What are some interesting topics in geology

Example of a field exercise

Day 5: Karlovy Vary Pluton

On the fifth day we saw the most important rock types of the Karlovy Vary Pluton complex, the type locality for the Karlovy Vary twins. We then spent most of the day in the metasediments, which are exposed near the pluton, and discussed the possible contact metamorphosis in these rocks. Here again there was the opportunity to practice using the structural compasses.

 

5.1 Road between Karlsbad & Březová u Karlových Var

The first stop on the 5th day of the field exercise led us to a road outcrop between Karlovy Vary and Březová to the south along the Tepla. Here you can see the southern part of a granitic intrusion that is exposed along the road over a distance of approx. 200 m. The rocks of this pluton are summarized under the name "Karlsbader Crystalline Complex".

An intrusion, also known as pluton, is magma that got stuck below the earth's surface and has slowly cooled down there over several million years. The intrusion of the Karlovy Vary crystalline complex is called "granitic" due to its chemistry and its main components are potassium feldspar, plagioclase, quartz, biotite and muscovite.

Due to the size of the entire outcrop, one can see clear differences in grain size and mineral composition in the rock in different areas of the outcrop (Fig. 21).

 

Furthermore, subvertical faults that have formed fissures are repeatedly visible throughout the course of the outcrop. Mineralization is often found in these fissures, indicating hydrothermal activity after the pluton cooled down. These mineralizations mostly consist of calcium carbonate (CaCO3), which is still missing today at the hydrothermal springs in Karlovy Vary.

The intrusion age, i.e. the time at which the magma began to cool, is around 330-320 million years, which corresponds to the late phase of the Variscan orogeny.

 

5.2 Kraslice

The second stop took us to Kraslice, or Graslitz in German. Here we looked at the rocks that are in direct contact with the Karlovy Vary Pluton (Fig. 22).

The sediments there were deposited as molasses during the Cambrian and Ordovician, 540–450 million years ago and form a siliciclastic sequence of sometimes more and sometimes less clay or sand-rich material. Clay and sand not only differ in grain size, but also have different mineral constituents.

 

This can be seen in the metamorphosis of the sediments, as different minerals are formed depending on the chemistry and the characteristics of structural features vary. A more detailed analysis of the chemism and the structures in the phyllites, which are pending here, are part of a current master's thesis. Several phases of metamorphosis probably had an effect on the rocks that are to be carved out.

It is already known that the rocks were initially metamorphically overprinted by the subduction of the Saxothuringian Ocean and were pressed out of the accretion wedge as a kind of blanket in the course of the further subduction.

The greatest changes in the sediments in the course of the subduction took place at approx. 9-10 kbar and a temperature of 450 ° C approx. 340 million years ago. Since the intrusion of the Karlovy Vary Pluton was about 320 million years ago, it can be assumed that the metasediments were again subjected to contact metamorphism - mainly by temperatures of the pluton of 650 to 700 ° C.

 

To overview