site 12

Where is the site located?

GPS: 49° 52′ 27.64″ N, 15° 51′ 8.06″ E

The site lies in the central part of the Iron Mountains National Geopark, in an area where the mountain range is terminated, passing to the NE into the flat region of the East Bohemian Table.

What is the geological position of the site?

The site is located in the Bohemian Massif, in the marginal area of the Central Bohemian Region, where Paleozoic magmatic rocks are exposed. Their present appearance results from endogenous geological forces – orogenic processes shaping the subsurface magmatic body – as well as exogenous geological forces which exhumed the body.

What happened at this site in the past?

– 350–300 million years

Two giant lithospheric plates – Gondwana in the south and Laurasia in the north – collided in the latest Devonian and in the Carboniferous at the end of the Paleozoic. This collision resulted in an orogeny (a mountain-building process) called the Variscan (Hercynian) Orogeny. An extensive high mountain range was formed, now stretching across Europe from the territory of Spain and southern England across France, Germany to Bohemia. The uplift of the mountain range was accompanied by volcanic activity, metamorphism of the original rocks, and the rise of huge bodies of deep-seated magmatic rocks – plutons – into the Earth crust. At this time, a solid basement of the Bohemian Massif was constituted.

What does the site display today?

Two geological units can be observed in the near vicinity of Žumberk. One of them is the northernmost part of the Železné hory plutonic complex, an extensive magmatic body in the core of the Iron Mountains. On its NE margin, this body is lined by subvolcanic rocks of the Lukavice Series, which can be considered the outer envelope of the pluton.

In this area, the Železné hory pluton consists of the Žumberk granite – a rock characterized by its reddish colour. It is also called “red graniteˮ. It comprises the Nasavrky pluton, which was probably formed during the Variscan Orogeny in Late Paleozoic times.

Dominant rocks of the Lukavice Series are porphyries and porphyroids. Their characteristic feature is intense deformation by strong orogenic pressures, which gives them the appearance of slates in places. These rocks locally contain considerable amounts of pyrite (gold-shining iron sulphide).

What was affected by man?

The red Žumberk granite has been exploited since very early times. Several relics of abandoned quarries lie in the close neighbourhood, such as the flooded quarry in the village centre.

Stone is now exploited in two multi-level open-pit mines in the valley of the Ležák Stream. The quarries have altered and scarred the original landscape. At the same time, however, they revealed interesting geological phenomena. Rocks of the Lukavice Series were rather exploited for the high pyrite contents. Exploitation clustered around the Lukavice village, which also hosted the oldest works in the Austrian-Hungarian Empire for chemical processing of pyrite.

What was discovered?

The study of geological conditions of this area has been in progress since 1940s. After having studied the representation of individual rock types, geologists concluded that the Žumberk granite is biotite granite (it contains dark mica), passing to a porphyritic facies (containing phenocrysts of feldspar – light silicate mineral) in some portions. Locally it can be characterized as glassy, with feldspar and quartz phenocrysts. In addition, the actual granitic body comprises other types of rocks, such as greyish-black granites with hornblende (a black silicate mineral) and numerous dark, sharply bounded inclusions. This suggests that the magmatic body is not uniform and underwent several successive stages of origin.

The Žumberk granite is a massive rock with locally typically developed blocky jointing. It is mostly very strongly tectonized, with mylonite zones (zones of intense crushing of original rocks by orogenic pressures) at contacts between the individual rock types.
The age of the Žumberk granite has been determined at 320 million years, which ranks this rock to the late stages of the Variscan Orogeny.

The near-surface parts of the quarry display the style of granite weathering, including colour changes.

Rocks of the Lukavice Series are originally of volcanic origin. Nevertheless, they were strongly pressurized by the subsequent orogenic processes. Later on, they became altered due to their pervasion by hot fluids (a process called hydrothermal alteration) and enriched in other minerals. For example, they became impregnated with pyrite. The rocks have not been precisely dated yet; it can be presumed, however, that they also originated within the Variscan orogenic cycle, although direct evidence is still missing.

“The Iron Mountains – a geologically significant region” project of 2014

Two information panels were manufactured within the project of “The Iron Mountains – a geologically significant region”. The main active quarries are located some 500 m to the northwest, along the road towards Lukavice in the Lažák Stream valley. The quarries are active and can be visited only after a prior consent by the company who operates the quarries.

> Various “granites” from the Žumberk area (Daniel Smutek)