks compose a huge part of the Earth’s crust they are less understood than sedimentary or igneous rocks because the depth of the Earth where they form is not accessible for direct study.

For example, if you want to know how the cross-bedding in sandstones forms, go to a river delta and take a look! If you want to know how the odd hexagonal lava pillars take their shape, visit a volcano and observe the cooling of lava. But how can you understand eclogite? The depth of 50 km is probably a bit far to reach.

The metamorphic evolution in Bergen area: (1) Granulite metamorphism during Grenvillian orogeny (900 Ma ago). P≤10 kbar, T= 800-900⁰C. (2) Eclogitisation in Caledonian orogeny Caledonian orogeny 425-460 Ma. P>19 kbar, T=700-750⁰C. (3) Amphibolitisation 420 Ma. P>8 kbar, T=600-650⁰C (base diagram from http://science.jrank.org/pages/47852/metamorphism-metamorphic-facies-metamorphic-rocks.html)

Fortunately there are other powerful tools to apply in this case, such as field observations in outcrops, microscopy examinations, chemical analysis, computer modeling and seismic wave velocity studies. Except of the last one I will try to incorporate all of these methods in my master thesis which is focused on metamorphic rocks in Bergen area.

But let’s start with my pretty outcrops which, I am not going to deny, was a very important reason for choosing this topic for the thesis.

The area

There are 3 main types of metamorphic rocks in my area: granulites, eclogites and amphibolites. All of them can be produced from the same (or different) types of rock just by changing pressure and temperature conditions and fluid content when the rocks goes deeper in the Earth. In Bergen area the source rock probably was something of gabbroic anorthosite composition. Then during multiple tectonic events the rocks were buried or exhumed. The picture above shows the sequence of metamorphic conditions Bergen area has undergone (based on Austrheim and Griffin, 1985; Boundy et al., 1992).

The metamorphic facies 

Granulite facies. The matrix is made of plagioclase (white) and surrounds corona structures which are composed of olivine or pyroxene in the core (black) and garnet rim (red).

Eclogite facies. There is still plagioclase (white) in the matrix because the eclogitisation was not completed and also omphacite (light green). Garnet (red) is disintegrated to individual grains.

 

Amphibolite facies. Matrix consist of plagioclase (white) and corona structures consist of talc or chlorite in the core (light gray) and amphibole in the rim (dark gray). Structurally similar to granulites, mineralogically different.

Each of these events overprinted the previous ones but neither eclogitisation nor amphibolitisation was fully penetrative. They are mostly localised in distinct shear zones. Probably around fractures where fluids got in and accelerated reactions.

So at the end these processes result in amazing outcrops where the transformations between different rocks are captured “in motion” revealing how the minerals and structures change into each other when the rocks experience metamorphism.

More of the pretty pictures:

Granulite blocks embraced by amphibolite facies shear zones

Rotated granulite blocks. Sometimes granulites have pronounced layering marked by elongated coronas. In this case it allows to distinguish rotational domains

Amphibolite shear zone. Note how fluid these rocks look. They should be very ductile at the time of amphibolitisation. Probably large parts of the earth crust is like this at 25 km depth where the amphibolite facies were formed

Initiation of the shear zones in granulites. See how the granulite minerals change the size, shape and colour with deformation and reaction

The shear zones were observed also on large scale

References:

Austrheim H., Griffin W.L. (1985). Shear deformation and eclogite formation within granulite-facies anorthosites of the Bergen Arcs, western Norway. Chemical Geology, 50, 267-281.

Boundy T.M., Fountain D.M., Austrheim H. (1992). Structural development and petrofabrics of eclogite facies shear zones, Bergen Arcs, western Norway: implications for deep crustal deformational processes. Journal of Metamorphic Geology, 10, 127-146.

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