Excursion to Northern Bohemia

July 1-4, 1999

Brief excursion guide

Dobroslav Matìjka, Charles University, Prague

Vojtìch Janoušek, Czech Geological Survey, Prague

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EXCURSION PROGRAMME

Thursday 1st

Lunch: Prague – uranium mine Stráž pod Ralskem (chemical leaching of U-ores, ecological aspects) – regional museum in Turnov (regional geology, gemstones and jewellery with emphasis on semiprecious

stones from the Bohemian Paradise region, Czech garnets)

Dinner and accommodation: Liberec (Reichenberg)

Friday 2nd – working quarry Doubravice (Permian basaltoids – melaphyres – with amygdales infilled by semiprecious

stones – quartz, agate, amethyst, smoky quartz, jasper etc. ) – abandoned quarry Pelechov (Tertiary nepheline basanite lava flow with Upper Mantle, spinel lherzolite nodules)

Lunch: Kozákov hill (Tertiary volcano – a viewpoint for much of the Bohemian Paradise) – abandoned quarry Jílové (roofing phyllites) – working quarry Černá Studnice (Tanvald two-mica granite) – working quarry Hraničná (Liberec biotite granite, mingling phenomena, pegmatites)

Dinner and accommodation: Liberec (Reichenberg)

Saturday 3rd – Bílé kameny (morphologically remarkable Cretaceous sandstones close to the Lusatian Fault) – Jítrava (moraine dating back to the continental glaciation)

Lunch: Stráž pod Ralskem – abandoned quarry Panská skála (columnar jointing in a basaltic lava flow, natural preserve area) – working quarry Soutěsky (famous locality for zeolites, namely natrolite, in Tertiary basaltoids, Tertiary tuffs) – abandoned quarry Dobkovice (dyke rocks of the Roztoky volcanic centre)

Dinner and accommodation: Ústí n.L.

Sunday 4th – Vrkoč (columnar jointing in a basalt dyke, natural preserve area) – working quarry Kubo (Late Carboniferous rhyolitic ignimbrite) – museum of Bohemian garnets in Třebenice (garnet mining, garnet jewellery)

Lunch: Třebenice

The authors would like to acknowledge the help from Jiří Adamovič, Milan Čermák, Marek Diepold,Emil Jelínek, Jiří Kühn, Jan Slezák and Tomáš Wiesner. We are obliged to DIAMO, Ligranit Inc., Železnice Doubravice Ltd,Weiss Ltd., Kubo Ltd. and R. Zajíček. Special thanx are due to Zbyněk and Martina Vencelides; shame and utter (economic) damnation on the TARMAC Inc. for requesting admission fee 100 CZK per student and quarry.

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INTRODUCTION

Geology of the southwestern part of the Krkonoše Crystalline Unit The West Sudetes constituting the NE margin of the Bohemian Massif are a heterogeneous mosaic of terranes with different pre-Variscan sedimentary and tectonometamorphic history juxtaposed during the Variscan docking of Gondwana-derived crustal fragments with Baltica/Avalonia. In the European Variscan belt this unit is often interpreted as the easternmost part of the Saxothuringian zone.

The Krkonoše–Jizera Crystalline Unit represents the westernmost part the W Sudetes situated between the Elbe line in the southwest, Lusatian anticlinorial zone in the W, Intrasudetic fault in the N and NE and Lesczyniec shear zone in the E (Fig. 1).

ŽacléřVrchlabí

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Figure 1. Simplified geological map of the Krkonoše-Jizera Crystalline Unit (Kachlík and Patočka 1999): Basement unit: 1. Late Proterozoic Machnín Group (metagraywackes, metapelites), 2. Cadomian Zawidow Granodiorite; 3. mica schists to gneisses in the Jizera Orthogneiss; 4. Jizera Orthogneiss (510-480 Ma); 5. Rumburk Granite (510 Ma); Paraautochthnous slices: 6. Early Palaeozoic phyllites, graphite phyllites (with metabasite, quartzite and marble intercalations - Silurian Ockerkalk), Devonian fossils; 7. Early Palaeozoic (?) Ordovician phyllites with quartzite intercalations overthrust over sequence with Late Devonian fauna; 8. quartzites, 9. phyllites with intercalations of Middle to Late Devonian marbles; 10. Late Devonian to Early Carboniferous flysch deposits with intercalations of metabasalts, acid volcanics and marbles with Famenian to Early Tournaisian Fauna. Allochthonous units: 11. (?) Cambrian–Ordovician volcanosedimentary unit (metatuffites, roofing phyllites, with metadiabase sills and dykes, rare metagabbros and picrites, phyllonitized granites; roofing phyllites with (?) Ordovician ichnofauna; 12. Železný Brod Volcanic Complex (metabasaltic pillow lavas, metatuffs and acid metavolcanics in the upper part with intercalations of marbles and mixed volcanogenic quartzites); 13. sericite phyllites with intercalations of marbles and quartzites, product of basic volcanism waning out towards top of the sequence); 14. phyllonitized granites and orthogneisses; Late Variscan granites: 15. Krkonoše-Jizera Pluton (310 Ma); Platform sediments: 16. Permo-Carboniferous deposits of the Krkonoše piedmont basin; 17. deposits of the Czech Cretaceous basin; Neovolcanic rocks: basanites, olivine basalts (Pliocene)

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Based on recent investigations, the following tectonostratigraphic units are distinguished in the Krkonoše-Jizera Crystalline Unit (going from bottom to top of the structural sequence):

(1) An autochthonous unit which is composed of Cadomian Lusatian granitoids dated at 540-587 Ma (Kröner et al., 1994), and the associated country rocks. The autochthonous unit experienced greenschist facies metamorphism of Cadomian age and a non-penetrative Variscan overprint.

(2) A paraautochthonous to allochthonous unit of very low-grade metamorphosed Early to Late Palaeozoic volcano-sedimentary suite showing close similarity with the Thuringian facies; these rocks are void of Variscan HP-LT metamorphic features and show only weak late Variscan greenschist overprint.

(3) An allochthonous composite unit. The major part of this unit is occupied by large antiform of the Jizera and Krkonoše gneisses. In the core of the antiform the Late Variscan Krkonoše-Jizera granite pluton was emplaced. The southern and eastern rims of the Krkonoše-Jizera Crystalline Unit consist of Early Palaeozoic volcanosedimentary sequences of the South and East Krkonoše Complexes. The major late Variscan shearing and thrusting which produced NW-SE directed linear fabric of the Krkonoše-Jizera Crystalline Unit progressed in the times between 340 and 320 Ma (Marheine et al. 1999a; Marheine et al. 1999b) and was followed by the Krkonoše-Jizera granite intrusion dated at 328±12 (Pin et al. 1987), which cooled down at 313±3 Ma (Marheine et al. 1999a; Marheine et al. 1999b).

DM - modified after Kachlík et al. (in print)

The Krkonoše-Jizerské hory Pluton Variscan granites (the Krkonoše-Jizerské hory Pluton) intruded the earliest Precambrian rocks forming the core of the Krkonoše–Jizerské hory crystalline complex. According to Klomínský (1969) on the Bohemian side of the pluton significantly prevails porphyritic medium-grained biotite granite to granodiorite and in the deeper levels porphyritic coarse-grained biotite granite. Medium-grained biotite granite and fine-grained biotite granite occur in subordinate amounts. They build mainly the high ridges of the Krkonoše Mts. In the northern surroundings of the town Liberec small bodies of biotite-hornblende granodiorite have been delimited. A special position in the pluton has the Tanvald two-mica granite which form an inconsistent rim along the western to southwestern margin of the pluton. Dykes of aplites, aplite granite, pegmatite, granite porphyry and lamprophyre are relatively rare on the Bohemian side of the pluton. They show mostly NW-SE trends. Preferred orientation of feldspar phenocrysts, biotite and aplite schlieren (probably remnants of uncompletely assimilated mica schist or gneissic rocks) and basic inclusions mark dome structures under which the roots of the granite pluton are presumed (Cloos 1925, Klomínský 1969). On the basis of gravimetric measurements estimated thickness of the tongue-shaped pluton on the Polish side seems be approximately 4-5 km respectively up to 10 km on the Bohemian side of the Krkonoše Mts. The granite intrusion reflects and preserves structures of the preexisting crystalline schists. The width of the contact aureole depends greatly on the inclination of the granite contact. At the southern border of the granite in the Krkonoše Mts. where the contact plane dips about 40-60° to the south, the width of the contact aureole reaches several hundreds up to 1500 meters. Westwards, along the Tanvald two-mica granite as well as along the northern border of the granite pluton the contact aureole is considerably narrower - the contact plane here is probably steep. The effects of contact metamorphism are absent along the deep-seated faults which delimit the granite pluton in the west and north-east. The Krkonoše-Jizerské hory Pluton is one of typical representants of the Variscan granitoids of the Bohemian Massif. Early Upper Carbonian ages of granite evolution (328 to 313 Ma - see above) correspond to the uplift of granite and of the whole crystalline area recorded in the formation of big conglomerate fans in the adjoining Permo-Carboniferous basins in the time of the Upper Carboniferous. The intrusion of the granite about on the Lower/Upper Carboniferous boundary probably corresponds to the period of stress relaxation which immediately followed the last prominent folding in the Krkonoše-Jizerské hory area. The granite intrusion used an old E-W trending tectonic zone (the South Lusatian Fault Zone).

DM - modified after Chaloupský et al. (1989)

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Intraplate Cainozoic volcanism in the Bohemian Massif An extensive rift system (extending from French Massif Central to N Moravia) formed in response to the Alpine collision between Europe and Africa in Early Tertiary times. Rift-related asthenospheric mantle upwelling (or, less likely, heating by several hot spots) resulted in generation of large volumes of mantle-derived magmas that originated as mixtures between asthenospheric- and lithospheric mantle-derived melts with negligible crustal contamination (Wilson and Downes, 1991). The main phase of volcanic activity took place in the Miocene–Pliocene times, with Pleistocene volcanism being restricted to only a few widely scattered districts.

In the Bohemian Massif, the SW–NE trending Ohře (Eger) Rift (Fig. 2) corresponds to a reactivated first-scale Variscan tectonic boundary separating the Saxothuringian (to the NW) and Moldanubian (to the SE) basement terranes. Continuation of the rift system towards the SE represents the Labe (Elbe) Tectono-Volcanic Zone. Two major Cainozoic volcanic centres, České středohoří Mts. and Doupovské hory Mts., developed on these deep-seated tectonic zones. According to Ulrych et al. eds (1998 and references therein), the intraplate magmatism connected with the Ohře Rift development can be divided into two main series:

1. ultramafic ultra-alkaline pre-rift series (79–51 Ma) — unimodal melililtic volcanism

(precursor of continental rifting) 2. alkaline rift series

2a coexisting mostly bimodal (basanite–trachyte and olivine nephelinite–phonolite) and mostly unimodal (foidite) rock series in Doupovské hory Mts. (42–16 Ma)

2b unimodal (foidite, in České středohoří) and coexisting contrasting strongly (basanite/tephrite–phonolite) and weakly (trachybasalt/trachyandesite–trachyte/rhyolite) alkaline series (12–8 Ma)

2c unimodal (foidite series) – W Bohemia and N Moravia (4.6–0.26 Ma)

The České středohoří Mts is the largest and the most complicated volcanic complex on the territory of the Czech Republic. In here, the volcanic activity lasted from 35 to 15 Ma, with a distinct peak at 30–25 Ma (Wilson et al.¸ 1994 ). According to the succession scheme given by Ulrych et al. eds (1998), the volcanic activity started with degassing of the magma chamber. At that time formed maars and diatremes, still preserved in the southern part of the mountain range. With the continuing subsidence in the rift zone, numerous deep-seated faults developed

Figure 2. Principal tectonic structures of the Bohemian Massif reactivated in connection with the Cainozoic intraplate volcanism (DH = Doupovské hory Mts., CS= České středohoří Mts.) (Ulrych et al eds.,1998)

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and the explosive phase was replaced by areal effusions of olivine basalts. The lava flows that reached shallow lakes were brecciated, forming hyaloclastites. Continued volcanic activity resulted in formation of a large composite volcano, with the main volcanic centre located at the intersection of the Ohře Rift and Labe Tectono-Volcanic Zone, where prevalent pyroclastic rocks alternated with mainly tephritic lavas. At this stage also formed large intrusive bodies of essexite and sodalite syenite. The last volcanic activity, represented by the unimodal (foidite) series, is restricted to the North Bohemian Brown Coal Basin.

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Figure 3. Anatomy of a region with active volcanism (Oxford–Duden Pictorial English Dictionary, 2nd edition, Oxford University Press) 13 – shield volcano, 14 – lava plateau, 15 – composite volcano, 16 – volcanic crater, 17 – volcanic vent, 18 – lava stream (lava flow), 19 – tuff, 20 – subterranean volcano 21– geyser, 22 – jet of hot water/steam, 23 – sinter terraces, 24 – cone, 25 – maar, 26 – tuff deposit, 27 – breccia (diatreme), 28 – vent

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U1 tephrite (ol < 10%) basanite (ol > 10%)U2 phonotephriteU3 tephriphonolitePh phonolite

Figure 4. IUGS TAS diagram for classification of volcanic rocks

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1. Stráž pod Ralskem DIAMO, area of the chemical mining, 2 km SSW of Stráž pod Ralskem In the mining field, uranium is leached from Cenomanian sandstones underlying Turonian sediments. Both formations are partially isolated by a Lower Turonian semi-pervious horizon. Uranium minerals are incorporated in the cement of sandstones. H2SO4 is the leaching medium but other acids as well as ammonia are used to improve technological properties of leaching solution and for processing of uranium-bearing liquids to ammonium diuranate as final product. Now the chemical mining has been terminated being changed to "rescue mining" removing contaminants from groundwater, however, this process should be finished only in 2030.

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2. Doubravice Working quarry 4 km S of Lomnice nad Popelkou The Permian basaltoid volcanic rocks — called locally melaphyres —are widespread in the region of the so-called Bohemian Paradise (Fig. 5). They are typically reddish in colour and contain abundant and often large amygdales (cavities with secondary infilling left after degassing of the magma) that, together with cracks, were infilled by products of secondary hydrothermal alterations of the parental rocks, relatively rich in silica. The localities throughout this region are renown for numerous occurrences of various semiprecious stones, mainly modifications of SiO2, such as rock crystal, amethyst, smoky quartz, agate, chalcedony (including carnelian and moss agate), opal and jasper. But the secondary mineral paragenesis is even richer, and includes also chlorites, calcite, hematite, goethite, zeolites, and native copper.

The tradition of collecting and polishing agates and jaspers in this region dates back to medieval times, and the jewellery produced here has been, until the discovery of deposits of larger and more colourful stones in Brazil and Uruguay (polished mainly in workshops in Idar–Oberstein, Germany), word-famous. Even nowadays, the jewellery is produced here from local agate, jasper as well as pyropes from České středohoří Mts. In this context is especially noteworthy the production of the Turnov co-operative “Granát”.

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Figure 5. Geological map of main occurrences of secondary minerals in volcanic (mainly Permian and Tertiary basaltic) rocks in Northern Bohemia (Bernard et al., 1981). 1 – granitoids, 2 – crystalline basement, 3 – Permian sediments, 4 – Permian basaltoids (melaphyres), 5, 6 – porphyries, 7 – Cretaceous cover, 8 – Tertiary basaltoids, 9 – 11 mineral localities

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3. Pelechov at Kozákov Disused quarry, 2 km ESE of Železný Brod At this locality can be observed nepheline basanite lava flow that originated in the Neogene (4–6 Ma) Kozákov Hill volcano (744 m). The nepheline basanite is a rather primitive rock, as documented by its high 143Nd/144Nd = 0.512796, and low 87Sr/86Sr = 0.70313 that afford for little crustal contamination and instead still reflect its asthenospheric mantle source (Bendl et al., 1993).

Some 10 vol. % of the Kozákov lava flows are made of upper mantle material, either still preserved in the form of rounded ellipsoidal xenoliths (2–3 %) or xenocrystic olivine (7–8 %) (Medaris et al., 1997). The great majority of xenoliths are spinel lherzolites and harzburgites (96%), subordinate are dunites (3%) and pyroxenites (1%) (Jakeš and Vokurka, 1987).

The spinel lherzolite xenoliths up to 70 cm across (Bernard et al., 1981) consist of olivine (91.5% of Fo component) orthopyroxene (bronzite), clinopyroxene (Cr-diopside), spinel (Cr-rich, picotite). Recently were recognised lherzolite xenoliths that contain spinel–pyroxene symplectite. These symplectites are interpreted as resulting from a reaction of pre-existing garnet with olivine during re-equilibration of garnet peridotites at shallower depths, already within the spinel

peridotite stability field (Medaris et al., 1997).

Some of the olivines from the

Kozákov lherzolite nodules are of a gem quality. The largest cut stone from here, 15.7 carats in weight, can be seen on exhibit in the National Museum in Prague (Bernard et al., 1981).

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Figure 6. Geobarometry of the symplectite-bearing lherzolite xenoliths from the Kozákov area (Medaris et al., 1997). SplPd/GrtPd is a boundary between the stability fields of spinel and garnet peridotites

S-1 A Lherzolite

(17) Smčí SiO2 43.47 44.78 TiO2 2.83 0.06 Al2O3 13.04 1.76 Cr2O3 - 0.35 Fe2O3 2.00 0.83 FeO 9.98 7.67 MnO 0.19 0.25 MgO 9.26 41.88 CaO 10.94 1.80 Na2O 3.16 0.08 K2O 1.39 - P2O5 2.03 - H2O+ 0.55 - LOI 0.68 SUM 100.14

Table 1 A chemical composition of basanitic rocks, hosting lherzolite xenoliths in Northern Bohemia (average of 17 nodule-free samples); S-1 whole-rock geochemical composition of a spinel lherzolite xenolith from the Kozákov area (Jakeš and Vokurka, 1987 and references therein)

Figure 7. IUGS classification of ultramafic rocks without amphibole

4. Roofing phyllites (? Ordovician) of the Železný Brod Crystalline Unit Abandoned quarry in the roofing phyllites, Jílové at Držkov, 6 km N of Železný Brod The so-called roofing phyllites are represented by thinly shistozed and mostly laminated grey to grey-greenish and violet graphite bearing sericite-chlorite phyllites with parallel planes of foliation. Such type of roofing phyllites is preserved mainly in the NE flanks of a large anticline moderately dipping to the NE (Fig. §§). In the highly strained domains and in the areas of alternation of competent (basic tuff layers) and incompetent (sedimentary layers) open to close folds are developed.

The roofing phyllites crop out in the N–NE limb of a large scale asymmetric antiformal structure, representing the oldest volcano-sedimentary part of the Železný Brod Crystalline unit (Fig. §§). They are overlain by grey to black graphitic phyllites with common quartzitic laminae, passing into light sericite quartzite and quartzitic phyllites. The tectonically modified boundary of both units is marked also by occurrences of metagabbros, coarse-grained metadiabases and picritic ultrabasites (Fig. 8).

The roofing phyllites consist of alternating fine-grained quartzitic layers with domains rich in sericite, chlorite with disseminated graphite or ore minerals. Columnar crystals of dark grey randomly to preferably oriented chloritoid are common constituents of these rocks. Stilpnomelane is sometimes present as tiny needles in recrystallized matrix in pressure shadows behind chloritoid crystals. The chloritoids and stilpmnomelane are often in highly strained samples partially replaced by chlorite an sericite during later greenschist equilibrations. Composite metamorphic foliation mostly follows primary sedimentary bedding (ichnofossils could be preserved on the foliation planes), but in fold hinges new cleavage originates oblique to perpendicular to primary layering. Pelocarbbranches of ichnofossils document flattening to plain strain deformatio

The original sediments were apparently relatively monotolayers, rich in basic volcanic admixture. The enrichment Al and Fe com

The roofing phyllites bears a relatively rich assemblage of icsee (Chlupáč, 1997, for details). The occurrences of spiral (Spirodestellatus) - (Fig. 9) and delicate meandering (Dictyodora) traces point environment of the Nereites ichnofacies. Such an ichnoassemblagphyllites. The occurrences of the above mentioned ichnotaxa suggest th

Figure 9. Star-like ichnofossil (probably Teichichnus stellatus BALDWIN) from the roofing phyllite. Jílové near Železný Brod. Scale bars = 30 mm (from Chlupáč 1993)

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Figure 8. Geological map of the surrounding of Loužnice village, NE from Železný Brod (Kachlík 1998) Quaternary: 1. Fluvial sediments, 2. deluvial sediments, Železný Brod Crystalline Unit: Subunit 1 (Kachlík, Patočka 1998): 3. roofing phyllites, (Ordovician?) , 4. metagabbro, 5. sericite-chlorite phyllite, metatuffite, Subunit 2: 6. graphitic phyllite, 7. Pyroxene-bearing coarse-grained metadiabases, 8. quartzitic phyllite, 9.graphite-bearing quartzitic banded phyllite, 10. metamorphosed ultrabasite (picrite).

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onate concretions and cross-sections of n. nous pelitic to tuffitic shales, with silty es from synchronous volcanism.

hnofossils (Chaloupský a Chlupáč, 1984), smos) star-like (Lorenzinia, “Teilichnus” out to an oxygenated low energy deep sea e rules out Proterozoic age of roofing at phyllites are not older than Ordovician.

DM - modified after Kachlík (in print)

5. Černá Studnice A small quarry in operation on the W slope of Černá Studnice Hill, 3 km SE of Jablonec nad Nisou At this locality, one of two main granite types building the Krkonoše-Jizerské hory Pluton, the two-mica granite (after a near town this rock is also described as the Tanvald granite) is quarried.

The rock is medium-grained (up to coarse-grained in the central part), light grey to light yellow in colour, with only rare schlieren and basic enclaves. It is composed of K-feldspar (microcline), sodic plagioclase, quartz, muscovite and biotite in nearly equal proportion, with accessory garnet, zircon, sphene, epidote, apatite, magnetite. Unlike biotite granite, two-mica granite contains also accessory andalusite and monazite. S-type character of the rock is supported by A/CNK values as well (Tab. 2).

Accompanying dyke rocks of the two-mica granite are rather uniform. They are mostly aplites, rarely pegmatites; two dykes of granite porphyry were also described in the quarry visited.

Two-mica granite forms a long body 3 km wide as a rim of the soutwestern part of a large body of (porphyritic) biotite granite (Liberec granite) and is exwith the biotite type seems to be sharp and steep. Thbiotite type. This can be suggested also from rare peprobably derived from the biotite granite. The rock is used as building as well as decorative mat

6. Hraničná A quarry in operation 5 km N of Jablonec nad Nisou Main type in the Krkonoše-Jizerské hory Pluton, biotcan be distinguished on the basis of the grain sizeporphyritic coarse-grained variety in deeper parts ofKrkonoše part of the pluton, also non-porphyritic, maplitic) varieties are developed.

At a glance the porphyritic coarse-grained biphenocrysts (up to 3 x 2 cm) of light pink K-feldsgroundmass is composed of K-feldspar, sodic plagioczircon, apatite, magnetite.

Various inhomogeneities can be observed in country rocks usually exhibiting sharp contact with well. The size of these enclaves varies extremely fromas individual types (the biotite-amphibole Fojtka granoThe Liberec granite is one of the most decorative ston

SiO2

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Fe2O3

FeO MnO MgO CaO Na2O K2O P2O5

CO2

H2O+ H2O- F Total A/CNK Table 2: AJizerské homedium–gcoarse–gragranite, 4 grained mgranodiori

1 2 3 4 5 6 70.98 72.87 74.37 75.02 64.82 73.88

0.48 0.3 0.24 0.18 0.89 0.04 14.09 13.45 13.15 13 14.71 14.56

0.77 0.43 0.66 0.5 0.88 0.47 1.81 1.55 1.11 0.83 3.83 0.66 0.06 0.06 0.04 0.04 0.09 0.08 0.81 0.41 0.48 0.33 1.87 0.11 1.91 1.54 1.02 0.85 3.64 0.41 3.39 3.35 3.37 3.33 3.44 4.05 4.47 4.65 4.48 4.65 3.81 4.46 0.12 0.1 0.07 0.05 0.26 0.14

- - 0.01 0.01 - 0.1 0.81 0.72 0.69 0.75 1.19 0.77 0.13 0.04 0.09 0.13 0.07 0.03

- - 0.02 0.02 - - 99.83 99.47 99.81 99.69 99.5 99.76 1.01 1.01 1.07 1.08 0.90 1.19

verage chemical composition of granites of the Krkonoše-ry Pluton (Chaloupský et al., 1989): 1 porphyritic

rained biotite granite to granodiorite, 2 porphyritic ined granite (Liberec granite), 3 medium-grained biotite

fine–grained biotite granite and aplitic granite, 5 fine–elanocratic porphyritic biotite–amfibolite granite (Fojtka te), 6 two–mica granite (Tanvald granite)

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posed in the western part of the pluton as well. The contact e two-mica granite is generally considered older than the

netrations of fine-grained granites, aplites and pegmatites,

erial.

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ite granite, is exposed in this quarry. Varieties of this type . Porphyritic medium-grained granite prevails passing to the body (exposed in the quarry presented here). In the edium-grained as well as fine-grained (locally passing to

otite granite (termed as Liberec granite) is characterized by par, often rimmed by white plagioclase. Coarse-grained lase (albite-oligoclase) and biotite, accessories are sphene,

the granite. They are mostly considered relics of crystalline the granite but schlieren (mostly slab-like) are present as a few centimeters to huge blocks forming rocks classified diorite). Pockets of pegmatitic material are also present.

es quarried in the Czech Republic.

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7. Bílé kameny (= White Stones) Morphologically prominent denudation relics of Upper Cretaceous sandstones, 14 km NW of Liberec, natural preserve area Between several hundred meters and almost 1 km of Upper Cretaceous platform sediments cover most of the basement in the northern Bohemia, forming the so-called Bohemian Cretaceous Basin. These sediments are mainly sandstones, marlstones and claystones, with proportion of clayey sediments being generally higher in the W part of the basin. The sedimentation started by Cenomanian (c. 97 Ma) clayey lacustrine sediments. With progressive subsidence, sea transgression has taken place and the sedimentation became mainly psammitic in character. The sea covered this region until Santonian (Mišík et al., 1985), i.e. for some 12 Ma. Since then, the Cretaceous cover was segmented by faults and subjected to erosion.

The small „rock city“ Bílé kameny is referred to sometimes as Sloní kameny („Elephants’ Stones“) as it resembles resting elephants by rounded shapes and greyish–whitish colour. The rock city is formed by Upper Cretaceous sandstones tilted due to Tertiary movements on the nearby Lusatian Fault. This normal fault was responsible for relative uplift of the NE block, from which Cretaceous sediments were completely removed by erosion. and, therefore, to the NE of this important tectonic line crop out only older metamorphic rocks of the Ještěd Crystalline Complex. The composition of matrix of the sandstone varies; towards the bottom, the proportion of kaolinite is lower and hence the sandstone shows more pronounced bedding and is distinctly darker in colour. In sandstone are well developed some weathering phenomena, such as fissures, small caves, and even a miniature tunnel, c. 4 m long (Kühn, 1999).

In a nearby disused sand pit can be observed glacial sediments, brought to this region from Northern

Europe by the continental glacier. The glacier has reached the northernmost parts of the Czech Republic during two glacial stages, termed Elster and Saale. Some exotic boulders can be observed within the moraine, among others of flint from the shores of Baltic Sea and rocks of the Baltic Shield.

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Figure 10. Europe during the last glacial stage. 1: tundra, 2: steppe, 3: coniferous forests, 4: broad leaf forests (Mišík et al., 1985 and references therein)

8. Panská skála at Kamenický Šenov Disused quarry 9 km NW of Česká Lípa, natural preserve area This conspicuous rock formation (580 m) — called due to its appearance also “Stone Organ” — is a fine example of columnar jointing in basaltic rocks. The regular (mostly hexagonal) jointing pattern developed in response to cooling of a lava flow, the individual columns being perpendicular to the cooled surface. The columns are nearly vertical, up to 12 m long and some 20–40 cm in diameter (Kühn, 1999).

This striking geological phenomenon was uncovered by a quarry, dating back to 18 century (Kühn, 1999). Its preservation was possible due to intervention of Prof. J.E. Hibsch, who was a key figure in early studies of volcanic rocks in the České středohoří region. In years 1891–1930 he conducted extensive geological mapping of the whole region on the scale 1:25 000 (Glöckner, 1995).

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9. Soutěsky Working quarry 4 km SE of Děčín In this quarry can be seen a profile through volcanic products of the České středohoří volcanic centre. The lowermost part is formed by re-deposited coarse-grained volcaniclastics of olivine basaltic rocks, covered by olivine basalt extrusions. The main basaltic rock, subject to quarrying, is a columnar-jointed intrusion of analcite basanite (Cajz ed., 1996).

Basaltoids contain locally large amygdales infilled by a rich paragenesis of zeolite minerals, well-known among the mineral collectors. The most famous are natrolite needles up to 5 cm in length, analcite in crystals up to 1 cm across, apophyllite crystals up to 1 cm across; common is also calcite (Bernard et al., 1981).

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SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO BaO Na2O K2O P2O5 CO2 H2O+ F S H2O- SUM Cr Cu Nb Ni Pb Rb Sr Y Zn Zr Be

Photo 12. „Stone Organs“ – columnar jointing in a basaltic lava flow, Panská skála at Kamenický Šenov (Kühn, 1999)

Table 3: A chemical composition of olivithe Soutěsky quarry (Shrbený, unp

CS142 ol alk basalt

Soutěsky 43.22

2.97 15.07

4.23 6.98 0.18 7.59

11.49 0.703

3.16 1.43 0.67 0.06 2.73 0.08 0.01 0.47

101.04 172

83 80 98 10 28

730 26

106 176

2

Figure 11 Columnar jointing in a basaltic rock (Homola, 1971)

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ne basalt from ublished data)

10. Dobkovice Abandoned quarry 5 km S of Děčín In this disused quarry (now used by a small sawmill) crops out olivine nephelinite, penetrated by younger dyke rocks. The bottom of the quarry walls is obscured by rubble but rock blocks still can be seen. Main rock type, olivine nephelinite, is a dark grey rock with a microporphyritic texture containing olivine crystals up to 1 mm across. Rock is made up by olivine (8 %), pyroxene–augite (52 %), magnetite (12 %) and nepheline (25 %), locally is present also volcanic glass.

Apart from the olivine nephelinite, also dykes up to 2 m wide can be seen in this quarry. Generally two types can be observed — light dykes of trachytic composition, and dark ones of camptonites to monchiquites. They are considered to having been derived from an alkaline magma, whose products — essexites and sodalite syenites — are exposed in the central part of the České středohoří Mts. (Roztoky volcanic centre).

Dark minerals of trachytes are represented by pyroxene, amphibole (kaersutite) as well as biotite in various proportions; groundmass is dominated by feldspar (50–70 %, sanidine prevailing over sodic plagioclase), with lesser proportion of magnetite. Accessoric are titanite and apatite.

Camptonite and monchiquite are very nice rocks of dark grey colour (that changes due to weathering to lighter grey) exhibiting well-developed phenocrysts of augite (having diopside-salite composition) as well as amygdales filled by zeolite minerals. Matrix is composed by augite (45 %), rather basic plagioclase (34 %), magnetite (10 %). The rest is made up by biotite, amphibole, glass and analcime.

Similar dykes can be studied in the working quarry Těchlovice on the opposite side of the Labe River.

DM

Monchiquite Camptonite Gauteite Phenocrysts Ti-pyroxene (diopside-salite), kaersutite, rare

plagioclase, biotite, ± leucite Ti-pyroxene, am± plagioclase an

glass » feldspar feldspar » glass Groundmass kaersutite, clinopyroxene, biotite, accessory Fe-Ti oxides, pyrite, apatite

feldspar, glass, clinopyroxene, kaersutite, biotitsodalite, accesso

Secondary minerals

zeolites (esp. natrolite), analcime, calcite, sericite, limonite, clay minerals

zeolites, analcimminerals

Remarks mafic minerals > Table 5. Nomenclature of common dyke rocks of the Roztoky volcanic centre ( 11. Vrkoč 4 km S of Ústí n. L., natural preserve area Famous basaltic dyke showing an inverted fan-shaped columnar jointing. It penoverlaying olivine basalt lava flows and has been uncovered due to its higher rpeculiar orientation of the columns is due to the presence during the solidificabeing the dyke margins and the third the actual surface of the lava flow(Cajz ed., 1996).

SiTAFeFeMMCNKP2

CHH

TRSrBZCCTmanD

M C C G O2 45.84 49.95 49.14 51.65 iO2 3.37 2.04 1.86 1.93 l2O3 15.75 17.35 17.50 17.18

2O3 5.23 4.36 4.71 3.54 O 5.43 3.35 2.78 2.95 nO 0.15 0.20 0.01 0.12 gO 4.12 2.52 2.26 1.87 aO 8.02 6.54 5.82 4.62 a2O 4.53 5.18 5.06 1.87

2O 4.19 4.46 4.60 8.95 O5 0.52 0.57 0.36 0.41 O2 0.49 0.88 0.85 0.89

2O+ 2.48 2.56 0.30 2.88

2O- 3.87

otal 100.12 99.96 99.12 98.86 b 110 206 835 1237 a 800 1480 r 467 376 r 34 20 o 22 20 able 4: Chemical composition of onchiquite (M), camptonite (C) d gauteite (G) from the

obkovice quarry (Tvrdý, 1986)

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Dyke trachyte phibole, d biotite

feldspar, clinopyroxene and amphibole in small amounts

e, ± ries

feldspar, green clinopyroxene, ± sodalite, accessories

e, calcite, limonite, sericite, clay

25 % mafic minerals < 25 %

afterTvrdý, 1986)

etrates Cretaceous sediments and esistance against the erosion. The tion of three cooled surfaces, two pouring from this fissure vent

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12. Malé Žernoseky - the Kubo quarry [after Ulrych et al. eds. (1998)] Small working quarry 3.5 km N of Lovosice Rocks of the pre-Cretaceous basement are exposed in the “Bohemian Gate” (Porta Bohemica) area, a narrow and deep incision of the Labe River. The boundary between gneisses of the Krušné hory Mts. Crystalline Complex of the Saxothuringicum (in the N) and the phyllites of the Teplá-Barrandian Zone of the Bohemicum (in the S) is marked by metabasites. These rocks are covered by the Upper Carboniferous ignimbritic rhyolite. The younger tectonic manifestation of the Saxothuringicum-Bohemicum contact, the Litoměřice Deep-seated Fault, trends WSW-ENE in this area. The uplift of the post-Cretaceous horst-like structure of the same direction (the Oparno-elevation) is responsible for the present high position of the crystalline complexes.

The erosional relict of Upper Carboniferous ignimbritic rhyolite is several tens of metres thick and is accompanied with small beds of conglomerates of identical age. At the top of the uplifted block, the relict of Quaternary sediments of the Labe River terrace is preserved. The rhyolite is extracted for building stone only in the Kubo quarry. In the La-Téne period millstones for hand-operated crushing of corn were produced from this material and some remnants of them are still present in the old quarry. Vertical columnar jointing, very rare in such rock type, was caused by the behaviour of rhyolite mass similar to that in lava flows. Primary pyroclastic material is strongly consolidated due to

welding. Hemicrystalline matrix of the ignimbrite often encloses broken crystals of alkali feldspar, quartz, plagioclase and less frequent fragments of crystalline rocks. Lapilli are rimmed by fluidal glassy material. Relics of pumice are visible by naked eye and the fiamme (former pumice thermally collapsed into the glass) were also described. The ignimbrite is apparently a product of very hot pyroclastic flow — nuée ardente. This occurrence is an erosional relic of a formerly wider ignimbrite sheet produced from the Altenberg caldera and pertains to its lowermost ignimbrite sequence. The precise eruption age of the ignimbrites is not yet known. The newly obtained data from boreholes and geological survey allow to define the extent of the caldera beneath Cretaceous and Miocene sediments (Mlčoch, 1994). The caldera is a deeply eroded structure about 18 × 35 km in size. It is filled with the Teplice rhyolite body, with sunken block of gneisses — Altenberg block, and it is limited by dykes of granite porphyries (see the Fig. 13). Its age can be estimated at Westphalian C/D based on radiometric dating and phytopaleontology of the associated sediments. Two analyses of the Kubo quarry ignimbrite are given in the Table 6.

wt. % 1 2 SiO2 79.51 77.29 TiO2 0.22 0.20 Al2O3 11.37 12.01 Fe2O3 1.61 2.44 FeO 0.26 0.35 MnO 0.02 0.02 MgO 0.20 traces CaO 0.77 0.63 Na2O 0.49 0.84 K2O 2.34 3.00 P2O5 0.07 0.07 H2O+ 3.13 2.73 CO2 0.02 0.07 SiO2 0.03 0.02 H2O- 0.08 0.08 Total 100.12 99.75

Table6: Chemical composition of ignimbritic rhyolite from the Kubo quarry (Skoček, 1965)

Figure 13. Geological sketch map of the Altenberg–Teplice caldera in plan view and cross–section. Note the position of the Kubo quarry.

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13. Třebenice (Museum of Bohemian Garnet)

Disused church 6 km SW of Lovosice The so-called Bohemian garnets (gemstone quality pyropes) have been collected and subsequently mined in the Třebenice area at southern slopes of České středohoří Mts. since ancient times; they were valued especially in 14th century, during the reign of the king Charles IV, because of their dark red colour with a slight violet hue (caused by elevated Cr contents).

Pyropes come from garnet lherzolites that form lenses in granulites of the local crystalline basement. The basement is overlain by Upper Carboniferous (in places) and Upper Cretaceous sediments. The garnets were brought up in xenoliths enclosed within Tertiary explosive breccias (or diatremes). In the region are known several such diatremes, of which that on Linhorka Hill, 14 km W of Třebenice, is the best known (Fig. 14).

However, pyropes cannot be — at economically viable cost — recovered from the volcanogenic breccias. Instead, secondary deposits within Quaternary deluvial gravels are utilized. These gravel beds, representing Quaternary landslides (Fig. 15), have thickness ranging between 3–6 m and several dm (Bouška, 1980).

Garnets occur together with an interesting association of minerals, e.g., zircon, corundum, spinel, topaz, beryl, olivine, magnetite, ilmenite, amphibole and augite.

In the Třebenice museum, devoted to the Bohemian garnets, is shown the history of mining as well as

geological mode of their occurrence. On exhibit should be the famous Empire garnet set of Ulrike von Levetzow (necklace, brooch, earrings, ring and bracelets), the last lover of Johann Wolfgang von Goethe.

Shown is also the biggest known cut pyrope, with dimensions 12.3 × 8.6 mm. It is indeed of exceptional size, keeping in mind that nowadays grains more than c. 5 mm across are rather rare.

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0 m

S N

200

400

600

Figure 14. Geological cross section through Linhorka pyrope-bearing diatreme (after Kopecký, 1973). 1 – Tertiary diatreme with peridotite fragments, 2 – Upper Cretaceous limestones and sandstones, 3– granulite with lenses of pyroxene-bearing garnet peridotites, 4 – boreholes

Figure 15. Geological sketch showing the aerial extent of pyrope-bearing gravels in environs of Třebenice (Bouška, 1980)

Figure 16. Common cuts used for “Bohemian garnets” (a) brilliant (b) rosette

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