What is an eclogite?

Eclogite is a very attractive, highly metamorphic rock mainly consisting of two minerals: the green-coloured omphacite and the deeply-red garnet. It can also contain other minerals in smaller, but still noteworthy quantities: kyanite, zoisite and corundum.

During the Cretaceous, rocks in the European lithospheric plate were pushed under the Adriatic plate (part of the former African lithospheric plate) in a process that is known as intracontinental subduction. Among these rocks were gabbros and basalts, which, once transported all the way to the depths of Earth’s mantle, fused into eclogites. Eventually, part of the subducted lithospheric crust broke off and sank into the asthenosphere. This caused an immediate rise in the buoyancy of the remaining plate which stopped sinking and rose back towards Earth’s surface. However, it took many additional millions of years of erosion and thinning of the crust in the region of Pohorje for the eclogites to become exposed to the light of day, sometime in the Tertiary.

Where in Slovenia can we find eclogites and how do we recognize them?

Eclogites can be found in the Pohorje area, where they occur in the form of small lenses or bands trapped within other metamorphic rocks – mostly gneisses, schists and amphibolites. Individual lenses of eclogite can be found both on the southern and northern slope of Pohorje, however they are present in noticeably higher concentrations in the immediate vicinity of Slovenska Bistrica.

How did eclogites of Pohorje form?

The eclogites of Pohorje were formed very deep inside Earth during the process of ultrahigh-pressure metamorphism. The former igneous rocks composed of gabbro and basalt sank to great depths, where under extreme pressure and temperature fragile minerals were disintegrated and new, more durable ones formed. Thus the original igneous rock was transformed into the metamorphic rock known as eclogite.

Thorough petrological and microchemical research has shown that eclogites from Pohorje were exposed to extremely high lithostatic pressures and temperatures (4 GPa, upwards of 900 °C) during their formation. This corresponds to a depth of at least 120 km. The maximum thickness of Earth’s crust, even under the world’s highest mountains, is less than 70 km, which means that the original rocks (basalts and gabbros) from which the Pohorje eclogites were formed were subducted all the way to the mantle. And just how did gabbros and basalts end up so deep in Earth’s interior? Sometime during the Cretaceous period, the approaching European and Adriatic (African) plates closed off all sea basins due to subduction. However, since the two continents continued to collide into each other, the convergence was compensated by the formation of a new, intracontinental subduction zone in the region of Eastern Alps, which Pohorje is a part of. Here the crust, which also contained the original gabbros and basalts, was pushed in a south-easterly direction all the way to the Earth’s mantle. Later, through a process of tectonic extraction, they were again brought to the surface.

What does an eclogite look like under a microscope?

Even rocks that are extremely interesting to the naked eye reveal all of their secrets only once they are carefully examined under a microscope. When thinly ground plates of rock (30 µm) are shone through with polarised light and observed under a microscope, we discover a complex mosaic of different minerals and structures that make up the eclogite.

But is there more to see?

If we want to learn more about eclogites (what pressure and temperatures they were exposed to when they formed? How deep inside the Earth were they? How were they brought back to the surface and what processes took place during this?), we have to use much more precise and expensive techniques. Usually one of the first methods employed is observation of thinly ground and smoothly polished stone tiles under an electron microscope, coupled with simultaneous microchemical measurements.


Do eclogites in Pohorje contain diamonds?

Detailed analyses showed eclogite rocks formed under 4 GPa of pressure and at temperatures exceeding 900 °C. This corresponds to a depth of at least 120 km. However, despite the fact that these conditions certainly allow for diamond crystallisation, unfortunately (or fortunately) there will never be diamond mines in Pohorje. Generally in these types of rocks, microdiamonds appear in the form of tiny inserts in mechanically extremely resistant minerals, such as garnets or zircons. Microdiamonds can be detected only under an electron microscope, and for their unequivocal identification, even more precise analytical devices are needed. In the case of Pohorje eclogites, which are already very attractive rocks in their own right, no diamonds have ever been found. What was found, however, was graphite, the structure of which suggested that it actually formed from diamonds in a reverse reaction. This means that microdiamonds definitely crystallized in the eclogite rocks from Pohorje, but did not survive their way back to the surface. With the process taking too long, they disintegrated into graphite before it was finished. Besides, even if at some point in the future we did manage to find diamonds in these rocks, they would surely be too small in quantity to make their exploitation worthwhile.

Do eclogites have any useful value?

Because of its beauty, some people think that eclogite might serve as a building material, but in reality there is not nearly enough of it in Pohorje for such use. Some jewellers and gemmologists are using it as raw material for small ornaments and jewellery pieces. In this way, the diamond-bearing rock becomes itself a kind of “special gem”.
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